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This page contains content that we have migrated from Jetty 7 or Jetty 8 documentation into the correct format, but we have not yet audited it for technical accuracy in with Jetty 9. Be aware that examples or information contained on this page may be incorrect. Please check back soon as we continue improving the documentation, or submit corrections yourself to this page through Github. Thank you.
Class loading in a web container is slightly more complex than a normal Java application. The normal configuration is that each web context (web application or WAR file) has its own classloader, which has the system classloader as its parent. Such a classloader hierarchy is normal in Java, however the servlet specification complicates the hierarchy because it requires the following:
Classes contained within WEB-INF/lib or WEB-INF/classes have priority over classes on the parent classloader. This is the opposite of the normal behaviour of a Java 2 classloader.
System classes such as java.lang.String are excluded
from the webapp priority, and you may not replace them with classes in
WEB-INF/lib or WEB-INF/ classes.
Unfortunately the specification does not clearly state what classes are
System classes, and it is unclear if all javax
classes should be treated as System classes.
Server implementation classes like Server should be hidden from the web application and should not be available in any classloader. Unfortunately the specification does not state what classes are Server classes, and it is unclear if common libraries like the Xerces parser should be treated as Implementation classes.
Jetty provides configuration options to control the three webapp class loading issues identified above.
You can configure webapp classloading by several methods on the
WebAppContext.
You can call these methods directly if you are working with the Jetty API,
or you can inject methods from a context XML file if you are using the
Context Provider (???). You CANNOT
set these methods from a jetty-web.xml file, as it
executes after the classloader configuration is set.
The method org.eclipse.jetty.webapp.WebAppContext.setParentLoaderPriority(boolean) allows control over the priority given to webapp classes over system classes. If you set it to false (the default), Jetty uses standard webapp classloading priority. However, if in this mode some classes that are dependencies of other classes are loaded from the parent classloader (due to settings of system classes below), ambiguities might arise as both the webapp and system classloader versions can end up being loaded.
If set to true, Jetty uses normal JavaSE classloading priority, and gives priority to the parent/system classloader. This avoids the issues of multiple versions of a class within a webapp, but the version the parent/system loader provides must be the right version for all webapps you configure in this way.
You can call the methods org.eclipse.jetty.webapp.WebAppContext.setSystemClasses(String Array) or org.eclipse.jetty.webapp.WebAppContext.addSystemClass(String) to allow fine control over which classes are considered System classes.
A web application can see a System class.
A WEB-INF class cannot replace a System class.
The default system classes are:
Table 30.1. Default System Classes
| System Classes | |
|---|---|
| java. | Java SE classes (per servlet spec v2.5 / SRV.9.7.2). |
| javax. | Java SE classes (per servlet spec v2.5 / SRV.9.7.2). |
| org.xml. | Needed by javax.xml. |
| org.w3c. | Needed by javax.xml. |
| org.eclipse.jetty.continuation. | Webapp can see and not change continuation classes. |
| org.eclipse.jetty.jndi. | Webapp can see and not change naming classes. |
| org.eclipse.jetty.jaas. | Webapp can see and not change JAAS classes. |
| org.eclipse.jetty.websocket. | WebSocket is a Jetty extension. |
| org.eclipse.jetty.servlet.DefaultServlet | Webapp can see and not change default servlet. |
Absolute classname can be passed, names ending with . are treated as packages names, and names starting with - are treated as negative matches and must be listed before any enclosing packages.
You can call the methods org.eclipse.jetty.webapp.WebAppContext.setServerClasses(String Array) or org.eclipse.jetty.webapp.WebAppContext.addServerClass(String) to allow fine control over which classes are considered Server classes.
A web application cannot see a Server class.
A WEB-INF class can replace a Server class.
The default server classes are:
Table 30.2. Default Server Classes
| Server Classes | |
|---|---|
| -org.eclipse.jetty.continuation. | Don't hide continuation classes. |
| -org.eclipse.jetty.jndi. | Don't hide naming classes. |
| -org.eclipse.jetty.jaas. | Don't hide jaas classes. |
| -org.eclipse.jetty.servlets. | Don't hide utility servlet classes if provided. |
| -org.eclipse.jetty.servlet.DefaultServlet | Don't hide default servlet. |
| -org.eclipse.jetty.servlet.listener. | Don't hide utility listeners |
| -org.eclipse.jetty.websocket. | Don't hide websocket extension. |
| org.eclipse.jetty. | Do hide all other Jetty classes. |
You can add extra classpaths to Jetty in several ways.
If you are using Start Features, at
startup the jetty runtime automatically loads option Jars from the top
level $jetty.home/lib directory. The default
settings include:
Adding Jars under $jetty.home/lib/ext to
the system classpath. You can place additional Jars here.
Adding the directory
$jetty.home/resources to the classpath (may
contain classes or other resources).
Adding a single path defined by the command line parameter path.
You can add an additional classpath to a context classloader by calling org.eclipse.jetty.webapp.WebAppContext.setExtraClasspath(String) with a comma-separated list of paths. You can do so directly to the API via a context XML file such as the following:
<Configure class="org.eclipse.jetty.webapp.WebAppContext"> ... <Set name="extraClasspath>../my/classes,../my/jars/special.jar,../my/jars/other.jar> </Set> ...
If none of the alternatives already described meet your needs, you can always provide a custom classloader for your webapp. We recommend, but do not require, that your custom loader subclasses WebAppClassLoader. You configure the classloader for the webapp like so:
MyCleverClassLoader myCleverClassLoader = new MyCleverClassLoader(); ... WebAppContext webapp = new WebAppContext(); ... webapp.setClassLoader(myCleverClassLoader);
You can also accomplish this in a context xml file.
If you start a Jetty server using a custom class loader–consider the Jetty classes not being available to the system class loader, only your custom class loader–you may run into class loading issues when the WebAppClassLoader kicks in. By default the WebAppClassLoader uses the system class loader as its parent, hence the problem. This is easy to fix, like so:
context.setClassLoader(new WebAppClassLoader(this.getClass().getClassLoader(), context));
or
context.setClassLoader(new WebAppClassLoader(new MyCustomClassLoader(), context));
See an error or something missing? Contribute to this documentation at Github!