Question

I build a sample program demonstrate memory leak in java.

public class MemoryLeakTest {
     static int depth = 0;
     int number=0;
     MemoryLeakTest mobj;

     MemoryLeakTest(){
      number = depth;
      if(depth < 6500){
          depth++;
          mobj = new MemoryLeakTest();
      }
     }

     protected void finalize(){
      System.out.println(number + " released.");
     }

    public static void main(String[] args) {
        try{
             System.out.println(ManagementFactory.getMemoryMXBean().getHeapMemoryUsage());
             System.out.println("Free Memory in starting "+ Runtime.getRuntime().freeMemory());
             MemoryLeakTest testObj = new MemoryLeakTest();
             System.out.println("Free Memory in end "+ Runtime.getRuntime().freeMemory());
             System.out.println(ManagementFactory.getMemoryMXBean().getHeapMemoryUsage());
        }
        catch(Exception exp){}
        finally{
            System.out.println("Free Memory"+ Runtime.getRuntime().freeMemory());
            System.out.println(ManagementFactory.getMemoryMXBean().getHeapMemoryUsage());
        }
    }

}

I run it by changing value of N in if(depth < N). An here is the result;

when depth is 1000

init = 16777216(16384K) used = 288808(282K) committed = 16252928(15872K) max = 259522560(253440K)
Free Memory in starting 15964120
Free Memory in end 15964120
init = 16777216(16384K) used = 288808(282K) committed = 16252928(15872K) max = 259522560(253440K)
Free Memory 15964120
init = 16777216(16384K) used = 288808(282K) committed = 16252928(15872K) max = 259522560(253440K)

when depth is 1500

init = 16777216(16384K) used = 288808(282K) committed = 16252928(15872K) max = 259522560(253440K)
Free Memory in starting 15964120
Free Memory in end 15964120
init = 16777216(16384K) used = 288808(282K) committed = 16252928(15872K) max = 259522560(253440K)
Free Memory 15873528
init = 16777216(16384K) used = 379400(370K) committed = 16252928(15872K) max = 259522560(253440K)

when depth is 6000

init = 16777216(16384K) used = 288808(282K) committed = 16252928(15872K) max = 259522560(253440K)
Free Memory in starting 15964120
Free Memory in end 15692784
init = 16777216(16384K) used = 560144(547K) committed = 16252928(15872K) max = 259522560(253440K)
Free Memory 15692784
init = 16777216(16384K) used = 560144(547K) committed = 16252928(15872K) max = 259522560(253440K)

when depth is 6500 (Exception in thread “main” java.lang.StackOverflowError)

init = 16777216(16384K) used = 288808(282K) committed = 16252928(15872K) max = 259522560(253440K)
Free Memory in starting 15964120
Free Memory in end 15676656
init = 16777216(16384K) used = 576272(562K) committed = 16252928(15872K) max = 259522560(253440K)

My questions are;

1. It is not calling finalize(). Is it memory leak?
2. There is not change in free memory up to N=1000. But when N=1500 there is 2 different
   values for used memory at the end of the program ie 282K and 370K.
   Why does it so?
3. When N=6500, JVM generates error. So why last 2
   statements of try{} are executed.

Answer 1

Your program won’t “leak” as Java will take care of anything “dangling” out there. That’s the benefit of a garbage-collected language.

But what you do have is a StackOverFlow error. Basically, the stack (which is the chain of functions you’re in, and how deep that is) is much MUCH smaller than the heap. The heap is “more or less” the size of main memory. Each thread’s stack is much much smaller. Basically you’re reaching that limit by doing your “Depth” thing.

If you want to test “leaks” (or the idea that you won’t have any eventually) try something more like this:

public class MemoryLeakTest {
     int number=0;
     public MemoryLeakTest mobj;

     MemoryLeakTest(int num){
        number = num;
     }

     protected void finalize(){
        System.out.println(number + " released.");
     }

    public static void main(String[] args) {
        try{
             System.out.println(ManagementFactory.getMemoryMXBean().getHeapMemoryUsage());
             System.out.println("Free Memory in starting "+ Runtime.getRuntime().freeMemory());
             MemoryLeakTest first = new MemoryLeakTest(0);  // Keep a reference to one of them
             MemoryLeakTest current = first;
             for(int i = 1; i < Int.Parse(args[0]); i++) // forgive me, Java's been a while.  This may be C#.  But parse the first arg for your number of objects
             {
                 current.mobj = new MemoryLeakTest(i);
                 current = current.mobj;
             }
             System.out.println("Free Memory in end "+ Runtime.getRuntime().freeMemory());
             System.out.println(ManagementFactory.getMemoryMXBean().getHeapMemoryUsage());
        }
        catch(Exception exp){}
        finally{
            System.out.println("Free Memory"+ Runtime.getRuntime().freeMemory());
            System.out.println(ManagementFactory.getMemoryMXBean().getHeapMemoryUsage());
        }
    }

}

That will give you a “chain” of objects all in memory until first goes out of scope.