Solution for the ClassCastException due to ClassLoader issue

问题

I have two ClassLoaders which loads the same class. So, obviously these can't cast to one another. But I need to access an object created in the other ClassLoader.

I have access to both ClassLoaders. How can I use that object in the other class? I don't need to cast the object to match to the current ClassLoader.

But the issue is that the returned object's type is Object. So, I have to cast down that object to access some methods. How can I do that? Normal cast like the following causes ClassCastException, which I already know.

Mojo mojo = (Mojo) descriptor.getMojo();

descriptor#getMojo() returns an object of type Mojo but the method returns Object. How can do this?

Let me know if you need further info.

I've read all the theories about classloading, but none has specified a proper solution for this.

回答1:

AFAIK, no, you can't cast an object of a class loaded by one class-loader in another class loader.

One solution would be to create a "common" class-loader which loads the classes to be used by your custom classloaders. So in your case, you'd have a new classloader which would load the given class and your custom classloaders would extend this classloader.
Another solution would be to pass around the "serialized" state between the two classloaders. Serialize one instance to a byte array and reconstruct the object in the other classloader by de-serializing the object stream.

回答2:

Reflection isn't that bad, and is appropriate here.
Is this a Maven plugin, BTW?

You'll want something like:

Mojo mojo = (Mojo)descriptor.getClass().getMethod("getMojo").invoke(descriptor);

I'm leaving out a lot - particularly exception handling - but this should lead you to the Javadoc you need. It's quite good, but read carefully.

If you also have two Mojo classes, the cast will break, and you'll have to do more reflection to do whatever you need to do with the evil-twin Mojo.

回答3:

Why you have 2 CloassLoaders, which loads the same class? This could be a programatic issue. It sounds like you are caching ClassLoader somewhere and re-use them in a wrong way. If this is not the case try a MultiClassLoader.

Create a MultiClassLoader which includes multiple other classloader. These MultiClassLoader you can use to load all Classes you wish. But you have to create these MultiClassLoader at the very beginning and not when the classes are loaded.

public class MultiClassLoader extends ClassLoader

You would have a collection of classloaders and in the findClass(...) you iterate over all these registered loaders.

protected Class findClass(String aName) throws ClassNotFoundException {
   for (Iterator iter = multiLoaders.iterator(); iter.hasNext();) {
      ClassLoader tmpLoader = (ClassLoader)iter.next();
      try {
         return tmpLoader.loadClass(aName);
      } catch (ClassNotFoundException e) {
      }
   }
   throw new ClassNotFoundException(aName);
}

回答4:

The easiest way is to use reflection. This allow you to dó anything you Can dó in "normal" code.

回答5:

I think better option to just store byte array instead of object. While deserliazing, get byte array back and convert into object.

I had the same issue and byte array approach worked.

ByteArrayOutputStream bos = new ByteArrayOutputStream();
    ObjectOutput out = null;
    try {
        out = new ObjectOutputStream(bos);
        out.writeObject(cachedValue);
        byte b[] = bos.toByteArray();

        //Store in DB, file wherever here using b[]. I am not writing code to store it as it may vary in your requirement.

    } catch (IOException e) {
        e.printStackTrace();
    }

Read from byte array:

ByteArrayInputStream bis = new ByteArrayInputStream(<<read byte[] where you stored earlier>>);
    ObjectInput in = null;

    try {
        in = new ObjectInputStream(bis);
        <Your Class >cachedRes = ( Your Class) in.readObject();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }

来源：https://stackoverflow.com/questions/7074532/solution-for-the-classcastexception-due-to-classloader-issue

标签

java

classloader

classcastexception