网站建站公司模板,网站建设与管理教学视频,临沂免费模板建站,seo网站优化方大家好#xff0c;我是烤鸭: 前几天看马士兵老师的并发的课#xff0c;里边讲到了 synchronize 锁的膨胀过程#xff0c;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#xff0c;由重量级锁调整为膨胀过程。分别是 偏向锁 轻量级锁#xff0…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 --
dependencygroupIdorg.openjdk.jol/groupIdartifactIdjol-core/artifactIdversion0.10/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:BiasedLockingStartupDelay0Thread.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_sourceapp
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。
