偏向锁是个什么东东?

  上次我们说了对象头是个什么东东?,但是从我们跑的代码输出却看不到偏向锁这个东东。为啥对象实例化出来之后,对象头里是不支持偏向的呢?其实是JVM搞的鬼,JVM虽然默认启用偏向锁,但启动后4秒内并不支持。可以通过-XX:BiasedLockingStartupDelay=0参数将JVM启动后支持偏向锁的延迟时间设置为0,这样就可以看到偏向锁的输出了:

偏向锁是个什么东东?

 

 

  代码也改动一下:

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 {

//        TimeUnit.SECONDS.sleep(5); // 启用偏向锁,要么开启jvm参数-XX:BiasedLockingStartupDelay=0,要么等待5秒

        // 实例化对象
        HelloWorld helloWorld = HelloWorld.builder().name("wlf").build();

        // 1、普通对象,无锁
        System.out.println("无锁,可偏向:");
        printfObjMarkWord(helloWorld);

        // 2、单线程首次加锁,偏向锁
        System.out.println("偏向锁:");
        new Thread(() -> {
            System.out.println("首次偏向:");
            synchronized (helloWorld) {
                printfObjMarkWord(helloWorld);
            }

            System.out.println("第二次,重偏向:");
            synchronized (helloWorld) {
                printfObjMarkWord(helloWorld);
                try {
                    TimeUnit.SECONDS.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
        TimeUnit.SECONDS.sleep(1);

        // 3、多个线程加锁,膨胀为重量锁
        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("-------------------------");

    }

}

 

  输出结果:

无锁,可偏向:
-------------------------
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
com.wlf.springcloudgateway.javabean.HelloWorld object internals:
 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)                           88 49 1c 17 (10001000 01001001 00011100 00010111) (387729800)
     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

-------------------------
偏向锁:
首次偏向:
-------------------------
com.wlf.springcloudgateway.javabean.HelloWorld object internals:
 OFFSET  SIZE               TYPE DESCRIPTION                               VALUE
      0     4                    (object header)                           05 00 93 19 (00000101 00000000 10010011 00011001) (429064197)
      4     4                    (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4                    (object header)                           88 49 1c 17 (10001000 01001001 00011100 00010111) (387729800)
     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

-------------------------
第二次,重偏向:
-------------------------
com.wlf.springcloudgateway.javabean.HelloWorld object internals:
 OFFSET  SIZE               TYPE DESCRIPTION                               VALUE
      0     4                    (object header)                           05 00 93 19 (00000101 00000000 10010011 00011001) (429064197)
      4     4                    (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4                    (object header)                           88 49 1c 17 (10001000 01001001 00011100 00010111) (387729800)
     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)                           9a 7b c2 02 (10011010 01111011 11000010 00000010) (46300058)
      4     4                    (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4                    (object header)                           88 49 1c 17 (10001000 01001001 00011100 00010111) (387729800)
     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)                           9a 7b c2 02 (10011010 01111011 11000010 00000010) (46300058)
      4     4                    (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4                    (object header)                           88 49 1c 17 (10001000 01001001 00011100 00010111) (387729800)
     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

 

  我们看到偏向锁在对象实例化后打开了(偏向锁状态为1,锁状态为01),第一次使用锁的时候就是用的偏向锁,持有锁的线程ID被写入对象头中,第二次该线程再次使用偏向锁,线程ID不变。随后我们使用两个其他线程来竞争该锁,偏向锁膨胀为重量锁(锁状态10),原线程ID变更为持有锁的线程指针。

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