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\ No newline at end of file diff --git a/docs/java/concurrent/images/interview-questions/synchronized-release-lock-block.png b/docs/java/concurrent/images/interview-questions/synchronized-release-lock-block.png new file mode 100644 index 00000000..c80a0030 Binary files /dev/null and b/docs/java/concurrent/images/interview-questions/synchronized-release-lock-block.png differ diff --git a/docs/java/concurrent/java并发进阶常见面试题总结.md b/docs/java/concurrent/java并发进阶常见面试题总结.md index 518e3cd1..91755431 100644 --- a/docs/java/concurrent/java并发进阶常见面试题总结.md +++ b/docs/java/concurrent/java并发进阶常见面试题总结.md @@ -125,9 +125,7 @@ public class SynchronizedDemo { ![synchronized关键字原理](https://my-blog-to-use.oss-cn-beijing.aliyuncs.com/2019-6/synchronized关键字原理.png) -从上面我们可以看出： - -**`synchronized` 同步语句块的实现使用的是 `monitorenter` 和 `monitorexit` 指令，其中 `monitorenter` 指令指向同步代码块的开始位置，`monitorexit` 指令则指明同步代码块的结束位置。** +从上面我们可以看出：**`synchronized` 同步语句块的实现使用的是 `monitorenter` 和 `monitorexit` 指令，其中 `monitorenter` 指令指向同步代码块的开始位置，`monitorexit` 指令则指明同步代码块的结束位置。** 当执行 `monitorenter` 指令时，线程试图获取锁也就是获取 **对象监视器 `monitor`** 的持有权。 @@ -137,7 +135,15 @@ public class SynchronizedDemo { 在执行`monitorenter`时，会尝试获取对象的锁，如果锁的计数器为 0 则表示锁可以被获取，获取后将锁计数器设为 1 也就是加 1。 -在执行 `monitorexit` 指令后，将锁计数器设为 0，表明锁被释放。如果获取对象锁失败，那当前线程就要阻塞等待，直到锁被另外一个线程释放为止。 +![执行 monitorenter 获取锁](./images/interview-questions/synchronized-get-lock-code-block.png) + +对象锁的的拥有者线程才可以执行 `monitorexit` 指令来释放锁。在执行 `monitorexit` 指令后，将锁计数器设为 0，表明锁被释放，其他线程可以尝试获取锁。 + + + +![执行 monitorexit 释放锁](./images/interview-questions/synchronized-release-lock-block.png) + +如果获取对象锁失败，那当前线程就要阻塞等待，直到锁被另外一个线程释放为止。 #### 1.3.2. synchronized 修饰方法的的情况 @@ -154,6 +160,8 @@ public class SynchronizedDemo2 { `synchronized` 修饰的方法并没有 `monitorenter` 指令和 `monitorexit` 指令，取得代之的确实是 `ACC_SYNCHRONIZED` 标识，该标识指明了该方法是一个同步方法。JVM 通过该 `ACC_SYNCHRONIZED` 访问标志来辨别一个方法是否声明为同步方法，从而执行相应的同步调用。 +如果是实例方法，JVM 会尝试获取实例对象的锁。如果是静态方法，JVM 会尝试获取当前 class 的锁。 + #### 1.3.3.总结 `synchronized` 同步语句块的实现使用的是 `monitorenter` 和 `monitorexit` 指令，其中 `monitorenter` 指令指向同步代码块的开始位置，`monitorexit` 指令则指明同步代码块的结束位置。 @@ -164,6 +172,8 @@ public class SynchronizedDemo2 { 相关推荐：[Java锁与线程的那些事 - 有赞技术团队](https://tech.youzan.com/javasuo-yu-xian-cheng-de-na-xie-shi/) 。 +🧗🏻进阶一下：学有余力的小伙伴可以抽时间详细研究一下对象监视器 `monitor`。 + ### 1.4. 说说 JDK1.6 之后的 synchronized 关键字底层做了哪些优化，可以详细介绍一下这些优化吗 JDK1.6 对锁的实现引入了大量的优化，如偏向锁、轻量级锁、自旋锁、适应性自旋锁、锁消除、锁粗化等技术来减少锁操作的开销。