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?