程序中涉及到对一些共享资源的读和写操作,且写操作没有读操作那么频繁。在没有写操作的时候,两个线程同时读一个资源没有任何问题,所以应该允许多个线程能在同时读取共享资源。但是如果有一个线程想去写这些共享资源,就不应该再有其它线程对该资源进行读或写(译者注:也就是说:读-读能共存,读-写不能共存,写-写不能共存)。这就需要一个读/写锁来解决这个问题。
按照上面的叙述,简单的实现出一个读/写锁
public class ReadWriteLock{ private int readers = ; private int writers = ; private int writeRequests = ; public synchronized void lockRead() throws InterruptedException{ while(writers > || writeRequests > ){ wait(); } readers++; } public synchronized void unlockRead(){ readers--; notifyAll(); } public synchronized void lockWrite() throws InterruptedException{ writeRequests++; while(readers > || writers > ){ wait(); } writeRequests--; writers++; } public synchronized void unlockWrite() throws InterruptedException{ writers--; notifyAll(); } }
ReadWriteLock类中,读锁和写锁各有一个获取锁和释放锁的方法。
可重入的ReadWriteLock的完整实现
下面是完整的ReadWriteLock实现。为了便于代码的阅读与理解,简单对上面的代码做了重构。重构后的代码如下。
public class ReadWriteLock{ private Map<Thread, Integer> readingThreads = new HashMap<Thread, Integer>(); private int writeAccesses = ; private int writeRequests = ; private Thread writingThread = null; public synchronized void lockRead() throws InterruptedException{ Thread callingThread = Thread.currentThread(); while(! canGrantReadAccess(callingThread)){ wait(); } readingThreads.put(callingThread, (getReadAccessCount(callingThread) + )); } 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 == ){ readingThreads.remove(callingThread); } else { readingThreads.put(callingThread, (accessCount -)); } 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 == ){ 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 ; return accessCount.intValue(); } private boolean hasReaders(){ return readingThreads.size() > ; } private boolean isReader(Thread callingThread){ return readingThreads.get(callingThread) != null; } private boolean isOnlyReader(Thread callingThread){ return readingThreads.size() == && readingThreads.get(callingThread) != null; } private boolean hasWriter(){ return writingThread != null; } private boolean isWriter(Thread callingThread){ return writingThread == callingThread; } private boolean hasWriteRequests(){ return this.writeRequests > ; } }
应用:线程安全并且高并发状态下的map实现
class RWDictionary {
private final Map<String, Data> m = new TreeMap<String, Data>();
private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
private final Lock r = rwl.readLock();
private final Lock w = rwl.writeLock(); public Data get(String key) {
r.lock();
try { return m.get(key); }
finally { r.unlock(); }
}
public String[] allKeys() {
r.lock();
try { return m.keySet().toArray(); }
finally { r.unlock(); }
}
public Data put(String key, Data value) {
w.lock();
try { return m.put(key, value); }
finally { w.unlock(); }
}
public void clear() {
w.lock();
try { m.clear(); }
finally { w.unlock(); }
}
}