When a class loading request is presented to a class loader, it first asks its parent class loader to fulfill the request. The parent class loader, in turn, asks its parent for the class until the request reaches the top of the hierarchy. If the class loader at the top of the hierarchy cannot fulfill the request to load a class, then the child class loader that called it is responsible for loading the class. If the child is also unable to load the class, the request continues back down the hierarchy until a class loader fulfils it or a ClassNotFoundException is produced by the last 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. Design and packaging of an application will determine the
behavior of class loading. WebSphere provides the ability to
change/modify the class loading behavior.
WebSphere Class Loaders:
WebSphere application server has 3 class loaders.-
Application server class loader
The application server class loader policy affects all applications that are deployed on the server. -
Enterprise application class loader
An application class loader is the parent class of an Enterprise application (EAR) and all modules within it. An application class loader groups enterprise bean (EJB) modules, shared libraries, and dependency Java archive (JAR) files associated to an application. Dependency JAR files are JAR files that contain code which can be used by both enterprise beans and servlets. -
Web module class loader
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.
1. Application server class loader
Go to Servers –> Application Servers –> Server name
Classloader Policy
Single: Applications are not isolated from each other. Uses a single application class loader to load all of the EJB modules, shared libraries, and JAR files which are contained in all applications installed into the JVM.
Multiple: Applications are isolated from each other. Gives each application its own class loader to load the EJB modules, shared libraries, and JAR files.
Class loading mode
parent first: Sets the loading of classes to its parent class loader before attempting to load the class from its local class path. This is the default value for Class loading mode
parent last: Tells the class loader to start with loading classes from its local class path before asking its parent.
For each application server in the system, you 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 that is used for loading the EJB modules, dependency JAR files, and shared libraries for that application.
Note: If you’ve multiple application running on the same server (JVM) and if their classes are conflicting each other or Also some times it can happen that, application classes may conflict with the WebSphere classes. then we change the class loading mode option from default.
2. Application Class Loader
In general Enterprise applications (EAR) will have multiple web, ejbs or sometimes may include application client modules. Enterprise applications can also override settings within the containedmodules deployment descriptors to combine or deploy them. By placing JAR files in the enterprise application instead of the global class path of an application server, they are also within the application and thus they get deployed along with the application. The concept is that an EAR file encapsulates all its required resources and hence it can be pre-configured using some java techniques.
Go to Applications –> Enterprise applications –> Application name –> Class loading and updation
Class loader order
Classes loaded with parent class loader first
Sets the loading of classes to its parent class loader before attempting to load the class from its local class path.
Classes loaded with application class loader first
Tells the class loader to start with loading classes from its local class path before asking its parent
WAR class loader policy
Class loader for each WAR file in application
A separate class loader is assigned to each WAR file.
Single class loader for application
One class loader is assigned to all WAR files.
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.
3. Web module class loader
Every web module will have 2 folders, WEB-INF/classes and WEB-INF/lib. The classes folder may contain Java classes within the web application. Then we can specify a class loader to looks at this folder to load those classes. Remember these classes are only for that specific web module.
Class loader order
Classes loaded with parent class loader first
If set, the class loader searches the application’s class loader for the class.
Classes loaded with application class loader first
If set, the class loader searches within the WAR class loader first to load a class.
Class-loader isolation
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 serverClass loader isolation combinations
| Type of Isolation | Application server Class loader | WAR class loader |
| Full | Multiple | WAR |
| Partial | Multiple | Application |
| Minimum | Single | Application |
you might be wondering, where is the 4th option (Application server
class loader – Single + WAR class loader WAR). Once you set the
application class loader to Single, there is only one class loader for
entire server. That’s why this option was not mentioned
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