Question

I have read a lot about Java classloaders, but so far I have failed to find an answer for this simple question:

I have two versions of com.abc.Hello.class in jars v1.jar and v2.jar. I want to use both in my application. What is the simplest way of doing this ?

I don’t expect to be that simple, but something along these lines would be awesome :

Classloader myClassLoader = [magic that includes v1.jar and ignores v2.jar]
Hello hello = myclassLoader.load[com.abc.Hello]

And in a different class :

Classloader myClassLoader = [magic that includes v2.jar and ignores v1.jar]
Hello hello = myclassLoader.load[com.abc.Hello]

I would like to avoid using OSGi.

Answer 1

You’re going the right way. You must take some things into account.

The normal thing is classes that exist in parent classloaders are used. So if you want two versions those classes must not be there.

But if you want to interact you can use reflection, or even better a common interface. So I’ll do this:

common.jar:
BaseInterface

v1.jar:
SomeImplementation implements BaseInterface

v2.jar:
OtherImplementation implements BaseInterface

command-line:
java -classpath common.jar YourMainClass
// you don't put v1 nor v2 into the parent classloader classpath

Then in your program:

loader1 = new URLClassLoader(new URL[] {new File("v1.jar").toURL()}, Thread.currentThread().getContextClassLoader());
loader2 = new URLClassLoader(new URL[] {new File("v2.jar").toURL()}, Thread.currentThread().getContextClassLoader());

Class<?> c1 = loader1.loadClass("com.abc.Hello");
Class<?> c2 = loader2.loadClass("com.abc.Hello");

BaseInterface i1 = (BaseInterface) c1.newInstance();
BaseInterface i2 = (BaseInterface) c2.newInstance();