Class loaders find and load class files. For a deployed application to run properly, the class loaders that affect the application and its modules must be configured so that the application can find the files and resources that it needs. Diagnosing problems with class loaders can be complicated and time-consuming. To diagnose and fix the problems more quickly, use the administrative console Class Loader Viewer to examine class loaders and the classes loaded by each class loader. This topic assumes that you have installed an application on a server supported by WebSphere Application Server and you want to examine class loaders used by the application or its modules. The modules can be Web modules (.war files) or enterprise bean (EJB) modules (.jar files). The Class Loader Viewer enables you to examine class loaders in a runtime environment.
This topic also assumes that you have enabled the class loader viewer service. Click Servers > Application servers > server_name > Class Loader Viewer Service, enable the service and restart the server.
The runtime environment of WebSphere Application Server uses the following class loaders to find and load new classes for an application in the following order:
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. 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.
If the class loaders that load the artifacts of an application are not configured properly, the JVM might throw a class loading exception when starting or running that application. Class loading exceptions describes the types of exceptions caused by improperly configured class loaders and suggests ways to use the Class Loader Viewer to correct configurations of class loaders. The types of exceptions include:
The steps that follow describe generally how to use the Class Loader Viewer to examine class loaders and correct problems with application or class loader configurations.
Click Troubleshooting > Class Loader Viewer to access the Enterprise Applications Topology page.
On the Enterprise Applications Topology page, select a module to access the Class Loader Viewer page. The page lists the class loaders visible to Web and EJB modules in an installed enterprise application. This page helps you to determine which class loaders loaded files of a module and to diagnose problems with class loaders.
The delegation hierarchy is determined by the class loader delegation mode, or class loader order, specified for an application or Web module. The mode value can be either Classes loaded with parent class loader first or Classes loaded with application class loader first. Refer to the Configure class loaders step for more information.
On the Class Loader Viewer page, click Table View. The Table View page displays the following information:
| Class loader attribute | Description |
|---|---|
| Delegation | Indicates whether the class loader delegates the loading of the module to its parent class loader. A value of true implies that the class loader of the parent application is being used (Parent first or Classes loaded with parent class loader first). A value of false implies that the module class loader is being used (Parent last or Classes loaded with application class loader first). Refer to the Configure class loaders step for more information. |
| Classpath | Lists the paths over which the class loader searches for classes and resources. |
| Classes | Lists the names of classes loaded in the JVM by this class loader. |
The Table View option does not return a value when out-of-memory errors are generated. The out-of-memory errors might be related to a memory leak. To examine information about class loaders in a table, resolve the out-of-memory problem, and then click Table View again.
The search is case-sensitive. Class loading exceptions describes several uses of the Search page.
Class loader configuration determines which class loader loads the classes and resource files for an application or Web module. Application and WAR module class loader configuration settings include Class loader order and WAR class loader policy.
A Class loader order value can be either Classes loaded with parent class loader first or Classes loaded with application class loader first. The default is Classes loaded with parent class loader first. A class loader with the Classes loaded with parent class loader first mode delegates loading a class or resource to its immediate parent class loader before searching its classpath.
When troubleshooting class loading problems, you might need to override classes visible to a parent class loader. To override such classes with those specific to an application, set the Class loader order to Classes loaded with application class loader first on the class loader that contains the application classes on its classpath. An application can override classes visible to a parent class loader, but doing so can result in a ClassCastException or UnsatisfiedLinkError if there is a mixed use of overridden classes and non-overridden classes.
For example, under default class loader policies, a Web module has its own Web module (WAR) class loader to load its artifacts, which are typically in the WEB-INF/classes and WEB-INF/lib directories. An application module class loader is the immediate parent of this WAR class loader. To ensure that the Web module class loader searches these paths for a particular class or resource first, before delegating the load operation to the application module class loader, set the Class loader order of the Web module to Classes loaded with application class loader first.
Class loader policies determine the structure of the application and WAR module class loaders. Under the default policies, every running application EAR has its own application module class loader, and every Web module has its own WAR module class loader. The default policies ensure J2EE compliance regarding visibility and isolation among application artifacts. Changing the default policies is not suggested when troubleshooting class loading problems.
If you continue to have class loader problems,
refer to Class loading exceptions and to Class loading.