在java并发包java.util.concurrent中，除了重入锁ReentrantLock外，读写锁ReentrantReadWriteLock也很常用。在实际开发场景中，在使用共享资源时，可能读操作远远多于写操作。这种情况下，如果对这部分共享资源能够让多个线程读的时候不受阻塞，仅仅在写的时候保证安全性，这样效率会得到显著提升。读写锁ReentrantReadWriteLock便适用于这种场景。
再描述一下进入读锁和写锁的条件。
进入读锁：
1.没有其他线程的写锁
2.有写请求且请求线程就是持有锁的线程
进入写锁：
1.没有其他线程读锁
2.没有其他线程写锁
本篇从源码方面，简要分析ReentrantReadWriteLock的实现原理，以及展示一下它的使用效果。
源码
这是ReentrantReadWriteLock维护的一对锁
/** Inner class providing readlock */ private final ReentrantReadWriteLock.ReadLock readerLock; /** Inner class providing writelock */ private final ReentrantReadWriteLock.WriteLock writerLock;
ReentrantReadWriteLock的构造器中，同时实例化读写锁，同时与ReentrantLock相同，也有公平锁和非公平锁之分
public ReentrantReadWriteLock(boolean fair) { sync = fair ? new FairSync() : new NonfairSync(); readerLock = new ReadLock(this); writerLock = new WriteLock(this); }
写锁
获取锁
public void lock() { sync.acquire(1); } //这里与ReentrantLock相同 public final void acquire(int arg) { if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) selfInterrupt(); } protected final boolean tryAcquire(int acquires) { Thread current = Thread.currentThread(); int c = getState(); int w = exclusiveCount(c); if (c != 0) { // (Note: if c != 0 and w == 0 then shared count != 0) if (w == 0 || current != getExclusiveOwnerThread()) return false; if (w + exclusiveCount(acquires) > MAX_COUNT) throw new Error("Maximum lock count exceeded"); // Reentrant acquire setState(c + acquires); return true; } if (writerShouldBlock() || !compareAndSetState(c, c + acquires)) return false; setExclusiveOwnerThread(current); return true; }
这里解析tryAcquire()方法。
- 获取当前线程
- 获取状态
- 获取写线程数
- 若state不为0，表示锁已被持有。再判断，如果写线程数为0，则读锁被占用，返回false；如果写线程数不为0，且独占线程不是当前线程，表示写锁被其他线程占用没返回false
- 如果写锁重入数大于最大值MAX_COUNT，抛错
- 写锁重入，返回true
- state为0，根据公平锁还是非公平锁判断是否阻塞线程。不需要阻塞就CAS更新state
- 当前线程设为独占线程，获取写锁，返回true
释放锁
public void unlock() { sync.release(1); } public final boolean release(int arg) { if (tryRelease(arg)) { Node h = head; if (h != null && h.waitStatus != 0) unparkSuccessor(h); return true; } return false; } protected final boolean tryRelease(int releases) { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); int nextc = getState() - releases; boolean free = exclusiveCount(nextc) == 0; if (free) setExclusiveOwnerThread(null); setState(nextc); return free; }
分析tryRelease()方法
- 判断持有写锁的线程是否当前线程，不是则抛错
- state减1
- 以新state计算写锁数量，如果为0，表示完全释放；
- 完全释放就设置独占线程为null
- 如果独占线程数量不是0，还是更新state，这里就表示多次重入写锁后，释放了一次
读锁
获取锁
public void lock() { sync.acquireShared(1); } public final void acquireShared(int arg) { if (tryAcquireShared(arg) < 0) doAcquireShared(arg); } protected final int tryAcquireShared(int unused) { Thread current = Thread.currentThread(); int c = getState(); if (exclusiveCount(c) != 0 && getExclusiveOwnerThread() != current) return -1; int r = sharedCount(c); if (!readerShouldBlock() && r < MAX_COUNT && compareAndSetState(c, c + SHARED_UNIT)) { if (r == 0) { firstReader = current; firstReaderHoldCount = 1; } else if (firstReader == current) { firstReaderHoldCount++; } else { HoldCounter rh = cachedHoldCounter; if (rh == null || rh.tid != getThreadId(current)) cachedHoldCounter = rh = readHolds.get(); else if (rh.count == 0) readHolds.set(rh); rh.count++; } return 1; } return fullTryAcquireShared(current); }
这里分析tryAcquireShared()方法
- 获取当前线程
- 获取state
- 如果写锁数量不为0，且独占线程不是本线程，获得读锁失败。因为写锁被其他线程占用
- 获取读锁数量
- 根据公平锁或者非公平锁判断是否应该被阻塞，判断读锁数量是否小于最大值MAX_COUNT，再尝试CAS更新state
- 以上判断都通过且更新state也成功后，如果读锁为0，记录第一个读线程和此线程占用读锁数量
- 如果第一个读线程是本线程，表示此时是读锁的重入，则把此线程占用读锁数量+1
- 如果读锁数量不为0，且此线程也不是第一个读线程，则找到当前线程的计数器，并计数+1
- 如果在阻塞判断，读锁数量判断和CAS更新是否成功这部分没有通过，则进入fullTryAcquireShared()方法，逻辑与上面的获取类似，以无限循环方式保证操作成功，不赘述。
释放锁
public void unlock() { sync.releaseShared(1); } public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; } protected final boolean tryReleaseShared(int unused) { Thread current = Thread.currentThread(); if (firstReader == current) { // assert firstReaderHoldCount > 0; if (firstReaderHoldCount == 1) firstReader = null; else firstReaderHoldCount--; } else { HoldCounter rh = cachedHoldCounter; if (rh == null || rh.tid != getThreadId(current)) rh = readHolds.get(); int count = rh.count; if (count <= 1) { readHolds.remove(); if (count <= 0) throw unmatchedUnlockException(); } --rh.count; } for (;;) { int c = getState(); int nextc = c - SHARED_UNIT; if (compareAndSetState(c, nextc)) // Releasing the read lock has no effect on readers, // but it may allow waiting writers to proceed if // both read and write locks are now free. return nextc == 0; } }
分析tryReleaseShared()方法
- 获取当前线程
- 如果当前线程是第一个读线程，则释放firstReader或者第一个读线程的锁计数-1
- 不是就获得当前线程的计数器。根据计数选择删除此计数器或者减少计数
- 无限循环更新state
获取锁和释放锁的源码部分代码就分析放到这里，接下来用代码时间看看ReentrantReadWriteLock的使用效果测试。
public class ReadWriteLockTest { private static ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock(); private static ExecutorService executorService = Executors.newCachedThreadPool(); //读操作 public static void read(){ try {
//加读锁 readWriteLock.readLock().lock(); System.out.println(Thread.currentThread().getName() + " is reading " + System.currentTimeMillis()); Thread.sleep(1000); } catch (InterruptedException e){ }finally { readWriteLock.readLock().unlock(); } } //写操作 public static void write() { try {
//加写锁 readWriteLock.writeLock().lock(); System.out.println(Thread.currentThread().getName() + " is writing "+ System.currentTimeMillis()); Thread.sleep(1000); } catch (InterruptedException e){ }finally { readWriteLock.writeLock().unlock(); } } public static void main(String[] args) { for (int i = 0; i < 3; i++) { executorService.execute(new Runnable() { @Override public void run() { ReadWriteLockTest.read(); } }); } for (int i = 0; i < 3; i++) { executorService.execute(new Runnable() { @Override public void run() { ReadWriteLockTest.write(); } }); } } }
执行结果如下:
pool-1-thread-2 is reading 1549002279198 pool-1-thread-1 is reading 1549002279198 pool-1-thread-3 is reading 1549002279198 pool-1-thread-4 is writing 1549002280208 pool-1-thread-5 is writing 1549002281214 pool-1-thread-6 is writing 1549002282224
可以看到，thread1，2，3在读时，是同时执行。thread4，5，6在写操作是，都差不多间隔1000毫秒。