面试：什么情况下，轻量级锁会升级为重量级锁？

上代码：

  1. public class MarkWordTest {
  2.     @Test
  3.     public  void test8() throws InterruptedException {
  4.         Thread.sleep(5000);
  5.         MyObject obj = new MyObject();
  6.         Thread t0 = new Thread(() -> {
  7.             synchronized (obj) {
  8.                 try {
  9.                     Thread.sleep(1000);
  10.                 } catch (InterruptedException e) {
  11.                     e.printStackTrace();
  12.                 }
  13.                 System.out.println(ClassLayout.parseInstance(obj).toPrintable() + "-------------------线程1 重量级锁 结束");
  14.             }
  15.         });
  16.         Thread t1 = new Thread(() -> {
  17.             synchronized (obj) {
  18.                 System.out.println(ClassLayout.parseInstance(obj).toPrintable()+ "-------------------线程2 重量级锁 结束");
  19.             }
  20.         });
  21.         t0.start();
  22.         t1.start();
  24.         Thread.sleep(20*1000);
  25.     }
  26. }
  1. 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esis\objenesis\2.6\objenesis-2.6.jar;D:\env\maven\repo\org\mockito\mockito-junit-jupiter\3.3.3\mockito-junit-jupiter-3.3.3.jar;D:\env\maven\repo\org\skyscreamer\jsonassert\1.5.0\jsonassert-1.5.0.jar;D:\env\maven\repo\com\vaadin\external\google\android-json\0.0.20131108.vaadin1\android-json-0.0.20131108.vaadin1.jar;D:\env\maven\repo\org\springframework\spring-core\5.2.7.RELEASE\spring-core-5.2.7.RELEASE.jar;D:\env\maven\repo\org\springframework\spring-jcl\5.2.7.RELEASE\spring-jcl-5.2.7.RELEASE.jar;D:\env\maven\repo\org\springframework\spring-test\5.2.7.RELEASE\spring-test-5.2.7.RELEASE.jar;D:\env\maven\repo\org\xmlunit\xmlunit-core\2.7.0\xmlunit-core-2.7.0.jar;D:\env\maven\repo\io\springfox\springfox-swagger2\2.9.2\springfox-swagger2-2.9.2.jar;D:\env\maven\repo\io\swagger\swagger-annotations\1.5.20\swagger-annotations-1.5.20.jar;D:\env\maven\repo\io\swagger\swagger-models\1.5.20\swagger-models-1.5.20.jar;D:\env\maven\repo\com\fasterxml\jackson\core\jackson-annotations\2.11.0\jackson-annotations-2.11.0.jar;D:\env\maven\repo\io\springfox\springfox-spi\2.9.2\springfox-spi-2.9.2.jar;D:\env\maven\repo\io\springfox\springfox-core\2.9.2\springfox-core-2.9.2.jar;D:\env\maven\repo\io\springfox\springfox-schema\2.9.2\springfox-schema-2.9.2.jar;D:\env\maven\repo\io\springfox\springfox-swagger-common\2.9.2\springfox-swagger-common-2.9.2.jar;D:\env\maven\repo\io\springfox\springfox-spring-web\2.9.2\springfox-spring-web-2.9.2.jar;D:\env\maven\repo\com\google\guava\guava\20.0\guava-20.0.jar;D:\env\maven\repo\com\fasterxml\classmate\1.5.1\classmate-1.5.1.jar;D:\env\maven\repo\org\slf4j\slf4j-api\1.7.30\slf4j-api-1.7.30.jar;D:\env\maven\repo\org\springframework\plugin\spring-plugin-core\1.2.0.RELEASE\spring-plugin-core-1.2.0.RELEASE.jar;D:\env\maven\repo\org\springframework\plugin\spring-plugin-metadata\1.2.0.RELEASE\spring-plugin-metadata-1.2.0.RELEASE.jar;D:\env\maven\repo\org\mapstruct\mapstruct\1.2.0.Final\mapstruct-1.2.0.Final.jar;D:\env\maven\repo\io\springfox\springfox-swagger-ui\2.9.2\springfox-swagger-ui-2.9.2.jar;D:\env\maven\repo\org\openjdk\jol\jol-core\0.10\jol-core-0.10.jar;D:\env\maven\repo\org\projectlombok\lombok\1.16.10\lombok-1.16.10.jar" com.intellij.rt.junit.JUnitStarter -ideVersion5 -junit4 com.oyb.jvm.test05.lesson09.MarkWordTest
  2. com.oyb.jvm.test03.lesson07.MyObject object internals:
  3. OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
  4. 0     4        (object header)                           1a 34 b3 1c (00011010 00110100 10110011 00011100) (481506330)
  5. 4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
  6. 8     4        (object header)                           a8 dd 00 f8 (10101000 11011101 00000000 11111000) (-134160984)
  7. 12     4        (loss due to the next object alignment)
  8. Instance size: 16 bytes
  9. Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
  10. -------------------线程1 重量级锁 结束
  11. com.oyb.jvm.test03.lesson07.MyObject object internals:
  12. OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
  13. 0     4        (object header)                           1a 34 b3 1c (00011010 00110100 10110011 00011100) (481506330)
  14. 4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
  15. 8     4        (object header)                           a8 dd 00 f8 (10101000 11011101 00000000 11111000) (-134160984)
  16. 12     4        (loss due to the next object alignment)
  17. Instance size: 16 bytes
  18. Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
  19. -------------------线程2 重量级锁 结束

什么是重量级锁？
自旋一直失败（达到一定程度），依然拿不到锁，就会阻塞，就会升级为重量级锁。
重量级锁依赖于操作系统的互斥量（mutext）实现的，而操作系统中线程间状态的转换需要相对比较长的时间
所以重量级锁效率很低，但被阻塞的线程不会消耗CPU。
重量级锁也是悲观锁的一种，自旋锁、轻量级锁与偏向锁属于乐观锁。

总结：

偏向锁的对象头MarkWord格式：偏向锁为0，锁类型为10
image.png