Java Tip 130: Do you know your data size?

Don't pay the price for hidden class fields

By Vladimir Roubtsov, JavaWorld.com, 08/16/02

Once we got the prototype running, we naturally decided to profile the data memory footprint after it had been parsed and loaded from disk. The unsatisfactory initial results, however, prompted me to search for explanations.

Note: You can download this article's source code from Resources.

The tool

Since Java purposefully hides many aspects of memory management, discovering how much memory your objects consume takes some work. You could use the Runtime.freeMemory() method to measure heap size differences before and after several objects have been allocated. Several articles, such as Ramchander Varadarajan's "Question of the Week No. 107" (Sun Microsystems, September 2000) and Tony Sintes's "Memory Matters" (JavaWorld, December 2001), detail that idea. Unfortunately, the former article's solution fails because the implementation employs a wrong Runtime method, while the latter article's solution has its own imperfections:

• A single call to Runtime.freeMemory() proves insufficient because a JVM may decide to increase its current heap size at any time (especially when it runs garbage collection). Unless the total heap size is already at the -Xmx maximum size, we should use Runtime.totalMemory()-Runtime.freeMemory() as the used heap size.
• Executing a single Runtime.gc() call may not prove sufficiently aggressive for requesting garbage collection. We could, for example, request object finalizers to run as well. And since Runtime.gc() is not documented to block until collection completes, it is a good idea to wait until the perceived heap size stabilizes.
• If the profiled class creates any static data as part of its per-class class initialization (including static class and field initializers), the heap memory used for the first class instance may include that data. We should ignore heap space consumed by the first class instance.

Considering those problems, I present Sizeof, a tool with which I snoop at various Java core and application classes:

public class Sizeof
{
    public static void main (String [] args) throws Exception
    {
        // Warm up all classes/methods we will use
        runGC ();
        usedMemory ();
        // Array to keep strong references to allocated objects
        final int count = 100000;
        Object [] objects = new Object [count];
        
        long heap1 = 0;
        // Allocate count+1 objects, discard the first one
        for (int i = -1; i < count; ++ i)
        {
            Object object = null;
            
            // Instantiate your data here and assign it to object
            
            object = new Object ();
            //object = new Integer (i);
            //object = new Long (i);
            //object = new String ();
            //object = new byte [128][1]
            
            if (i >= 0)
                objects [i] = object;
            else
            {
                object = null; // Discard the warm up object
                runGC ();
                heap1 = usedMemory (); // Take a before heap snapshot
            }
        }
        runGC ();
        long heap2 = usedMemory (); // Take an after heap snapshot:
        
        final int size = Math.round (((float)(heap2 - heap1))/count);
        System.out.println ("'before' heap: " + heap1 +
                            ", 'after' heap: " + heap2);
        System.out.println ("heap delta: " + (heap2 - heap1) +
            ", {" + objects [0].getClass () + "} size = " + size + " bytes");
        for (int i = 0; i < count; ++ i) objects [i] = null;
        objects = null;
    }
    private static void runGC () throws Exception
    {
        // It helps to call Runtime.gc()
        // using several method calls:
        for (int r = 0; r < 4; ++ r) _runGC ();
    }
    private static void _runGC () throws Exception
    {
        long usedMem1 = usedMemory (), usedMem2 = Long.MAX_VALUE;
        for (int i = 0; (usedMem1 < usedMem2) && (i < 500); ++ i)
        {
            s_runtime.runFinalization ();
            s_runtime.gc ();
            Thread.currentThread ().yield ();
            
            usedMem2 = usedMem1;
            usedMem1 = usedMemory ();
        }
    }
    private static long usedMemory ()
    {
        return s_runtime.totalMemory () - s_runtime.freeMemory ();
    }
    
    private static final Runtime s_runtime = Runtime.getRuntime ();
} // End of class

Sizeof's key methods are runGC() and usedMemory(). I use a runGC() wrapper method to call _runGC() several times because it appears to make the method more aggressive. (I am not sure why, but it's possible creating and destroying a method call-stack frame causes a change in the reachability root set and prompts the garbage collector to work harder. Moreover, consuming a large fraction of the heap space to create enough work for the garbage collector to kick in also helps. In general, it is hard to ensure everything is collected. The exact details depend on the JVM and garbage collection algorithm.)

Note carefully the places where I invoke runGC(). You can edit the code between the heap1 and heap2 declarations to instantiate anything of interest.

Also note how Sizeof prints the object size: the transitive closure of data required by all count class instances, divided by count. For most classes, the result will be memory consumed by a single class instance, including all of its owned fields. That memory footprint value differs from data provided by many commercial profilers that report shallow memory footprints (for example, if an object has an int[] field, its memory consumption will appear separately).

The results

Let's apply this simple tool to a few classes, then see if the results match our expectations.

Note: The following results are based on Sun's JDK 1.3.1 for Windows. Due to what is and is not guaranteed by the Java language and JVM specifications, you cannot apply these specific results to other platforms or other Java implementations.

java.lang.Object

Well, the root of all objects just had to be my first case. For java.lang.Object, I get:

'before' heap: 510696, 'after' heap: 1310696
heap delta: 800000, {class java.lang.Object} size = 8 bytes

So, a plain Object takes 8 bytes; of course, no one should expect the size to be 0, as every instance must carry around fields that support base operations like equals(), hashCode(), wait()/notify(), and so on.

java.lang.Integer

My colleagues and I frequently wrap native ints into Integer instances so we can store them in Java collections. How much does it cost us in memory?

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