Java Object 有多大? (转帖)

public class ObjectMemorySize {
	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();
			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
}