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The Interesting Leak

Does you appllication use a WeakHashMap with a String key? Pause. Here is something you need to know, while working with such a WeakHashMap. This article is extracted from couple of posts made on my blog.

Part-1

This again is one of those interesting observations I ran into during some memory optimizations CXF. Coincidently here also its a WeakHashMap.
Consider the code snippet below.

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]

Part-2

This again is a follow up of my previous post - The Interesting Leak.
I was quite amused after reading Markus' follow-up post An interesting leak when using WeakHashMaps.
What surprised me most was If you put

 ("abc" + "def").intern();

as key in the WeakHashMap, it doesn't get GC'd whereas

(new String("abc") + "def").intern()

as key leads to the entry being garbage collected.
Huh!
Tried all combinations. No clarity. Last resort - Pinged Rajiv.
And so went the conversation -

Me : If you put ("abc" + "def").intern(); as key, it doesnt get GC'd but if you put (new String("abc") + "def").intern() it gets GC'd
Rajiv : Decompile and see if "abc"+"def" is being converted to "abcdef" by javac
Me: Yes it is. So?
[This could be the clue. Am still thinking.. tick tick tick]

Rajiv: Check if "abcdef"== (new String("abc") + "def").intern()
Me: It is... printed the identitity hashcodes.
Rajiv: In the class you have both "abcdef" and (new String("abc") + "def").intern() and still
(new String("abc") + "def").intern() gets gc'ed?
Me: God! Then it doesn't get gc'd.
[Now Rajiv cracks it -]

Rajiv: "I think intern is weak map and constant pool has a strong ref"
Me: ohh!
Me: In that case (new String("abc") ).intern(); should get GC'd right? But we saw it doesn't. The maya happens only when someString is '+'d to (new String("abc")) and then the resultant String is interned.
Me: Just (new String("abc")).intern() doesnt get GC'd.
Rajiv: When you say (new String("abc")).intern() there is a string "abc" in constant pool.
Me: Yes "abc" in constant pool would be the literal we created and passed as argument to the String constructor.
Rajiv: (new String("abc")).intern() returns that string. So wont get gc'ed
Me: Oh yeah. Got it!
Me: So only when you do a "+" you get a String which is not there in constant pool and hence it gets GC'd ...
Rajiv: ya right.

I had earlier thought of intern pool and constant pool to be the same. But Rajiv 's prediction of intern being a weak map and constant pool holding a strong ref looks quite convincing.
Oo la.. That solved our mystery.

Originally Posted at: http://thoughts.bharathganesh.com

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