Java Garbage Collection for String literals

Tonight I had a ponder about Javas garbage collection mechanism and what would make an object held by a WeakReference be garbage collected. The answer was quite simple, create a instance of a variable, create a WeakReference for it, null the instance and call System.gc(). I'm guessing that the instance would be garbage collected on the next natural sweep but because I was using a unit test I needed it to be done straight away.

However, garbage collection on string literals is far more interesting. See the unit tests.

import static org.junit.Assert.*;

import java.lang.ref.WeakReference;

import org.junit.Test;

public class ReferenceTest
{
    @Test
    public void testWeakReference()
    {
 Object reference = new Object();
 
 WeakReference<Object> weakRef = 
     new WeakReference<Object>(reference);
 
 assertNotNull(reference);
 
 reference = null;
 
 // reference will not be null 
 // without garbage collection here
 System.gc();
 
 assertNull(weakRef.get());
    }
    
    @Test
    public void testWeakStringReference()
    {
 String reference = new String();
 
 WeakReference<String> weakRef = 
     new WeakReference<String>(reference);
 
 assertNotNull(reference);
 
 reference = null;
 
 // reference will not be null 
 // without garbage collection here
 System.gc();
 
 assertNull(weakRef.get());
    }
    
    @Test
    public void testWeakStringLiteralReference()
    {
 String reference = "my reference";
 
 WeakReference<String> weakRef = 
     new WeakReference<String>(reference);
 
 assertNotNull(reference);
 
 reference = null;
 
 // does not matter if garbage collection 
 // is here or not but lets do it anyway
 System.gc();
 
 // !!!
 assertNotNull(weakRef.get());
    }
}

All of these tests pass. Notice the last test, even after the string instance has been nulled the WeakReference still has a not null value. After a bit of googling I found that string literals are referenced from Javas 'String Literal Pool' which is a cache for string literals so is not eligible for garbage collection!