jvm两种方式获取对象所占用的内存

在开发过程中，我们有时需要来获取某个对象的大小，以方便我们参考，来决定开发的技术方案。jvm中提供了两种方式来获取一个对象的大小。

通过Instrumentation来计算对象的大小

• 编写计算代码：

package com.java.basic;

import java.lang.instrument.Instrumentation;
import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.IdentityHashMap;
import java.util.Map;
import java.util.Stack;

public class SizeOfAgent  
{  
    private static Instrumentation inst;  

    /** initializes agent */  
    public static void premain(String agentArgs, Instrumentation instP)   
    {  
        inst = instP;  
    }  

    /** 
     * Returns object size without member sub-objects. 
     * @param o object to get size of 
     * @return object size 
     */  
    public static long sizeOf(Object o)   
    {  
        if(inst == null)   
        {  
            throw new IllegalStateException("Can not access instrumentation environment.\n" +  
                            "Please check if jar file containing SizeOfAgent class is \n" +  
                            "specified in the java's \"-javaagent\" command line argument.");  
        }  
        return inst.getObjectSize(o);  
    }  

    /** 
     * Calculates full size of object iterating over 
     * its hierarchy graph. 
     * @param obj object to calculate size of 
     * @return object size 
     */  
    public static long fullSizeOf(Object obj)   
    {  
        Map visited = new IdentityHashMap();  
        Stack