JUC高并发编程 - 读写锁 - 《Java服务端技术》

无锁
synchronized和ReentrantLock
ReentrantReadWriteLock
读写锁ReentrantReadWriteLock
锁降级：将写入锁降级为读锁

一个资源可以被多个线程访问，或者可以被一个写线程访问，但是不能同时存在读写线程，读写互斥，读读共享的

无锁

多线程抢夺资源，乱

synchronized和ReentrantLock

读读和写写都只能一个来操作

ReentrantReadWriteLock

读读可以共享
写写只能一个操作

读写锁ReentrantReadWriteLock

package com.daijunyi.lock;
import java.util.HashMap;
import java.util.Random;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantReadWriteLock;
class Cache{
    private volatile HashMap<String,Object> map = new HashMap<>();
    //读写锁
    private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
    public void put(String key,String value){
        rwLock.writeLock().lock();
        try {
            System.out.println(Thread.currentThread().getName()+"开始做存储操作");
            TimeUnit.SECONDS.sleep(new Random().nextInt(5));
            map.put(key,value);
            System.out.println(Thread.currentThread().getName()+"+++存储完成");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            rwLock.writeLock().unlock();
        }
    }
    public Object get(String key){
        rwLock.readLock().lock();
        Object value = null;
        try {
            System.out.println(Thread.currentThread().getName()+"---读操作");
            TimeUnit.SECONDS.sleep(new Random().nextInt(4));
            value = map.get(key);
            System.out.println(Thread.currentThread().getName()+"读操作完成");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            rwLock.readLock().unlock();
        }
        return value;
    }
}
//读写锁
public class ReadWriteDemo {
    public static void main(String[] args) {
        Cache cache = new Cache();
        //写操作
        for (int i=0;i<4;i++){
            final String num = String.valueOf(i);
            new Thread(()->{
                cache.put(num,num);
            },num).start();
        }
        //读操作
        for (int i=0;i<4;i++){
            final String num = String.valueOf(i);
            new Thread(()->{
                Object o = cache.get(num);
                System.out.println("读操作"+o);
            },num).start();
        }
    }
}

运行结果

/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/bin/java -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/tools.jar:/Users/djy/WorkFile/learn/juc/picker/target/classes com.daijunyi.lock.ReadWriteDemo
0开始做存储操作
0+++存储完成
1开始做存储操作
1+++存储完成
2开始做存储操作
2+++存储完成
3开始做存储操作
3+++存储完成
0---读操作
1---读操作
2---读操作
3---读操作
2读操作完成
读操作2
3读操作完成
读操作3
0读操作完成
读操作0
1读操作完成
读操作1

Process finished with exit code 0

锁降级：将写入锁降级为读锁

锁降级的必要性：
锁降级中读锁的获取是否必要呢？答案是必要的。主要是为了保证数据的可见性，如果当前线程不获取读锁而是直接释放写锁，假设此刻另一个线程（记作线程T）获取了写锁并修改了数据，那么当前线程无法感知线程T的数据更新。如果当前线程获取读锁，即遵循锁降级的步骤，则线程T将会被阻塞，直到当前线程使用数据并释放读锁之后，线程T才能获取写锁进行数据更新。
这时因为可能存在一个事务线程不希望自己的操作被别的线程中断，而这个事务操作可能分成多部分操作更新不同的数据（或表）甚至非常耗时。如果长时间用写锁独占，显然对于某些高响应的应用是不允许的，所以在完成部分写操作后，退而使用读锁降级，来允许响应其他进程的读操作。只有当全部事务完成后才真正释放锁。
所以总结下锁降级的意义应该就是：在一边读一边写的情况下提高性能。

package com.daijunyi.lock;

import java.util.concurrent.locks.ReentrantReadWriteLock;

//演示读写锁降级
public class ReadWriteDemo2 {

    public static void main(String[] args) {
        //可重入读写锁对象创建
        ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
        ReentrantReadWriteLock.ReadLock readLock = rwLock.readLock();
        ReentrantReadWriteLock.WriteLock writeLock = rwLock.writeLock();

        //锁降级
        //1 获取到写锁
        writeLock.lock();
        System.out.println("获取到写锁，做写入操作");

        //获取读锁
        readLock.lock();
        System.out.println("获取读锁");

        //释放写锁
        writeLock.unlock();
        System.out.println("释放写锁");

        //释放读锁
        readLock.unlock();
        System.out.println("释放读锁");
    }

}

运行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/bin/java -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/lib/tools.jar:/Users/djy/WorkFile/learn/juc/picker/target/classes com.daijunyi.lock.ReadWriteDemo2
获取到写锁
获取读锁
释放写锁
释放读锁

Process finished with exit code 0