Why is heap memory going up together with metaspace in Java 8?

Question

I'm doing a little test to understand how metaspace memory (Java 8 onwards) works. When I create 100,000 classes dynamically the metaspace memory is growing (obviously) but heap memory is going up too. Can someone explain to me why this occurs?

PS: I'm running the test with 128 MB of heap and 128 MB of metaspace.

@Test
public void metaspaceTest() throws CannotCompileException, InterruptedException {

ClassPool cp = ClassPool.getDefault();
System.out.println("started");

for (int i = 0; i <= 100000; i++) {
    Class c = cp.makeClass("br.com.test.GeneratedClass" + i).toClass();
    Thread.sleep(1);
    if (i % 10000 == 0) {
    System.out.println(i);
    }
}

System.out.println("finished");
}

See the images below:

Answer 1

Your class pool uses heap memory. It's got hashtables and lists and other things. It also uses java reflection code which uses heap memory. Heap memory that is not gc'ed is probably all those data structures in class pool, a couple hashtables, linked lists, array lists, pools, etc... For instance, every single class you create is stored in a hashtable by the class pool. That is a 100,000 element hash table.

Secondly, if there are any static initializers in the classes you are creating, that will use heap memory. Static initializers include the static field initializations and the static block code.