对象头，顾名思义，就是对象的头。对象是实例化出来的，实例化的前提是必须有类这个模板。举个不大恰当的例子，人类就是个类，你我他就是人类实例化出来的对象。我们的头，自然就是对象头。我们的头有口鼻眼耳，对象头也有一些东西，主要包含两部分：Mark Word（标记字）和Class Pointer（类指针），如果是数组对象还得再加一项Array Length（数组长度）。
对象头为啥要有这些东东？Mark Word用来标记运行时信息，Class Pointer用来指向生成该对象所在的类，Array Length告诉我们数组的长度。后两项简单易懂，最复杂的是这个Mark Word了，看下JVM的markWord.hpp是怎么规定它的：
// The markWord describes the header of an object.
//
// Bit-format of an object header (most significant first, big endian layout below):
//
// 32 bits: // -------- // hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object) // JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object) // // 64 bits: // -------- // unused:25 hash:31 -->| unused_gap:1 age:4 biased_lock:1 lock:2 (normal object) // JavaThread*:54 epoch:2 unused_gap:1 age:4 biased_lock:1 lock:2 (biased object)
它说markWord描述了对象头，采用大端点表示，markWord包含了哈希码、GC分代年龄、偏向锁、锁状态和用来滥竽充数的间隔位。32位和62位机器存储这些东东的大小各不相同。如果对象不再是普通对象，变成了一个偏向锁，那么markWord所含内容的哈希码就变成了持有锁的线程指针、偏向时间戳，其他不变。
最后再说这个锁状态：
// - the two lock bits are used to describe three states: locked/unlocked and monitor. // // [ptr | 00] locked ptr points to real header on stack // [header | 0 | 01] unlocked regular object header // [ptr | 10] monitor inflated lock (header is wapped out) // [ptr | 11] marked used to mark an object
00——轻量锁
01——无锁，偏向锁就再往前一位看，0：无锁，1：偏向锁
10——监视器锁（moniter)，也就是重量锁
11——GC标记
好了，举例说明：
1、pom.xml新增jar包：
<dependency> <groupId>org.openjdk.jol</groupId> <artifactId>jol-core</artifactId> <version>0.14</version> </dependency>
2、新增测试类：
/* * Licensed to Elasticsearch under one or more contributor * license agreements. See the NOTICE file distributed with * this work for additional information regarding copyright * ownership. Elasticsearch licenses this file to you under * the Apache License, Version 2.0 (the "License"); you may * not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, * software distributed under the License is distributed on an * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY * KIND, either express or implied. See the License for the * specific language governing permissions and limitations * under the License. */ package com.wlf.springcloudgateway.concurrent; import com.wlf.springcloudgateway.javabean.HelloWorld; import org.openjdk.jol.info.ClassLayout; import java.util.concurrent.TimeUnit; public class ObjectMarkWord { public static void main(String[] args) throws InterruptedException { // 实例化对象 HelloWorld helloWorld = HelloWorld.builder().name("wlf").build(); // 1、普通对象，无锁 System.out.println("普通对象："); printfObjMarkWord(helloWorld); // 2、实例化数组，打印数组对象信息，无锁 System.out.println("数组对象："); String[] address = new String[]{"hangzhou", "nanjing", "shenzhen"}; printfObjMarkWord(address); // 3、单线程首次加锁，轻量锁 System.out.println("轻量锁："); new Thread(() -> { synchronized (helloWorld) { printfObjMarkWord(helloWorld); } }).start(); TimeUnit.SECONDS.sleep(2); // 4、多个线程加锁，升级重量锁 System.out.println("重量锁："); for (int i = 0; i < 2; i++) { new Thread(() -> { synchronized (helloWorld) { printfObjMarkWord(helloWorld); } }).start(); } } private static void printfObjMarkWord(Object obj) { System.out.println("-------------------------"); System.out.println(ClassLayout.parseInstance(obj).toPrintable()); System.out.println("-------------------------"); } }
在跑之前给IDEA去掉指针压缩：Run -> Edit Configurations -> VM:options那一栏加上-XX:-UseCompressedOops
输出结果：
普通对象： ------------------------- com.wlf.springcloudgateway.javabean.HelloWorld object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 88 49 0e 17 (10001000 01001001 00001110 00010111) (386812296) 12 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 16 8 java.lang.String HelloWorld.name (object) Instance size: 24 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total ------------------------- 数组对象： ------------------------- [Ljava.lang.String; object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 20 e9 dd 16 (00100000 11101001 11011101 00010110) (383641888) 12 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 16 4 (object header) 03 00 00 00 (00000011 00000000 00000000 00000000) (3) 20 4 (alignment/padding gap) 24 24 java.lang.String String;.<elements> N/A Instance size: 48 bytes Space losses: 4 bytes internal + 0 bytes external = 4 bytes total ------------------------- 轻量锁： ------------------------- com.wlf.springcloudgateway.javabean.HelloWorld object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 60 f4 05 1a (01100000 11110100 00000101 00011010) (436597856) 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 88 49 0e 17 (10001000 01001001 00001110 00010111) (386812296) 12 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 16 8 java.lang.String HelloWorld.name (object) Instance size: 24 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total ------------------------- 重量锁： ------------------------- com.wlf.springcloudgateway.javabean.HelloWorld object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) ca da 66 17 (11001010 11011010 01100110 00010111) (392616650) 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 88 49 0e 17 (10001000 01001001 00001110 00010111) (386812296) 12 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 16 8 java.lang.String HelloWorld.name (object) Instance size: 24 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total ------------------------- ------------------------- com.wlf.springcloudgateway.javabean.HelloWorld object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) ca da 66 17 (11001010 11011010 01100110 00010111) (392616650) 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 88 49 0e 17 (10001000 01001001 00001110 00010111) (386812296) 12 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 16 8 java.lang.String HelloWorld.name (object) Instance size: 24 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total ------------------------- Process finished with exit code 0
我本机是64位的，所以markWord、class pointer、array length都是8字节：
至于有锁、无锁，看标黄的00、01，同一个对象的class pointer是一致的，从标黄处也能看出来。
原文：https://www.cnblogs.com/wuxun1997/p/14075463.html