公平锁与非公平锁，为什么要使用
公平锁与非公平锁
首先，我们来看下什么是公平锁和非公平锁，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。
合适的时机
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。
为了能够更好的理解公平锁和非公平锁，我们举一个生活中的例子，假设我们还在学校读书，去食堂排队买饭，我排在队列的第二个，我前面还有一位同学，但此时我脑子里想的不是午饭，而是上午的一道数学题并陷入深思，所以当前面的同学打完饭之后轮到我时我走神了，并也没注意到现在轮到我了，此时前面的同学突然又回来插队，说“不好意思，阿姨麻烦给我加个鸡腿”，像这样的行为就可以类比我们的公平锁和非公平锁。
设置非公平的好处
Java 设计者设计非公平锁，是为了提高整体的运行效率。
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
场景
公平场景
下面我们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。
非公平场景
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。
公平锁与非公平锁的优缺点
| 优势 | 劣势 | |
|---|---|---|
| 公平锁 | 各线程公平平等，每个线程等待一段时间后，总有执行机会 | 更慢，吞吐量小 |
| 非公平锁 | 更快，吞吐量大 | 有可能产生线程饥饿（某些线程长时间得不到执行） |
公平锁的优点在于各个线程公平平等，每个线程等待一段时间后，都有执行的机会，而它的缺点就在于整体执行速度更慢，吞吐量更小，相反非公平锁的优势就在于整体执行速度更快，吞吐量更大，但同时也可能产生线程饥饿问题，也就是说如果一直有线程插队，那么在等待队列中的线程可能长时间得不到运行。
示例
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 {
// 非公平锁
private final Lock lock = new ReentrantLock(false);
public void printJob(Object doc) {
lock.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 {
lock.unlock();
}
}
}
公平锁结果
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 0: The document has been printed
Thread 1: PrintQueue: Printing a Job during 5 seconds
Thread 1: The document has been printed
Thread 2: PrintQueue: Printing a Job during 2 seconds
Thread 2: The document has been printed
Thread 3: PrintQueue: Printing a Job during 9 seconds
Thread 3: The document has been printed
Thread 4: PrintQueue: Printing a Job during 6 seconds
Thread 4: The document has been printed
Thread 5: PrintQueue: Printing a Job during 8 seconds
Thread 5: The document has been printed
Thread 6: PrintQueue: Printing a Job during 2 seconds
Thread 6: The document has been printed
Thread 7: PrintQueue: Printing a Job during 5 seconds
Thread 7: The document has been printed
Thread 8: PrintQueue: Printing a Job during 1 seconds
Thread 8: The document has been printed
Thread 9: PrintQueue: Printing a Job during 9 seconds
Thread 9: The document has been printed
线程直接获取锁的顺序是完全公平的，先到先得
非公平锁结果
Thread 0: Going to print a job
Thread 0: PrintQueue: Printing a Job during 3 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: 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 0 seconds
Thread 2: The document has been printed
Thread 3: PrintQueue: Printing a Job during 8 seconds
Thread 3: The document has been printed
Thread 4: PrintQueue: Printing a Job during 9 seconds
Thread 4: The document has been printed
Thread 5: PrintQueue: Printing a Job during 8 seconds
Thread 5: The document has been printed
Thread 6: PrintQueue: Printing a Job during 0 seconds
Thread 7: PrintQueue: Printing a Job during 1 seconds
Thread 6: The document has been printed
Thread 7: The document has been printed
Thread 8: PrintQueue: Printing a Job during 7 seconds
Thread 8: The document has been printed
Thread 9: PrintQueue: Printing a Job during 3 seconds
Thread 9: The document has been printed
6 7顺序不一样，存在插队的问题
源码分析
Sync.java
@ReservedStackAccess
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
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;
}
公平锁
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
非公平锁
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
*/
@ReservedStackAccess
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;
}
}
hasQueuedPredecessors()
通过对比，我们可以发现这两个方法其实整体思路都是很类似的，而最大的不同点就是在于非公平锁缺少了一个hasQueuedPredecessors的判断。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。
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
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
最后我们做一下总结。非公平锁在特定的情况下可以插队，这虽然破坏了整体的执行顺序，但是却也因此在更宏观的层面上提升了程序的运行效率。
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