上次我們說了對象頭是個什么東東？，但是從我們跑的代碼輸出卻看不到偏向鎖這個東東。為啥對象實例化出來之后，對象頭里是不支持偏向的呢？其實是JVM搞的鬼，JVM雖然默認啟用偏向鎖，但啟動后4秒內並不支持。可以通過-XX:BiasedLockingStartupDelay=0參數將JVM啟動后支持偏向鎖的延遲時間設置為0，這樣就可以看到偏向鎖的輸出了：
代碼也改動一下：
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); // 啟用偏向鎖，要么開啟jvm參數-XX:BiasedLockingStartupDelay=0，要么等待5秒 // 實例化對象 HelloWorld helloWorld = HelloWorld.builder().name("wlf").build(); // 1、普通對象，無鎖 System.out.println("無鎖，可偏向："); printfObjMarkWord(helloWorld); // 2、單線程首次加鎖，偏向鎖 System.out.println("偏向鎖："); 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、多個線程加鎖，膨脹為重量鎖 System.out.println("重量鎖："); 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("-------------------------"); } }
輸出結果：
無鎖，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 ------------------------- 偏向鎖： 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 ------------------------- 重量鎖： ------------------------- 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
我們看到偏向鎖在對象實例化后打開了（偏向鎖狀態為1，鎖狀態為01），第一次使用鎖的時候就是用的偏向鎖，持有鎖的線程ID被寫入對象頭中，第二次該線程再次使用偏向鎖，線程ID不變。隨后我們使用兩個其他線程來競爭該鎖，偏向鎖膨脹為重量鎖（鎖狀態10），原線程ID變更為持有鎖的線程指針。