4.2 ReentrantReadWriteLock读写锁的理解和简单示例

ReentrantReadWriteLock读写锁的特点：

1、允许多个读线程同时访问，读访问时，写锁不能访问

2、当有写线程访问时，其他的读和写线程都不能够访问

3、编程方式相对于使用等待通知机制：写的时候，其他的读写都阻塞，保证每次读取到的都是最新的数据

4、在读多于写的情况下，读写锁能够提供比排它锁更好的并发性和吞吐量

ReentrantReadWriteLock其实实现的是ReadWriteLock接口

使用的标准方式

readLock.lock();
try {
	//DoSomeThing
}finally {
	readLock.unlock();
}

示例一：一个线程读的时候，其他的读线程也能读

public class MyReentrantWriteReadLock {
    static ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    static ReentrantReadWriteLock.ReadLock readLock = lock.readLock();
    static ReentrantReadWriteLock.WriteLock writeLock = lock.writeLock();

    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(4);

        new Thread(()->{
            readLock.lock();
            try {
                for (Integer item : list) {
                    Thread.sleep(200);
                    System.out.println(Thread.currentThread().getName()+":"+item);
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                readLock.unlock();
            }
        }).start();

        new Thread(()->{
            readLock.lock();
            try {
                for (Integer item : list) {
                    Thread.sleep(200);
                    System.out.println(Thread.currentThread().getName()+":"+item);
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                readLock.unlock();
            }
        }).start();
    }
}

执行结果：两个线程交替执行

Thread-0:1
Thread-1:1
Thread-0:2
Thread-1:2
Thread-0:3
Thread-1:3
Thread-0:4
Thread-1:4

示例二：一个线程读，其他的写线程阻塞，一个线程写，其他读写都阻塞

public class MyReentrantWriteReadLock2 {
    static ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    static ReentrantReadWriteLock.ReadLock readLock = lock.readLock();
    static ReentrantReadWriteLock.WriteLock writeLock = lock.writeLock();

    public static void main(String[] args) throws InterruptedException {
        List list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(4);

        //先获取读锁，读取完毕之后才能写
        new Thread(()->{
            readLock.lock();
            try {
                Thread.sleep(1000);
                for (Integer item : list) {
                    System.out.println(Thread.currentThread().getName()+":"+item);
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                readLock.unlock();
            }
        }).start();

        Thread.sleep(10);
        //写锁
        new Thread(()->{
            writeLock.lock();
            try {
                Thread.sleep(1000);
                System.out.println("先获取读锁，读锁执行完之后，写锁才开始");
                list.add(5);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                writeLock.unlock();
            }
        }).start();
        Thread.sleep(10);
        new Thread(()->{
            readLock.lock();
            try {
                System.out.println("写锁执行完之后，读锁才开始");
                for (Integer item : list) {
                    System.out.println(Thread.currentThread().getName()+":"+item);
                }
            //} catch (InterruptedException e) {
            //    e.printStackTrace();
            }finally {
                readLock.unlock();
            }
        }).start();
    }
}

执行结果：

Thread-0:1
Thread-0:2
Thread-0:3
Thread-0:4
现获取读锁，读锁执行完之后，写锁才开始
写锁执行完之后，读锁才开始
Thread-2:1
Thread-2:2
Thread-2:3
Thread-2:4
Thread-2:5