jdk 源码系列之ReentrantLock

原創

2020-11-10 18:58

If the current thread is the holder of this lock then the hold\n * count is decremented. If the hold count is now zero then the lock\n * is released. If the current thread is not the holder of this\n * lock then {@link IllegalMonitorStateException} is thrown.\n *\n * @throws IllegalMonitorStateException if the current thread does not\n * hold this lock\n */\n public void unlock() {\n // 携带 1\n sync.release(1);\n }\n\n\n public final boolean release(int arg) {\n // 释放成功则为 true 否则是 false\n if (tryRelease(arg)) {\n Node h = head;\n if (h != null && h.waitStatus != 0)\n // 唤醒队列后面的线程\n unparkSuccessor(h);\n // 释放成功\n return true;\n }\n // 释放失败\n return false;\n }\n\n\n\n protected final boolean tryRelease(int releases) {\n\n // c = 0\n int c = getState() - releases;\n // 如果不是当前线程和不是独占线程\n if (Thread.currentThread() != getExclusiveOwnerThread())\n // 直接抛出错\n throw new IllegalMonitorStateException();\n boolean free = false;\n if (c == 0) {\n // 释放锁\n free = true;\n // 独占线程设置null\n setExclusiveOwnerThread(null);\n }\n // 0\n setState(c);\n return free;\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"锁的释放，比较简单。将锁状态重新设置回 0，同时独占线程也设置null，之后唤醒后面的队列里面的线程，完成释放。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"源码总结"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"ReentrantLock"},{"type":"text","text":" 创建的时候，默认是非公平锁，不过你也可以在构造的时候，也可以创建一个公平锁。其中通过 "},{"type":"text","marks":[{"type":"strong"}],"text":"CAS"},{"type":"text","text":" 改变 "},{"type":"text","marks":[{"type":"strong"}],"text":"state"},{"type":"text","text":" 的状态来改变锁的数值， 0 表示有锁可以获取，1 表示锁已被获取，来设置锁的独占线程。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在公平锁的机制中，请求锁的线程会直接排到一个队列中（通过一个双向链表来模拟的队列）的最后一个，去获取锁。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"非公平锁的机制中，请求锁的线程首先会先通过 "},{"type":"text","marks":[{"type":"strong"}],"text":"CAS"},{"type":"text","text":" 来改变 "},{"type":"text","marks":[{"type":"strong"}],"text":"state"},{"type":"text","text":" 的锁状态，如果可以改变（0 -> 1），则直接获取到锁，将自身设置成独占锁。这样的好处就减少了一些进队列、加载队列、唤醒线程等性能消耗。如果未能修改到 "},{"type":"text","marks":[{"type":"strong"}],"text":"state"},{"type":"text","text":" 的状态，也会变成公平锁的机制，进入到队列的最后一个，等待到它去获取锁。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"锁的释放，将 "},{"type":"text","marks":[{"type":"strong"}],"text":"state"},{"type":"text","text":" 重新设置回 0，同时独占线程（你也可以认为这是持有锁的线程对象）设置null，之后唤醒排在它下个的线程。这一系列步骤做完，则宣告锁的释放。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"优缺点 "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"非公平锁"},{"type":"text","text":"，确实性能比较高。不过也有一个显而易见的缺点，我们可以想象一下，当你在排队吃饭的时候，轮到你吃饭的时候，这时候突然来一个人插在你前面，提前打饭了，导致你打饭时间变长了，如果这时候在有几个人在也突然插到你前打饭，又会继续导致你打饭时间变得更长。那如果放到线程里面，突然其他线程提前获取到了锁，那会导致当前线程获取到锁时间变长，而导致线程阻塞，迟迟未获取到锁。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以得根据业务去选择合适的锁类型，进行上锁，尽可能的避免有一些重要的业务因为上锁而阻塞到。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"声明"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"作者: Sinsy 本文链接：https://blog.sincehub.cn/2020/11/10/jdk-reentrantLock/ "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"版权声明：本文为博主原创文章，遵循 "},{"type":"link","attrs":{"href":"https://creativecommons.org/licenses/by-sa/4.0/deed.zh","title":null},"content":[{"type":"text","text":"CC 4.0 BY-SA"}]},{"type":"text","text":" 版权协议，转载请附上原文声明。如您有任何商业合作或者授权方面的协商，请给我留言：[email protected]"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"引用"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[1] "},{"type":"link","attrs":{"href":"https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/Lock.html","title":null},"content":[{"type":"text","text":"Lock (Java Platform SE 8 )"}]}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}