公平锁指的是按照线程请求的顺序，来分配锁；而非公平锁指的是不完全按照请求的顺序，在一定情况下，可以允许插队。但需要注意这里的非公平并不是指完全的随机，不是说线程可以任意插队，而是仅仅“在合适的时机”插队
什么时候是合适的时机呢？
假设当前线程在请求获取锁的时候，恰巧前一个持有锁的线程释放了这把锁，那么当前申请锁的线程就可以不顾已经等待的线程而选择立刻插队。但是如果当前线程请求的时候，前一个线程并没有在那一时刻释放锁，那么当前线程还是一样会进入等待队列
为什么要设置非公平策略呢?
我们都知道非公平是 ReentrantLock的默认策略，如果我们不加以设置的话默认就是非公平的，难道我的这些排队的时间都白白浪费了吗，为什么别人比我有优先权呢？毕竟公平是一种很好的行为，而非公平是一种不好的行为
让我们考虑一种情况，假设线程 A 持有一把锁，线程 B 请求这把锁，由于线程 A 已经持有这把锁了，所以线程 B 会陷入等待，在等待的时候线程 B 会被挂起，也就是进入阻塞状态，那么当线程 A 释放锁的时候，本该轮到线程 B 苏醒获取锁，但如果此时突然有一个线程 C 插队请求这把锁，那么根据非公平的策略，会把这把锁给线程 C，这是因为唤醒线程 B 是需要很大开销的，很有可能在唤醒之前，线程 C 已经拿到了这把锁并且执行完任务释放了这把锁。相比于等待唤醒线程 B 的漫长过程，插队的行为会让线程 C 本身跳过陷入阻塞的过程，如果在锁代码中执行的内容不多的话，线程 C 就可以很快完成任务，并且在线程 B 被完全唤醒之前，就把这个锁交出去，这样是一个双赢的局面，对于线程 C 而言，不需要等待提高了它的效率，而对于线程 B 而言，它获得锁的时间并没有推迟，因为等它被唤醒的时候，线程 C 早就释放锁了，因为线程 C 的执行速度相比于线程 B 的唤醒速度，是很快的，所以 Java 设计非公平锁，是为了提高整体的运行效率
用图示来说明公平和非公平的场景，先来看公平的情况。假设我们创建了一个公平锁，此时有 4 个线程按顺序来请求公平锁，线程 1 在拿到这把锁之后，线程 2、3、4 会在等待队列中开始等待，然后等线程 1 释放锁之后，线程 2、3、4 会依次去获取这把锁，线程 2 先获取到的原因是它等待的时间最长
假设线程 1 在解锁的时候，突然有线程 5 尝试获取这把锁，那么根据我们的非公平策略，线程 5 是可以拿到这把锁的，尽管它没有进入等待队列，而且线程 2、3、4 等待的时间都比线程 5 要长，但是从整体效率考虑，这把锁此时还是会交给线程 5 持有
/**
* 描述：演示公平锁，分别展示公平和不公平的情况，非公平锁会让现在持有锁的线程优先再次获取到锁
*/
public class FairAndUnfair {
public static void main(String args[]) {
PrintQueue printQueue = new PrintQueue();
Thread thread[] = new Thread[10];
for (int i = 0; i < 10; i++) {
thread[i] = new Thread(new Job(printQueue), "Thread " + i);
}
for (int i = 0; i < 10; i++) {
thread[i].start();
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
class Job implements Runnable {
private PrintQueue printQueue;
public Job(PrintQueue printQueue) {
this.printQueue = printQueue;
}
@Override
public void run() {
System.out.printf("%s: Going to print a job\n", Thread.currentThread().getName());
printQueue.printJob(new Object());
System.out.printf("%s: The document has been printed\n", Thread.currentThread().getName());
}
}
class PrintQueue {
private final Lock queueLock = new ReentrantLock(false);
public void printJob(Object document) {
queueLock.lock();
try {
Long duration = (long) (Math.random() * 10000);
System.out.printf("%s: PrintQueue: Printing a Job during %d seconds\n",
Thread.currentThread().getName(), (duration / 1000));
Thread.sleep(duration);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
queueLock.unlock();
}
queueLock.lock();
try {
Long duration = (long) (Math.random() * 10000);
System.out.printf("%s: PrintQueue: Printing a Job during %d seconds\n",
Thread.currentThread().getName(), (duration / 1000));
Thread.sleep(duration);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
queueLock.unlock();
}
}
}
可以通过改变 new ReentrantLock(false) 中的参数来设置公平/非公平锁，以上代码在公平的情况下的输出
Thread 0: Going to print a job
Thread 0: PrintQueue: Printing a Job during 5 seconds
Thread 1: Going to print a job
Thread 2: Going to print a job
Thread 3: Going to print a job
Thread 4: Going to print a job
Thread 5: Going to print a job
Thread 6: Going to print a job
Thread 7: Going to print a job
Thread 8: Going to print a job
Thread 9: Going to print a job
Thread 1: PrintQueue: Printing a Job during 3 seconds
Thread 2: PrintQueue: Printing a Job during 4 seconds
Thread 3: PrintQueue: Printing a Job during 3 seconds
Thread 4: PrintQueue: Printing a Job during 9 seconds
Thread 5: PrintQueue: Printing a Job during 5 seconds
Thread 6: PrintQueue: Printing a Job during 7 seconds
Thread 7: PrintQueue: Printing a Job during 3 seconds
Thread 8: PrintQueue: Printing a Job during 9 seconds
Thread 9: PrintQueue: Printing a Job during 5 seconds
Thread 0: PrintQueue: Printing a Job during 8 seconds
Thread 0: The document has been printed
Thread 1: PrintQueue: Printing a Job during 1 seconds
Thread 1: The document has been printed
Thread 2: PrintQueue: Printing a Job during 8 seconds
Thread 2: The document has been printed
Thread 3: PrintQueue: Printing a Job during 2 seconds
Thread 3: The document has been printed
Thread 4: PrintQueue: Printing a Job during 0 seconds
Thread 4: The document has been printed
Thread 5: PrintQueue: Printing a Job during 7 seconds
Thread 5: The document has been printed
Thread 6: PrintQueue: Printing a Job during 3 seconds
Thread 6: The document has been printed
Thread 7: PrintQueue: Printing a Job during 9 seconds
Thread 7: The document has been printed
Thread 8: PrintQueue: Printing a Job during 5 seconds
Thread 8: The document has been printed
Thread 9: PrintQueue: Printing a Job during 9 seconds
Thread 9: The document has been printed
而以上代码在非公平的情况下的输出是这样的
Thread 0: Going to print a job
Thread 0: PrintQueue: Printing a Job during 6 seconds
Thread 1: Going to print a job
Thread 2: Going to print a job
Thread 3: Going to print a job
Thread 4: Going to print a job
Thread 5: Going to print a job
Thread 6: Going to print a job
Thread 7: Going to print a job
Thread 8: Going to print a job
Thread 9: Going to print a job
Thread 0: PrintQueue: Printing a Job during 8 seconds
Thread 0: The document has been printed
Thread 1: PrintQueue: Printing a Job during 9 seconds
Thread 1: PrintQueue: Printing a Job during 8 seconds
Thread 1: The document has been printed
Thread 2: PrintQueue: Printing a Job during 6 seconds
Thread 2: PrintQueue: Printing a Job during 4 seconds
Thread 2: The document has been printed
Thread 3: PrintQueue: Printing a Job during 9 seconds
Thread 3: PrintQueue: Printing a Job during 8 seconds
Thread 3: The document has been printed
Thread 4: PrintQueue: Printing a Job during 4 seconds
Thread 4: PrintQueue: Printing a Job during 2 seconds
Thread 4: The document has been printed
Thread 5: PrintQueue: Printing a Job during 2 seconds
Thread 5: PrintQueue: Printing a Job during 5 seconds
Thread 5: The document has been printed
Thread 6: PrintQueue: Printing a Job during 2 seconds
Thread 6: PrintQueue: Printing a Job during 6 seconds
Thread 6: The document has been printed
Thread 7: PrintQueue: Printing a Job during 6 seconds
Thread 7: PrintQueue: Printing a Job during 4 seconds
Thread 7: The document has been printed
Thread 8: PrintQueue: Printing a Job during 3 seconds
Thread 8: PrintQueue: Printing a Job during 6 seconds
Thread 8: The document has been printed
Thread 9: PrintQueue: Printing a Job during 3 seconds
Thread 9: PrintQueue: Printing a Job during 5 seconds
Thread 9: The document has been printed
可以看出，非公平情况下，存在抢锁“插队”的现象，比如Thread 0 在释放锁后又能优先获取到锁，虽然此时在等待队列中已经有 Thread 1 ~ Thread 9 在排队了
| 优势 | 劣势 | |
|---|---|---|
| 公平锁 | 各线程公平平等，每一个线程在等待一段时间后，总有执行的机会 | 更慢，吞吐量更少 |
| 非公平锁 | 更快，吞吐量更大 | 有可能产生线程饥饿，也就是某些线程在长时间内，始终得不到执行 |
下面我们来分析公平和非公平锁的源码，具体看下它们是怎样实现的，可以看到在 ReentrantLock 类包含一个 Sync 类，这个类继承自AQS（AbstractQueuedSynchronizer），代码如下
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
/** Synchronizer providing all implementation mechanics */
private final Sync sync;
Sync 类的代码
abstract static class Sync extends AbstractQueuedSynchronizer {...}
Sync 有公平锁 FairSync 和非公平锁 NonfairSync两个子类
static final class NonfairSync extends Sync {...}
static final class FairSync extends Sync {...}
公平锁的锁获取源码如下
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() && //这里判断了 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;
}
非公平锁的锁获取源码如下
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) { //这里没有判断 hasQueuedPredecessors()
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;
}
通过对比，我们可以明显的看出公平锁与非公平锁的 lock() 方法唯一的区别就在于公平锁在获取锁时多了一个限制条件：hasQueuedPredecessors() 为 false，这个方法就是判断在等待队列中是否已经有线程在排队了。这也就是公平锁和非公平锁的核心区别，如果是公平锁，那么一旦已经有线程在排队了，当前线程就不再尝试获取锁；对于非公平锁而言，无论是否已经有线程在排队，都会尝试获取一下锁，获取不到的话，再去排队
注意：针对 tryLock() 方法，它不遵守设定的公平原则
例如，当有线程执行 tryLock() 方法的时候，一旦有线程释放了锁，那么这个正在 tryLock 的线程就能获取到锁，即使设置的是公平锁模式，即使在它之前已经有其他正在等待队列中等待的线程，简单地说就是 tryLock 可以插队
看源码就会发现
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
这里调用的就是 nonfairTryAcquire()，表明了是不公平的，和锁本身是否是公平锁无关
公平锁就是会按照多个线程申请锁的顺序来获取锁，从而实现公平的特性。非公平锁加锁时不考虑排队等待情况，直接尝试获取锁，所以存在后申请却先获得锁的情况，但由此也提高了整体的效率