Problems with Ant optional tasks SSHExec and SCP. Classpath issue?

Question

I'm in the process of modifying an Ant script (currently in use from within MyEclipse) to work from the command line. I'm doing this so anyone can check out the project and build it without MyEclipse. The problem I'm running into is that MyEclipse includes the dependencies behind the scenes. It does this by looking at the workspace's Ant configuration and compiling the classpath based on the selected libraries in the preferences dialog. Long story short, I need to take those dependencies and make the script smart enough to include them on its own, without the help of MyEclipse.

The tasks that are giving me a headache are the sshexec and scp tasks. They are optional ant tasks that require a version of jsch to run. I removed jsch from MyEclipse's Ant classpath and added it to a lib folder in the project itself (lib/dev). MyEclipse immediately complained that the SSHExec class could not find the dependent class, com.jcraft.jsch.UserInfo which is part of jsch-0.1.44.jar.

I don't see a way to set the classpath for Ant from within the build script. I have the following code, which adds a path element to the script, but I don't think Ant uses this unless explicitly associated to a task or another element.

<path id="web-jars">
  <fileset dir="${web-lib}">
    <include name="**/*.jar" />
  </fileset>
  <fileset dir="${app-lib}"> <!-- this is where jsch resides --> 
    <include name="**/*.jar" />
  </fileset>
</path>

It seems that I need to use taskdef to define the sshexec and scp tasks:

<taskdef name="sshexec" classname="org.apache.tools.ant.taskdefs.optional.ssh.SSHExec"
    classpathref="web-jars"/>

MyEclipse complains about this, "taskdef A class needed by class org.apache.tools.ant.taskdefs.optional.ssh.SSHExec cannot be found: com/jcraft/jsch/UserInfo"

It's clearly in the classpathref, web-jars. And I can't run anything in the script because of this malformed or misconfigured taskdef.

Answer 1

The problem here is that the SSHExec class is loaded from a classloader which itself has no access to your web-jars class loader. Supplying this classpath for the taskdef does not change this. Each class can only load classes from its own classloader and any parent class loaders, but the web-jars classloader is not a parent class loader of SSHExec's class loader (it is likely the other way around, since SSHExec seems to be found here).

It looks like this:

 ClassLoader    web-jars  ------------->   application CL ------------->  bootstrap CL

 taskdef 
       =>   look for SSHExec here
            => look first in parent class loader
                                     => look for SSHExec here
                                     => look first in parent class loader
                                                                     => look for SSHExec here
                                                                     => not found
                                     => look in our own classpath
                                     => found, load the class
                                     => it somehow uses interface UserInfo
                                     => look for UserInfo here
                                     => look first in parent class loader
                                                                    => look for UserInfo here
                                                                    => not found
                                     => look in our own classpath
                                     => not found, throw exception.

The VM has no idea to look for UserInfo (and the other JSch classes) in the web-jars classloader.

I suppose the SSHExec task is somewhere in the usual ant classpath, i.e. loaded by the application class loader. Then removing SSHExec from ant's classpath (or adding jsch.jar to it) seems to be the only solution here.