1、非公平锁不能保证锁的获取是按照请求锁的顺序进行的。这可能会导致某个或某些线程永远得不到锁。
2、CPU唤醒线程的费用可以降低，整体吞吐效率会很高。但是可能会有线程长时间甚至永远得不到锁，导致饿死。
/**
* Sync object for non-fair locks
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
/**
* Sync object for fair locks
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {
acquire(1);
}
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
}
知识点扩展：
非公平锁，顾名思义，各个线程获取到锁的顺序，不一定和它们申请的先后顺序一致，有可能后来的线程，反而先获取到了锁。
在实现上，公平锁在进行lock时，首先会进行tryAcquire()操作。在tryAcquire中，会判断等待队列中是否已经有别的线程在等待了。如果队列中已经有别的线程了，则tryAcquire失败，则将自己加入队列。如果队列中没有别的线程，则进行获取锁的操作。
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
**/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
非公平锁，在进行lock时，会直接尝试进行加锁，如果成功，则获取到锁，如果失败，则进行和公平锁相同的动作。
