Class Loader & ClassNotFound exception in WebSphere

A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class. Given the name of a class, a class loader should attempt to locate or generate data that constitutes a definition for the class. A typical strategy is to transform the name into a file name and then read a class file of that name from a file system.
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

Look for the above options.
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 contained
modules 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

you can see the above options
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

Specifies whether the class loader searches in the parent class loader or in the application class loader first to load a class. The standard for development kit class loaders and product class loaders is Classes loaded with parent class loader first. By specifying Classes loaded with application class loader first, your application can override classes contained in the parent class loader, but this action can potentially result in ClassCastException or LinkageErrors if you have mixed use of overridden classes and non-overridden classes.
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 server

Class 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 

http://publib.boulder.ibm.com/infocenter/wasinfo/v6r1/index.jsp?topic=/com.ibm.websphere.nd.doc/info/ae/ae/crun_classload.html 

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