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.
Before you begin
This topic assumes that you have installed an application
on a server supported by the product 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 , enable the service
and restart the server.
About this task
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:
- The bootstrap, extensions, and CLASSPATH class loaders created
by the Java virtual machine
- A WebSphere extensions class loader
- One or more application module class loaders that load elements
of enterprise applications running in the server
- Zero or more web module class loaders

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 Java
virtual machine (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:
Use the class loader viewer to examine class loaders and correct
problems with application or class loader configurations.
Procedure
- Examine a tree view that lists all installed applications
and their modules. The modules can be web modules (.war files)
or EJB modules (.jar files).
Click to access the Enterprise applications topology page.
- Examine the class loader delegation hierarchy.
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
value can be either Classes loaded with parent class loader
first or Classes loaded with local class loader
first (parent last). Refer to the Configure class loaders step
for more information.
- Export information on class loaders.
- On the Class loader viewer page, click Export.
- Select to open a browser or editor on the class loader
information or to save the information to disk in XML format.
- Click OK, and specify any additional
information requested by the system.
- Display information about class loaders visible to the
module in an HTML table format.
On the Class loader viewer
page, click Table View. The Table View
page displays the following information:
Table 1. Table View page. Information available on class
loader attributes.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 (Classes
loaded with parent class loader first). A value of false implies
that the module class loader is being used (Classes loaded
with local class loader first (parent last)). 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.
- Search class loaders.
On the Class loader
viewer page, click
Search to access the
Search page, on which
you can search class loaders for the following:
- Specific strings
- Specific .jar files
- The names of files in a specific directory
- The names of files loaded by a specific class loader
The search is case-sensitive.
Class loading exceptions describes
several uses of the Search page.
- Configure class loaders. You
can configure class loaders for the following:
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 local class loader first (parent last). 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 local class loader
first (parent last) 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 local class loader first (parent last).
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 Java
EE compliance regarding visibility and isolation among application
artifacts. Changing the default policies is not suggested when troubleshooting
class loading problems.
What to do next
If you continue to have class loader
problems, refer to Class loading exceptions and to Class loading.