Question

I’ve a very simple class which has one integer variable. I just print the value of variable ‘i’ to the screen and increment it, and make the thread sleep for 1 second. When I run a profiler against this method, the memory usage increases slowly even though I’m not creating any new variables. After executing this code for around 16 hours, I see that the memory usage had increased to 4 MB (initially 1 MB when I started the program). I’m a novice in Java. Could any one please help explain where am I going wrong, or why the memory usage is gradually increasing even when there are no new variables created? Thanks in advance.

I’m using netbeans 7.1 and its profiler to view the memory usage.

    public static void main(String[] args)
    {
        try
        {
            int i = 1;
            while(true)
            {
                System.out.println(i);
                i++;
                Thread.sleep(1000);
            }
        }
        catch(InterruptedException ex)
        {
            System.out.print(ex.toString());
        }
    }

Initial memory usage when the program started : 1569852 Bytes.

Memory usage after executing the loop for 16 hours : 4095829 Bytes

Answer 1

It is not necessarily a memory leak. When the GC runs, the objects that are allocated (I presume) in the System.out.println(i); statement will be collected. A memory leak in Java is when memory fills up with useless objects that can’t be reclaimed by the GC.

The println(i) is using Integer.toString(int) to convert the int to a String, and that is allocating a new String each time. That is not a leak, because the String will become unreachable and a candidate for GC’ing once it has been copied to the output buffer.

Other possible sources of memory allocation:

• Thread.sleep could be allocating objects under the covers.

• Some private JVM thread could be causing this.

• The “java agent” code that the profiler is using to monitor the JVM state could be causing this. It has to assemble and send data over a socket to the profiler application, and that could well involve allocating Java objects. It may also be accumulating stuff in the JVM’s heap or non-heap memory.

But it doesn’t really matter so long as the space can be reclaimed if / when the GC runs. If it can’t, then you may have found a JVM bug or a bug in the profiler that you are using. (Try replacing the loop with one very long sleep and see if the “leak” is still there.) And it probably doesn’t matter if this is a slow leak caused by profiling … because you don’t normally run production code with profiling enabled for that long.

---

Note: calling System.gc() is not guaranteed to cause the GC to run. Read the javadoc.