Java多线程Lock对象之公平锁和非公平锁

在Lock对象加锁时，存在两种获取锁的机制，一种是按照线程排队等锁的顺序来获取，一种是随机获取锁，前者叫公平锁，后者叫非公平锁。公平锁的实现是内部维护了一个队列，因此效率上会比非公平锁低5-10倍。

资源共享类：

package com.lenovo.plm.dms.p16;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Service {
    
    private Lock lock;
    
    public Service(boolean isFair){
        lock = new ReentrantLock(isFair);
    }
    
    public void service(){
        
        lock.lock();
        System.out.println(Thread.currentThread().getName() + " is running");
        lock.unlock();
    }

}

线程类：

package com.lenovo.plm.dms.p16;

public class MyThread extends Thread {

    private Service service;
    
    public MyThread(Service service){
        this.service = service;
    }
    
    @Override
    public void run() {
        // TODO Auto-generated method stub
        super.run();
        System.out.println(Thread.currentThread().getName() + " begin to run...");
        service.service();
    }
    
}

运行类如下：

package com.lenovo.plm.dms.p16;

public class Main {
    
    public static void main(String[] args) {
        Service service = new Service(false);
        MyThread[] t = new MyThread[10];
        
        for(int i=0;i<10;i++){
            t[i] = new MyThread(service);
        }
        
        for(int i=0;i<10;i++){
            t[i].start();
        }

    }
}

运行结果如下：

Thread-1 begin to run...
Thread-5 begin to run...
Thread-4 begin to run...
Thread-7 begin to run...
Thread-2 begin to run...
Thread-3 begin to run...
Thread-0 begin to run...
Thread-6 begin to run...
Thread-1 is running
Thread-8 begin to run...
Thread-9 begin to run...
Thread-8 is running
Thread-4 is running
Thread-5 is running
Thread-7 is running
Thread-2 is running
Thread-3 is running
Thread-0 is running
Thread-6 is running
Thread-9 is running

线程在等锁和获取锁的顺序是一样的，因此这是一个公平锁，如果将上述ReentrackLock构造函数传入false，则就是一个非公平锁。