偏向锁是什么

我不是女神ヾ 2022-12-29 14:28 179阅读 0赞

偏向锁操作流程偏向锁，顾名思义，它会偏向于第一个访问锁的线程，如果在接下来的运行过程中，该锁没有被其他的线程访问，则持有偏向锁的线程将永远不需要触发同步

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：

![b14e00828914dec8999b454358b6e4f5.png][]

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。

文章转自：[https://www.cnblogs.com/wuxun1997/archive/2004/01/13/14098987.html][https_www.cnblogs.com_wuxun1997_archive_2004_01_13_14098987.html]

[b14e00828914dec8999b454358b6e4f5.png]: /images/20221120/0af6fedd83134cd987053182565a10b2.png
[https_www.cnblogs.com_wuxun1997_archive_2004_01_13_14098987.html]: https://www.cnblogs.com/wuxun1997/archive/2004/01/13/14098987.html