dynamically loading classes from a ImageJ plugin

Question

I am having problems dynamically loading a java class from a dynamically loaded class.

My class is an ImageJ plugin and it loads other modules dynamically through classloader. I have NoClassDefFoundError when loading the module that references to something that is in the same package as the ImageJ plugin.

What I exactly do is:

1. ImageJ loads plugin (A)
2. (A) gets system class loader
3. (A) add jar url to class loader
4. (A) try to load the desired class (B) in the jar using Class.forName

I am unable to load class B because I get a NoClassDefFoundError caused by B pointing to a class in A that was not found in the current classloader.

I think I need to use the same classloader ImageJ used for loading plugins on the first place to be able to load my modules and still find references to jars previously loaded by ImageJ. I would like to do this without having to recompile ImageJ.

Is there any way to fix this problem without having to indagate ImageJ code or having to modify it?

pseudo classes example

package a;
class A extends PlugInFrame {}


package a;
class C extends MyOwnPlugIn {}

---

package b;
import a;
class B extends C {}

Answer 1

The easiest way is to refrain from loading the class dynamically, but directly, by using it.

Absent that method, the best way is to make sure that the .jar files are all in ImageJ's plugins/ folder (or if you use Fiji, non-plugin .jar files live in jars/) and use ImageJ's plugin classloader: IJ.getClassLoader().loadClass("blub");

Note that your step 3 is a big no-no: you should never add elements to the system class loader's classpath.

Note also that all the ImageJ experts can be reached via imagej@list.nih.gov, not by posting on StackOverflow.

Answer 2

I'm not sure whether you need to be loading these classes dynamically but to get A's classloader

ClassLoader classLoader = A.getClass().getClassLoader()

Then use that to instatiate B bearing in mind that B needs to have a no argument constructor.

Why don't you build you build your plugin jar file incorporating B? I've created a bunch of plugins andgenerally I add dependancies and then build the plugin jar file incorporating all dependencies that it needs.