J2EE Classloading Best Practices

Learn valuable tips and tricks in classloading through the J2EE.

Here are some common classloading best practices that we have been following and hope useful to many.

  • Declare dependencies: Make dependencies explicit. Hidden or unknown dependencies will be left behind when you move your application to another environment. Specify the dependencies in the Manifest Class-Path entry of EJB-JAR or WAR file on the libraries packaged in the same application.
  • Group dependencies: Ensure that all dependencies are visible at the same level or above. If you must move a library, make sure all dependencies are still visible.
  • Minimize visibility: Dependency libraries should be placed at the lowest visibility level that satisfies all dependencies.
  • Share libraries: Avoid duplicating libraries. Duplication of libraries may make debugging classloading issues a nightmare. Share classes across a set of applications by using the configuration facilities provided by your application server. For example, use the tag in the global application.xml file in OC4J to share classes across all applications.
  • Keep configurations portable: Choose configuration options in the following order:
    • Standard J2EE options
    • Options that can be expressed within your EAR file
    • Server-level options
    • J2SE extension options
  • Use the correct loader: If you call Class.forName(), always explicitly pass the loader returned by Thread.currentThread().getContextClassLoader. If you are loading a properties file, use Thread.currentThread().getContextClassLoader().getResourceAsStream().
  • Avoid hardcoding of a specific parser in your application: Use JAXP API to access the underlying XML/XSL parsers in a vendor-neutral way instead of hard coding a specific XML parser in your application.
  • Avoid hot redeployment: Hot deployment depends upon classloaders and hot (re)depployment may introduce classloading errors. Avoid hot deployment if there is to much dependencies between applications and external libraries.

For details on Classloading in J2EE please take a look at the Classloading technical paper in OTN

About the author


Debu Panda Blog: I'm Debu Panda (debabrata.panda@oracle.com) and I'm the Product Manager responsible for EJB Container in the Oracle Application Server development team. I've around 13 years of experience in IT mainly in the Database and Application Server technologies. I've authored articles in several journals and presented in several conferences.

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