How can I create a memory leak in Java?

Question

I just had an interview, and I was asked to create a memory leak with Java.

Needless to say, I felt pretty dumb having no clue on how to eve

Answer 1

Threads are not collected until they terminate. They serve as roots of garbage collection. They are one of the few objects that won't be reclaimed simply by forgetting about them or clearing references to them.

Consider: the basic pattern to terminate a worker thread is to set some condition variable seen by the thread. The thread can check the variable periodically and use that as a signal to terminate. If the variable is not declared volatile, then the change to the variable might not be seen by the thread, so it won't know to terminate. Or imagine if some threads want to update a shared object, but deadlock while trying to lock on it.

If you only have a handful of threads these bugs will probably be obvious because your program will stop working properly. If you have a thread pool that creates more threads as needed, then the obsolete/stuck threads might not be noticed, and will accumulate indefinitely, causing a memory leak. Threads are likely to use other data in your application, so will also prevent anything they directly reference from ever being collected.

As a toy example:

static void leakMe(final Object object) {
    new Thread() {
        public void run() {
            Object o = object;
            for (;;) {
                try {
                    sleep(Long.MAX_VALUE);
                } catch (InterruptedException e) {}
            }
        }
    }.start();
}

Call System.gc() all you like, but the object passed to leakMe will never die.

(*edited*)