java并发库 Lock 公平锁和非公平锁

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

//公平获取锁  java.util.concurrent.locks.ReentrantLock$FairSync.java

protected final boolean tryAcquire( int acquires) {

final Thread current = Thread.currentThread();

int c = getState();

//状态为0，说明当前没有线程占有锁

if (c == 0 ) {

//如果当前线程是等待队列的第一个或者等待队列为空，则通过cas指令设置state为1，当前线程获得锁

if (isFirst(current) &&

compareAndSetState( 0 , acquires)) {

setExclusiveOwnerThread(current);

return true ;

}

}

//如果当前线程本身就持有锁，那么叠加状态值，持续获得锁

else if (current == getExclusiveOwnerThread()) {

int nextc = c + acquires;

if (nextc < 0 )

throw new Error( "Maximum lock count exceeded" );

setState(nextc);

return true ;

}

//以上条件都不满足，那么线程进入等待队列。

return false ;

}

//非公平获取锁  java.util.concurrent.locks.ReentrantLock$UnFairSync.java

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;

}