JConsole 中的内存泄漏是什么样的？

Question

I am trying to use JConsole to see if I have a memory leak but do not know what to look for. If I had to guess, the memory usage should always go up, despite garbage collection, like this:

As you can see in my other SO question, I'm seeing a jagged edge, where the memory usage goes up—even if the browser is closed and no requests are hitting my local Tomcat server—and then go down.

What does a memory leak in Java "look like" in JConsole?

Answer 1

I think the best way to learn what a memory leak looks like is to do an experiment. Try something like this to create a memory leak:

    Collection<Object> data = new LinkedList<>();
    while(true) {
        long[] canBeGarbageCollected = new long[10000];
        long[] canNotBeGarbageCollected = new long[100];
        data.add(Arrays.asList(canNotBeGarbageCollected));
    }

Besides JConsole you also have VisualVM that has a nice plugin called "Visual GC". Java Mission Control with Flight Recording can give you a lot of details. A very powerful tool exists on the command line as well:

jstat -gcutil -h20 $PID 1000

This will show that a memory leak will eventually lead to 100% Old space utilization (O) and the CPU will spend more and more time doing full garbage collections (FGCT)