I recently had a memory leak in my Trekwar game, and to find it (or at least confirm it’s there) I used the WeakReference object in Java..

This is how I proceeded in locating the memory leak, this method could easily be adapted to other programs.

Trekwar is turn based, so each turn the client downloads a Galaxy object from the server. There are many references to this objects inside action listeners, threads, etc..

WeakReference
To make a long story short, a weak reference is like a normal reference, except it will not prevent the object it points to from being garbage collected.

This means that is you  have a normal and a weak reference to object A, the weak reference will be null if you remove the “hard” reference.

1) In the client main executable (the class that holds the method that updates the map from the server). Import, declare and make a list of weak references

Code block

import java.lang.ref.WeakReference;
private ArrayList<WeakReference<Galaxy>> weakRefs;
 
weakRefs = new ArrayList<WeakReference<Galaxy>>();

2) Find the place in your program where a new object is being added, in my place this is the localGalaxy object. Create a weak reference to this object, and list all your weak references.

Code block

localGalaxy = (Galaxy) objStream.readObject();
weakRefs.add(new WeakReference(localGalaxy));
for(int i = 0; i < weakRefs.size(); i++) {
System.out.print("ref # " + i + " = ");
try {
Galaxy testG = (Galaxy)weakRefs.get(i).get();
System.out.println(testG + ", tick = " + testG.currentTick);
}
catch(NullPointerException npe) {
System.out.println("null");
}
}

And you’re done!

Now when you run the program, you should see something like this each time the method is called

Code block

ref # 0 = org.aakretech.trekwar2.common.Galaxy@11ed166, tick = 37
ref # 1 = org.aakretech.trekwar2.common.Galaxy@9801f4, tick = 37
ref # 2 = org.aakretech.trekwar2.common.Galaxy@1c5d81c, tick = 38
ref # 3 = org.aakretech.trekwar2.common.Galaxy@101f8f4, tick = 39
ref # 4 = org.aakretech.trekwar2.common.Galaxy@1ed1e7e, tick = 40
ref # 5 = org.aakretech.trekwar2.common.Galaxy@e0a7ea, tick = 41

Unless you want to wait until your GC decides it’s time to go, use a tool like VisualVM to force the Garbage Collector to run.

After the GC has run, the output should look like this:

Code block

ref # 0 = null
ref # 1 = null
ref # 2 = null
ref # 3 = null
ref # 4 = null
ref # 5 = null
ref # 6 = org.aakretech.trekwar2.common.Galaxy@2b3c91, tick = 42

Now, this means that the object IS being garbage collected, and there is no memory leak.. If you run this and the object you are monitoring (in this example the localGalaxy object) is not GC’ed because of stray references, you can tell because none of the references will point to null.