Java 锁系列（二）——ReentrantLock 和 ReentrantReadWriteLock基础运用

Lock 接口提供了与 synchronized 相似的功能，和 synchronized 不同的是，Lock 在使用的时候需要显示的获取和释放锁。虽然牺牲了隐式获取释放锁的便捷性，但是对于锁的操作具有更强的可操作性、可控制性以及提供可中断操作和超时获取锁等机制。本文将描述 ReentrantLock 和 ReadWriteLock 锁的基础使用。

一、ReentrantLock 运用

package com.nineya.test;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.ReentrantLock;

public class ReentrantLockMain {
    private int num;
    private int numLock;
    ReentrantLock lock = new ReentrantLock();

    public void execute() {
        num++;
    }

    public void executeLock() {
        lock.lock();
        numLock++;
        lock.unlock();
    }

    public void print() {
        System.out.println("num = " + num + ", numLock = " + numLock);
        num = 0;
        numLock = 0;
    }

    public static void main(String[] args) throws InterruptedException {
        ReentrantLockMain main = new ReentrantLockMain();
        ExecutorService executor = Executors.newCachedThreadPool();
        for (int j = 0; j < 5; j++) {
            for (int i = 0; i < 1000; i++) {
                executor.execute(main::execute);
                executor.execute(main::executeLock);
            }
            Thread.sleep(1000);
            main.print();
        }
    }
}

程序运行结果如下：

num = 986, numLock = 1000
num = 990, numLock = 1000
num = 998, numLock = 1000
num = 995, numLock = 1000
num = 995, numLock = 1000

众所周知，i++ 并不是原子操作，其执行过程可以分成读内存到寄存器、在寄存器中自增、写回内存三个部分，并发执行时就可能导致部分自增操作被覆盖，所以未加锁时 num 小于实际的值，加锁后的 numLock 值为正常。

二、ReadWriteLock 运用

ReadWriteLock 是独占锁和共享锁的实现，适用于读多写少的场景。

package com.nineya.test;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReadWriteLockMain {
    private int num;
    private int numLock;
    ReadWriteLock lock = new ReentrantReadWriteLock();

    public int getNum() {
        return num;
    }

    public void read() {
        int a = getNum();
        int b = getNum();
        if (a != b) {
            System.out.println("num 改变: a = " + a + ", b = " + b);
        }
    }

    public void write() {
        num++;
    }

    public int getNumLock() {
        return numLock;
    }

    public void readLock(){
        lock.readLock().lock();
        int a = getNumLock();
        int b = getNumLock();
        if (a != b) {
            System.out.println("numLock 改变: a = " + a + ", b = " + b);
        }
        lock.readLock().unlock();
    }

    public void writeLock() {
        lock.writeLock().lock();
        numLock++;
        lock.writeLock().unlock();
    }

    public void print() {
        System.out.println("num = " + num + ", numLock = " + numLock);
        num = 0;
        numLock = 0;
    }

    public static void main(String[] args) throws InterruptedException {
        ReadWriteLockMain main = new ReadWriteLockMain();
        ExecutorService executor = Executors.newCachedThreadPool();
        for (int j = 0; j < 5; j++) {
            for (int i = 0; i < 1000; i++) {
                executor.execute(main::write);
                executor.execute(main::writeLock);
                executor.execute(main::read);
                executor.execute(main::readLock);
            }
            Thread.sleep(1000);
            main.print();
        }
    }
}

程序运行结果如下：

num 改变: a = 277, b = 278
num 改变: a = 322, b = 323
num 改变: a = 376, b = 377
num 改变: a = 422, b = 423
num = 997, numLock = 1000
num = 999, numLock = 1000
num = 998, numLock = 1000
num = 997, numLock = 1000
num = 999, numLock = 1000

可以看到，num 由于没有添加读锁，在两次值之间 num 的值发生了改变，添加读锁则可以解决该问题。

三、其他接口方法