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。