Do not search highest classloader

[jerrinot]

--Problem--
Short Version:
Our classloading assumes a hiearchical parent-first delegation model.
This assumption is not always right.

Long Version:
Imagine this classloader hiearchy: childCL -> parentCL
Both childCl and parentCL can access the same JAR. The childCL does
not simply delegate to its parent, it loads (&define) classes on its own.

Now Hazelcast is loaded by the childCL. It's loading hooks with service
definitions when it's starting. It finds META-INF/service/c.h.hook resource
can be loaded by the childCl. However when the findHighestReachableClassLoader()
is used then it will also find the parentCL can load the resource as well (because both classloaders point to the same physical JAR thus they both have a resource with the same URL available) ->
Hazelcast will use the parentCl to load the hook. However the classes loaded
by parentCL are not the same as classes loaded by childCL -> ClassCastException.

Solution:
Do not search for the highest available classloader.

Problem with the solution:
We now might have multiple classloaders loading the same resource
-> Duplicate registration

Solution of the problem:
De-duplication in the ClassIterator.

[mmedenjak]

Looks fine, except for one detail

[mmedenjak]

IntelliJ is warning me this line can cause NPE. I believe it can, since the ArrayList can contain null and it could be added to the list as a result of this:

        ClassLoader tccl = Thread.currentThread().getContextClassLoader();
        if (tccl != classLoader) {
            classLoaders.add(tccl);
        }

Where getContextClassLoader can return null:

     * @return  the context ClassLoader for this Thread, or {@code null}
     *          indicating the system class loader (or, failing that, the
     *          bootstrap class loader)

[mmedenjak]

I was just thinking about this - this would eliminate classes that are equal. That includes the check if the class was loaded by the same classloader. So this check only eliminates same classes loaded by different classloaders? So for example, we had one classloader A loading the service definitions from one JAR and a different classloader B loading the service defintions from a different JAR. If both of them have the same parent and delegate to this parent, this check would eliminate duplicate service definitions from different JARs?