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：
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。
