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
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//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;
??????? }
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