Java-技术专题-AQS和Volatile和Synchronized实现原理

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"本文参考："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://links.jianshu.com/go?to=https%3A%2F%2Fblog.csdn.net%2Fu010670411%2Farticle%2Fdetails%2F87921138","title":null},"content":[{"type":"text","text":"JUC学习（八）：AQS的CLH队列"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://www.jianshu.com/p/4682a6b0802d","title":null},"content":[{"type":"text","text":"并发编程——详解 AQS CLH 锁"}]},{"type":"link","attrs":{"href":"https://www.jianshu.com/p/8a58d8335270","title":null},"content":[{"type":"text","text":"JMM和底层实现原理"}]}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"AQS"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":" "},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"ReentrantLock类关于lock接口的操作都交给了内部类Sync类来实现，Sync类又有两个子类"},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#9254DE","name":"purple"}},{"type":"strong"}],"text":"NonFairSync，FairSync"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"，公平锁和不公平锁；"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"abstract static class Sync extends AbstractQueuedSynchronizer\n static final class NonfairSync extends Sync\n\tstatic final class FairSync extends Sync"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"AQS重要成员变量"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" private transient volatile Node tail; // CLH队列\n private volatile int state; // 锁的状态"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"AQS使用的设计模式："}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":" "},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"模板方法设计模式："}],"marks":[{"type":"strong"}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"定义一个代码模板结构，相同部分在父类实现，不同部分由子类实现"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "},{"type":"link","attrs":{"href":"https://links.jianshu.com/go?to=https%3A%2F%2Fblog.csdn.net%2Fcarson_ho%2Farticle%2Fdetails%2F54910518%3Futm_medium%3Ddistribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-1.nonecase%26depth_1-utm_source%3Ddistribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-1.nonecase","title":null},"content":[{"type":"text","text":"模板方法模式（Template Method） - 最易懂的设计模式解析"}]},{"type":"text","marks":[{"type":"strong"}],"text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":" 安卓中的View，Activity都使用了模板方法设计模式，View类规范了所有View需要实现的行为，View的子类可以在"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"onMeasure"},{"type":"text","marks":[{"type":"strong"}],"text":"，"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"onLayout，onDraw"},{"type":"text","marks":[{"type":"strong"}],"text":"中扩展各自不同的行为；体现了设计模式的"},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"strong"}],"text":"开闭原则"}],"marks":[{"type":"strong"}]},{"type":"text","marks":[{"type":"strong"}],"text":"AQS抽象类为子类提供了"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"tryAcquire"},{"type":"text","marks":[{"type":"strong"}],"text":"，"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"tryRelease"},{"type":"text","marks":[{"type":"strong"}],"text":"去扩展自己的不同行为"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"NonfairSync："}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"NonfairSync.lock()"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"final void lock() {\n if (compareAndSetState(0, 1))\n setExclusiveOwnerThread(Thread.currentThread());\n else\n acquire(1);\n}\n\nprotected final boolean compareAndSetState(int expect, int update) {\n // See below for intrinsics setup to support this\n return unsafe.compareAndSwapInt(this, stateOffset, expect, update);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":" 首先通过CAS尝试将AQS的state由0变为1，如果成功，说明当前锁没有被线程持有，调用setExclusiveOwnerThread（）设置当前线程持有当前锁即可；"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 如果失败了说明当前锁被持有，调用acquire（1）；"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"acquire()"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public final void acquire(int arg) {\n if (!tryAcquire(arg) &&\n acquireQueued(addWaiter(Node.EXCLUSIVE), arg))\n selfInterrupt();\n }"}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在acquire（）中，首先调用了tryAcquire（）尝试获取"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"NonFairSync的tryAcquire的实现"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" final boolean nonfairTryAcquire(int acquires) {\n final Thread current = Thread.currentThread();\n int c = getState();\n if (c == 0) {\n if (compareAndSetState(0, acquires)) {\n setExclusiveOwnerThread(current);\n return true;\n }\n }\n else if (current == getExclusiveOwnerThread()) {\n int nextc = c + acquires;\n if (nextc < 0) // overflow\n throw new Error(\"Maximum lock count exceeded\");\n setState(nextc);\n return true;\n }\n return false;\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 首先判断AQS的"},{"type":"text","marks":[{"type":"strong"}],"text":"state"},{"type":"text","text":"是否为"},{"type":"text","marks":[{"type":"strong"}],"text":"0"},{"type":"text","text":"，如果是"},{"type":"text","marks":[{"type":"strong"}],"text":"0"},{"type":"text","text":"，则当前锁未被持有，设置当前线程即可，如果不为0，说明当前锁被持有，并且有另一个线程尝试进入，则将AQS的"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"state+1"},{"type":"text","text":"（类似"},{"type":"text","marks":[{"type":"strong"}],"text":"synchronized"},{"type":"text","text":"的"},{"type":"text","marks":[{"type":"strong"}],"text":"monitor"},{"type":"text","text":"的进入数）；"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"addWaiter（）"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" private Node addWaiter(Node mode) {\n Node node = new Node(Thread.currentThread(), mode);\n // Try the fast path of enq; backup to full enq on failure\n Node pred = tail;\n if (pred != null) {\n node.prev = pred;\n if (compareAndSetTail(pred, node)) {\n pred.next = node;\n return node;\n }\n }\n enq(node);\n return node;\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "},{"type":"text","marks":[{"type":"strong"}],"text":"addWaiter"},{"type":"text","text":"的作用是将为"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"获得锁被阻塞的线程打包成Node添加到tail链表（队列）中保存起来"},{"type":"text","text":"，添加链表节点的过程使用了CAS添加；"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/be/be1156567ac4e731da5c0ff12d7eace2.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":8,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"UnFairSync lock"}]},{"type":"paragraph","attrs":{"indent":8,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":" 回顾一下UnFairSync的lock()过程："},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"首先尝试通过CAS将锁的状态（AQS的state）由0变为1；如果成功说明锁未被持有，设置当前线程持有即可，如果失败，说明锁已经被持有，调用acquire（1）；在acquire（）中，"},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"1"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"首先调用tryAcquire（）再次尝试获取锁，如果失败将锁的state+1，"},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"2"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"其次调用addWaiter（）将当前线程包装成Node放入等待队列"},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"AQS的tail"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"中，"},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"3"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"调用当线程的interrupt（）尝试中断；"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"unLock（）："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void unlock() {\n sync.release(1);\n }\n\n public final boolean release(int arg) {\n if (tryRelease(arg)) {\n Node h = head;\n if (h != null && h.waitStatus != 0)\n unparkSuccessor(h);\n return true;\n }\n return false;\n }\n\n protected final boolean tryRelease(int releases) {\n int c = getState() - releases;\n if (Thread.currentThread() != getExclusiveOwnerThread())\n throw new IllegalMonitorStateException();\n boolean free = false;\n if (c == 0) {\n free = true;\n setExclusiveOwnerThread(null);\n }\n setState(c);\n return free;\n }\n"}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"unLock()做的事情：将锁的state-1，如果state==0了，在等待队列中唤醒一个线程；"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"公平锁和不公平锁的区别："}]}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"FairLock.lock（）"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" final void lock() {\n acquire(1);\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"公平锁的lock方法是直接调用acquire（）；"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"tryAcquire的实现："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" /**\n * Performs non-fair tryLock. tryAcquire is implemented in\n * subclasses, but both need nonfair try for trylock method.\n */\n final boolean nonfairTryAcquire(int acquires) {\n final Thread current = Thread.currentThread();\n int c = getState();\n if (c == 0) {\n if (compareAndSetState(0, acquires)) {\n setExclusiveOwnerThread(current);\n return true;\n }\n }\n else if (current == getExclusiveOwnerThread()) {\n int nextc = c + acquires;\n if (nextc < 0) // overflow\n throw new Error(\"Maximum lock count exceeded\");\n setState(nextc);\n return true;\n }\n return false;\n }\n /**\n * Fair version of tryAcquire. Don't grant access unless\n * recursive call or no waiters or is first.\n */\n protected final boolean tryAcquire(int acquires) {\n final Thread current = Thread.currentThread();\n int c = getState();\n if (c == 0) {\n if (!hasQueuedPredecessors() &&\n compareAndSetState(0, acquires)) {\n setExclusiveOwnerThread(current);\n return true;\n }\n }\n else if (current == getExclusiveOwnerThread()) {\n int nextc = c + acquires;\n if (nextc < 0)\n throw new Error(\"Maximum lock count exceeded\");\n setState(nextc);\n return true;\n }\n return false;\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":" "},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"在调用tryAcquire（）尝试获取锁时，公平和不公平锁的实现稍微不同，公平锁会在state == 0的时候直接设置当前线程去持有锁，而非公平锁会调用"},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"hasQueuedPredecessors()"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"去判断tail等待队列中有没有比当前线程等待时间更长的线程，如果有，就不会设置当前线程去持有锁；"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"AQS内部的CLH自旋锁"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":" CLH是一个基于链表（队列）的自旋（公平）锁，每一个等待锁的线程封装成节点，不断自旋判断前一个节点的状态，如果前一个节点释放锁就结束自旋；"}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 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tryAcquire()中判断是否是当前线程持有，如果是则将state通过CAS +1；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#BAE7A1","name":"green"}},{"type":"strong"}],"text":"在释放锁时，只有state为0时，锁才会正真释放，可以被其他线程持有；"}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"JMM ("},{"type":"codeinline","content":[{"type":"text","text":"JAVA Memory Model"}]},{"type":"text","text":")"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong"}],"text":" 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stringBuffer = new StringBuffer();\n stringBuffer.append(\"加锁----释放锁\");\n stringBuffer.append(\"加锁----释放锁\");\n stringBuffer.append(\"加锁----释放锁\");\n \n @Override\n public synchronized StringBuffer append(String str) {\n toStringCache = null;\n super.append(str);\n return this;\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面的操作就会进行synchronized锁粗化的优化"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#FDED8A","name":"yellow"}},{"type":"strong"}],"text":"自适应自旋："}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#FFBEBF","name":"pink"}},{"type":"strong"}],"text":"自旋时间由前一次在同一个锁的自旋时间和锁的拥有者状态来决定，如果虚拟机判断获得这个锁的可能性很大，就会增加自旋时间，如果觉得很难获得锁，可能会省去自旋这一步节约CPU；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#FDED8A","name":"yellow"}},{"type":"strong"}],"text":"偏向锁："}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#FDED8A","name":"yellow"}},{"type":"strong"}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#FFBEBF","name":"pink"}},{"type":"strong"}],"text":"这个锁会偏向于第一个持有它的线程，如果在运行过程中，同步锁只有一个线程访问，不存在多线程争用的情况，则线程是不需要触发同步的，减少加锁／解锁的一些CAS操作（比如等待队列的一些CAS操作），这种情况下，就会给线程加一个偏向锁。 如果在运行过程中，遇到了其他线程抢占锁，则持有偏向锁的线程会被挂起，JVM会消除它身上的偏向锁，将锁恢复到标准的轻量级锁；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#FDED8A","name":"yellow"}},{"type":"strong"}],"text":"轻量级锁："}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#FDED8A","name":"yellow"}},{"type":"strong"}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#FFBEBF","name":"pink"}},{"type":"strong"}],"text":"由偏向锁转化来，相对于传统的重量级锁，不会阻塞线程，而是通过自旋进行等待；以CPU为代价，避免线程的上下文切换，追求响应速度；"},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"bgcolor","attrs":{"color":"#FFBEBF","name":"pink"}},{"type":"strong"}],"text":"偏向锁切换到轻量级锁会Stop 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