java多线程-ReadWriteLock

淡淡的烟草味﹌ 2021-12-22 19:57 222阅读 0赞

大纲：

1.  ReadWriteLock接口
2.  ReentrantReadWriteLock
3.  ReentrantReadWriteLock使用

一、ReadWriteLock

public interface ReadWriteLock {
        Lock readLock();
        Lock writeLock();
    }

1.  readLock获得读锁。
2.  writeLock获得写锁。

二、ReentrantReadWriteLock是ReadWriteLock的实现类

1.  ReentrantReadWriteLock内部维护了两个实现Lock接口的内部类ReadLock读锁，与WriteLock写锁。
2.  仅写锁支持condition。
3.  锁降级：写锁降级为读锁，获取写锁、获取读锁、释放写锁。
4.  锁升级：不支持读锁升级为写锁。
5.  线程间互斥关系：读读共享锁、读写独享锁、写写独享锁
    
        public class Met {
            ReadWriteLock lock = new ReentrantReadWriteLock();
            public void read(Thread thread){
                try {
                    lock.readLock().lock();
                    System.out.println(thread.getName()+"read lock");
                    for(int i =0;i<10;i++){ System.out.println(thread.getName()+":"+i); } } finally { System.out.println(Thread.currentThread().getName()+"read unlock"); lock.readLock().unlock(); } } public void write(Thread thread){ try { lock.writeLock().lock(); System.out.println(thread.getName()+"write lock"); for(int i =0;i<10;i++){ System.out.println(thread.getName()+":"+i); } } finally { System.out.println(Thread.currentThread().getName()+"write unlock"); lock.writeLock().unlock(); } } }
    
        public class ReadRunnable implements Runnable {
            Met met;
            ReadRunnable(Met met){
                this.met = met;
            }
            @Override
            public void run() {
                met.read(Thread.currentThread());
            }
        }
    
        public class WriteRunnable implements Runnable {
            Met met;
            WriteRunnable(Met met){
                this.met = met;
            }
            @Override
            public void run() {
                met.write(Thread.currentThread());
            }
        }
    
        class TestReadWrite {
            public static void main(String[] args) {
                Met met = new Met();
                ReadRunnable read = new ReadRunnable(met);
                WriteRunnable write = new WriteRunnable(met);
                Thread threadread0 = new Thread(read); Thread threadread1 = new Thread(read); Thread threadwrite0 = new Thread(write); threadread0.start(); threadread1.start(); threadwrite0.start(); } }

三、ReentrantReadWriteLock使用

官方demo：更新缓存后的锁降级，保持读锁。

public class CachedData {
        ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
        String data = "未更新"; //缓存数据
        volatile boolean isValid; //缓存数据是否有效
        void processCachedData(){
            readWriteLock.readLock().lock();
            System.out.println(Thread.currentThread().getName()+"readLock.lock");
            //校验缓存数据是否有效,若无效跟新data，有效直接使用
            if(!isValid){
                System.out.println(Thread.currentThread().getName()+"readLock.unlock");
                readWriteLock.readLock().unlock();
                readWriteLock.writeLock().lock();
                System.out.println(Thread.currentThread().getName()+"writeLock.lock");
                //再次校验
                if(!isValid){
                    data = "已更新";
                    isValid = true;
                    System.out.println(Thread.currentThread().getName()+">>>refresh data");
                }
                //写锁降级为读锁：释放写锁前获取读锁
                readWriteLock.readLock().lock();
                System.out.println(Thread.currentThread().getName()+"readLock.lock");
                System.out.println(Thread.currentThread().getName()+"writeLock.unlock");
                //写锁被释放，读锁依旧
                readWriteLock.writeLock().unlock();
            }
            System.out.println(Thread.currentThread().getName()+">>>use data "+data);
            System.out.println(Thread.currentThread().getName()+"readLock.unlock");
            readWriteLock.readLock().unlock();
        }
    }

public class ReadWriteRunnable implements Runnable {
        CachedData task;
        public ReadWriteRunnable(CachedData task){
            this.task = task;
        }
        @Override
        public void run() {
            task.processCachedData();
        }
    }

class TestReadWrite {
        public static void main(String[] args) {
            CachedData task = new CachedData();
            ReadWriteRunnable readWriteRunnable = new ReadWriteRunnable(task);
            new Thread(readWriteRunnable).start();
            new Thread(readWriteRunnable).start();
            new Thread(readWriteRunnable).start();
            new Thread(readWriteRunnable).start();
            new Thread(readWriteRunnable).start();
            new Thread(readWriteRunnable).start();
        }
    }

转载于:https://www.cnblogs.com/liuboyuan/p/10316411.html