Java多线程再学习，温故知新（六）公平锁

川长思鸟来 2022-03-24 14:40 220阅读 0赞

## 公平锁 ##

公平是针对锁的获取而言的，如果一个锁是公平的，那么锁的获取顺序就应该符合请求的绝对时间顺序。可以通过控制队列来实现一个公平锁，线程进入队列后，每次都唤醒队列中的第一个线程。

![20190122112733486.png][]

## 公平锁的实现 ##

import java.util.ArrayList;
    import java.util.List;
    
    public class FairLock {
    	private boolean isLocked = false;
    	private Thread lockingThread = null;
    	private List<QueueObject> waitingThreads = new ArrayList<QueueObject>();
    	boolean isLockedForThisThread = true;
    
    	public void lock() throws InterruptedException {
    		QueueObject queueObject = new QueueObject();
    		synchronized (this) {
    			waitingThreads.add(queueObject);
    		}
    		// 可重入锁实现
    		while (isLockedForThisThread) {
    			synchronized (this) {
    				isLockedForThisThread = isLocked || waitingThreads.get(0) != queueObject;
    				if (!isLockedForThisThread) {
    					isLocked = true;
    					waitingThreads.remove(queueObject);
    					lockingThread = Thread.currentThread();
    					return;
    				}
    			}
    
    			try {
    				queueObject.doWait();
    			} catch (InterruptedException e) {
    				synchronized (this) {
    					waitingThreads.remove(queueObject);
    				}
    				throw e;
    			}
    		}
    	}
    
    	public synchronized void unlock() {
    		if (this.lockingThread != Thread.currentThread()) {
    			throw new IllegalMonitorStateException("Calling thread has not locked this lock");
    		}
    		isLocked = false;
    		lockingThread = null;
    		if (waitingThreads.size() > 0) {
    			// 将队列的第一个元素叫醒
    			waitingThreads.get(0).doNotify();
    		}
    	}
    }

队列实现类

public class QueueObject {
    
    	private boolean isNotified = false;
    
    	public synchronized void doWait() throws InterruptedException {
    		// 只要没被叫醒就一直等待
    		while (!isNotified) {
    			this.wait();
    		}
    		this.isNotified = false;
    	}
    
    	public synchronized void doNotify() {
    		this.isNotified = true;
    		// 当前等待的就被叫醒
    		this.notify();
    	}
    
    	public boolean equals(Object o) {
    		return this == o;
    	}
    }

[20190122112733486.png]: /images/20220323/fbe2549a425d40378ec49a2a105e5d66.png