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# 7 Developing Web Applications

This chapter describes how web applications are supported in the Oracle GlassFish Server.

The following topics are addressed here:

- Using Servlets

- Using JavaServer Pages

- Creating and Managing Sessions

- Using Comet

- Advanced Web Application Features

For general information about web applications, see "The Web Tier" in The Java EE 6 Tutorial.

Note:

The Web Profile of the GlassFish Server supports the EJB 3.1 Lite specification, which allows enterprise beans within web applications, among other features. The full GlassFish Server supports the entire EJB 3.1 specification. For details, see JSR 318.

## Using Servlets

GlassFish Server supports the Java Servlet Specification version 3.0.

Note:

Servlet API version 3.0 is fully backward compatible with versions 2.3, 2.4, and 2.5, so all existing servlets should work without modification or recompilation.

To develop servlets, use the Java Servlet API. For information about using the Java Servlet API, see the documentation at `http://www.oracle.com/technetwork/java/javaee/servlet/index.html`.

The GlassFish Server provides the `wscompile` and `wsdeploy` tools to help you implement a web service endpoint as a servlet. For more information about these tools, see the Oracle GlassFish Server Reference Manual.

This section describes how to create effective servlets to control application interactions running on a GlassFish Server, including standard-based servlets. In addition, this section describes the GlassFish Server features to use to augment the standards.

The following topics are addressed here:

- Caching Servlet Results

- About the Servlet Engine

### Caching Servlet Results

The GlassFish Server can cache the results of invoking a servlet, a JSP, or any URL pattern to make subsequent invocations of the same servlet, JSP, or URL pattern faster. The GlassFish Server caches the request results for a specific amount of time. In this way, if another data call occurs, the GlassFish Server can return the cached data instead of performing the operation again. For example, if your servlet returns a stock quote that updates every 5 minutes, you set the cache to expire after 300 seconds.

Whether to cache results and how to cache them depends on the data involved. For example, it makes no sense to cache the results of a quiz submission, because the input to the servlet is different each time. However, it makes sense to cache a high level report showing demographic data taken from quiz results that is updated once an hour.

To define how a GlassFish Server web application handles response caching, you edit specific fields in the `glassfish-web.xml` file.

Note:

A servlet that uses caching is not portable.

For Javadoc tool pages relevant to caching servlet results, go to `http://glassfish.java.net/nonav/docs/v3/api/` and click on the `com.sun.appserv.web.cache` package.

For information about JSP caching, see JSP Caching.

The following topics are addressed here:

- Caching Features

- Default Cache Configuration

- Caching Example

- The CacheKeyGenerator Interface

#### Caching Features

The GlassFish Server has the following web application response caching capabilities:

- Caching is configurable based on the servlet name or the URI.

- When caching is based on the URI, this includes user specified parameters in the query string. For example, a response from `/garden/catalog?category=roses` is different from a response from `/garden/catalog?category=lilies`. These responses are stored under different keys in the cache.

- Cache size, entry timeout, and other caching behaviors are configurable.

- Entry timeout is measured from the time an entry is created or refreshed. To override this timeout for an individual cache mapping, specify the `cache-mapping` subelement `timeout`.

- To determine caching criteria programmatically, write a class that implements the com.sun.appserv.web.cache.CacheHelper interface. For example, if only a servlet knows when a back end data source was last modified, you can write a helper class to retrieve the last modified timestamp from the data source and decide whether to cache the response based on that timestamp.

- To determine cache key generation programmatically, write a class that implements the com.sun.appserv.web.cache.CacheKeyGenerator interface. See The CacheKeyGenerator Interface.

- All non-ASCII request parameter values specified in cache key elements must be URL encoded. The caching subsystem attempts to match the raw parameter values in the request query string.

- Since newly updated classes impact what gets cached, the web container clears the cache during dynamic deployment or reloading of classes.

- The following `HttpServletRequest` request attributes are exposed.

  - `com.sun.appserv.web.cachedServletName`, the cached servlet target

  - `com.sun.appserv.web.cachedURLPattern`, the URL pattern being cached

- Results produced by resources that are the target of a `RequestDispatcher.include()` or `RequestDispatcher.forward()` call are cached if caching has been enabled for those resources. For details, see "cache-mapping" in Oracle GlassFish Server Application Deployment Guide and "dispatcher" in Oracle GlassFish Server Application Deployment Guide. These are elements in the `glassfish-web.xml` file.

#### Default Cache Configuration

If you enable caching but do not provide any special configuration for a servlet or JSP, the default cache configuration is as follows:

- The default cache timeout is 30 seconds.

- Only the HTTP GET method is eligible for caching.

- HTTP requests with cookies or sessions automatically disable caching.

- No special consideration is given to `Pragma:`, `Cache-control:`, or `Vary:` headers.

- The default key consists of the Servlet Path (minus `pathInfo` and the query string).

- A "least recently used" list is maintained to evict cache entries if the maximum cache size is exceeded.

- Key generation concatenates the servlet path with key field values, if any are specified.

- Results produced by resources that are the target of a `RequestDispatcher.include()` or `RequestDispatcher.forward()` call are never cached.

#### Caching Example

Here is an example cache element in the `glassfish-web.xml` file:

    <cache max-capacity="8192" timeout="60">
    <cache-helper name="myHelper" class-name="MyCacheHelper"/>
    <cache-mapping>
        <servlet-name>myservlet</servlet-name>
        <timeout name="timefield">120</timeout>
        <http-method>GET</http-method>
        <http-method>POST</http-method>
    </cache-mapping>
    <cache-mapping>
        <url-pattern> /catalog/* </url-pattern>
        <!-- cache the best selling category; cache the responses to
           -- this resource only when the given parameters exist. Cache
           -- only when the catalog parameter has 'lilies' or 'roses'
           -- but no other catalog varieties:
          -- /orchard/catalog?best&category='lilies'
          -- /orchard/catalog?best&category='roses'
          -- but not the result of
           -- /orchard/catalog?best&category='wild'
        -->
        <constraint-field name='best' scope='request.parameter'/>
        <constraint-field name='category' scope='request.parameter'>
            <value> roses </value>
            <value> lilies </value>
        </constraint-field>
         <!-- Specify that a particular field is of given range but the
           -- field doesn't need to be present in all the requests -->
        <constraint-field name='SKUnum' scope='request.parameter'>
            <value match-expr='in-range'> 1000 - 2000 </value>
        </constraint-field>
        <!-- cache when the category matches with any value other than
           -- a specific value -->
        <constraint-field name="category" scope="request.parameter>
            <value match-expr="equals" cache-on-match-failure="true">
           bogus
            </value>
        </constraint-field>
    </cache-mapping>
    <cache-mapping>
        <servlet-name> InfoServlet </servlet-name>
        <cache-helper-ref>myHelper</cache-helper-ref>
    </cache-mapping>
    </cache>

For more information about the `glassfish-web.xml` caching settings, see "cache" in Oracle GlassFish Server Application Deployment Guide.

#### The CacheKeyGenerator Interface

The built-in default CacheHelper implementation allows web applications to customize the key generation. An application component (in a servlet or JSP) can set up a custom CacheKeyGenerator implementation as an attribute in the `ServletContext`.

The name of the context attribute is configurable as the `value` of the `cacheKeyGeneratorAttrName` property in the `default-helper` element of the `glassfish-web.xml` deployment descriptor. For more information, see "default-helper" in Oracle GlassFish Server Application Deployment Guide.

### About the Servlet Engine

Servlets exist in and are managed by the servlet engine in the GlassFish Server. The servlet engine is an internal object that handles all servlet meta functions. These functions include instantiation, initialization, destruction, access from other components, and configuration management.

The following topics are addressed here:

- Instantiating and Removing Servlets

- Request Handling

#### Instantiating and Removing Servlets

After the servlet engine instantiates the servlet, the servlet engine calls the servlet's `init` method to perform any necessary initialization. You can override this method to perform an initialization function for the servlet's life, such as initializing a counter.

When a servlet is removed from service, the servlet engine calls the `destroy` method in the servlet so that the servlet can perform any final tasks and deallocate resources. You can override this method to write log messages or clean up any lingering connections that won't be caught in garbage collection.

#### Request Handling

When a request is made, the GlassFish Server hands the incoming data to the servlet engine. The servlet engine processes the request's input data, such as form data, cookies, session information, and URL name-value pairs, into an `HttpServletRequest` request object type.

The servlet engine also creates an `HttpServletResponse` response object type. The engine then passes both as parameters to the servlet's `service` method.

In an HTTP servlet, the default `service` method routes requests to another method based on the HTTP transfer method: `POST`, `GET`, `DELETE`, `HEAD`, `OPTIONS`, `PUT`, or `TRACE`. For example, HTTP `POST` requests are sent to the `doPost` method, HTTP `GET` requests are sent to the `doGet` method, and so on. This enables the servlet to process request data differently, depending on which transfer method is used. Since the routing takes place in the service method, you generally do not override `service` in an HTTP servlet. Instead, override `doGet`, `doPost`, and so on, depending on the request type you expect.

To perform the tasks to answer a request, override the `service` method for generic servlets, and the `doGet` or `doPost` methods for HTTP servlets. Very often, this means accessing EJB components to perform business transactions, then collating the information in the request object or in a JDBC `ResultSet` object.

## Using JavaServer Pages

The GlassFish Server supports the following JSP features:

- JavaServer Pages (JSP) Specification

- Precompilation of JSP files, which is especially useful for production servers

- JSP tag libraries and standard portable tags

For information about creating JSP files, see the JavaServer Pages web site at `http://www.oracle.com/technetwork/java/javaee/jsp/index.html`.

For information about Java Beans, see the JavaBeans web page at `http://www.oracle.com/technetwork/java/javase/tech/index-jsp-138795.html`.

This section describes how to use JavaServer Pages (JSP files) as page templates in a GlassFish Server web application.

The following topics are addressed here:

- JSP Tag Libraries and Standard Portable Tags

- JSP Caching

- Options for Compiling JSP Files

### JSP Tag Libraries and Standard Portable Tags

GlassFish Server supports tag libraries and standard portable tags. For more information, see the JavaServer Pages Standard Tag Library (JSTL) page at `http://www.oracle.com/technetwork/java/index-jsp-135995.html`.

Web applications don't need to bundle copies of the `jsf-impl.jar` or `appserv-jstl.jar` JSP tag libraries (in as-install`/lib`) to use JavaServer Faces technology or JSTL, respectively. These tag libraries are automatically available to all web applications.

However, the as-install`/lib/jspcachtags.jar` tag library for JSP caching is not automatically available to web applications. See JSP Caching, next.

### JSP Caching

JSP caching lets you cache tag invocation results within the Java engine. Each can be cached using different cache criteria. For example, suppose you have invocations to view stock quotes, weather information, and so on. The stock quote result can be cached for 10 minutes, the weather report result for 30 minutes, and so on.

The following topics are addressed here:

- Enabling JSP Caching

- Caching Scope

- The `cache` Tag

- The `flush` Tag

For more information about response caching as it pertains to servlets, see Caching Servlet Results.

#### Enabling JSP Caching

To globally enable JSP caching, set the `jspCachingEnabled` property to `true`. The default is `false`. For example:

    asadmin set server-config.web-container.property.jspCachingEnabled="true"

For more information about the `asadmin set` command, see the Oracle GlassFish Server Reference Manual.

To enable JSP caching for a single web application, follow these steps:

1. Extract the `META-INF/jspcachtags.tld` file from the as-install`/modules/web-glue.jar` file.

2. Create a new JAR file (for example, `jspcachtags.jar`) containing just the `META-INF/jspcachtags.tld` file previously extracted.

3. Bundle this new JAR file in the `WEB-INF/lib` directory of your web application.

Note:

Web applications that use JSP caching without bundling the tag library are not portable.

Refer to GlassFish Server tags in JSP files as follows:

    <%@ taglib prefix="prefix" uri="http://glassfish.org/taglibs/cache" %>

Subsequently, the cache tags are available as `<`prefix`:cache>` and `<`prefix`:flush>`. For example, if your prefix is `mypfx`, the cache tags are available as `<mypfx:cache>` and `<mypfx:flush>`.

#### Caching Scope

JSP caching is available in three different scopes: `request`, `session`, and `application`. The default is `application`. To use a cache in `request` scope, a web application must specify the `com.sun.appserv.web.taglibs.cache.CacheRequestListener` in its `web.xml` deployment descriptor, as follows:

    <listener>
       <listener-class>
          com.sun.appserv.web.taglibs.cache.CacheRequestListener
       </listener-class>
    </listener>

Likewise, for a web application to utilize a cache in `session` scope, it must specify the `com.sun.appserv.web.taglibs.cache.CacheSessionListener` in its `web.xml` deployment descriptor, as follows:

    <listener>
       <listener-class>
          com.sun.appserv.web.taglibs.cache.CacheSessionListener
       </listener-class>
    </listener>

To utilize a cache in `application` scope, a web application need not specify any listener. The `com.sun.appserv.web.taglibs.cache.CacheContextListener` is already specified in the `jspcachtags.tld` file.

#### The `cache` Tag

The cache tag caches the body between the beginning and ending tags according to the attributes specified. The first time the tag is encountered, the body content is executed and cached. Each subsequent time it is run, the cached content is checked to see if it needs to be refreshed and if so, it is executed again, and the cached data is refreshed. Otherwise, the cached data is served.

##### Attributes of `cache`

The following table describes attributes for the `cache` tag.

Table 7-1 The `cache` Attributes

Attribute | Default | Description
--- | --- | ---
`key` | ServletPath`_`Suffix | (optional) The name used by the container to access the cached entry. The cache key is suffixed to the servlet path to generate a key to access the cached entry. If no key is specified, a number is generated according to the position of the tag in the page.
`timeout` | `60s` | (optional) The time in seconds after which the body of the tag is executed and the cache is refreshed. By default, this value is interpreted in seconds. To specify a different unit of time, add a suffix to the timeout value as follows: `s` for seconds, `m` for minutes, `h` for hours, `d` for days. For example, `2h` specifies two hours.
`nocache` | `false` | (optional) If set to `true`, the body content is executed and served as if there were no `cache` tag. This offers a way to programmatically decide whether the cached response is sent or whether the body has to be executed, though the response is not cached.
`refresh` | `false` | (optional) If set to `true`, the body content is executed and the response is cached again. This lets you programmatically refresh the cache immediately regardless of the `timeout` setting.
`scope` | `application` | (optional) The scope of the cache. Can be `request`, `session`, or `application`. See Caching Scope.

##### Example of `cache`

The following example represents a cached JSP file:

    <%@ taglib prefix="mypfx" uri="http://glassfish.org/taglibs/cache" %>
    <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
    <mypfx:cache                 key="${sessionScope.loginId}"
                nocache="${param.nocache}"
                refresh="${param.refresh}"
                timeout="10m">
    <c:choose>
        <c:when test="${param.page == 'frontPage'}">
            <%-- get headlines from database --%>
        </c:when>
        <c:otherwise>
            ...
        </c:otherwise>
    </c:choose>
    </mypfx:cache>
    <mypfx:cache timeout="1h">
    <h2> Local News </h2>
        <%-- get the headline news and cache them --%>
    </mypfx:cache>

#### The `flush` Tag

Forces the cache to be flushed. If a `key` is specified, only the entry with that key is flushed. If no key is specified, the entire cache is flushed.

##### Attributes of `flush`

The following table describes attributes for the `flush` tag.

Table 7-2 The `flush` Attributes

Attribute | Default | Description
--- | --- | ---
`key` | ServletPath`_`Suffix | (optional) The name used by the container to access the cached entry. The cache key is suffixed to the servlet path to generate a key to access the cached entry. If no key is specified, a number is generated according to the position of the tag in the page.
`scope` | `application` | (optional) The scope of the cache. Can be `request`, `session`, or `application`. See Caching Scope.

##### Examples of `flush`

To flush the entry with `key="foobar"`:

    <mypfx:flush key="foobar"/>

To flush the entire cache:

    <c:if test="${empty sessionScope.clearCache}">
       <mypfx:flush />
    </c:if>

### Options for Compiling JSP Files

GlassFish Server provides the following ways of compiling JSP source files into servlets:

- JSP files are automatically compiled at runtime.

- The `asadmin deploy` command has a `--precompilejsp` option. For details, see the Oracle GlassFish Server Reference Manual.

- The `jspc` command line tool allows you to precompile JSP files at the command line. For details, see the Oracle GlassFish Server Reference Manual.

## Creating and Managing Sessions

This section describes how to create and manage HTTP sessions that allows users and transaction information to persist between interactions.

The following topics are addressed here:

- Configuring Sessions

- Session Managers

### Configuring Sessions

The following topics are addressed here:

- HTTP Sessions, Cookies, and URL Rewriting

- Coordinating Session Access

- Saving Sessions During Redeployment

- Logging Session Attributes

- Distributed Sessions and Persistence

#### HTTP Sessions, Cookies, and URL Rewriting

To configure whether and how HTTP sessions use cookies and URL rewriting, edit the `session-properties` and `cookie-properties` elements in the `glassfish-web.xml` file for an individual web application. For more about the properties you can configure, see "session-properties" in Oracle GlassFish Server Application Deployment Guide and "cookie-properties" in Oracle GlassFish Server Application Deployment Guide.

For information about configuring default session properties for the entire web container, see Using the `default-web.xml` File and the Oracle GlassFish Server High Availability Administration Guide.

#### Coordinating Session Access

Make sure that multiple threads don't simultaneously modify the same session object in conflicting ways. If the persistence type is `replicated` (see The `replicated` Persistence Type), the following message in the log file indicates that this might be happening:

    Primary Key Constraint violation while saving session session_id

This is especially likely to occur in web applications that use HTML frames where multiple servlets are executing simultaneously on behalf of the same client. A good solution is to ensure that one of the servlets modifies the session and the others have read-only access.

#### Saving Sessions During Redeployment

Whenever a redeployment is done, the sessions at that transit time become invalid unless you use the `--keepstate=true` option of the `asadmin redeploy` command. For example:

    asadmin redeploy --keepstate=true --name hello.war

For details, see the Oracle GlassFish Server Reference Manual.

The default for `--keepstate` is false. This option is supported only on the default server instance, named `server`. It is not supported and ignored for any other target.

For web applications, this feature is applicable only if in the `glassfish-web-app.xml` file the `persistence-type` attribute of the `session-manager` element is `file`.

If any active web session fails to be preserved or restored, none of the sessions will be available when the redeployment is complete. However, the redeployment continues and a warning is logged.

The new class loader of the redeployed application is used to deserialize any sessions previously saved. The usual restrictions about serialization and deserialization apply. For example, any application-specific class referenced by a session attribute may evolve only in a backward-compatible fashion. For more information about class loaders, see Class Loaders.

#### Logging Session Attributes

You can write session attribute values to an access log. The access log format token `%session.`name`%` logs one of the following:

- The value of the session attribute with the name name

- `NULL-SESSION-ATTRIBUTE-`name if the named attribute does not exist in the session

- `NULL-SESSION` if no session exists

For more information about access logging and format tokens, see online help for the Access Log tab of the HTTP Service page in the Administration Console.

#### Distributed Sessions and Persistence

A distributed HTTP session can run in multiple GlassFish Server instances, provided the following criteria are met:

- Each server instance has the same distributable web application deployed to it. The `web-app` element of the `web.xml` deployment descriptor file must have the `distributable` subelement specified.

- The web application uses high-availability session persistence. If a non-distributable web application is configured to use high-availability session persistence, a warning is written to the server log, and the session persistence type reverts to `memory`. See The `replicated` Persistence Type.

- All objects bound into a distributed session must be of the types listed in Table 7-3.

- The web application must be deployed using the `deploy` or `deploydir` command with the `--availabilityenabled` option set to `true`. See the Oracle GlassFish Server Reference Manual.

Note:

Contrary to the Servlet 3.0 specification, GlassFish Server does not throw an `IllegalArgumentException` if an object type not supported for failover is bound into a distributed session.

Keep the distributed session size as small as possible. Session size has a direct impact on overall system throughput.

In the event of an instance or hardware failure, another server instance can take over a distributed session, with the following limitations:

- If a distributable web application references a Java EE component or resource, the reference might be lost. See Table 7-3 for a list of the types of references that `HTTPSession` failover supports.

- References to open files or network connections are lost.

For information about how to work around these limitations, see the Oracle GlassFish Server Deployment Planning Guide.

In the following table, No indicates that failover for the object type might not work in all cases and that no failover support is provided. However, failover might work in some cases for that object type. For example, failover might work because the class implementing that type is serializable.

For more information about the `InitialContext`, see Accessing the Naming Context. For more information about transaction recovery, see Using the Transaction Service. For more information about Administered Objects, see "Administering JMS Physical Destinations" in Oracle GlassFish Server Administration Guide.

Table 7-3 Object Types Supported for Java EE Web Application Session State Failover

Java Object Type | Failover Support
--- | ---
Colocated or distributed stateless session, stateful session, or entity bean reference | Yes
JNDI context | Yes, `InitialContext` and `java:comp/env`
UserTransaction | Yes, but if the instance that fails is never restarted, any prepared global transactions are lost and might not be correctly rolled back or committed.
JDBC DataSource | No
Java Message Service (JMS) ConnectionFactory, Destination | No
JavaMail Session | No
Connection Factory | No
Administered Object | No
Web service reference | No
Serializable Java types | Yes
Extended persistence context | No

### Session Managers

A session manager automatically creates new session objects whenever a new session starts. In some circumstances, clients do not join the session, for example, if the session manager uses cookies and the client does not accept cookies.

GlassFish Server offers these session management options, determined by the `session-manager` element's `persistence-type` attribute in the `glassfish-web.xml` file:

- The `memory` Persistence Type, the default

- The `file` Persistence Type, which uses a file to store session data

- The `replicated` Persistence Type, which uses other servers in the cluster for session persistence

- The `coherence-web` Persistence Type, which uses Coherence*Web for session persistence

Note:

If the session manager configuration contains an error, the error is written to the server log and the default (`memory`) configuration is used.

For more information, see "session-manager" in Oracle GlassFish Server Application Deployment Guide.

#### The `memory` Persistence Type

This persistence type is not designed for a production environment that requires session persistence. It provides no session persistence. However, you can configure it so that the session state in memory is written to the file system prior to server shutdown.

To specify the `memory` persistence type for a specific web application, edit the `glassfish-web.xml` file as in the following example. The `persistence-type` attribute is optional, but must be set to `memory` if included. This overrides the web container availability settings for the web application.

    <glassfish-web-app>
    ...

    <session-config>
        <session-manager persistence-type="memory" />
            <manager-properties>
                <property name="sessionFilename" value="sessionstate" />
            </manager-properties>
        </session-manager>
        ...
    </session-config>
    ...
    </glassfish-web-app>

The only manager property that the `memory` persistence type supports is `sessionFilename`, which is listed under "manager-properties" in Oracle GlassFish Server Application Deployment Guide. The `sessionFilename` property specifies the name of the file where sessions are serialized and persisted if the web application or the server is stopped. To disable this behavior, specify an empty string as the value of `sessionFilename`. The default value is an empty string.

For more information about the `glassfish-web.xml` file, see the Oracle GlassFish Server Application Deployment Guide.

#### The `file` Persistence Type

This persistence type provides session persistence to the local file system, and allows a single server domain to recover the session state after a failure and restart. The session state is persisted in the background, and the rate at which this occurs is configurable. The store also provides passivation and activation of the session state to help control the amount of memory used. This option is not supported in a production environment. However, it is useful for a development system with a single server instance.

Note:

Make sure the `delete` option is set in the `server.policy` file, or expired file-based sessions might not be deleted properly. For more information about `server.policy`, see The `server.policy` File.

To specify the `file` persistence type for a specific web application, edit the `glassfish-web.xml` file as in the following example. Note that `persistence-type` must be set to `file`. This overrides the web container availability settings for the web application.

    <glassfish-web-app>
    ...
    <session-config>
        <session-manager persistence-type="file">
            <store-properties>
                <property name="directory" value="sessiondir" />
            </store-properties>
        </session-manager>
        ...
    </session-config>
    ...
    </glassfish-web-app>

The `file` persistence type supports all the manager properties listed under "manager-properties" in Oracle GlassFish Server Application Deployment Guide except `sessionFilename`, and supports the `directory` store property listed under "store-properties" in Oracle GlassFish Server Application Deployment Guide.

For more information about the `glassfish-web.xml` file, see the Oracle GlassFish Server Application Deployment Guide.

#### The `replicated` Persistence Type

The replicated persistence type uses other servers in the cluster for session persistence. Clustered server instances replicate session state. Each backup instance stores the replicated data in memory. This allows sessions to be distributed. For details, see Distributed Sessions and Persistence. In addition, you can configure the frequency and scope of session persistence. The other servers are also used as the passivation and activation store. Use this option in a production environment that requires session persistence.

To use the replicated persistence type, you must enable availability. Select the Availability Service component under the relevant configuration in the Administration Console. Check the Availability Service box. To enable availability for the web container, select the Web Container Availability tab, then check the Availability Service box. All instances in an GlassFish Server cluster should have the same availability settings to ensure consistent behavior. For details, see the Oracle GlassFish Server High Availability Administration Guide.

To change settings such as persistence frequency and persistence scope for the entire web container, use the Persistence Frequency and Persistence Scope drop-down lists on the Web Container Availability tab in the Administration Console, or use the `asadmin set` command. For example:

    asadmin set
    server-config.availability-service.web-container-availability.persistence-frequency=time-based

For more information, see the description of the `asadmin set` command in the Oracle GlassFish Server Reference Manual.

To specify the `replicated` persistence type for a specific web application, edit the `glassfish-web.xml` file as in the following example. Note that `persistence-type` must be set to `replicated`. This overrides the web container availability settings for the web application.

    <glassfish-web-app>
    ...
    <session-config>
        <session-manager persistence-type="replicated">
            <manager-properties>
                <property name="persistenceFrequency" value="web-method" />
            </manager-properties>
            <store-properties>
                <property name="persistenceScope" value="session" />
            </store-properties>
        </session-manager>
        ...
    </session-config>
    ...
    </glassfish-web-app>

The `replicated` persistence type supports all the manager properties listed under "manager-properties" in Oracle GlassFish Server Application Deployment Guide except `sessionFilename`, and supports the `persistenceScope` store property listed under "store-properties" in Oracle GlassFish Server Application Deployment Guide.

For more information about the `glassfish-web.xml` file, see the Oracle GlassFish Server Application Deployment Guide.

To specify that web sessions for which high availability is enabled are first buffered and then replicated using a separate asynchronous thread, use the `--asyncreplication=true` option of the `asadmin deploy` command. For example:

    asadmin deploy --availabilityenabled=true --asyncreplication=true --name hello.war

If `--asyncreplication` is set to true (the default), performance is improved but availability is reduced. If the instance where states are buffered but not yet replicated fails, the states are lost. If set to false, performance is reduced but availability is guaranteed. States are not buffered but immediately transmitted to other instances in the cluster.

#### The `coherence-web` Persistence Type

Built on top of Oracle Coherence, Coherence*Web is an HTTP session management module dedicated to managing session state in clustered environments. Starting with Coherence 3.7 and GlassFish Server 3.1.2, there is a new feature of Coherence*Web called ActiveCache for GlassFish. ActiveCache for GlassFish provides Coherence*Web functionality in web applications deployed on GlassFish Servers. Within GlassFish Server, Coherence*Web functions as an additional web container persistence type, named `coherence-web`.

For information about how to configure and deploy Coherence*Web on GlassFish Server, see Using Coherence*Web with GlassFish Server.

## Using Comet

This section explains the Comet programming technique and how to create and deploy a Comet-enabled application with the Oracle GlassFish Server.

The following topics are addressed here:

- Introduction to Comet

- Grizzly Comet

- Bayeux Protocol

### Introduction to Comet

Comet is a programming technique that allows a web server to send updates to clients without requiring the clients to explicitly request them.

This kind of programming technique is called server push, which means that the server pushes data to the client. The opposite style is client pull, which means that the client must pull the data from the server, usually through a user-initiated event, such as a button click.

Web applications that use the Comet technique can deliver updates to clients in a more timely manner than those that use the client-pull style while avoiding the latency that results from clients frequently polling the server.

One of the many use cases for Comet is a chat room application. When the server receives a message from one of the chat clients, it needs to send the message to the other clients without requiring them to ask for it. With Comet, the server can deliver messages to the clients as they are posted rather than expecting the clients to poll the server for new messages.

To accomplish this scenario, a Comet application establishes a long-lived HTTP connection. This connection is suspended on the server side, waiting for an event to happen before resuming. This kind of connection remains open, allowing an application that uses the Comet technique to send updates to clients when they are available rather than expecting clients to reopen the connection to poll the server for updates.

#### The Grizzly Implementation of Comet

A limitation of the Comet technique is that you must use it with a web server that supports non-blocking connections to avoid poor performance. Non-blocking connections are those that do not need to allocate one thread for each request. If the web server were to use blocking connections then it might end up holding many thousands of threads, thereby hindering its scalability.

The GlassFish server includes the Grizzly HTTP Engine, which enables asynchronous request processing (ARP) by avoiding blocking connections. Grizzly's ARP implementation accomplishes this by using the Java NIO API.

With Java NIO, Grizzly enables greater performance and scalability by avoiding the limitations experienced by traditional web servers that must run a thread for each request. Instead, Grizzly's ARP mechanism makes efficient use of a thread pool system and also keeps the state of requests so that it can keep requests alive without holding a single thread for each of them.

Grizzly supports two different implementations of Comet:

- Grizzly Comet — Based on ARP, this includes a set of APIs that you use from a web component to enable Comet functionality in your web application. Grizzly Comet is specific to the Oracle GlassFish Server.

- Bayeux Protocol — Often referred to as `Cometd`, it consists of the JSON-based Bayeux message protocol, a set of Dojo or Ajax libraries, and an event handler. The Bayeux protocol uses a publish/subscribe model for server/client communication. The Bayeux protocol is portable, but it is container dependent if you want to invoke it from an Enterprise Java Beans (EJB ) component. The Grizzly implementation of `Cometd` consists of a servlet that you reference from your web application.

#### Client Technologies to Use With Comet

In addition to creating a web component that uses the Comet APIs, you need to enable your client to accept asynchronous updates from the web component. To accomplish this, you can use JavaScript, IFrames, or a framework, such as Dojo.

An IFrame is an HTML element that allows you to include other content in an HTML page. As a result, the client can embed updated content in the IFrame without having to reload the page.

The example in this tutorial employs a combination of JavaScript and IFrames to allow the client to accept asynchronous updates. A servlet included in the example writes out JavaScript code to one of the IFrames. The JavaScript code contains the updated content and invokes a function in the page that updates the appropriate elements in the page with the new content.

The next section explains the two kinds of connections that you can make to the server. While you can use any of the client technologies listed in this section with either kind of connection, it is more difficult to use JavaScript with an HTTP-streaming connection.

#### Types of Comet Connections

When working with Comet, as implemented in Grizzly, you have two different ways to handle client connections to the server:

- HTTP Streaming

- Long Polling

##### HTTP Streaming

The HTTP Streaming technique keeps a connection open indefinitely. It never closes, even after the server pushes data to the client.

In the case of HTTP streaming, the application sends a single request and receives responses as they come, reusing the same connection forever. This technique significantly reduces the network latency because the client and the server don't need to open and close the connection.

The basic life cycle of an application using HTTP-streaming is:

request > suspend > data available > write response > data available > write response

The client makes an initial request and then suspends the request, meaning that it waits for a response. Whenever data is available, the server writes it to the response.

##### Long Polling

The long-polling technique is a combination of server-push and client-pull because the client needs to resume the connection after a certain amount of time or after the server pushes an update to the client.

The basic life cycle of an application using long-polling is:

request > suspend > data available > write response > resume

The client makes an initial request and then suspends the request. When an update is available, the server writes it to the response. The connection closes, and the client optionally resumes the connection.

##### How to Choose the Type of Connection

If you anticipate that your web application will need to send frequent updates to the client, you should use the HTTP-streaming connection so that the client does not have to frequently reestablish a connection. If you anticipate less frequent updates, you should use the long-polling connection so that the web server does not need to keep a connection open when no updates are occurring. One caveat to using the HTTP-streaming connection is that if you are streaming through a proxy, the proxy can buffer the response from the server. So, be sure to test your application if you plan to use HTTP-streaming behind a proxy.

### Grizzly Comet

The following topics are addressed here:

- The Grizzly Comet API

- The Hidden Frame Example

- Creating a Comet-Enabled Application

- Developing the Web Component

- Creating the Client Pages

- Creating the Deployment Descriptor

- Deploying and Running a Comet-Enabled Application

#### The Grizzly Comet API

Grizzly's support for Comet includes a small set of APIs that make it easy to add Comet functionality to your web applications. The Grizzly Comet APIs that developers use most often are the following:

- `CometContext`: A Comet context, which is a shareable space to which applications subscribe to receive updates.

- `CometEngine`: The entry point to any component using Comet. Components can be servlets, JavaServer Pages ( JSP), JavaServer Faces components, or pure Java classes.

- `CometEvent`: Contains the state of the `CometContext` object

- `CometHandler`: The interface an application implements to be part of one or more Comet contexts.

The way a developer would use this API in a web component is to perform the following tasks:

1. Register the context path of the application with the `CometContext` object:

       CometEngine cometEngine =
           CometEngine.getEngine();
       CometContext cometContext =
           cometEngine.register(contextPath)

2. Register the CometHandler implementation with the `CometContext` object:

       cometContext.addCometHandler(handler)

3. Notify one or more CometHandler implementations when an event happens:

       cometContext.notify((Object)(handler))

#### The Hidden Frame Example

This rest of this tutorial uses the Hidden Frame example to explain how to develop Comet-enabled web applications. You can download the example from `grizzly.dev.java.net` at Hidden example download. From there, you can download a prebuilt WAR file as well as a JAR file containing the servlet code.

The Hidden Frame example is so called because it uses hidden IFrames. The example allows multiple clients to increment a counter on the server. When a client increments the counter, the server broadcasts the new count to the clients using the Comet technique.

The Hidden Frame example uses the long-polling technique, but you can easily modify it to use HTTP-streaming by removing two lines. See To Notify the Comet Handler of an Event and To Create a HTML Page That Updates and Displays the Content for more information on converting the example to use the HTTP-streaming technique.

The client side of the example uses hidden IFrames with embedded JavaScript tags to connect to the server and to asynchronously post content to and accept updates from the server.

The server side of the example consists of a single servlet that listens for updates from clients, updates the counter, and writes JavaScript code to the client that allows it to update the counter on its page.

See Deploying and Running a Comet-Enabled Application for instructions on how to deploy and run the example.

When you run the example, the following happens:

1. The `index.html` page opens.

2. The browser loads three frames: The first one accesses the servlet using an HTTP GET; the second one loads the `count.html` page, which displays the current count; and the third one loads the `button.html` page, which is used to send the POST request.

3. After clicking the button on the `button.html` page, the page submits a POST request to the servlet.

4. The `doPost` method calls the `onEvent` method of the Comet handler and redirects the incremented count along with some JavaScript to the `count.html` page on the client.

5. The `updateCount()` JavaScript function on the `count.html` page updates the counter on the page.

6. Because this example uses long-polling, the JavaScript code on `count.html` calls `doGet` again to resume the connection after the servlet pushes the update.

#### Creating a Comet-Enabled Application

This section uses the Hidden Frame example application to demonstrate how to develop a Comet application. The main tasks for creating a simple Comet-enabled application are the following:

#### Developing the Web Component

This section shows you how to create a Comet-enabled web component by giving you instructions for creating the servlet in the Hidden Frame example.

Developing the web component involves performing the following steps:

1. Create a web component to support Comet requests.

2. Register the component with the Comet engine.

3. Define a Comet handler that sends updates to the client.

4. Add the Comet handler to the Comet context.

5. Notify the Comet handler of an event using the Comet context.

##### To Create a Web Component to Support Comet

Follow this procedure.

1. Create an empty servlet class, like the following:

       import javax.servlet.*;

       public class HiddenCometServlet extends HttpServlet {
           private static final long serialVersionUID = 1L;
       private String contextPath = null;
           @Override
           public void init(ServletConfig config) throws ServletException {}

           @Override
           protected void doGet(HttpServletRequest req,
           HttpServletResponse res)
           throws ServletException, IOException {}

           @Override
           protected void doPost(HttpServletRequest req,
           HttpServletResponse res)
           throws ServletException, IOException {);
       }

2. Import the following Comet packages into the servlet class:

       import com.sun.grizzly.comet.CometContext;
       import com.sun.grizzly.comet.CometEngine;
       import com.sun.grizzly.comet.CometEvent;
       import com.sun.grizzly.comet.CometHandler;

3. Import these additional classes that you need for incrementing a counter and writing output to the clients:

       import java.io.IOException;
       import java.io.PrintWriter;
       import java.util.concurrent.atomic.AtomicInteger;

4. Add a private variable for the counter:

       private final AtomicInteger counter = new AtomicInteger();

##### To Register the Servlet With the Comet Engine

Follow this procedure.

1. In the servlet's `init` method, add the following code to get the component's context path:

       ServletContext context = config.getServletContext();
       contextPath = context.getContextPath() + "/hidden_comet";

2. Get an instance of the Comet engine by adding this line after the lines from Step 1:

        CometEngine engine = CometEngine.getEngine();

3. Register the component with the Comet engine by adding the following lines after those from Step 2:

       CometContext cometContext = engine.register(contextPath);
       cometContext.setExpirationDelay(30 * 1000);

##### To Define a Comet Handler to Send Updates to the Client

Follow this procedure.

1. Create a private class that implements CometHandler and add it to the servlet class:

       private class CounterHandler
           implements CometHandler<HttpServletResponse> {
           private HttpServletResponse response;
       }

2. Add the following methods to the class:

       public void onInitialize(CometEvent event)
           throws IOException {}

           public void onInterrupt(CometEvent event)
               throws IOException {
               removeThisFromContext();
           }

           public void onTerminate(CometEvent event)
               throws IOException {
               removeThisFromContext();
           }

           public void attach(HttpServletResponse attachment) {
                   this.response = attachment;
           }

           private void removeThisFromContext() throws IOException {
               response.getWriter().close();
               CometContext context =
                   CometEngine.getEngine().getCometContext(contextPath);
               context.removeCometHandler(this);
           }

   You need to provide implementations of these methods when implementing CometHandler. The `onInterrupt` and `onTerminate` methods execute when certain changes occur in the status of the underlying TCP communication. The `onInterrupt` method executes when communication is resumed. The `onTerminate` method executes when communication is closed. Both methods call `removeThisFromContext`, which removes the CometHandler object from the `CometContext` object.

##### To Add the Comet Handler to the Comet Context

Follow this procedure.

1. Get an instance of the Comet handler and attach the response to it by adding the following lines to the `doGet` method:

       CounterHandler handler = new CounterHandler();
       handler.attach(res);

2. Get the Comet context by adding the following lines to `doGet`:

       CometEngine engine = CometEngine.getEngine();
       CometContext context = engine.getCometContext(contextPath);

3. Add the Comet handler to the Comet context by adding this line to `doGet`:

       context.addCometHandler(handler);

##### To Notify the Comet Handler of an Event

Follow this procedure.

1. Add an `onEvent` method to the CometHandler implementation class to define what happens when an event occurs:

       public void onEvent(CometEvent event)
           throws IOException {
           if (CometEvent.NOTIFY == event.getType()) {
               int count = counter.get();
               PrintWriter writer = response.getWriter();
               writer.write("<script type='text/javascript'>" +
                   "parent.counter.updateCount('" + count + "')" +
                   "</script>\n");
               writer.flush();
               event.getCometContext().resumeCometHandler(this);
           }
       }

   This method first checks if the event type is `NOTIFY`, which means that the web component is notifying the CometHandler object that a client has incremented the count. If the event type is `NOTIFY`, the `onEvent` method gets the updated count, and writes out JavaScript to the client. The JavaScript includes a call to the `updateCount ()` function, which will update the count on the clients' pages.

   The last line resumes the Comet request and removes it from the list of active CometHandler objects. By this line, you can tell that this application uses the long-polling technique. If you were to delete this line, the application would use the HTTP-Streaming technique.

   For HTTP-Streaming, add the same code as for long-polling, except do not include the following line:

       event.getCometContext().resumeCometHandler(this);

   You don't include this line because you do not want to resume the request. Instead, you want the connection to remain open.

2. Increment the counter and forward the response by adding the following lines to the `doPost` method:

       counter.incrementAndGet();
       CometEngine engine = CometEngine.getEngine();
       CometContext<?> context =
           engine.getCometContext(contextPath);
       context.notify(null);
       req.getRequestDispatcher("count.html").forward(req, res);

   When a user clicks the button, the `doPost` method is called. The `doPost` method increments the counter. It then obtains the current `CometContext` object and calls its `notify` method. By calling `context.notify`, the `doPost` method triggers the `onEvent` method you created in the previous step. After `onEvent` executes, `doPost` forwards the response to the clients.

#### Creating the Client Pages

Developing the HTML pages for the client involves performing these steps:

1. Create a welcome HTML page, called `index.html`, that contains: one hidden frame for connecting to the servlet through an HTTP GET; one IFrame that embeds the `count.html` page, which contains the updated content; and one IFrame that embeds the `button.html` page, which is used for posting updates using HTTP POST.

2. Create the `count.html` page that contains an HTML element that displays the current count and the JavaScript for updating the HTML element with the new count.

3. Create the `button.html` page that contains a button for the users to submit updates.

##### To Create a HTML Welcome Page That Contains IFrames for Receiving and Sending Updates

Follow this procedure.

1. Create an HTML page called `index.html`.

2. Add the following content to the page:

       <html>
           <head>
               <title>Comet Example: Counter with Hidden Frame</title>
           </head>
           <body>
         </body>
       </html>

3. Add IFrames for connecting to the server and receiving and sending updates to `index.html` in between the `body` tags:

       <frameset>
           <iframe name="hidden" src="hidden_comet"
               frameborder="0" height="0" width="100%"></iframe>
           <iframe name="counter" src="count.html"
               frameborder="0" height="100%" width="100%"></iframe>
       <iframe name="button" src="button.html" frameborder="0" height="30%" widget="100%"></iframe>
       </frameset>

   The first frame, which is hidden, points to the servlet by referencing its context path. The second frame displays the content from `count.html`, which displays the current count. The second frame displays the content from `button.html`, which contains the submit button for incrementing the counter.

##### To Create a HTML Page That Updates and Displays the Content

Follow this procedure.

1. Create an HTML page called `count.html` and add the following content to it:

       <html>
           <head>
           </head>
               <body>
                   <center>
                       <h3>Comet Example: Counter with Hidden Frame</h3>
                       <p>
                       <b id="count">&nbsp;</b>
                       <p>
                   </center>
               </body>
       </html>

   This page displays the current count.

2. Add JavaScript code that updates the count in the page. Add the following lines in between the `head` tags of `count.html`:

       <script type='text/javascript'>
           function updateCount(c) {
               document.getElementById('count').innerHTML = c;
               parent.hidden.location.href = "hidden_comet";
           };
       </script>

   The JavaScript takes the updated count it receives from the servlet and updates the count element in the page. The last line in the `updateCount ()` function invokes the servlet's `doGet` method again to reestablish the connection.

   For HTTP-Streaming, add the same code as for long-polling, except for the following line:

       parent.hidden.location.href = "hidden_comet"

   This line invokes the `doGet` method of `CometServlet` again, which would reestablish the connection. In the case of HTTP-Streaming, you want the connection to remain open. Therefore, you don't include this line of code.

##### To Create the HTML Page That Allows Submitting Updates

Create an HTML page called `button.html` and add the following content to it:

    <html>
        <head>
        </head>
            <body>
                <center>
                    <form method="post" action="hidden_comet">
                        <input type="submit" value="Click">
                    </form>
                </center>
            </body>
    </html>

This page displays a form with a button that allows a user to update the count on the server. The servlet will then broadcast the updated count to all clients.

#### Creating the Deployment Descriptor

This section describes how to create a deployment descriptor to specify how your Comet-enabled web application should be deployed.

##### To Create the Deployment Descriptor

Create a file called `web.xml` and put the following contents in it:

    <?xml version="1.0" encoding="UTF-8"?>
        <web-app version="3.0"
            xmlns="http://java.sun.com/xml/ns/javaee"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation=
                "http://java.sun.com/xml/ns/javaee
                http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd ">

            <servlet>
                <servlet-name>HiddenCometServlet</servlet-name>
                <servlet-class>
                    com.sun.grizzly.samples.comet.HiddenCometServlet
                </servlet-class>
                <load-on-startup>0</load-on-startup>
            </servlet>
            <servlet-mapping>
                <servlet-name>HiddenCometServlet</servlet-name>
                <url-pattern>/hidden_comet</url-pattern>
            </servlet-mapping>
        </web-app>

This deployment descriptor contains a servlet declaration and mapping for `HiddenCometServlet`. The `load-on-startup` attribute must be set to 0 so that the Comet-enabled servlet will not load until the client makes a request to it.

#### Deploying and Running a Comet-Enabled Application

Before running a Comet-enabled application in the GlassFish Server, you need to enable Comet in the server. Then you can deploy the application just as you would any other web application.

When running the application, you need to connect to it from at least two different browsers to experience the effect of the servlet updating all clients in response to one client posting an update to the server.

##### Enabling Comet in the GlassFish Server

Before running a Comet-enabled application, you need to enable Comet in the HTTP listener for your application by setting a special attribute in the associated protocol configuration. The following example shows the `asadmin set` command that adds this attribute:

    asadmin set server-config.network-config.protocols.protocol.http-1.http.comet-support-enabled="true"

Substitute the name of the protocol for `http-1`.

##### To Deploy the Example

These instructions tell you how to deploy the Hidden Frame example.

1. Download grizzly-comet-hidden-1.7.3.1.war.

2. Run the following command to deploy the example:

       as-install/bin/asadmin deploy grizzly-comet-hidden-1.7.3.1.war

##### To Run the Example

These instructions tell you how to run the Hidden Frame example.

1. Open two web browsers, preferably two different brands of web browser.

2. Enter the following URL in both browsers:

       http://localhost:8080/grizzly-comet-hidden/index.html

3. When the first page loads in both browsers, click the button in one of the browsers and watch the count change in the other browser window.

### Bayeux Protocol

The Bayeux protocol, often referred to as `Cometd`, greatly simplifies the use of Comet. No server-side coding is needed for servers such as GlassFish Server that support the Bayeux protocol. Just enable Comet and the Bayeux protocol, then write and deploy the client.

The following topics are addressed here:

- Enabling Comet

- To Configure the `web.xml` File

- To Write, Deploy, and Run the Client

#### Enabling Comet

Before running a Comet-enabled application, you need to enable Comet in the HTTP listener for your application by setting a special attribute in the associated protocol configuration. The following example shows the `asadmin set` command that adds this attribute:

    asadmin set server-config.network-config.protocols.protocol.http-1.http.comet-support-enabled="true"

Substitute the name of the protocol for `http-1`.

#### To Configure the `web.xml` File

To enable the Bayeux protocol on the GlassFish Server, you must reference the `CometdServlet` in your web application's `web.xml` file. In addition, if your web application includes a servlet, set the `load-on-startup` value for your servlet to `0` (zero) so that it will not load until the client makes a request to it.

1. Open the `web.xml` file for your web application in a text editor.

2. Add the following XML code to the `web.xml` file:

       <servlet>
          <servlet-name>Grizzly Cometd Servlet</servlet-name>
          <servlet-class>
             com.sun.grizzly.cometd.servlet.CometdServlet
          </servlet-class>
          <init-param>
             <description>
                expirationDelay is the long delay before a request is
                resumed. -1 means never.
             </description>
             <param-name>expirationDelay</param-name>
             <param-value>-1</param-value>
          </init-param>
          <load-on-startup>1</load-on-startup>
       </servlet>
       <servlet-mapping>
          <servlet-name>Grizzly Cometd Servlet</servlet-name>
          <url-pattern>/cometd/*</url-pattern>
       </servlet-mapping>

   Note that the `load-on-startup` value for the `CometdServlet` is `1`.

3. If your web application includes a servlet, set the `load-on-startup` value to `0` for your servlet (not the `CometdServlet`) as follows:

       <servlet>
          ...
          <load-on-startup>0</load-on-startup>
       </servlet>

4. Save the `web.xml` file.

#### To Write, Deploy, and Run the Client

1. Add script tags to the HTML page. For example:

       <script type="text/javascript" src="chat.js"></script>

2. In the script, call the needed libraries. For example:

       dojo.require("dojo.io.cometd");

3. In the script, use `publish` and `subscribe` methods to send and receive messages. For example:

       cometd.subscribe("/chat/demo", false, room, "_chat");
       cometd.publish("/chat/demo", { user: room._username, chat: text});

4. Deploy the web application as you would any other web application. For example:

       asadmin deploy cometd-example.war

5. Run the application as you would any other web application.

   The context root for the example chat application is `/cometd` and the HTML page is `index.html`. So the URL might look like this:

       http://localhost:8080/cometd/index.html

See Also

For more information about deployment in the GlassFish Server, see the Oracle GlassFish Server Application Deployment Guide.

For more information about the Bayeux protocol, see Bayeux Protocol.

For more information about the Dojo toolkit, see `http://dojotoolkit.org/`.

For information about REpresentational State Transfer (RESTful) web services and Comet, see RESTful Web Services and Comet.

## Advanced Web Application Features

The following topics are addressed here:

- Internationalization Issues

- Virtual Server Properties

- Class Loader Delegation

- Using the `default-web.xml` File

- Configuring Logging and Monitoring in the Web Container

- Configuring Idempotent URL Requests

- Header Management

- Configuring Valves and Catalina Listeners

- Alternate Document Roots

- Using a context.xml File

- Enabling WebDav

- Using SSI

- Using CGI

### Internationalization Issues

The following topics are addressed here:

- The Server's Default Locale

- Servlet Character Encoding

#### The Server's Default Locale

To set the default locale of the entire GlassFish Server, which determines the locale of the Administration Console, the logs, and so on, use the Administration Console. Select the domain component. Then type a value in the Locale field. For details, click the Help button in the Administration Console.

#### Servlet Character Encoding

This section explains how the GlassFish Server determines the character encoding for the servlet request and the servlet response. For encodings you can use, see `https://download.oracle.com/javase/6/docs/technotes/guides/intl/encoding.doc.html`.

##### Servlet Request

When processing a servlet request, the server uses the following order of precedence, first to last, to determine the request character encoding:

- The `getCharacterEncoding` method

- A hidden field in the form, specified by the `form-hint-field` attribute of the `parameter-encoding` element in the `glassfish-web.xml` file

- The `default-charset` attribute of the `parameter-encoding` element in the `glassfish-web.xml` file

- The default, which is `ISO-8859-1`

For details about the `parameter-encoding` element, see "parameter-encoding" in Oracle GlassFish Server Application Deployment Guide.

##### Servlet Response

When processing a servlet response, the server uses the following order of precedence, first to last, to determine the response character encoding:

- The `setCharacterEncoding` or `setContentType` method

- The `setLocale` method

- The default, which is `ISO-8859-1`

### Virtual Server Properties

You can set virtual server properties in the following ways:

- You can define virtual server properties using the `asadmin create-virtual-server` command. For example:

      asadmin create-virtual-server --hosts localhost --property authRealm=ldap MyVS

  For details and a complete list of virtual server properties, see `create-virtual-server`(1).

- You can define virtual server properties using the `asadmin set` command. For example:

      asadmin set server-config.http-service.virtual-server.MyVS.property.authRealm="ldap"

  For details, see `set`(1).

- You can define virtual server properties using the Administration Console. Select the HTTP Service component under the relevant configuration, select Virtual Servers, and select the desired virtual server. Select Add Property, enter the property name and value, check the enable box, and select Save. For details and a complete list of virtual server properties, click the Help button in the Administration Console.

Some virtual server properties can be set for a specific web application. For details, see "glassfish-web-app" in Oracle GlassFish Server Application Deployment Guide.

### Class Loader Delegation

The Servlet specification recommends that a web application class loader look in the local class loader before delegating to its parent. To make the web application class loader follow the delegation model in the Servlet specification, set `delegate="false"` in the `class-loader` element of the `glassfish-web.xml` file. It's safe to do this only for a web module that does not interact with any other modules.

The default value is `delegate="true"`, which causes the web application class loader to delegate in the same manner as the other class loaders. Use `delegate="true"` for a web application that accesses EJB components or that acts as a web service client or endpoint. For details about `glassfish-web.xml`, see the Oracle GlassFish Server Application Deployment Guide.

For a number of packages, including `java.*` and `javax.*`, symbol resolution is always delegated to the parent class loader regardless of the `delegate` setting. This prevents applications from overriding core Java runtime classes or changing the API versions of specifications that are part of the Java EE platform.

For general information about class loaders, see Class Loaders.

### Using the `default-web.xml` File

You can use the `default-web.xml` file to define features such as filters and security constraints that apply to all web applications.

For example, directory listings are disabled by default for added security. To enable directory listings, in your domain's `default-web.xml` file, search for the definition of the servlet whose `servlet-name` is equal to `default`, and set the value of the `init-param` named `listings` to `true`. Then redeploy your web application if it has already been deployed, or restart the server.

    <init-param>
       <param-name>listings</param-name>
       <param-value>true</param-value>
    </init-param>

If `listings` is set to `true`, you can also determine how directory listings are sorted. Set the value of the `init-param` named `sortedBy` to `NAME`, `SIZE`, or `LAST_MODIFIED`. Then redeploy your web application if it has already been deployed, or restart the server.

    <init-param>
       <param-name>sortedBy</param-name>
       <param-value>LAST_MODIFIED</param-value>
    </init-param>

The `mime-mapping` elements in `default-web.xml` are global and inherited by all web applications. You can override these mappings or define your own using `mime-mapping` elements in your web application's `web.xml` file. For more information about `mime-mapping` elements, see the Servlet specification.

You can use the Administration Console to edit the `default-web.xml` file. For details, click the Help button in the Administration Console. As an alternative, you can edit the file directly using the following steps.

#### To Use the `default-web.xml` File

1. Place the JAR file for the filter, security constraint, or other feature in the domain-dir`/lib` directory.

2. Edit the domain-dir`/config/default-web.xml` file to refer to the JAR file.

3. Restart the server.

### Configuring Logging and Monitoring in the Web Container

For information about configuring logging and monitoring in the web container using the Administration Console, click the Help button in the Administration Console. Select Logger Settings under the relevant configuration, or select the Stand-Alone Instances component, select the instance from the table, and select the Monitor tab.

### Configuring Idempotent URL Requests

An idempotent request is one that does not cause any change or inconsistency in an application when retried. To enhance the availability of your applications deployed on an GlassFish Server cluster, configure the load balancer to retry failed idempotent HTTP requests on all the GlassFish Server instances in a cluster. This option can be used for read-only requests, for example, to retry a search request.

The following topics are addressed here:

- Specifying an Idempotent URL

- Characteristics of an Idempotent URL

#### Specifying an Idempotent URL

To configure idempotent URL response, specify the URLs that can be safely retried in `idempotent-url-pattern` elements in the `glassfish-web.xml` file. For example:

    <idempotent-url-pattern url-pattern="sun_java/*" no-of-retries="10"/>

For details, see "idempotent-url-pattern" in Oracle GlassFish Server Application Deployment Guide.

If none of the server instances can successfully serve the request, an error page is returned.

#### Characteristics of an Idempotent URL

Since all requests for a given session are sent to the same application server instance, and if that GlassFish Server instance is unreachable, the load balancer returns an error message. Normally, the request is not retried on another GlassFish Server instance. However, if the URL pattern matches that specified in the `glassfish-web.xml` file, the request is implicitly retried on another GlassFish Server instance in the cluster.

In HTTP, some methods (such as GET) are idempotent, while other methods (such as POST) are not. In effect, retrying an idempotent URL should not cause values to change on the server or in the database. The only difference should be a change in the response received by the user.

Examples of idempotent requests include search engine queries and database queries. The underlying principle is that the retry does not cause an update or modification of data.

A search engine, for example, sends HTTP requests with the same URL pattern to the load balancer. Specifying the URL pattern of the search request to the load balancer ensures that HTTP requests with the specified URL pattern are implicitly retried on another GlassFish Server instance.

For example, if the request URL sent to the GlassFish Server is of the type `/search/`something`.html`, then the URL pattern can be specified as `/search/*`.

Examples of non-idempotent requests include banking transactions and online shopping. If you retry such requests, money might be transferred twice from your account.

### Header Management

In all Editions of the GlassFish Server, the `Enumeration` from `request.getHeaders()` contains multiple elements (one element per request header) instead of a single, aggregated value.

The header names used in `HttpServletResponse.add` XXX `Header()` and `HttpServletResponse.set` XXX `Header()` are returned as they were created.

### Configuring Valves and Catalina Listeners

You can configure custom valves and Catalina listeners for web modules or virtual servers by defining properties. A valve class must implement the org.apache.catalina.Valve interface from Tomcat or previous GlassFish Server releases, or the org.glassfish.web.valve.GlassFishValve interface from the current GlassFish Server release. A listener class for a virtual server must implement the org.apache.catalina.ContainerListener or org.apache.catalina.LifecycleListener interface. A listener class for a web module must implement the org.apache.catalina.ContainerListener , org.apache.catalina.LifecycleListener, or org.apache.catalina.InstanceListener interface.

In the `glassfish-web.xml` file, valve and listener properties for a web module look like this:

    <glassfish-web-app ...>
       ...
       <property name="valve_1" value="org.glassfish.extension.Valve"/>
       <property name="listener_1" value="org.glassfish.extension.MyLifecycleListener"/>
    </glassfish-web-app>

You can define these same properties for a virtual server. For more information, see Virtual Server Properties.

### Alternate Document Roots

An alternate document root (docroot) allows a web application to serve requests for certain resources from outside its own docroot, based on whether those requests match one (or more) of the URI patterns of the web application's alternate docroots.

To specify an alternate docroot for a web application or a virtual server, use the `alternatedocroot_`n property, where n is a positive integer that allows specification of more than one. This property can be a subelement of a `glassfish-web-app` element in the `glassfish-web.xml` file or a virtual server property. For more information about these elements, see "glassfish-web-app" in Oracle GlassFish Server Application Deployment Guide.

A virtual server's alternate docroots are considered only if a request does not map to any of the web modules deployed on that virtual server. A web module's alternate docroots are considered only once a request has been mapped to that web module.

If a request matches an alternate docroot's URI pattern, it is mapped to the alternate docroot by appending the request URI (minus the web application's context root) to the alternate docroot's physical location (directory). If a request matches multiple URI patterns, the alternate docroot is determined according to the following precedence order:

- Exact match

- Longest path match

- Extension match

For example, the following properties specify three `glassfish-web.xml` docroots. The URI pattern of the first alternate docroot uses an exact match, whereas the URI patterns of the second and third alternate docroots use extension and longest path prefix matches, respectively.

    <property name="alternatedocroot_1" value="from=/my.jpg dir=/srv/images/jpg"/>
    <property name="alternatedocroot_2" value="from=*.jpg dir=/srv/images/jpg"/>
    <property name="alternatedocroot_3" value="from=/jpg/* dir=/src/images"/>

The `value` of each alternate docroot has two components: The first component, `from`, specifies the alternate docroot's URI pattern, and the second component, `dir`, specifies the alternate docroot's physical location (directory).

Suppose the above examples belong to a web application deployed at `http://company22.com/myapp`. The first alternate docroot maps any requests with this URL:

    http://company22.com/myapp/my.jpg

To this resource:

    /svr/images/jpg/my.jpg

The second alternate docroot maps any requests with a `*.jpg` suffix, such as:

    http://company22.com/myapp/*.jpg

To this physical location:

    /svr/images/jpg

The third alternate docroot maps any requests whose URI starts with `/myapp/jpg/`, such as:

    http://company22.com/myapp/jpg/*

To the same directory as the second alternate docroot.

For example, the second alternate docroot maps this request:

    http://company22.com/myapp/abc/def/my.jpg

To:

    /srv/images/jpg/abc/def/my.jpg

The third alternate docroot maps:

    http://company22.com/myapp/jpg/abc/resource

To:

    /srv/images/jpg/abc/resource

If a request does not match any of the target web application's alternate docroots, or if the target web application does not specify any alternate docroots, the request is served from the web application's standard docroot, as usual.

### Using a context.xml File

You can define a `context.xml` file for all web applications, for web applications assigned to a specific virtual server, or for a specific web application.

To define a global `context.xml` file, place the file in the domain-dir`/config` directory and name it `context.xml`.

Use the `contextXmlDefault` property to specify the name and the location, relative to domain-dir, of the `context.xml` file for a specific virtual server. Specify this property in one of the following ways:

- In the Administration Console, open the HTTP Service component under the relevant configuration. Open the Virtual Servers component and scroll down to the bottom of the page. Enter `contextXmlDefault` as the property name and the path and file name relative to domain-dir as the property value.

- Use the `asadmin create-virtual-server` command. For example:

      asadmin create-virtual-server --property contextXmlDefault=config/vs1ctx.xml vs1

- Use the `asadmin set` command for an existing virtual server. For example:

      asadmin set server-config.http-service.virtual-server.vs1.property.contextXmlDefault=config/myctx.xml

To define a `context.xml` file for a specific web application, place the file in the `META-INF` directory and name it `context.xml`.

For more information about virtual server properties, see Virtual Server Properties. For more information about the `context.xml` file, see The Context Container. Context parameters, environment entries, and resource definitions in `context.xml` are supported in the GlassFish Server.

### Enabling WebDav

To enable WebDav in the GlassFish Server, you edit the `web.xml` and `glassfish-web.xml` files as follows.

First, enable the WebDav servlet in your `web.xml` file:

    <servlet>
       <servlet-name>webdav</servlet-name>
       <servlet-class>org.apache.catalina.servlets.WebdavServlet</servlet-class>
       <init-param>
          <param-name>debug</param-name>
          <param-value>0</param-value>
       </init-param>
       <init-param>
          <param-name>listings</param-name>
          <param-value>true</param-value>
       </init-param>
       <init-param>
          <param-name>readonly</param-name>
          <param-value>false</param-value>
       </init-param>
    </servlet>

Then define the servlet mapping associated with your WebDav servlet in your `web.xml` file:

    <servlet-mapping>
       <servlet-name>webdav</servlet-name>
       <url-pattern>/webdav/*</url-pattern>
    </servlet-mapping>

To protect the WebDav servlet so other users can't modify it, add a security constraint in your `web.xml` file:

    <security-constraint>
       <web-resource-collection>
          <web-resource-name>Login Resources</web-resource-name>
          <url-pattern>/webdav/*</url-pattern>
       </web-resource-collection>
       <auth-constraint>
          <role-name>Admin</role-name>
       </auth-constraint>
       <user-data-constraint>
          <transport-guarantee>NONE</transport-guarantee>
       </user-data-constraint>
       <login-config>
          <auth-method>BASIC</auth-method>
          <realm-name>default</realm-name>
       </login-config>
       <security-role>
          <role-name>Admin</role-name>
       </security-role>
    </security-constraint>

Then define a security role mapping in your `glassfish-web.xml` file:

    <security-role-mapping>
       <role-name>Admin</role-name>
       <group-name>Admin</group-name>
    </security-role-mapping>

If you are using the `file` realm, create a user and password. For example:

    asadmin create-file-user --groups Admin --authrealmname default admin

Enable the security manager as described in Enabling and Disabling the Security Manager.

You can now use any WebDav client by connecting to the WebDav servlet URL, which has this format:

    http://host:port/context-root/webdav/file

For example:

    http://localhost:80/glassfish-webdav/webdav/index.html

You can add the WebDav servlet to your `default-web.xml` file to enable it for all applications, but you can't set up a security role mapping to protect it.

### Using SSI

To enable SSI (server-side includes) processing for a specific web module, add the `SSIServlet` to your `web.xml` file as follows:

    <web-app>
       <servlet>
          <servlet-name>ssi</servlet-name>
          <servlet-class>org.apache.catalina.ssi.SSIServlet</servlet-class>
       </servlet>
       ...
       <servlet-mapping>
          <servlet-name>ssi</servlet-name>
          <url-pattern>*.shtml</url-pattern>
       </servlet-mapping>
       ...
       <mime-mapping>
          <extension>shtml</extension>
          <mime-type>text/html</mime-type>
       </mime-mapping>
    </web-app>

To enable SSI processing for all web modules, un-comment the corresponding sections in the `default-web.xml` file.

If the `mime-mapping` is not specified in `web.xml`, GlassFish Server attempts to determine the MIME type from `default-web.xml` or the operating system default.

You can configure the following `init-param` values for the `SSIServlet`.

Table 7-4 `SSIServlet` `init-param` Values

init-param | Type | Default | Description
--- | --- | --- | ---
buffered | `boolean` | `false` | Specifies whether the output should be buffered.
debug | `int` | `0` (for no debugging) | Specifies the debugging level.
expires | `Long` | `Expires` header in HTTP response not set | Specifies the expiration time in seconds.
inputEncoding | `String` | operating system encoding | Specifies encoding for the SSI input if there is no URL content encoding specified.
isVirtualWebappRelative | `boolean` | `false` (relative to the given SSI file) | Specifies whether the virtual path of the `#include` directive is relative to the `content-root`.
outputEncoding | `String` | UTF-8 | Specifies encoding for the SSI output.

For more information about SSI, see `http://httpd.apache.org/docs/2.2/mod/mod_include.html`.

### Using CGI

To enable CGI (common gateway interface) processing for a specific web module, add the `CGIServlet` to your `web.xml` file as follows:

    <web-app>
       <servlet>
          <servlet-name>cgi</servlet-name>
          <servlet-class>org.apache.catalina.servlets.CGIServlet</servlet-class>
       </servlet>
       ...
       <servlet-mapping>
          <servlet-name>cgi</servlet-name>
          <url-pattern>/cgi-bin/*</url-pattern>
       </servlet-mapping>
    </web-app>

To enable CGI processing for all web modules, un-comment the corresponding sections in the `default-web.xml` file.

Package the CGI program under the `cgiPathPrefix`. The default `cgiPathPrefix` is `WEB-INF/cgi`. For security, it is highly recommended that the contents and binaries of CGI programs be prohibited from direct viewing or download. For information about hiding directory listings, see Using the `default-web.xml` File.

Invoke the CGI program using a URL of the following format:

    http://host:8080/context-root/cgi-bin/cgi-name

For example:

    http://localhost:8080/mycontext/cgi-bin/hello

You can configure the following `init-param` values for the `CGIServlet`.

Table 7-5 `CGIServlet` `init-param` Values

init-param | Type | Default | Description
--- | --- | --- | ---
cgiPathPrefix | `String` | `WEB-INF/cgi` | Specifies the subdirectory containing the CGI programs.
debug | `int` | `0` (for no debugging) | Specifies the debugging level.
executable | `String` | `perl` | Specifies the executable for running the CGI script.
parameterEncoding | `String` | `System.getProperty``("file.encoding",` `"UTF-8")` | Specifies the parameter's encoding.
passShellEnvironment | `boolean` | `false` | Specifies whether to pass shell environment properties to the CGI program.

To work with a native executable, do the following:

1. Set the value of the `init-param` named executable to an empty `String` in the `web.xml` file.

2. Make sure the executable has its executable bits set correctly.

3. Use directory deployment to deploy the web module. Do not deploy it as a WAR file, because the executable bit information is lost during the process of `jar` and `unjar`. For more information about directory deployment, see the Oracle GlassFish Server Application Deployment Guide.