public class ReadWriteLock{
private boolean canGrantReadAccess(Thread callingThread){
if(isWriter(callingThread)) return true;
if(writingThread != null) return false;
if(isReader(callingThread) return true;
if(writeRequests > 0) return false;
return true;
}
}
完整的重进入ReadWriteLock
下面是一个完成的重进入ReadWriteLock实现。我对访问条件做了一些小的修正以便阅读，同时更容易使你相信它是正确的。
public class ReadWriteLock{
private Map
readingThreads =
new HashMap
(); private int writeAccesses = 0; private int writeRequests = 0; private Thread writingThread = null; public synchronized void lockRead() throws InterruptedException{ Thread callingThread = Thread.currentThread(); while(! canGrantReadAccess(callingThread)){ wait(); } readingThreads.put(callingThread, (getReadAccessCount(callingThread) + 1)); } private boolean canGrantReadAccess(Thread callingThread){ if( isWriter(callingThread) ) return true; if( hasWriter() ) return false; if( isReader(callingThread) ) return true; if( hasWriteRequests() ) return false; return true; } public synchronized void unlockRead(){ Thread callingThread = Thread.currentThread(); if(!isReader(callingThread)){ throw new IllegalMonitorStateException("Calling Thread does not" + " hold a read lock on this ReadWriteLock"); } int accessCount = getReadAccessCount(callingThread); if(accessCount == 1){ readingThreads.remove(callingThread); } else { readingThreads.put(callingThread, (accessCount -1)); } notifyAll(); } public synchronized void lockWrite() throws InterruptedException{ writeRequests++; Thread callingThread = Thread.currentThread(); while(! canGrantWriteAccess(callingThread)){ wait(); } writeRequests--; writeAccesses++; writingThread = callingThread; } public synchronized void unlockWrite() throws InterruptedException{ if(!isWriter(Thread.currentThread()){ throw new IllegalMonitorStateException("Calling Thread does not" + " hold the write lock on this ReadWriteLock"); } writeAccesses--; if(writeAccesses == 0){ writingThread = null; } notifyAll(); } private boolean canGrantWriteAccess(Thread callingThread){ if(isOnlyReader(callingThread)) return true; if(hasReaders()) return false; if(writingThread == null) return true; if(!isWriter(callingThread)) return false; return true; } private int getReadAccessCount(Thread callingThread){ Integer accessCount = readingThreads.get(callingThread); if(accessCount == null) return 0; return accessCount.intValue(); } private boolean hasReaders(){ return readingThreads.size() > 0; } private boolean isReader(Thread callingThread){ return readingThreads.get(callingThread) != null; } private boolean isOnlyReader(Thread callingThread){ return readingThreads.size() == 1 && readingThreads.get(callingThread) != null; } private boolean hasWriter(){ return writingThread != null; } private boolean isWriter(Thread callingThread){ return writingThread == callingThread; } private boolean hasWriteRequests(){ return this.writeRequests > 0; } }
在finally语句中调用unlock()方法
当使用ReadWriteLock来保护一个关键的代码片段，同时该关键片段有可能抛出异常时，在finally语句中调用readUnlock()和writeUnlock()方法就显得很重要了。这样做是为了确保对应的ReadWriteLock被解锁(unlocked)，同时其他的线程才能对其进行锁定。下面是一个例子：
lock.lockWrite();
try{
//do critical section code, which may throw exception
} finally {
lock.unlockWrite();
}
上面这个小的代码结构保证了当关键代码抛出异常时能够