synchronized 锁的升级过程

测试代码

为了更清楚的看 synchronized 的锁的升级的过程,我们用代码来打印对象的布局,使用的类库是:

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<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.14</version>
<scope>provided</scope>
</dependency>

具体代码如下:

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static A obj;
// -XX:BiasedLockingStartupDelay=0 偏向锁开关
// -XX:+PrintFlagsInitial 打印所有参数
public static void main(String[] args) throws InterruptedException {
obj = new A();
// Thread.sleep(60000);
System.out.println(ClassLayout.parseInstance(obj).toPrintable());
printHeader();
// obj.hashCode();
printHeader();

Thread thread = new Thread(() -> {
printHeader();
});
Thread thread2 = new Thread(() -> {
printHeader();
});
thread.start();
thread2.start();
thread.join();
thread2.join();
}

private static void printHeader() {
synchronized (obj) {
System.out.println(ClassLayout.parseInstance(obj).toPrintable());
System.out.println("========");
}

}

日志打印结果如下:

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) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

由于 java 的对象是小端存储,所以真实的展示和实际书写是相反的方向,实际的例子中,展示的 001 代表锁的内容。

默认状态

1. 未加锁 ,锁状态是 001(无锁不可偏向)

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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) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

2. 主线程进入,锁升级为 00(轻量级锁)

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) c0 f5 81 02 (11000000 11110101 10000001 00000010) (42071488)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

3. 线程 1 进入,无其他线程竞争 00

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) c0 f5 81 02 (11000000 11110101 10000001 00000010) (42071488)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

4. 线程 2 等线程 1 执行玩后进入,无其他线程竞争 00

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) 38 f6 6c 1f (00111000 11110110 01101100 00011111) (527234616)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

5. 线程 1 和线程 2 同时进入 ,存在竞争 ,升级位重量级锁 10

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) 1a e7 ec 02 (00011010 11100111 11101100 00000010) (49080090)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

◆ 打开偏向锁开关

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-XX:BiasedLockingStartupDelay=0 

1. 未加锁 锁状态 101,无锁可偏向,实际是偏向锁但是没有存储线程 ID,所以还是无锁状态

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 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) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

2. 主线程进入,锁状态 101,存储了线程 ID,转成真实的偏向锁。

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) 05 40 41 03 (00000101 01000000 01000001 00000011) (54607877)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

3. 线程 1 进入,无其他线程竞争,锁状态 00 (轻量锁)

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) d8 f3 7e 1e (11011000 11110011 01111110 00011110) (511636440)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

4. 线程 2 进入,无其他线程竞争,锁状态 00 (轻量锁)

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) a8 ef 7e 1e (10101000 11101111 01111110 00011110) (511635368)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

5. 线程 1 和线程 2 同时进入 ,存在竞争 10

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) 5a e5 d1 02 (01011010 11100101 11010001 00000010) (47310170)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

◆ 打开偏向锁开关—-同时运行对象的HashCode

1. 未加锁 锁状态 101,无锁可偏向,实际是偏向锁但是没有存储线程 ID,所以还是无锁状态

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 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) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

2. 运行 hashcode 后,锁状态升级位无锁不可偏向 ,原本存放线程 ID 的位置被 hashcode 覆盖

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) 05 40 62 02 (00000101 01000000 01100010 00000010) (39993349)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

3. 线程 1 进入,无锁竞争,锁升级为轻量锁 000

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) 90 f1 93 00 (10010000 11110001 10010011 00000000) (9695632)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

4. 线程 2 进入,存在锁竞争,锁升级为轻量锁 010

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OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0 4 (object header) 0a e5 71 02 (00001010 11100101 01110001 00000010) (41018634)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 47 c1 00 f8 (01000111 11000001 00000000 11111000) (-134168249)
12 4 (loss due to the next object alignment)

◆ 锁升级基本流程

image-20210321220112848

作者

付威的网络博客

发布于

2021-03-06

更新于

2021-03-06

许可协议

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