演示 ReadWriteLock 用法的示例

Question

我试图理解 ReadWriteLock 的概念并实现了下面的简单示例。据我了解，只要一个线程有 readlock，另一个线程就可以获取 readlock。但是对于具有 writelock 的线程，没有其他线程可以获取 write/read lock，直到另一个线程释放锁。然而，从下面程序的输出中，我注意到在 WriterOdd 释放锁之前（打印释放锁的输出），Reader 线程进入并读取 number 字符串.是不是释放writelock后reader线程进来，然后打印释放writelock的打印语句？

public static void main(String[] args) {

    new Thread(new Reader(), "Reader 1").start();
    new Thread(new Reader(), "Reader 2").start();
    new Thread(new WriterEven(), "Writer Even").start();
    new Thread(new WriterOdd(), "Writer Odd").start();

}

public static class Reader implements Runnable {

    public void run() {

        for (int i = 1; i <= 6; i++) {
            rwl.readLock().lock();
            // rl.lock();
            System.out.println(Thread.currentThread().getName() + "  is reading" + number);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } finally {
                rwl.readLock().unlock();
            }
        }

    }

}

public static class WriterEven implements Runnable {

    public void run() {
        System.out.println(Thread.currentThread().getName() + " is trying  writing");
        rwl.writeLock().lock();
        System.out.println(Thread.currentThread().getName() + " got the lock");
        for (int i = 2; i <= 6; i += 2) {

            System.out.println(Thread.currentThread().getName() + " is writing");
            number = number.concat(" " + i);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } finally {

            }

        }
        rwl.writeLock().unlock();

        System.out.println(Thread.currentThread().getName() + " left the lock");

    }

}

public static class WriterOdd implements Runnable {

    public void run() {
        System.out.println(Thread.currentThread().getName() + " is trying  writing");
        rwl.writeLock().lock();
        System.out.println(Thread.currentThread().getName() + " got the lock");
        for (int i = 1; i <= 5; i += 2) {

            System.out.println(Thread.currentThread().getName() + " is writing");
            number = number.concat(" " + i);
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } finally {

            }

        }
        rwl.writeLock().unlock();
        System.out.println(Thread.currentThread().getName() + " left the lock");

    }

}

输出：

Reader 2  is reading0
Reader 1  is reading0
Writer Even is trying  writing
Writer Odd is trying  writing
Writer Even got the lock
Writer Even is writing
Writer Even is writing
Writer Even is writing
Writer Odd got the lock
Writer Odd is writing
Writer Even left the lock
Writer Odd is writing
Writer Odd is writing
Reader 1  is reading0 2 4 6 1 3 5
Reader 2  is reading0 2 4 6 1 3 5
Writer Odd left the lock

Answer 1

写入有关释放写锁的控制台消息的代码行在释放锁之后。

rwl.writeLock().unlock();
// here readers can read already
// because they are free to aquire lock
System.out.println(Thread.currentThread().getName() + " left the lock");

演示 ReadWriteLock 用法的示例

Example demonstrating the use of ReadWriteLock

java

multithreading

locking

reentrantlock