計算java對象大小

/**
 * 
 * http://hi.baidu.com/whzkinger/blog/item/84e82b01295a500d7bec2c0e.html
 * 
 */
public class TestSizeOf {

    private static final Runtime s_runtime = Runtime.getRuntime();

    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.print("after heap(" + heap2 + ")-before heap(" + heap1 + ")=" + (heap2 - heap1)
                         + "\nclassName=" + objects[0].getClass().getName() + "\nsize=" + 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();
        long 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();
    }
}