淺談JAVA并發之ReentrantLock
結合上面的ReentrantLock類圖，ReentrantLock實現了Lock接口，它的內部類Sync繼承自AQS，絕大部分使用AQS的子類需要自定義的方法存在Sync中。而ReentrantLock有公平與非公平的區別，即’是否先阻塞就先獲取資源’，它的主要實現就是FairSync與NonfairSync，后面會從源碼角度看看它們的區別。
2. 源碼剖析Sync是ReentrantLock控制同步的基礎。它的子類分為了公平與非公平。使用AQS的state代表獲取鎖的數量
abstract static class Sync extends AbstractQueuedSynchronizer { private static final long serialVersionUID = -5179523762034025860L; /*** Performs {@link Lock#lock}. The main reason for subclassing* is to allow fast path for nonfair version.*/ abstract void lock(); ...}
我們可以看出內部類Sync是一個抽象類，繼承它的子類(FairSync與NonfairSync)需要實現抽象方法lock。
下面我們先從非公平鎖的角度來看看獲取資源與釋放資源的原理
故事就從就兩個變量開始:
// 獲取一個非公平的獨占鎖/*** public ReentrantLock() {* sync = new ReentrantLock.NonfairSync();* }*/private Lock lock = new ReentrantLock();// 獲取條件變量private Condition condition = lock.newCondition();2.1 上鎖(獲取資源)
lock.lock()
public void lock() { sync.lock();}
static final class NonfairSync extends Sync { private static final long serialVersionUID = 7316153563782823691L; // 獲取資源 final void lock() {// 若此時沒有線程獲取到資源，直接設置當前線程獨占訪問資源。if (compareAndSetState(0, 1)) setExclusiveOwnerThread(Thread.currentThread());else // AQS的方法 acquire(1); } protected final boolean tryAcquire(int acquires) {// 實現在父類Sync中return nonfairTryAcquire(acquires); }}
AQS的acquire
public final void acquire(int arg) { if (!tryAcquire(arg) &&acquireQueued(addWaiter(Node.EXCLUSIVE), arg))selfInterrupt();}
// Sync實現的非公平的tryAcquirefinal boolean nonfairTryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); // 此時若沒有線程獲取到資源，當前線程就直接占用該資源 if (c == 0) {if (compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true;} } // 若當前線程已經占用了該資源，可以再次獲取該資源 ->這個行為就是可重入鎖的支撐 else if (current == getExclusiveOwnerThread()) {int nextc = c + acquires;if (nextc < 0) // overflow throw new Error('Maximum lock count exceeded');setState(nextc);return true; } return false;}
嘗試獲取資源的過程是非常簡單的，這里再貼一下acquire的流程
lock.unlock();
public void unlock() { // AQS的方法 sync.release(1);}
AQS的release
public final boolean release(int arg) { if (tryRelease(arg)) {Node h = head;if (h != null && h.waitStatus != 0) unparkSuccessor(h);return true; } return false;}
release的流程已經剖析過了，接下來看看tryRelease的實現
protected final boolean tryRelease(int releases) { int c = getState() - releases; // 這里可以看出若沒有持有鎖，就釋放資源，就會報錯 if (Thread.currentThread() != getExclusiveOwnerThread())throw new IllegalMonitorStateException(); boolean free = false; if (c == 0) {free = true;setExclusiveOwnerThread(null); } setState(c); return free;}
tryRelease的實現也很簡單，這里再貼一下release的流程圖
公平鎖與非公平鎖，即’是否先阻塞就先獲取資源’, ReentrantLock中公平與否的控制就在tryAcquire中。下面我們看看，公平鎖的tryAcquire
static final class FairSync extends Sync {private static final long serialVersionUID = -3000897897090466540L;final void lock() { acquire(1);}protected final boolean tryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) {// (2.3.1)// sync queue中是否存在前驅結點if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true;} } else if (current == getExclusiveOwnerThread()) {int nextc = c + acquires;if (nextc < 0) throw new Error('Maximum lock count exceeded');setState(nextc);return true; } return false;} }
區別在代碼(2.3.1)
hasQueuedPredecessors
判斷當前線程的前面有無其他線程排隊；若當前線程在隊列頭部或者隊列為空返回false
public final boolean hasQueuedPredecessors() { // The correctness of this depends on head being initialized // before tail and on head.next being accurate if the current // thread is first in queue. Node t = tail; // Read fields in reverse initialization order Node h = head; Node s; return h != t &&((s = h.next) == null || s.thread != Thread.currentThread());}
結合下面的入隊代碼(enq), 我們分析hasQueuedPredecessors為true的情況:
1.h != t ,表示此時queue不為空; (s = h.next) == null, 表示另一個結點已經運行了下面的步驟(2)，還沒來得及運行步驟(3)。簡言之，就是B線程想要獲取鎖的同時，A線程獲取鎖失敗剛好在入隊(B入隊的同時，之前占有的資源的線程，剛好釋放資源)
2.h != t 且 (s = h.next) ！= null，表示此時至少有一個結點在sync queue中；s.thread != Thread.currentThread()，這個情況比較復雜，設想一下有這三個結點 A -> B C， A此時獲取到資源，而B此時因為獲取資源失敗正在sync queue阻塞，C還沒有獲取資源(還沒有執行tryAcquire)。
時刻一：A釋放資源成功后(執行tryRelease成功)，B此時還沒有成功獲取資源(C執行s = h.next時，B還在sync queue中且是老二)
時刻二: C此時執行hasQueuedPredecessors，s.thread != Thread.currentThread()成立，此時s.thread表示的是B
private Node enq(final Node node) { for (;;) {Node t = tail;if (t == null) { // Must initialize if (compareAndSetHead(new Node())) // (1) 第一次初始化tail = head;} else { node.prev = t; if (compareAndSetTail(t, node)) { // (2) 設置queue的tailt.next = node; // (3)return t; }} }}
Note that 1. because cancellations due to interrupts and timeouts may occur at any time, a true return does not guarantee that some other thread will acquire before the current thread（虛假true）. 2. Likewise, it is possible for another thread to win a race to enqueue after this method has returned false, due to the queue being empty（虛假false）.
這位大佬對hasQueuedPredecessors進行詳細的分析，他文中解釋了虛假true以及虛假false。我這里簡單解釋一下:
1.虛假true, 當兩個線程都執行tryAcquire，都執行到hasQueuedPredecessors，都返回true，但是只有一個線程執行compareAndSetState(0, acquires)成功
2.虛假false，當一個線程A執行doAcquireInterruptibly，發生了中斷，還沒有清除掉該結點時；此時，線程B執行hasQueuedPredecessors時，返回true
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