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Sun Java System Application Server Enterprise Edition 8.2 Developer's Guide

Classloaders

Understanding Application Server classloaders can help you determine where and how you can position supporting JAR and resource files for your modules and applications.

In a Java Virtual Machine (JVM), the classloaders dynamically load a specific Java class file needed for resolving a dependency. For example, when an instance of java.util.Enumeration needs to be created, one of the classloaders loads the relevant class into the environment. This section includes the following topics:

The Classloader Hierarchy

Classloaders in the Application Server runtime follow a hierarchy that is illustrated in the following figure and fully described in Table 3–3.

Figure 3–5 Classloader runtime hierarchy

Figure shows the classloader runtime hierarchy.

Table 3–3 Sun Java System Application Server Classloaders

Classloader  

Description  

Bootstrap

The Bootstrap Classloader loads all the JDK classes. It is parent to the System Classloader. 

System

The System Classloader loads most of the core Application Server classes. It is parent to the Common Classloader. It is created based on the classpath-prefix, as-install/lib, server-classpath, and classpath-suffix attributes of the java-config element in the domain.xml file. The environment classpath is included if env-classpath-ignored="false" is set in the java-config element.

Add the classes to the classpath-prefix, server-classpath, or classpath-suffix attribute of the domain administration server (DAS) in addition to the corresponding attribute on the server instances that use the classes. The default name for the DAS configuration is server-config.

Common

The Common Classloader loads into the system classpath classes in the domain-dir/lib/classes directory, followed by JAR and ZIP files in the domain-dir/lib directory. It is parent to the Connector Classloader. No special classpath settings are required. The existence of these directories is optional; if they don’t exist, the Common Classloader is not created.

Connector

The Connector Classloader is a single class loader instance that loads individually deployed connector modules, which are shared across all applications. It is parent to the LifeCycleModule Classloader and the EJB Classloader. 

LifeCycleModule

The LifeCycleModule Classloader is the parent class loader for lifecycle modules. Each lifecycle module’s classpath is used to construct its own class loader. 

EJB

The EJB Classloader loads the enabled EJB classes in a specific enabled EJB module or J2EE application. One instance of this class loader is present in each class loader universe. The EJB Classloader is created with a list of URLs that point to the locations of the classes it needs to load. It is parent to the Web Classloader. 

Web

The Web Classloader loads the servlets and other classes in a specific enabled web module or J2EE application. One instance of this class loader is present in each class loader universe. The Web Classloader is created with a list of URLs that point to the locations of the classes it needs to load. It is parent to the JSP Engine Classloader. 

JSP Engine

The JSP Engine Classloader loads compiled JSP classes of enabled JSP files. One instance of this class loader is present in each class loader universe. The JSP Engine Classloader is created with a list of URLs that point to the locations of the classes it needs to load. 

Note that this is not a Java inheritance hierarchy, but a delegation hierarchy. In the delegation design, a class loader delegates classloading to its parent before attempting to load a class itself. A class loader parent can be either the System Classloader or another custom class loader. If the parent class loader can’t load a class, the findClass() method is called on the class loader subclass. In effect, a class loader is responsible for loading only the classes not available to the parent.

The Servlet specification recommends that the Web Classloader look in the local class loader before delegating to its parent. You can make the Web Classloader follow the delegation model in the Servlet specification by setting delegate="false" in the class-loader element of the sun-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 Classloader to delegate in the same manner as the other classloaders. You must use delegate="true" for a web application that accesses EJB components or that acts as a web service client or endpoint. For details about sun-web.xml, see The sun-web.xml File.

Classloader Universes

Access to components within applications and modules installed on the server occurs within the context of isolated class loader universes, each of which has its own EJB, Web, and JSP Engine classloaders.

Note –

A resource such as a file that is accessed by a servlet, JSP, or EJB component must be in a directory pointed to by the class loader’s classpath. For example, the web class loader’s classpath includes these directories:

module-name/WEB-INF/classes
module-name/WEB-INF/lib

If a servlet accesses a resource, it must be in one of these directories or it is not loaded.

Note –

In iPlanet Application Server 6.x, individually deployed modules shared the same class loader. In subsequent Application Server versions, each individually deployed module has its own class loader universe.

Circumventing Classloader Isolation

Since each application or individually deployed module class loader universe is isolated, an application or module cannot load classes from another application or module. This prevents two similarly named classes in different applications from interfering with each other.

To circumvent this limitation for libraries, utility classes, or individually deployed modules accessed by more than one application, you can include the relevant path to the required classes in one of these ways:

Using the System class loader or Common class loader requires a server restart and makes a library accessible to any other application or module across the domain.

Using the System Classloader

To use the System Classloader, do one of the following, then restart the server:

Using the System Classloader makes an application or module accessible to any other application or module across the domain.

Add the classes to the classpath-suffix attribute of the DAS in addition to the classpath-suffix attribute on the server instances that use the classes. The default name for the DAS configuration is server-config.

Using the Common Classloader

To use the Common Classloader, copy the JAR and ZIP files into the domain-dir/lib directory or copy the .class files into the domain-dir/lib/classes directory, then restart the server.

Using the Common Classloader makes an application or module accessible to any other application or module across the domain.

For example, this is the recommended way of adding JDBC drivers to the Application Server. For a list of the JDBC drivers currently supported by the Application Server, see the Sun Java System Application Server Enterprise Edition 8.2 Release Notes. For configurations of supported and other drivers, see Configurations for Specific JDBC Drivers.

Sharing Libraries Across a Cluster

To share libraries across a specific cluster instead of the entire domain, copy the JAR files to the domain-dir/config/cluster-config-name/lib directory. Then add the path to the JAR files to the System class loader as explained in Using the System Classloader.

Using the Java Optional Package Mechanism

To use the Java optional package mechanism, copy the JAR and ZIP files into the domain-dir/lib/ext directory, then restart the server.

Using the Java optional package mechanism makes an application or module accessible to any other application or module across the domain.

Packaging the Client JAR for One Application in Another Application

By packaging the client JAR for one application in a second application, you allow an EJB or web component in the second application to call an EJB component in the first (dependent) application, without making either of them accessible to any other application or module.

As an alternative for a production environment, you can have the Common Classloader load client JAR of the dependent application as described in Using the Common Classloader restart the server to make the dependent application accessible, and it is accessible across the domain.

ProcedureTo package the client JAR for one application in another application

  1. Deploy the dependent application.

  2. Add the dependent application’s client JAR file to the calling application.

    • For a calling EJB component, add the client JAR file at the same level as the EJB component. Then add a Class-Path entry to the MANIFEST.MF file of the calling EJB component. The Class-Path entry has this syntax:

      Class-Path: filepath1.jar filepath2.jar ...

      Each filepath is relative to the directory or JAR file containing the MANIFEST.MF file. For details, see the J2EE specification, section 8.1.1.2, “Dependencies.”

    • For a calling web component, add the client JAR file under the WEB-INF/lib directory.

  3. If you need to package the client JAR with both the EJB and web components, set delegate="true" in the class-loader element of the sun-web.xml file.

    This changes the Web Classloader so it follows the standard class loader delegation model and delegates to its parent before attempting to load a class itself.

    For most applications, packaging the client JAR file with the calling EJB component is sufficient. You do not need to package the client JAR file with both the EJB and web components unless the web component is directly calling the EJB component in the dependent application.

  4. Deploy the calling application.

    The calling EJB or web component must specify in its sun-ejb-jar.xml or sun-web.xml file the JNDI name of the EJB component in the dependent application. Using an ejb-link mapping does not work when the EJB component being called resides in another application.