Java的锁—读写锁(WriteReadLock)

对某一数据进行线程安全的读写操作，那么就要利用锁来进行线程同步，习惯的做法是，读的时候加锁，禁止其他的读写操作，写的时候，更是要对其他读写操作禁止。那么读写锁就是要更细粒度的进行锁的操作。读的时候，允许其他的线程的读操作，禁止其他线程的写操作等。那么具体的总结如下表：

比较极端的例子，如果系统中写的次数远远小于读的次数，那么这更能突出读写锁的优势：

public class WriteReadLockDemo {

    private static Lock lock = new ReentrantLock();
    private static ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
    private static Lock readLock = readWriteLock.readLock();
    private static Lock writeLock = readWriteLock.writeLock();

    private int value;

    public Object handleRead(Lock lock) throws InterruptedException {
        try {
            lock.lock();
            Thread.sleep(1000);
            return value;
        } finally {
            lock.unlock();
        }
    }

    public void handdleWrite(Lock lock, int index) throws InterruptedException {
        try {
            lock.lock();
            Thread.sleep(1000);
            value = index;
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) {
        WriteReadLockDemo writeReadLockDemo = new WriteReadLockDemo();
        Runnable readRunnable = () -> {
            try {
                // 下面两行是普通锁和读写锁
// writeReadLockDemo.handleRead(readLock);
                    writeReadLockDemo.handleRead(lock);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        };

        Runnable writeRunnable = () -> {
            try {
                // 下面两行是普通锁和读写锁
// writeReadLockDemo.handdleWrite(writeLock, new Random().nextInt());
                    writeReadLockDemo.handdleWrite(lock, new Random().nextInt());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        };

        for (int i = 0; i < 18; i++) {
            new Thread(readRunnable).start();
        }

        for (int i = 18; i < 20; i++) {
            new Thread(writeRunnable).start();
        }

    }

}

假设读写操作都很耗时，每个读或写操作都需要1秒。

main方法里面，两个for循环，分别启动18个读线程，2个写线程。程序的读方法，写方法分别有一个注释，是表示用普通锁和读写锁，用读写锁2秒执行完成，而利用普通锁则需要20秒。