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Oracle GlassFish Server 3.1 Application Development Guide
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Part I Development Tasks and Tools

1.  Setting Up a Development Environment

2.  Class Loaders

The Class Loader Hierarchy


Using the Java Optional Package Mechanism

Using the Endorsed Standards Override Mechanism

Class Loader Universes

Application-Specific Class Loading

Circumventing Class Loader Isolation

Using the Common Class Loader

Sharing Libraries Across a Cluster

Packaging the Client JAR for One Application in Another Application

To Package the Client JAR for One Application in Another Application

3.  Debugging Applications

Part II Developing Applications and Application Components

4.  Securing Applications

5.  Developing Web Services

6.  Using the Java Persistence API

7.  Developing Web Applications

8.  Using Enterprise JavaBeans Technology

9.  Using Container-Managed Persistence

10.  Developing Java Clients

11.  Developing Connectors

12.  Developing Lifecycle Listeners

13.  Developing OSGi-enabled Java EE Applications

Part III Using Services and APIs

14.  Using the JDBC API for Database Access

15.  Using the Transaction Service

16.  Using the Java Naming and Directory Interface

17.  Using the Java Message Service

18.  Using the JavaMail API



Note that the class loader hierarchy is not a Java inheritance hierarchy, but a delegation hierarchy. In the delegation design, a class loader delegates class loading to its parent before attempting to load a class itself. If the parent class loader cannot load a class, the class loader attempts to load the class itself. In effect, a class loader is responsible for loading only the classes not available to the parent. Classes loaded by a class loader higher in the hierarchy cannot refer to classes available lower in the hierarchy.

The Java Servlet specification recommends that a web module's class loader look in the local class loader before delegating to its parent. You can make this class loader follow the delegation inversion model in the Servlet specification by setting delegate="false" in the class-loader element of the glassfish-web.xml file. It is safe to do this only for a web module that does not interact with any other modules. For details, see class-loader in Oracle GlassFish Server 3.1 Application Deployment Guide.

The default value is delegate="true", which causes a web module's class loader to delegate in the same manner as the other class loaders. 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 glassfish-web.xml, see Oracle GlassFish Server 3.1 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.