FreeMarker is a Java-based template engine that is a great alternative to JSP. FreeMarker is ideal for situations where your action results can possibly be loaded from outside a Servlet container. For example, if you wished to support plugins in your application, you might wish to use FreeMarker so that the plugins could provide the entire action class and view in a single jar that is loaded from the classloader.
For more information on FreeMarker itself, please visit the FreeMarker website
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FreeMarker is very similar to Velocity, as both are template languages that can be used outside of a Servlet container. The framework utilizes FreeMarker because FreeMarker has better error reporting. However, both are good alternatives to JSP. |
Getting Started
Getting started with FreeMarker is as simple as ensuring all the dependencies are included in your project's classpath. Typically, the only dependency is freemarker.jar. Other than that, struts-default.xml already configures the FreeMarker Result needed to process your application's templates.
<action name="test" class="com.acme.TestAction"> <result name="success" type="freemarker">test-success.ftl</result> </action>
Then in test-success.ftl:
<html> <head> <title>Hello</title> </head> <body> Hello, ${name} </body> </html>
Where name is a property on your action. That's it! Read the rest of this document for details on how templates are loaded, variables are resolved, and tags can be used.
Servlet / JSP Scoped Objects
The following are ways to obtained Application scope attributes, Session scope attributes, Request scope attributes, Request parameters, and framework Context scope parameters:-
Application Scope Attribute
Assuming there's an attribute with name myApplicationAttribute in the Application scope.
<#if Application.myApplicationAttribute?exists> ${Application.myApplicationAttribute} </#if>
or
<@s.property value="%{#application.myApplicationAttribute}" />
Session Scope Attribute
Assuming there's an attribute with name mySessionAttribute in the Session scope.
<#if Session.mySessionAttribute?exists> ${Session.mySessionAttribute} </#if>
or
<@s.property value="%{#session.mySessionAttribute}" />
Request Scope Attribute
Assuming there's an attribute with name 'myRequestAttribute' in the Request scope.
<#if Request.myRequestAttribute?exists> ${Request.myRequestAttribute} </#if>
or
<@s.property value="%{#request.myRequestAttribute}" />
Request Parameter
Assuming there's a request parameter myParameter (eg. http://host/myApp/myAction.action?myParameter=one).
<#if Parameters.myParameter?exists> ${Parameters.myParameter} </#if>
or
<@s.property value="%{#parameters.myParameter}" />
Context parameter
Assuming there's a parameter with the name myContextParam in framework context.
${stack.findValue('#myContextParam')}
or
<@s.property value="%{#myContextParam}" />
Template Loading
The framework looks for FreeMarker templates in two locations (in this order):
- Web application
- Class path
This ordering makes it ideal for providing templates inside a fully-built jar, but allowing for overrides of those templates to be defined in your web application. In fact, this is how you can override the default UI tags and Form Tags included with the framework.
In addition, you can specify a location (directory on your file system) through the templatePath or TemplatePath context variable (in the {{web.xml)}. If a variable is specified, the content of the directory it points to will be searched first.
| This variable is currently NOT relative to the root of your application. |
Variable Resolution
When using FreeMarker with the framework, variables are looked up in several different places, in this order:
- Built-in variables
- Value stack
- Action context
- Request scope
- Session scope
- Application scope
Note that the action context is looked up after the value stack. This means that you can reference the variable without the typical preceding has marker (#) like you would have to when using the JSP s:property tag. This is a nice convenience, though be careful because there is a small chance it could trip you up.
<@s.url id="url" value="http://www.yahoo.com"/> Click <a xhref="${url}">here</a>!
The built-in variables that Struts-FreeMarker integration provides are:
| Name | Description |
|---|---|
| stack | The value stack itself, useful for calls like ${stack.findString('ognl expr')} |
| action | The action most recently executed |
| response | The HttpServletResponse |
| res | Same as response |
| request | The HttpServletRequest |
| req | Same as request |
| session | The HttpSession |
| application | The ServletContext |
| base | The request's context path |
Tag Support
FreeMarker includes complete tag support. See the FreeMarker Tags documentation for information on how to use the generic Struts Tags provided by Struts. In addition to this, you can use any JSP tag, like so:
<#assign mytag=JspTaglibs["/WEB-INF/mytag.tld"]> <@mytag.tagx attribute1="some ${value}"/>
Where mytag.tld is the JSP Tag Library Definition file for your tag library. Note: in order to use this support in FreeMarker, you must enable the JSPSupportServlet in web.xml:
<servlet> <servlet-name>JspSupportServlet</servlet-name> <servlet-class>org.apache.struts2.views.JspSupportServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet>
Tips and Tricks
There are some advanced features that may be useful when building Struts applications with FreeMarker.
Type Conversion and Locales
FreeMarker has built in support for formatting dates and numbers. The formatting rules are based on the locale associated with the action request, which is by default set in struts.properties but can be over-ridden using the I18n Interceptor. This is normally perfect for your needs, but it is important to remember that these formatting rules are handled by FreeMarker and not by the framework's Type Conversion support.
If you want the framework to handle the formatting according to the Type Conversion you have specified, you shouldn't use the normal ${...} syntax. Instead, you should use the property tag. The difference is that the property tag is specifically designed to take an OGNL expression, evaluate it, and then convert it to a String using any Type Conversion rules you have specified. The normal ${...} syntax will use a FreeMarker expression language, evaluate it, and then convert it to a String using the built in formatting rules.
The difference in how type conversion is handled under Freemarker is subtle but important to understand.
Extending
Sometimes you may with to extend the framework's FreeMarker support. For example, you might want to extend the Struts tags that come bundled with the framework.
To extend the Freemarker support, develop a class that extends org.apache.struts2.views.freemarker.FreemarkerManager, overriding methods as needed, and plugin the class through the struts.properties:
struts.freemarker.manager.classname = com.yourcompany.YourFreeMarkerManager
ObjectWrapper Settings
Once you get familiar with FreeMarker, you will find certain subtletieswith it that may become frustrating. The most common thing you'll likely run in to is the BeansWrapper provided by FreeMarker. If you don't know what this is, don't worry. However, if you do, know this:
The StrutsBeanWrapper extends the default FreeMarker BeansWrapper and provides almost no change in functionality, except for how it handles maps. Normally, FreeMarker has two modes of operation: either support for friendly map built-ins (?keys, ?values, etc) but only support for String keys; OR no special built-in support (ie: ?keys returns the methods on the map instead of the keys) but support for String and non-String keys alike. Struts provides an alternative implementation that gives us the best of both worlds.
It is possible that this special behavior may be confusing or can cause problems. Therefore, you can set the struts.freemarker.wrapper.altMap property in struts.properties to false, allowing the normal BeansWrapper logic to take place instead.
Syntax Notes
As of FreeMarker 2.3.4, an alternative syntax is supported. This alternative syntax is great if you find that your IDE (especially IntelliJ IDEA) makes it difficult to work with the default syntax. You can read more about this syntax here.