Are WeakHashMap cleared during a full GC?

Question

I encountered some troubles with WeakHashMap.

Consider this sample code:

List<byte[]> list = new ArrayList<byte[]>();

Map<String, Calendar> map = new WeakHashMap<String, Calendar>();
String anObject = new String("string 1");
String anOtherObject = new String("string 2");

map.put(anObject, Calendar.getInstance());
map.put(anOtherObject, Calendar.getInstance());
// In order to test if the weakHashMap works, i remove the StrongReference in this object
anObject = null;
int i = 0;
while (map.size() == 2) {
   byte[] tab = new byte[10000];
   System.out.println("iteration " + i++ + "map size :" + map.size());
   list.add(tab);
}
System.out.println("Map size " + map.size());

This code works. Inside the loops, i'm creating object.When a minor GC occurs, the map size is equal to 1 at the 1360th iteration. All is OK.

Now when i comment this line:

//anObject = null; 

I expect to have an OutOfMemoryError because the mapSize is always equal to 2. However at the 26XXX th iteration, a full GC occurs and the map size is equal to 0. I dont understand why?

I thought that the map shouldn't have cleared because there are also strong references to both objects.

Answer 1

The just-in-time compiler analyzes the code, sees that anObject and anOtherObject are not used after the loop, and removes them from the local variable table or sets them to null, while the loop is still running. This is called OSR compilation.

Later the GC collects the strings because no strong references to them remain.

If you used anObject after the loop you'd still get an OutOfMemoryError.

Update: You'll find a more detailed discussion about OSR compilation in my blog.