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Class loaders

Class loaders find and load class files. Class loaders enable applications deployed on servers to access repositories of available classes and resources. Application developers and deployers must consider the location of class and resource files, and the class loaders used to access those files, to make the files available to deployed applications.


Class loaders used and the order of use

The product runtime environment uses the following class loaders to find and load new classes for an application in the following order:

  1. The bootstrap, extensions, and CLASSPATH class loaders created by the Java virtual machine

    The bootstrap class loader uses the boot class path (typically classes in jre/lib) to find and load classes. The extensions class loader uses the system property java.ext.dirs (typically jre/lib/ext) to find and load classes. The CLASSPATH class loader uses the CLASSPATH environment variable to find and load classes.

  2. A WebSphere extensions class loader

    The WebSphere extensions class loader loads the WAS classes required at run time. WAS classes are provided as a set of OSGi bundles. Each bundle is loaded by a separate class loader within a network of OSGi class loaders. The extensions class loader delegates to a gateway class loader to load classes from this OSGi class loader network. Packages exported from the OSGi class loader network are visible to applications through the gateway. For details, see OSGi class loader model.

    The Java EE APIs are provided in the javax.j2ee.*.jar bundles, which are loaded within the OSGi class loader network and made visible to applications through the gateway. Because classes deployed within OSGi bundles are not visible to the Java virtual machine class loaders, do not use the CLASSPATH environment variable or the java.ext.dirs and java.lang.classpath system properties to specify paths to libraries that depend on the Java EE APIs. Also, do not use CLASSPATH, java.ext.dirs, and java.lang.classpath to specify paths to application libraries because these libraries might cause linkage errors or unexpected server behaviors.

    The WebSphere extensions class loader uses a ws.ext.dirs system property to determine the path used to load classes and resources beyond those provided in OSGi bundles. Each directory in the ws.ext.dirs class path and every JAR file or compressed file in these directories is added to the class path used by this class loader.

    The WebSphere extensions class loader also loads resource provider classes into a server if an application module installed on the server refers to a resource associated with the provider and if the provider specifies the directory name of the resource drivers.

  3. One or more application module class loaders that load elements of enterprise applications running in the server

    The application elements can be web modules, EJB modules, resource adapter archives (RAR files), and dependency JAR files. Application class loaders follow Java EE class-loading rules to load classes and JAR files from an enterprise application. The product enables us to associate shared libraries with an application.

  4. Zero or more web module class loaders

    By default, web module class loaders load the contents of the WEB-INF/classes and WEB-INF/lib directories. Web module class loaders are children of application class loaders. We can specify that an application class loader load the contents of a web module rather than the web module class loader.

Each class loader is a child of the previous class loader. That is, the application module class loaders are children of the WebSphere extensions class loader, which is a child of the CLASSPATH Java class loader. Whenever a class needs to be loaded, the class loader usually delegates the request to its parent class loader. If none of the parent class loaders can find the class, the original class loader attempts to load the class. Requests can only go to a parent class loader; they cannot go to a child class loader. If the WebSphere extensions class loader is requested to find a class in a Java EE module, it cannot go to the application module class loader to find that class and a ClassNotFoundException error occurs. After a class is loaded by a class loader, any new classes that it tries to load reuse the same class loader or go up the precedence list until the class is found.


Class-loader isolation policies

The number and function of the application module class loaders depend on the class-loader policies that are specified in the server configuration. Class loaders provide multiple options for isolating applications and modules to enable different application packaging schemes to run on an application server.

Two class-loader policies control the isolation of applications and modules:

Class-loader policy Description
Application Application class loaders load EJB modules, dependency JAR files, embedded resource adapters, and application-scoped shared libraries. Depending on the application class-loader policy, an application class loader can be shared by multiple applications (Single) or unique for each application (Multiple). The application class-loader policy controls the isolation of applications running in the system. When set to Single, applications are not isolated. When set to Multiple, applications are isolated from each other.
WAR By default, web module class loaders load the contents of the WEB-INF/classes and WEB-INF/lib directories. The application class loader is the parent of the web module class loader. We can change the default behavior by changing the web application archive (WAR) class-loader policy of the application.

The WAR class-loader policy controls the isolation of web modules. If this policy is set to Application, then the Web module contents also are loaded by the application class loader (in addition to the EJB files, RAR files, dependency JAR files, and shared libraries). If the policy is set to Module, then each web module receives its own class loader whose parent is the application class loader.

Tip: The console and the underlying deployment.xml file use different names for WAR class-loader policy values. In the console, the WAR class-loader policy values are Application or Module. However, in the underlying deployment.xml file where the policy is set, the WAR class-loader policy values are Single instead of Application, or Multiple instead of Module. Application is the same as Single, and Module is the same as Multiple.

Restriction: WAS class loaders never load application client modules.

For each application server in the system, we can set the application class-loader policy to Single or Multiple. When the application class-loader policy is set to Single, then a single application class loader loads all EJB modules, dependency JAR files, and shared libraries in the system. When the application class-loader policy is set to Multiple, then each application receives its own class loader used for loading the EJB modules, dependency JAR files, and shared libraries for that application.

An application class loader loads classes from web modules if the application's WAR class-loader policy is set to Application. If the application's WAR class-loader policy is set to Module, then each WAR module receives its own class loader.

The following example shows that when the application class-loader policy is set to Single, a single application class loader loads all of the EJB modules, dependency JAR files, and shared libraries of all applications on the server. The single application class loader can also load web modules if an application has its WAR class-loader policy set to Application. Applications that have a WAR class-loader policy set to Module use a separate class loader for web modules.

Server's application class-loader policy: Single
Application's WAR class-loader policy: Module

Application 1
	Module: 	EJB1.jar
	Module:	WAR1.war
	 	MANIFEST Class-Path: Dependency1.jar
		WAR Classloader Policy = Module
Application 2  
	Module:  	EJB2.jar
		MANIFEST Class-Path: Dependency2.jar
	Module:	WAR2.war
		WAR Classloader Policy = Application 

The following example shows that when the application class-loader policy of an application server is set to Multiple, each application on the server has its own class loader. An application class loader also loads its web modules if the application WAR class-loader policy is set to Application. If the policy is set to Module, then a web module uses its own class loader.

Server's application class-loader policy: Multiple
Application's WAR class-loader policy: Module

Application 1
	Module: 	EJB1.jar
	Module:	WAR1.war   
		MANIFEST Class-Path: Dependency1.jar
		WAR Classloader Policy = Module
Application 2  
	Module:  	EJB2.jar
		MANIFEST Class-Path: Dependency2.jar
	Module:	WAR2.war
		WAR Classloader Policy = Application 


Class-loader modes

The class-loader delegation mode, also known as the class loader order, determines whether a class loader delegates the loading of classes to the parent class loader. The following values for class-loader mode are supported:

Class-loader mode Description
Parent first

Also known as Classes loaded with parent class loader first.

The Parent first class-loader mode causes the class loader to delegate the loading of classes to its parent class loader before attempting to load the class from its local class path. This value is the default for the class-loader policy and for standard JVM class loaders.
Parent last

Also known as Classes loaded with local class loader first or Application first.

The Parent last class-loader mode causes the class loader to attempt to load classes from its local class path before delegating the class loading to its parent. Using this policy, an application class loader can override and provide its own version of a class that exists in the parent class loader.

The following settings determine the mode of a class loader:


OSGi class loader model

OSGi uses metadata in the manifest file as its class loader mode. There is no global class path in OSGi. When bundles are installed into the OSGi Framework, their metadata is processed by the module layer and their declared external dependencies are reconciled against the versioned exports declared by other installed modules. The OSGi Framework works out all the dependencies, and calculates the independent required class path for each bundle. This approach resolves the shortcomings of plain Java class loading by ensuring that the following requirements are met:

For more information about OSGi, refer to the topic The OSGi Framework.


Related:

  • Enterprise (Java EE) applications
  • The OSGi Framework
  • Create shared libraries
  • Manage shared libraries
  • Enterprise application settings
  • Application server settings
  • Shared library collection