java 读写锁

Lock类除了提供了ReentrantLock的锁以外，还提供了ReentrantReadWriteLock的锁。读写锁分成两个锁，一个锁是读锁，一个锁是写锁。读锁与读锁之间是共享的，读锁与写锁之间是互斥的，写锁与写锁之间也是互斥的。
看下面的读读共享的例子：

public class ReadReadService {
    private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    public void read(){
        try{
            try{
                lock.readLock().lock();
                System.out.println("获得读锁" + Thread.currentThread().getName() +
                " " + System.currentTimeMillis());
                Thread.sleep(1000 * 10);
            }finally {
                lock.readLock().unlock();
            }
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }
}

测试的代码和结果如下：

        ReadReadService service = new ReadReadService();
        Thread a = new Thread(service::read);
        a.setName("A");

        Thread b = new Thread(service::read);
        b.setName("B");

        a.start();
        b.start();

结果如下：

获得读锁A 1485614976979
获得读锁B 1485614976981

两个线程几乎同时执行同步代码。
下面的例子是写写互斥的例子

public class WriteWriteService {
    private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    public void write(){
        try{
            try{
                lock.writeLock().lock();
                System.out.println("获得写锁" + Thread.currentThread().getName() +
                        " " +System.currentTimeMillis());
                Thread.sleep(1000 * 10);
            }finally {
                lock.writeLock().unlock();
            }
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }
}

测试代码和结果如下：

        WriteWriteService service = new WriteWriteService();
        Thread a = new Thread(service::write);
        a.setName("A");
        Thread b = new Thread(service::write);
        b.setName("B");

        a.start();
        b.start();
        Thread.sleep(1000 * 30);

结果如下：

获得写锁A 1485615316519
获得写锁B 1485615326524

两个线程同步执行代码

读写互斥的例子：

public class WriteReadService {

    private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    public void read(){
        try{
            try{
                lock.readLock().lock();
                System.out.println("获得读锁" + Thread.currentThread().getName()
                        + " " + System.currentTimeMillis());
                Thread.sleep(1000 * 10);
            }finally {
                lock.readLock().unlock();
            }
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }

    public void write(){
        try{
            try{
                lock.writeLock().lock();
                System.out.println("获得写锁" + Thread.currentThread().getName()
                        + " " + System.currentTimeMillis());
                Thread.sleep(1000 * 10);
            }finally {
                lock.writeLock().unlock();
            }
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }

}

测试的代码如下：

        WriteReadService service = new WriteReadService();
        Thread a = new Thread(service::write);
        a.setName("A");
        a.start();
        Thread.sleep(1000);

        Thread b = new Thread(service::read);
        b.setName("B");
        b.start();

        Thread.sleep(1000 * 30);

结果如下：

获得写锁A 1485615633790
获得读锁B 1485615643792

两个线程读写之间也是同步执行代码。