AQS中可重入锁ReentrantLock源码一加锁过程

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原博客地址blog.csdn.net/weixin_3931…

ReentrantLock类的结构

public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    /** Synchronizer providing all implementation mechanics */
    private final Sync sync;

    /**
     * Base of synchronization control for this lock. Subclassed
     * into fair and nonfair versions below. Uses AQS state to
     * represent the number of holds on the lock.
     */
    abstract static class Sync extends AbstractQueuedSynchronizer {
    	// 去除其他逻辑
    }
  }
 
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众所周知(没吃过猪肉,肯定看过猪),ReentrantLock是基于AQS(Abstract Queued Synchronizer)来实现的。通过看ReentrantLock的类结构其实也是通过组合了一个Sync类属性。所以核心的功能全部都是由Sync类来完成的。

Sync类的结构

image-20210311222850326

最顶层的AbstractOwnableSynchronizer类

A synchronizer that may be exclusively owned by a thread. This class provides a basis for creating locks and related synchronizers that may entail a notion of ownership. The AbstractOwnableSynchronizer class itself does not manage or use this information. However, subclasses and tools may use appropriately maintained values to help control and monitor access and provide diagnostics.

百度翻译:线程独占的同步器。此类提供了创建锁和相关同步器的基础,这些同步器可能包含所有权的概念。AbstractOwnableSynchronizer类本身不管理或使用此信息。但是,子类和工具可以使用适当维护的值来帮助控制和监视访问并提供诊断。

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image-20210311223623198

核心的AbstractQueuedSynchronizer类

Provides a framework for implementing blocking locks and related synchronizers (semaphores, events, etc) that rely on first-in-first-out (FIFO) wait queues. This class is designed to be a useful basis for most kinds of synchronizers that rely on a single atomic int value to represent state. Subclasses must define the protected methods that change this state, and which define what that state means in terms of this object being acquired or released. Given these, the other methods in this class carry out all queuing and blocking mechanics. Subclasses can maintain other state fields, but only the atomically updated int value manipulated using methods getState, setState and compareAndSetState is tracked with respect to synchronization.
Subclasses should be defined as non-public internal helper classes that are used to implement the synchronization properties of their enclosing class. Class AbstractQueuedSynchronizer does not implement any synchronization interface. Instead it defines methods such as acquireInterruptibly that can be invoked as appropriate by concrete locks and related synchronizers to implement their public methods.
This class supports either or both a default exclusive mode and a shared mode. When acquired in exclusive mode, attempted acquires by other threads cannot succeed. Shared mode acquires by multiple threads may (but need not) succeed. This class does not "understand" these differences except in the mechanical sense that when a shared mode acquire succeeds, the next waiting thread (if one exists) must also determine whether it can acquire as well. Threads waiting in the different modes share the same FIFO queue. Usually, implementation subclasses support only one of these modes, but both can come into play for example in a ReadWriteLock. Subclasses that support only exclusive or only shared modes need not define the methods supporting the unused mode.
This class defines a nested AbstractQueuedSynchronizer.ConditionObject class that can be used as a Condition implementation by subclasses supporting exclusive mode for which method isHeldExclusively reports whether synchronization is exclusively held with respect to the current thread, method release invoked with the current getState value fully releases this object, and acquire, given this saved state value, eventually restores this object to its previous acquired state. No AbstractQueuedSynchronizer method otherwise creates such a condition, so if this constraint cannot be met, do not use it. The behavior of AbstractQueuedSynchronizer.ConditionObject depends of course on the semantics of its synchronizer implementation.
This class provides inspection, instrumentation, and monitoring methods for the internal queue, as well as similar methods for condition objects. These can be exported as desired into classes using an AbstractQueuedSynchronizer for their synchronization mechanics.
Serialization of this class stores only the underlying atomic integer maintaining state, so deserialized objects have empty thread queues. Typical subclasses requiring serializability will define a readObject method that restores this to a known initial state upon deserialization.
Usage
To use this class as the basis of a synchronizer, redefine the following methods, as applicable, by inspecting and/or modifying the synchronization state using getState, setState and/or compareAndSetState:
tryAcquire
tryRelease
tryAcquireShared
tryReleaseShared
isHeldExclusively
Each of these methods by default throws UnsupportedOperationException. Implementations of these methods must be internally thread-safe, and should in general be short and not block. Defining these methods is the only supported means of using this class. All other methods are declared final because they cannot be independently varied.
You may also find the inherited methods from AbstractOwnableSynchronizer useful to keep track of the thread owning an exclusive synchronizer. You are encouraged to use them -- this enables monitoring and diagnostic tools to assist users in determining which threads hold locks.
Even though this class is based on an internal FIFO queue, it does not automatically enforce FIFO acquisition policies. The core of exclusive synchronization takes the form:
   Acquire:
       while (!tryAcquire(arg)) {
          enqueue thread if it is not already queued;
          possibly block current thread;
       }
  
   Release:
       if (tryRelease(arg))
          unblock the first queued thread;
   
(Shared mode is similar but may involve cascading signals.)
Because checks in acquire are invoked before enqueuing, a newly acquiring thread may barge ahead of others that are blocked and queued. However, you can, if desired, define tryAcquire and/or tryAcquireShared to disable barging by internally invoking one or more of the inspection methods, thereby providing a fair FIFO acquisition order. In particular, most fair synchronizers can define tryAcquire to return false if hasQueuedPredecessors (a method specifically designed to be used by fair synchronizers) returns true. Other variations are possible.
Throughput and scalability are generally highest for the default barging (also known as greedy, renouncement, and convoy-avoidance) strategy. While this is not guaranteed to be fair or starvation-free, earlier queued threads are allowed to recontend before later queued threads, and each recontention has an unbiased chance to succeed against incoming threads. Also, while acquires do not "spin" in the usual sense, they may perform multiple invocations of tryAcquire interspersed with other computations before blocking. This gives most of the benefits of spins when exclusive synchronization is only briefly held, without most of the liabilities when it isn't. If so desired, you can augment this by preceding calls to acquire methods with "fast-path" checks, possibly prechecking hasContended and/or hasQueuedThreads to only do so if the synchronizer is likely not to be contended.
This class provides an efficient and scalable basis for synchronization in part by specializing its range of use to synchronizers that can rely on int state, acquire, and release parameters, and an internal FIFO wait queue. When this does not suffice, you can build synchronizers from a lower level using atomic classes, your own custom java.util.Queue classes, and LockSupport blocking support.
Usage Examples
Here is a non-reentrant mutual exclusion lock class that uses the value zero to represent the unlocked state, and one to represent the locked state. While a non-reentrant lock does not strictly require recording of the current owner thread, this class does so anyway to make usage easier to monitor. It also supports conditions and exposes one of the instrumentation methods:
  
 class Mutex implements Lock, java.io.Serializable {

   // Our internal helper class
   private static class Sync extends AbstractQueuedSynchronizer {
     // Reports whether in locked state
     protected boolean isHeldExclusively() {
       return getState() == 1;
     }

     // Acquires the lock if state is zero
     public boolean tryAcquire(int acquires) {
       assert acquires == 1; // Otherwise unused
       if (compareAndSetState(0, 1)) {
         setExclusiveOwnerThread(Thread.currentThread());
         return true;
       }
       return false;
     }

     // Releases the lock by setting state to zero
     protected boolean tryRelease(int releases) {
       assert releases == 1; // Otherwise unused
       if (getState() == 0) throw new IllegalMonitorStateException();
       setExclusiveOwnerThread(null);
       setState(0);
       return true;
     }

     // Provides a Condition
     Condition newCondition() { return new ConditionObject(); }

     // Deserializes properly
     private void readObject(ObjectInputStream s)
         throws IOException, ClassNotFoundException {
       s.defaultReadObject();
       setState(0); // reset to unlocked state
     }
   }

   // The sync object does all the hard work. We just forward to it.
   private final Sync sync = new Sync();

   public void lock()                { sync.acquire(1); }
   public boolean tryLock()          { return sync.tryAcquire(1); }
   public void unlock()              { sync.release(1); }
   public Condition newCondition()   { return sync.newCondition(); }
   public boolean isLocked()         { return sync.isHeldExclusively(); }
   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
   public void lockInterruptibly() throws InterruptedException {
     sync.acquireInterruptibly(1);
   }
   public boolean tryLock(long timeout, TimeUnit unit)
       throws InterruptedException {
     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
   }
 }
Here is a latch class that is like a CountDownLatch except that it only requires a single signal to fire. Because a latch is non-exclusive, it uses the shared acquire and release methods.
  
 class BooleanLatch {

   private static class Sync extends AbstractQueuedSynchronizer {
     boolean isSignalled() { return getState() != 0; }

     protected int tryAcquireShared(int ignore) {
       return isSignalled() ? 1 : -1;
     }

     protected boolean tryReleaseShared(int ignore) {
       setState(1);
       return true;
     }
   }

   private final Sync sync = new Sync();
   public boolean isSignalled() { return sync.isSignalled(); }
   public void signal()         { sync.releaseShared(1); }
   public void await() throws InterruptedException {
     sync.acquireSharedInterruptibly(1);
   }
 }


// 就不翻译了。感兴趣自己百度翻译。就是提供了一个先入先出的CLH双向链表,基于这个链表做一大堆叭叭叭的操作(手动狗头)
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核心的属性

image-20210312103219165

Node节点

image-20210312103510036 核心属性:

  • prev :上一个节点
  • next:下一个节点
  • thread:当前node节点代表哪一个线程
  • waitStatus:当前节点的状态(后面重点说这块)

正文开始

加锁流程

lock() 方法

源码开始

ReentraLock加锁

流程:

  • ReentrantLock lock = new ReentrantLock(); 默认创建一个非公平锁
  • 非公平锁加锁的时候会走一次”特殊通道“,如果”特殊通道“没获取到锁,则走正常逻辑去加锁
  • 核心流程在正常逻辑里面

核心加锁流程

    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
// 这段代码还是要自己仔细的去看看,我们来拆解一下,下面是我YY的
    public  void acquire(int arg) {
        // 尝试去加锁
        boolean 是否加锁成功 = tryAcquire(arg);
        if(是否加锁成功){
            return;
        }
        // 创建Node节点(保存了当前线程)并且将Node节点放入队列
        AbstractQueuedSynchronizer.Node node = addWaiter(Node.EXCLUSIVE);
        
        // 在该方法对于已经排队的线程,死循环去获取锁(包括用于条件等待的唤醒),如果没有获取到锁,就被park,即睡眠。
        boolean 线程是否被打断 = acquireQueued(node, arg);
        
        if(线程是否被打断){
            //标记线程被打断
            selfInterrupt();
        }
    }
    /**
     * 其中我们先不要关心线程打断和条件等待
     * 1、尝试加锁CAS操作
     * 2、没获取到锁就创建一个Node节点并将节点放入CLH双向链表队列中
     * 3、死循环的去抢锁,如果抢不到则直接park等在别的线程来唤醒
     */
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tryAcquire(arg)尝试加锁

ReentraLock加锁全

原流程图地址:wangtingblog.oss-cn-beijing.aliyuncs.com/img/Reentra…

使用下面工具打开:draw.io/

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