cast across classloader?

java classloader
IttayD picture IttayD · Apr 7, 2010 · Viewed 14k times · Source

How can I do this:

class Foo {
  public static Foo get() throws Exception {
    ClassLoader cl = new URLClassLoader(new URL[]{"foo.jar"}, null); // Foo.class is in foo.jar
    return (Foo)cl.loadClass("Foo").newInstance(); // fails on class cast
  }
}

What I need is for the JVM to consider the Foo instance from cl as if it is an instance of Foo from the classloader of the executing code.

I have seen these approaches, none of them good for me (the above example is a toy example):

  1. Load the class (or a separate interface) by a class loader that is a parent of both the calling code and created classloader
  2. Serialize and deserialize the object.

Answer

Michael Borgwardt picture Michael Borgwardt · Apr 7, 2010

Not possible. Class identity consists of the fully qualified name and the class loader.

Casting an object to a class with the same name loaded by different classloaders is no different than trying to cast a String to Integer, because those classes really could be completely different despite having the same name.

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