在Java的ReentrantLock构造函数中提供了两种锁：创建公平锁和非公平锁（默认）。代码如下：
public ReentrantLock() {
sync = new NonfairSync();
}
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
此例可反应公平锁和非公平锁的差异:
package concurrency; import java.util.concurrent.locks.ReentrantLock; /**
* @author yuanxu
* Apr 11, 2017 9:59:24 PM
*/
public class ReentrantLockTest {
private static final ReentrantLock lock = new ReentrantLock();
private static final ReentrantLock fairlock = new ReentrantLock(true);
private int n; public static void main(String[] args) {
// TODO Auto-generated method stub
ReentrantLockTest rlt = new ReentrantLockTest();
for (int i=0; i<100; i++) {
Thread nonT = new Thread(new NonFairTestThread(rlt));
nonT.setName("nonFair[" + (i + 1) + "]");
nonT.start(); Thread fairT = new Thread(new FairTestThread(rlt));
fairT.setName("fair[" + (i + 1) + "]");
fairT.start();
}
} static class NonFairTestThread implements Runnable {
private ReentrantLockTest rlt; public NonFairTestThread(ReentrantLockTest rlt) {
this.rlt = rlt;
} public void run() {
lock.lock();
try {
rlt.setNum(rlt.getNum() + 1);
System.out.println(Thread.currentThread().getName()
+ " nonfairlock***************" + rlt.getNum());
} finally {
lock.unlock();
}
}
} static class FairTestThread implements Runnable {
private ReentrantLockTest rlt; public FairTestThread(ReentrantLockTest rlt) {
this.rlt = rlt;
} public void run() {
fairlock.lock();
try {
rlt.setNum(rlt.getNum() + 1);
System.out.println(Thread.currentThread().getName()
+ " fairlock=======" + rlt.getNum() + " "
+ fairlock.getHoldCount() + " queuelength="
+ fairlock.getQueueLength());
} finally {
fairlock.unlock();
}
}
} public void setNum(int n) {
this.n = n;
} public int getNum() {
return n;
} }
运行结果反映：
在公平的锁上，线程按照他们发出请求的顺序获取锁，但在非公平锁上，则允许‘插队’。
在公平的锁中，如果有另一个线程持有锁或者有其他线程在等待队列中等待这个所，那么新发出的请求的线程将被放入到队列中。而非公平锁上，只有当锁被某个线程持有时，新发出请求的线程才会被放入队列中。
非公平锁性能高于公平锁性能的原因： 在恢复一个被挂起的线程与该线程真正运行之间存在着严重的延迟。
参考:
http://blog.csdn.net/zmx729618/article/details/51593666
http://blog.csdn.net/qq_22929803/article/details/51458340
