jdk1.5并发包中ReentrantLock的创建可以指定构造函数的boolean类型来得到公平锁或非公平锁,关于两者区别,java并发编程实践里面有解释
公平锁: Threads acquire a fair lock in the order in which they requested it
非公平锁:a nonfair lock permits barging: threads requesting a lock can jump ahead of the queue
of waiting threads if the lock happens to be available when it is requested.
公平锁,就是很公平,在并发环境中,每个线程在获取锁时会先查看此锁维护的等待队列,如果为空,或者当前线程线程是等待队列的第一个,就占有锁,否则就会加入到等待队列中,以后会按照FIFO的规则从队列中取到自己
非公平锁比较粗鲁,上来就直接尝试占有锁,如果尝试失败,就再采用类似公平锁那种方式
protected final boolean tryAcquire( int acquires) { |
final Thread current = Thread.currentThread(); |
compareAndSetState( 0 , acquires)) { |
setExclusiveOwnerThread(current); |
else if (current == getExclusiveOwnerThread()) { |
int nextc = c + acquires; |
throw new Error( "Maximum lock count exceeded" ); |
}
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
//如果当前没有线程占有锁,当前线程直接通过cas指令占有锁,管他等待队列,就算自己排在队尾也是这样
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
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;
}