转自:
在一些java的业务场景中，多线程对共享资源的读写占比有时候不均衡，如：读操作非常频繁，而写操作非常少，
那么有时候希望在没有写操作时,多个线程可同时读一个共享资源，
如果一个线程希望对共享资源进行写操作时，则此时就不应该有其它的线程对共享资源进行操作
当一个线程想要读取资源,此时如果没有线程正在对该资源进行写操作且没有线程请求对该资源的写操作即可
注意事项:
当写操作比读操作更重要时，此时我们可通过提高写的优先级
因为如果写操作非常频繁,写操作未提升优先级，则会出现“饥饿”现象(写操作一直处于阻塞状态)
例:
public class ReadWriteLock{
private int readers = 0;
private int writers = 0;
private int writeRequests = 0;
public synchronized void lockRead()
throws InterruptedException{
while(writers > 0 || writeRequests > 0){
wait();
}
readers++;
}
public synchronized void unlockRead(){
readers--;
notifyAll();
}
public synchronized void lockWrite()
throws InterruptedException{
writeRequests++;
while(readers > 0 || writers > 0){
wait();
}
writeRequests--;
writers++;
}
public synchronized void unlockWrite()
throws InterruptedException{
writers--;
notifyAll();
}
}
在ReadWriteLock类中,读锁和写锁各有一个获取锁和释放锁的方法
读锁的实现在lockRead()中
当没有线程拥有写锁(writers==0),且没有线程在请求写锁(writeRequests ==0),所有想获得读锁的线程都能成功获取。
写锁的实现在lockWrite()中,当一个线程想获得写锁的时候
首先会把写锁请求数加1(writeRequests++),然后再去判断是否能够真能获得写锁,当没有线程持有读锁(readers==0),且没有线程持有写锁(writers==0)时就能获得写锁
需要注意的是,在两个释放锁的方法(unlockRead，unlockWrite)中,都调用了notifyAll方法,而不是notify
如果有线程在等待获取读锁，同时又有线程在等待获取写锁 如果这时其中一个等待读锁的线程被notify方法唤醒，但因为此时仍有请求写锁的线程存在(writeRequests>0),所以被唤醒的线程会再次进入阻塞状态。 然而,等待写锁的线程一个也没被唤醒，就像什么也没发生过一样（译者注：信号丢失现象）。如果用的是notifyAll方法，所有的线程都会被唤醒，然后判断能否获得其请求的锁。 ----------------------------------------------------------------------------------------------------------------------------------------------------------------- 用notifyAll还有一个好处。如果有多个读线程在等待读锁且没有线程在等待写锁时，调用unlockWrite()后，所有等待读锁的线程都能立马成功获取读锁----而不是一次只允许一个
上面实现的读/写锁(ReadWriteLock)是不可重入的,当一个已经持有写锁的线程再次请求写锁时,就会被阻塞。原因是已经有一个写线程了——就是它自己
Thread 1 获得了读锁 Thread 2 请求写锁，但因为Thread 1 持有了读锁，所以写锁请求被阻塞。 Thread 1 再想请求一次读锁，但因为Thread 2处于请求写锁的状态，所以想再次获取读锁也会被阻塞。
为了让ReadWriteLock可重入,需做相应的改进
为了让ReadWriteLock的读锁可重入，我们要先为读锁重入建立规则
实现思路：
使用一个map存储已持有锁的线程及对应线程获取读锁的次数，
然后读map中的信息,进行相应的判断
例:
public class ReadWriteLock{
private Map readingThreads =
new HashMap();
private int writers = 0;
private int writeRequests = 0;
public synchronized void lockRead()
throws InterruptedException{
Thread callingThread = Thread.currentThread();
while(! canGrantReadAccess(callingThread)){
wait();
}
readingThreads.put(callingThread,
(getAccessCount(callingThread) + 1));
}
public synchronized void unlockRead(){
Thread callingThread = Thread.currentThread();
int accessCount = getAccessCount(callingThread);
if(accessCount == 1) {
readingThreads.remove(callingThread);
} else {
readingThreads.put(callingThread, (accessCount -1));
}
notifyAll();
}
private boolean canGrantReadAccess(Thread callingThread){
if(writers > 0) return false;
if(isReader(callingThread) return true;
if(writeRequests > 0) return false;
return true;
}
private int getReadAccessCount(Thread callingThread){
Integer accessCount = readingThreads.get(callingThread);
if(accessCount == null) return 0;
return accessCount.intValue();
}
private boolean isReader(Thread callingThread){
return readingThreads.get(callingThread) != null;
}
}
当一个线程已经持有写锁，才允许写锁重入（再次获得写锁）
public class ReadWriteLock{
private Map readingThreads =
new HashMap();
private int writeAccesses = 0;
private int writeRequests = 0;
private Thread writingThread = null;
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{
writeAccesses--;
if(writeAccesses == 0){
writingThread = null;
}
notifyAll();
}
private boolean canGrantWriteAccess(Thread callingThread){
if(hasReaders()) return false;
if(writingThread == null) return true;
if(!isWriter(callingThread)) return false;
return true;
}
private boolean hasReaders(){
return readingThreads.size() > 0;
}
private boolean isWriter(Thread callingThread){
return writingThread == callingThread;
}
}
public class ReadWriteLock{
private Map readingThreads =
new HashMap();
private int writeAccesses = 0;
private int writeRequests = 0;
private Thread writingThread = null;
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{
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 boolean hasReaders(){
return readingThreads.size() > 0;
}
private boolean isWriter(Thread callingThread){
return writingThread == callingThread;
}
private boolean isOnlyReader(Thread thread){
return readers == 1 && readingThreads.get(callingThread) != null;
}
}
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;
}
}
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;
}
}