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(每个对象各不相同)，Class Pointer用来指向生成该对象所在的类(认祖归宗)，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(hash)、GC分代年龄(age)、偏向锁(biased_lock)、锁状态(lock)和用来滥竽充数的间隔位(64位里那些unused开头的字段)。32位和64位机器存储这些东东的位大小各不相同。如果对象不再是普通对象(normal object)，变成了一个偏向锁(biased object)，那么markWord所含内容的哈希码就变成了持有锁的线程指针(JavaThread*)、偏向时间戳(epoch)，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，存储内容是栈帧中指向锁对象的指针(ptr)
01——无锁，至于是否偏向锁，需要再往前一位看，0：无锁，1：偏向锁。至于所存储之物，上面已提到：如果是普通对象(0)，存储内容就是上哈希码、GC分代年龄；若是偏向锁(1)，则是拥有锁的线程指针、偏向时间戳、对象分代年龄
10——监视器锁(moniter)，也就是重量锁，存储内容为指向锁对象的指针(ptr)
11——GC标记
好了，理论说完，下面举例说明：
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：
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("-------------------------");
}
}
import lombok.Data;
@Data
@Builder
public class HelloWorld {
private String name;
}
在跑之前给IDEA去掉指针压缩(jdk8默认开启)：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：
有锁、无锁，看标黄的00、01，是否偏向锁，看标红的那一位。同一个对象的class pointer是一致的，从标黄处也能看出来。普通对象是无锁(01)的，首次加锁后变成了轻量锁(00，为啥不是偏向锁？参见)。接着再另起两个线程竞争锁，原来的轻量锁膨胀为重量锁。这里有个问题：为啥偏向锁没有出来冒泡？答案参见偏向锁是个什么东东？
