計算對象所佔的內存空間

package observer;

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();

}