显示锁ReentrantLock和Lock的使用_reentrantlock和lock用法-优快云博客

本文深入探讨了Java中ReentrantLock的特性和使用方法，对比了与synchronized关键字的区别，介绍了其高级功能如定时等待和可中断等待，并通过示例代码展示了如何在多线程环境中正确使用ReentrantLock。

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Jdk1.5以后，Doug Lea大神添加了新的多线程的包,在java.util.concurrent.locks下，有一组实现线程同步的接口Lock和类ReentrantLock.ReentrantLock也是Lock接口的唯一实现类.

Lock提供了一种无条件的、可轮询的、定时的以及可中断的锁获取操作,所有的加锁和解锁方法都是显示的.

public interface Lock {

　　// 用来获取锁，如果锁已经被其他线程获取，则一直等待，直到获取到锁
   void lock();

　　// 该方法获取锁时，可以响应中断，比如现在有两个线程，一个已经获取到了锁，另一个线程调用这个方法正在等待锁，但是此刻又不想让这个线程一直在这死等，可以通过调用线程的Thread.interrupted()方法，来中断线程的等待过程
　　void lockInterruptibly() throws InterruptedException;

　　// tryLock方法会返回布尔值，该方法会尝试着获取锁，如果获取到锁，就返回true，如果没有获取到锁，就返回false，但是该方法会立刻返回，而不会一直等待
   boolean tryLock();

　　// 这个方法和上面的tryLock差不多是一样的，只是会尝试指定的时间，如果在指定的时间内拿到了锁，则会返回true，如果在指定的时间内没有拿到锁，则会返回false
   boolean tryLock(long time, TimeUnit unit) throws InterruptedException;

　　// 释放锁
   void unlock();

　　// 实现线程通信，相当于wait和notify
   Condition newCondition();
}

ReentrantLock实现了Lock接口,并提供了和synchronized相同的互斥性和内存可见性,ReentrantLock也提供了可重入的加锁语义.使用Lock锁有标准的使用格式:

Lock l = ...;
l.lock();
try {
      　　// access the resource protected by this lock
     } finally {// 必须使用try，最后在finally里面释放锁
      　　l.unlock();
     }

下面给出自己的demo:

package com.springcloud.server.springserver.thread;

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

public class LockDemo {
    // new一个锁对象，注意此处必须声明成类对象，保持只有一把锁,ReentrantLock是Lock的唯一实现类
    Lock lock = new ReentrantLock();
    public void readFile(String fileMessage){
        lock.lock();// 上锁
        try{
            System.out.println(Thread.currentThread().getName()+"得到了锁，正在读取文件……");
            for(int i=0; i<fileMessage.length(); i++){
                System.out.print(fileMessage.charAt(i));
            }
            System.out.println();
            System.out.println("文件读取完毕！");
        }finally{
            lock.unlock();
            System.out.println(Thread.currentThread().getName()+"释放了锁！");
            System.out.println("------------------------");
        }
    }

    public void demo(final String fileMessage){
        // 创建数量可变的线程池
        ExecutorService service = Executors.newCachedThreadPool();
        // 提交5个任务
        for(int i=0; i<5; i++){
            service.execute(new Runnable() {
                @Override
                public void run() {
                    readFile(fileMessage);
                }
            });
        }
        // 释放线程池中的线程
        service.shutdown();
    }

    public static void main(String[] args) {
        LockDemo lockDemo = new LockDemo();
        lockDemo.demo("消息");
    }
}

输出结果:

"D:\JAVA Develop\jdk1.8\jdk8\bin\java.exe" "-javaagent:E:\idea\IntelliJ IDEA 2018.1.5\lib\idea_rt.jar=55424:E:\idea\IntelliJ IDEA 2018.1.5\bin" -Dfile.encoding=UTF-8 -classpath "D:\JAVA Develop\jdk1.8\jdk8\jre\lib\charsets.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\deploy.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\access-bridge-64.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\cldrdata.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\dnsns.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\jaccess.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\jfxrt.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\localedata.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\nashorn.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\sunec.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\sunjce_provider.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\sunmscapi.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\sunpkcs11.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\ext\zipfs.jar;D:\JAVA Develop\jdk1.8\jdk8\jre\lib\javaws.jar;D:\JAVA 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pository_sshold\org\springframework\spring-core\5.0.9.RELEASE\spring-core-5.0.9.RELEASE.jar;E:\repository\repository_sshold\org\springframework\spring-jcl\5.0.9.RELEASE\spring-jcl-5.0.9.RELEASE.jar;E:\repository\repository_sshold\org\springframework\spring-test\5.0.9.RELEASE\spring-test-5.0.9.RELEASE.jar;E:\repository\repository_sshold\org\xmlunit\xmlunit-core\2.5.1\xmlunit-core-2.5.1.jar;E:\repository\repository_sshold\org\slf4j\slf4j-api\1.7.25\slf4j-api-1.7.25.jar;E:\repository\repository_sshold\org\slf4j\slf4j-log4j12\1.7.25\slf4j-log4j12-1.7.25.jar;E:\repository\repository_sshold\log4j\log4j\1.2.17\log4j-1.2.17.jar" com.springcloud.server.springserver.thread.LockDemo
pool-1-thread-1得到了锁，正在读取文件……
消息
文件读取完毕！
pool-1-thread-1释放了锁！
------------------------
pool-1-thread-4得到了锁，正在读取文件……
消息
文件读取完毕！
pool-1-thread-4释放了锁！
------------------------
pool-1-thread-5得到了锁，正在读取文件……
消息
文件读取完毕！
pool-1-thread-5释放了锁！
------------------------
pool-1-thread-2得到了锁，正在读取文件……
消息
文件读取完毕！
pool-1-thread-2释放了锁！
------------------------
pool-1-thread-3得到了锁，正在读取文件……
消息
文件读取完毕！
pool-1-thread-3释放了锁！
------------------------

ReentrantLock在加锁语义上和内置的synchronized锁相同,此外它还提供了一些其它的功能,包括定时的锁等待、可中断的锁等待、公平性以及实现非块结构的加锁.性能的话稍稍优于内置锁.

与显示锁相比内置锁仍然具有很大的优势.内置锁为许多开发人员所熟悉,并且简洁紧凑.ReentrantLock的危险性比同步机制要高,如果忘记在finally块中调用unlock,代码表面可以运行,但是已经埋下了一颗定时炸弹.只有当内置锁不满足需求的时候,才考虑使用ReentrantLock.