java多线程-ReadWriteLock

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();
    }
}