SECTION: 250-Configuration TITLE: 200classloading QUESTION: I'm having class loading problems?

Class loading in a web container is slightly more complex than a normal java application.

The normal configuration is for each web context (webapplication or war file) is given it's own classloader, which has the system classloader as it's parent. Such a classloader hierarchy is normal in Java, however the servlet specification complicates the hierarchy by requiring that:

- Classes contained within `WEB-INF/lib` or `WEB-INF/classes` have priority over classes on the parent class loader. This is the opposite of the normal behaviour of a java 2 class loader.
- System classes such as java.lang.String may not be replaced by 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 should be hidden from the web application and should not be available in any class loader. Unfortunately the specification does not state what is a Server class and it is unclear if common libraries like the xerces parser should be treated as Implementation classes.

Jetty provides configuration options to control all three of these options. The method `org.mortbay.http.HttpContext.setClassLoaderJava2Compliant(boolean)` allows the normal java 2 behaviour to be used and all classes will be loaded from the system classpath if possible. This is very useful if the libraries that a web application uses are having problems loading classes that are both in a web application and on the system classpath.

The methods `setSystemClasses(String[])` and `setServerClasses(String[])` may be called on either the `org.mortbay.http.HttpServer` or `org.mortbay.http.HttpContext` class to configure the whole server or just a particular context. This allows fine control over what classes can be seen or overridden by a web application. Absolute classname can be passed, names ending with . are treated as packages names and names starting with - are treated as negative matches.

These configuration may be setup either in code, in `jetty.xml` or in a `jetty-web.xml` file. An example for `jetty.xml` is below:

      <Call name="addWebApplication">
        <Arg>/mywebapp</Arg>
        <Arg>./webapps/mywebapp.war</Arg>

        <Set name="classLoaderJava2Compliant">true</Set>

        <!-- System classes cannot be overriden by a HttpContext or webapp -->
        <Set name="systemClasses">
          <Array type="java.lang.String">
    	<Item>java.</Item>
    	<Item>javax.servlet.</Item>
    	<Item>javax.xml.</Item>
    	<Item>org.mortbay.</Item>
    	<Item>org.xml.</Item>
    	<Item>org.w3c.</Item>
          </Array>
        </Set>

        <!-- Server classes are hidden from a HttpContext or webapp -->
        <Set name="serverClasses">
          <Array type="java.lang.String">
    	<Item>-org.mortbay.http.PathMap</Item>
    	<Item>-org.mortbay.</Item>
    	<Item>org.mortbay.start.</Item>
    	<Item>org.mortbay.stop.</Item>
    	<Item>org.mortbay.jetty.Server</Item>
          </Array>
        </Set>
      </Call>