Weakhashmap : When value depends on key

Consider the following code:

import java.util.Map;
import java.util.WeakHashMap;

public class TestWeakHeakHashMap
{
 private String name = new String("java");
 private Map cache = new WeakHashMap<String, DependentObject>();

 public void testMethod()
 {
     cache.put(name, new DependentObject("1", name));

     //Discard the strong reference to the key
     name = null;
     while (true) {
         System.gc();
         /**
          * Verify Full GC with the -verbose:gc option
            Since there is no strong reference to the key, it is assumed that the
            entry has been removed from the WeakHashMap
          */
         System.out.println(cache.size());
     }
 }

 private class DependentObject
 {
     private String id;
     private String name;

     public DependentObject(String id, String name)
     {
         this.id = id;
         this.name = name;
     }
 }
}

Now when the testMethod() is run what do you expect the output to be? Since the strong reference to key is discarded, we assume that the entry from the map would be removed, and map would be empty after a full GC.
But that does not happen though.

Let us see what was the put operation on the WeakHashMap.

cache.put(name, new DependentObject("1", name));


Here the value DependentObject was holding the key name. This would mean that the value always strongly refers to the key, and hence the key would never be garbage collected. The entry would always remain the map.

This is what WeakHashMap API says - "The value objects in a WeakHashMap are held by ordinary strong references. Thus care should be taken to ensure that value objects do not strongly refer to their own keys, either directly or indirectly, since that will prevent the keys from being discarded."

Weakhashmap : Using string from literal pool as key

Consider the following code snippet:

public class TestWeakHashMap
{
   private String str1 = new String("newString1");
   private String str2 = "literalString2";
   private String str3 = "literalString3";
   private String str4 = new String("newString4");
   private Map map = new WeakHashMap();

   private void testGC() throws IOException
   {
       map.put(str1, new Object());
       map.put(str2, new Object());
       map.put(str3, new Object());
       map.put(str4, new Object());

       /**
        * Discard the strong reference to all the keys
        */
       str1 = null;
       str2 = null;
       str3 = null;
       str4 = null;

       while (true) {
           System.gc();
           /**
            * Verify Full GC with the -verbose:gc option
            * We expect the map to be emptied as the strong references to
            * all the keys are discarded.
            */
           System.out.println("map.size(); = " + map.size() + "  " + map);
       }
   }
}

What do we expect the size of the map to be after full GC? I initially thought it should be empty. But it turned out to be 2.

Look at the way the four Strings are initialized. Two of them are defined using the 'new' operator, whereas the other two are defined as literals. The Strings defined using the 'new' operator would be allocated in the Java heap, but the Strings defined defined as literals would be in the literal pool.
The Strings allocated in the literal pool (Perm Space) would never be garbage collected.
This would mean that String 'str2' and 'str3' would always be strongly referenced and the corresponding entry would never be removed from the WeakHashMap.

So next time you create a 'new String()' , put it as a key in a WeakHashMap, and later intern() the String, beware - Your key will always be strongly referenced.

Invoking intern() method on a String will add your String to the literal pool if some other String equal to this String does not exist in the pool

private String str5 = (str4+str1).intern();

WeakHashMap in java : Soft reference based hashmap

A WeakHashMap stores the keys using WeakReference objects, which means that as soon as the key is not referenced from somewhere else in your program, the entry may be removed and is available for garbage collection. Otherwise it is similar to HashMap.

One of the most common instances of memory leaks in Java is in hash maps, so Sun Microsystems (now Oracle) has provided a WeakHashMap to minimize memory usage in caches implemented with maps. A WeakHashMap stores the keys using WeakReference objects, which means that as soon as the key is not referenced from somewhere else in your program, the entry may be removed and is available for garbage collection. (Have a look at the JavaDocs for the java.util.WeakHashMap and java.lang.ref.WeakReference for more information). It is important to note that the WeakHashMap has a WeakReference to the key—rather than, as we would expect—the value.

Read about type of references here.

 

As the garbage collector may remove keys from the WeakHashMap and garbage collect the object, outputs from methods like size() , isEmpty() may vary with time. The size( ) method may return different values over time. The isEmpty( ) method may return false and then true.

 

Note: Value objects in the WeakHashMap will be garbage collected only if their key is removed and they have no other reference to them. It should be noted that if the value object has a reference to its own key object, the key objetc will not be garbage collected. This situation should be avoided.

WeakHashMap Example:

public class TestWeakHashMap {
   public static void main(String[] args) {
    WeakHashMap map=new WeakHashMap();

     String s1=new String("java");
     map.put(s1, "good");
     String s2=new String("java");
     map.put(s2,"ok");

   //Since s1.equals(s2) is true and hash is same, the earlier value
   //against key s1 ("good") in the map is replaced by the new one. ("ok")

     s1=null;

     System.gc();
     //Verify Full GC with the -verbose:gc option

    System.out.println(map.size());
  }
 }

Here s1 and s2 are two different objects on the heap. So in line 5, a new (key,value) pair with key s1 is put into the map. Later when a (key,value) with key s2 is being put into the map, it checks for equals on s1 and s2 and their hashcode. When it finds the equals returns true and hashCode is same, it replaces the value of the earlier entry with the new value. But the issue here is, WeakHashMap/HashMap does not replace the earlier key while adding a (key, value) pair whose key is actually a duplicate key in the map.
So even after putting an entry with key s2, the WeakHashMap has only one entry whose key refers to the object refered by s1 and not s2.

Now the object on the heap refered by s1, has one strong reference(through s1) and one weak reference through the WeakHashMap.
Later when I say s1=null, the object on the heap refered to by s1 lost the strong reference and when gc happens, the entry is removed from the map.

So thats how it works.

Also note WeakHashMap is only a wrapper over HashMap and the HashMap's put api says " If the map previously contained a mapping for this key, the old value is replaced by the specified value."

Also see -

• 
  
  Using string from literal pool as Key


• 
  
  When value depends on Key