Override a provided API with an alternative version

If an application provides (or uses a library that provides) classes that are also available in Liberty, by default the classes from Liberty are used. To change this so that the application uses the alternative versions of these classes, the application must be configured in server.xml, or an included file.

If a web application includes classes that are also present in the server runtime environment, we might want to control which copy of each of those classes is used by the application. For example, if different versions of the classes are present in both the application and the server runtime environment, we must ensure that the version packaged in the application is used.

By default, classes from the Liberty runtime environment are used by all Java EE applications. We can override this behavior using the class loader configuration delegation attribute. This configuration is specific to a particular application, or to a shared library that can be selected for use by an application.

Example

In the following example, an application called Scholar needs to use classes that it provides (or provided in a library that it uses), rather than using the copies of the classes available in Liberty.

• When the classes are packaged within the application, override the default parentFirst delegation behavior with a classloader element in the server.xml configuration file or a file that it includes:

  <application id="" name="Scholar" type="ear" location="scholar.ear">
    <classloader delegation="parentLast" />
  </application>

  This tells the application class loader to look at the Liberty classes only after looking in the application and its associated libraries for a class.

• When the classes are packaged in a shared library, add the delegation attribute to the classloader element that configures the use of the shared library as follows:

  <application id="" name="Scholar" type="ear" location="scholar.ear">
    <classloader delegation="parentLast" commonLibraryRef="mySharedLib"/>
  </application>
  
  <library id="mySharedLib">
    <fileset dir="${server.config.dir}/myLib" includes="*.jar" />
  </library>

We can also use the privateLibraryRef attribute for private libraries in an application.

See Sharing a library across multiple Java EE applications.