Servlets / Class Loaders

Question

          Hi,
          I have read the BEA documentation on the class loader hierarchy in Weblogic Server,
          and I have some questions regarding some behavior I am seeing.
          I am running Weblogic Server 7.0.
          I have an ear file that contains 3 web apps (wars) and several utility jars.
          The web apps' manifests contain the Class-Path entry for the utility jars. My
          understanding of this is that each web app SHOULD have its own class loader.
          Also, the utility jars will be scoped in a separate class loader and WILL NOT
          have visibility to the web app classes. The web app classes should have visibility
          to the utility jars.
          Is this correct????
          I added a static segment of code in each web app and printed the class loader
          for each servlet when it was loaded. I also printed the class loader from a class
          that is DEFINITELY contained in one of the utility jars. Here is the result:
          Utility Class ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04
          Utility Class Parent ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
          Servlet 1 ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04
          Servlet 1 Parent ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
          Servlet 2 ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04
          Servlet 2 Parent ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
          I'm a little confused.... I expected to see 3 different class loaders (i.e.
          one for each class above). I believe the above printout says that all 3 classes
          are being loaded by the SAME class loader instance (Launcher$AppClassLoader@b9d04).
          Am I interpreting this correctly? If so, what's going on?
          

Answer 1

Hi Mark,
          It sounds perfectly understandable.
          <Quote from edocs.bea.com>
          WebLogic Server classloading is centered on the concept of an application. An
          application is normally packaged in an Enterprise Archive (EAR) file
          containing application classes. Everything within an EAR file is considered
          part of the same application. Other applications include:
          An Enterprise JavaBean (EJB) JAR file
          A Web Application WAR file
          If you deploy an EJB JAR file and a Web Application WAR file separately, they
          are considered two applications. If they are deployed together
          within an EAR file, they are one application. You deploy components together
          in an EAR file for them to be considered part of the same
          application.
          </quote>
          So there is a classloader for the ear which is used to load everything in that
          ear and the parent classloader must be the one that started your weblogic
          server(or a thread which started your weblogic server).
          -Raj
          Slava Imeshev wrote:
          > Hi Mark,
          >
          > Check if that utility and webapp classes are not in the server classpath.
          >
          > Regards,
          >
          > Slava Imeshev
          >
          > "Mark Cotherman" <[email protected]> wrote in message
          > news:[email protected]...
          > >
          > > Hi,
          > >
          > > I have read the BEA documentation on the class loader hierarchy in
          > Weblogic Server,
          > > and I have some questions regarding some behavior I am seeing.
          > >
          > > I am running Weblogic Server 7.0.
          > >
          > > I have an ear file that contains 3 web apps (wars) and several utility
          > jars.
          > > The web apps' manifests contain the Class-Path entry for the utility jars.
          > My
          > > understanding of this is that each web app SHOULD have its own class
          > loader.
          > > Also, the utility jars will be scoped in a separate class loader and WILL
          > NOT
          > > have visibility to the web app classes. The web app classes should have
          > visibility
          > > to the utility jars.
          > >
          > > Is this correct????
          > >
          > > I added a static segment of code in each web app and printed the class
          > loader
          > > for each servlet when it was loaded. I also printed the class loader from
          > a class
          > > that is DEFINITELY contained in one of the utility jars. Here is the
          > result:
          > >
          > >
          > > Utility Class ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04
          > > Utility Class Parent ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
          > >
          > > Servlet 1 ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04
          > > Servlet 1 Parent ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
          > >
          > > Servlet 2 ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04
          > > Servlet 2 Parent ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
          > >
          > >
          > >
          > > I'm a little confused.... I expected to see 3 different class loaders
          > (i.e.
          > > one for each class above). I believe the above printout says that all 3
          > classes
          > > are being loaded by the SAME class loader instance
          > (Launcher$AppClassLoader@b9d04).
          > >
          > >
          > > Am I interpreting this correctly? If so, what's going on?
          > >