java对象占用heap的大小

一个对象占用的堆空间首先是和它有多少个成员变量直接相关

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)
        {
            A object = null;
            // Instantiate your data here and assign it to object
             object = new A ();
            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

class A{
	private String a="abbfffffffff";
	private  String b;
	private  String d;
	private  String e;
	private  String d1;
	private  String e1;
	private  String d2;
	private  String e2;
	public String getA() {
		return a;
	}
	public void setA(String a) {
		this.a = a;
	}
	public String getB() {
		return b;
	}
	public void setB(String b) {
		this.b = b;
	}
	public String getD() {
		return d;
	}
	public void setD(String d) {
		this.d = d;
	}
	public String getE() {
		return e;
	}
	public void setE(String e) {
		this.e = e;
	}
	
}

下面执行的结果是：
'before' heap: 552496, 'after' heap: 4552496
heap delta: 4000000, {class A} size = 40 bytes