从 class 文件看 synchronize 锁膨胀过程(偏向锁轻量级锁自旋锁重量级锁)

大家好，我是烤鸭:

前几天看马士兵老师的并发的课，里边讲到了 synchronize 锁的膨胀过程，今天想用代码演示一下。

1. 简单介绍

关于synchronize jdk 1.5 以后的优化，由重量级锁调整为膨胀过程。分别是偏向锁轻量级锁（自旋锁）重量级锁。

2. 用例编写

pom文件增加 jol的包，用于看对象头的信息。

<!-- https://mvnrepository.com/artifact/org.openjdk.jol/jol-core -->
<dependency><groupId>org.openjdk.jol</groupId><artifactId>jol-core</artifactId><version>0.10</version>
</dependency>

下面的注释已经写的挺清楚的了，关于锁几种状态的转换。

SyncSourceTest.java

package src.source;import org.openjdk.jol.info.ClassLayout;/*** Created by test on 2020/5/10*/
public class SyncSourceTest {static Object noLock;static Object biaseLock;static Object lightLock;static Object heavyLock;public static void main(String[] args) throws InterruptedException {noLock = new Object();// 无锁状态,由于print 方法是synchronize 修饰,其实打印语句就已经是加偏向锁了(如果满足下面的偏向锁条件)System.out.print("线程["+Thread.currentThread().getName()+"]:无锁状态对象布局:"+ClassLayout.parseInstance(noLock).toPrintable());// 偏向锁,由于JVM 默认偏向锁4s后开启,可以线程sleep.5 或者设置VM参数关闭延迟 -XX:BiasedLockingStartupDelay=0Thread.sleep(5000L);biaseLock = new Object();System.out.println("线程["+Thread.currentThread().getName()+"]:偏向锁状态对象布局:"+ClassLayout.parseInstance(biaseLock).toPrintable());// 轻量级锁,由于轻量级锁是偏向锁升级的,需要先给对象一个偏向锁,如果不加偏向锁，只有一个线程加锁变成偏向锁lightLock = new Object();synchronized (lightLock) {System.out.println("线程["+Thread.currentThread().getName()+"]:[轻量级锁提前加偏向锁]轻量级锁状态对象布局:"+ClassLayout.parseInstance(lightLock).toPrintable());}synLight();// 重量级锁heavyLock = new Object();synHeavy();}public static void synLight() throws InterruptedException {for (int i = 0; i < 1; i++) {getLightLock();}}public static void getLightLock() {new Thread(() -> {try {synchronized (lightLock){System.out.println("线程["+Thread.currentThread().getName()+"]:轻量级锁状态对象布局:"+ClassLayout.parseInstance(lightLock).toPrintable());}} catch (Exception e) {e.printStackTrace();}}).start();}public static void synHeavy() throws InterruptedException {for (int i = 0; i < 2; i++) {getHeavyLock();}}private static void getHeavyLock() {new Thread(() -> {try {synchronized (heavyLock){System.out.println("线程["+Thread.currentThread().getName()+"]:重量级锁状态对象布局:"+ClassLayout.parseInstance(heavyLock).toPrintable());}} catch (Exception e) {e.printStackTrace();}}).start();}
}

关于对象布局，我们就先不研究了，这里重点说一下对象头。

HotSpot虚拟机的对象头（Object Header）分为两部分，第一部分用于存储对象自身的运行时数据，如哈希码（HashCode）、GC分代年龄（Generational GC Age）等。这部分数据的长度在32位和64位的Java虚拟机中分别会占用32个或64个比特，官方称它为“Mark Word”。这部分是实现轻量级锁和偏向锁的关键。另外一部分用于存储指向方法区对象类型数据的指针，如果是数组对象，还会有一个额外的部分用于存储数组长度。

运行之后我们看一下输出信息：

线程[main]:无锁状态对象布局:java.lang.Object object internals:OFFSET  SIZE   TYPE DESCRIPTION                               VALUE0     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)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
线程[main]:偏向锁状态对象布局:java.lang.Object object internals:OFFSET  SIZE   TYPE DESCRIPTION                               VALUE0     4        (object header)                           05 00 00 00 (00000101 00000000 00000000 00000000) (5)4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total线程[main]:[轻量级锁提前加偏向锁]轻量级锁状态对象布局:java.lang.Object object internals:OFFSET  SIZE   TYPE DESCRIPTION                               VALUE0     4        (object header)                           05 28 e3 02 (00000101 00101000 11100011 00000010) (48441349)4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total线程[Thread-2]:重量级锁状态对象布局:java.lang.Object object internals:OFFSET  SIZE   TYPE DESCRIPTION                               VALUE0     4        (object header)                           ba ee 0d 26 (10111010 11101110 00001101 00100110) (638447290)4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total线程[Thread-0]:轻量级锁状态对象布局:java.lang.Object object internals:OFFSET  SIZE   TYPE DESCRIPTION                               VALUE0     4        (object header)                           88 f5 a4 29 (10001000 11110101 10100100 00101001) (698676616)4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total线程[Thread-1]:重量级锁状态对象布局:java.lang.Object object internals:OFFSET  SIZE   TYPE DESCRIPTION                               VALUE0     4        (object header)                           ba ee 0d 26 (10111010 11101110 00001101 00100110) (638447290)4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes totalProcess finished with exit code 0

可以看出无锁状态下各个状态下 mark word 后三位的值：

无锁：001

偏向：101

轻量级锁：000

重量级锁：010

本来想写一下，关于轻量级锁当前线程栈帧中 lock record 和 mark word的变化，无奈查了很多资料不知道在哪可以看到 lock record，有的说是显式或者隐式地创建lock record 空间，咱也不清楚了。更多关于轻量级锁的源码看这篇吧。

https://blog.csdn.net/z69183787/article/details/104502540?utm_source=app

3. 编译class文件

看下面的class文件，顺便说一下jvm的字节码指令。

可以看下 class文件里边对象的变化：

7: putstatic #3 // Field noLock:Ljava/lang/Object; 静态变量初始化时

39: getstatic #3 // Field noLock:Ljava/lang/Object; 获取静态变量

而到了偏向锁对象初始化之前，线程睡眠了5秒。

57: ldc2_w #16 // long 5000l ，5000入栈
60: invokestatic #18 // Method java/lang/Thread.sleep:(J)V ，执行 sleep

135: monitorenter 对应的这行代码：synchronized (lightLock)

184: monitorexit 加锁结束

190: monitorexit 后面又有一次加锁结束

原因是线程内部加锁后，调用 print 方法，又加了一次锁(重入锁)，所以需要释放两次。

D:\gitee\rep\leetcode-gradle\src\main\java\src\source> javac -classpath ".;D:\dev\repository\org\openjdk\jol\jol-core\0.10\jol-core-0.10.jar"  -encoding UTF-8 .\SyncSourceTest.javaD:\gitee\rep\leetcode-gradle\src\main\java\src\source> javap -c .\SyncSourceTest.class
Compiled from "SyncSourceTest.java"
public class src.source.SyncSourceTest {static java.lang.Object noLock;static java.lang.Object biaseLock;static java.lang.Object lightLock;static java.lang.Object heavyLock;public src.source.SyncSourceTest();Code:0: aload_01: invokespecial #1                  // Method java/lang/Object."<init>":()V4: returnpublic static void main(java.lang.String[]) throws java.lang.InterruptedException;Code:0: new           #2                  // class java/lang/Object3: dup4: invokespecial #1                  // Method java/lang/Object."<init>":()V7: putstatic     #3                  // Field noLock:Ljava/lang/Object;10: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;13: new           #5                  // class java/lang/StringBuilder16: dup17: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V20: ldc           #7                  // String 线程[22: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;25: invokestatic  #9                  // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;28: invokevirtual #10                 // Method java/lang/Thread.getName:()Ljava/lang/String;31: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;34: ldc           #11                 // String ]:无锁状态对象布局:36: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;39: getstatic     #3                  // Field noLock:Ljava/lang/Object;42: invokestatic  #12                 // Method org/openjdk/jol/info/ClassLayout.parseInstance:(Ljava/lang/Object;)Lorg/openjdk/jol/info/ClassLayout;45: invokevirtual #13                 // Method org/openjdk/jol/info/ClassLayout.toPrintable:()Ljava/lang/String;48: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;51: invokevirtual #14                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;54: invokevirtual #15                 // Method java/io/PrintStream.print:(Ljava/lang/String;)V57: ldc2_w        #16                 // long 5000l60: invokestatic  #18                 // Method java/lang/Thread.sleep:(J)V63: new           #2                  // class java/lang/Object66: dup67: invokespecial #1                  // Method java/lang/Object."<init>":()V70: putstatic     #19                 // Field biaseLock:Ljava/lang/Object;73: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;76: new           #5                  // class java/lang/StringBuilder79: dup80: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V83: ldc           #7                  // String 线程[85: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;88: invokestatic  #9                  // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;91: invokevirtual #10                 // Method java/lang/Thread.getName:()Ljava/lang/String;94: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;97: ldc           #20                 // String ]:偏向锁状态对象布局:99: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;102: getstatic     #19                 // Field biaseLock:Ljava/lang/Object;105: invokestatic  #12                 // Method org/openjdk/jol/info/ClassLayout.parseInstance:(Ljava/lang/Object;)Lorg/openjdk/jol/info/ClassLayout;108: invokevirtual #13                 // Method org/openjdk/jol/info/ClassLayout.toPrintable:()Ljava/lang/String;111: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;114: invokevirtual #14                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;117: invokevirtual #21                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V120: new           #2                  // class java/lang/Object123: dup124: invokespecial #1                  // Method java/lang/Object."<init>":()V127: putstatic     #22                 // Field lightLock:Ljava/lang/Object;130: getstatic     #22                 // Field lightLock:Ljava/lang/Object;133: dup134: astore_1135: monitorenter136: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;139: new           #5                  // class java/lang/StringBuilder142: dup143: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V146: ldc           #7                  // String 线程[148: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;151: invokestatic  #9                  // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;154: invokevirtual #10                 // Method java/lang/Thread.getName:()Ljava/lang/String;157: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;160: ldc           #23                 // String ]:[轻量级锁提前加偏向锁]轻量级锁状态对象布局:162: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;165: getstatic     #22                 // Field lightLock:Ljava/lang/Object;168: invokestatic  #12                 // Method org/openjdk/jol/info/ClassLayout.parseInstance:(Ljava/lang/Object;)Lorg/openjdk/jol/info/ClassLayout;171: invokevirtual #13                 // Method org/openjdk/jol/info/ClassLayout.toPrintable:()Ljava/lang/String;174: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;177: invokevirtual #14                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;180: invokevirtual #21                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V183: aload_1184: monitorexit185: goto          193188: astore_2189: aload_1190: monitorexit191: aload_2192: athrow193: invokestatic  #24                 // Method synLight:()V196: new           #2                  // class java/lang/Object199: dup200: invokespecial #1                  // Method java/lang/Object."<init>":()V203: putstatic     #25                 // Field heavyLock:Ljava/lang/Object;206: invokestatic  #26                 // Method synHeavy:()V209: returnException table:from    to  target type136   185   188   any188   191   188   anypublic static void synLight() throws java.lang.InterruptedException;Code:0: iconst_01: istore_02: iload_03: iconst_14: if_icmpge     167: invokestatic  #27                 // Method getLightLock:()V10: iinc          0, 113: goto          216: returnpublic static void getLightLock();Code:0: new           #28                 // class java/lang/Thread3: dup4: invokedynamic #29,  0             // InvokeDynamic #0:run:()Ljava/lang/Runnable;9: invokespecial #30                 // Method java/lang/Thread."<init>":(Ljava/lang/Runnable;)V12: invokevirtual #31                 // Method java/lang/Thread.start:()V15: returnpublic static void synHeavy() throws java.lang.InterruptedException;Code:0: iconst_01: istore_02: iload_03: iconst_24: if_icmpge     167: invokestatic  #32                 // Method getHeavyLock:()V10: iinc          0, 113: goto          216: return
}

4. 总结

有很多文章对 synchronize 分析过，我这里只是想使用代码演示一下各种场景，很多地方并没有深入到源码和原理层面。

简单来说，就是：

无锁：mark word 记录 hashcode和分代年龄。

单线程加锁（偏向锁），mark word 记录线程id。

偏向锁升级到轻量级锁，mark word 值替换为当前线程栈中的lock record 的指针。

轻量级锁到重量级锁：mark word 值重量级锁的指针。

其中自旋锁是轻量级锁到重量级锁发生的：

用CAS操作尝试把对象的Mark Word更新为指向Lock Record的指针，如果成功了，就是轻量级锁，失败了，就变成重量级锁。