Configure web module class loaders
We can set values that control the class-loading behavior of an installed web module.
This topic assumes that we installed a web module on an application server.
Configure the class loader order value of an installed web module. By default, a web module has its own web application archive (WAR) class loader to load the contents of the web module, which are in the WEB-INF/classes and WEB-INF/lib directories.
An application class loader is the parent of a WAR class loader. The WAR class-loader policy value of an application class loader determines whether the WAR class loader or the application class loader is used to load the contents of the web module.
The default WAR class loader policy value is Class loader for each WAR file in application. If the policy is set to Class loader for each WAR file in application, then each web module receives its own class loader whose parent is the application class loader. If the policy is set to Single class loader for application, then the application class loader loads the web module contents as well as the EJB modules, shared libraries, resource adapter archive (RAR) files, and dependency JAR files associated to an application. Thus, the configuration of the parent application class loader affects the WAR class loader. We can set the policy on the Class loading and update detection page of an administrative console.
Use the administrative console to configure the application and WAR class loaders.
If an application is running, changing an application setting causes the application to restart. On stand-alone servers, the application restarts after we save the change. On multiple-server products, the application restarts after we save the change and files synchronize on the node where the application is installed. To control when synchronization occurs on multiple-server products, deselect Synchronize changes with nodes on the Console preferences page.
Tasks
- If we have not done so already, configure the application class loader.
Sets such as Override class reloading settings for web and EJB modules, Polling interval for updated files and WAR class loader policy can affect web module class loading.
If WAR class loader policy is set to Class loader for each WAR file in application, then the web module receives its own class loader and the WAR class-loader policy of the web module defines the mode for a WAR class loader. If the policy is set to Single class loader for application, then the application class loader loads the web module contents.
- Specify the class loader order for the installed web module.
The web module class-loader mode specifies whether the class loader searches in the parent application class loader or in the WAR class loader first to load a class. The default is to search in the parent application class loader before searching in the WAR class loader to load a class.
Select either of the following values for Class loader order:
See Understanding Class Loaders Redbooks publication, especially the Changing the WAR class loader delegation mode step by step instructions.
Option Description Classes loaded with parent class loader first This option causes the class loader to prefer classes provided by the product over the classes that exist within the web module. This approach is standard for Development Kit class loaders and WebSphere Application Server class loaders. Classes loaded with local class loader first This option causes the class loader to prefer classes that exist in the web module over the classes provided by the product. If the same class exists in both the product and the web module, the class from the web module is loaded. If we specify the Classes loaded with local class loader first value, we might receive LinkageErrors or ClassCastException messages if we have mixed use of overridden classes and non-overridden classes.
- Click OK.
What to do next
Save the changes to the administrative configuration.
Related:
Class loaders Install enterprise application files Configure application class loaders Class loading Class loading and update detection settings Web module deployment settings WAS V7: Understanding Class Loaders