懂java中lock锁的请进

Question

 package thread;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class Foo {

    private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

    private final Lock r = rwl.readLock();

    private final Lock w = rwl.writeLock();

    public void read() {
        try {
            r.lock();
            Thread.sleep(1000);
            System.out.println("read...");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            r.unlock();
        }

    }

    public void wirte() {

        try {
            w.lock();
            Thread.sleep(1000);
            System.out.println("writing...");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            w.lock();
        }
    }
}

上面是我在书上看到的代码

书上说让我写一个实现runnable接口的类
然后测试读写锁

我没明白,谁能解释下

Answer 1

首先，你这个Foo对象必须是被多个线程共享的，然后定义两个任务，一个执行读操作，另一个任务执行写操作，然后定义2个写线程，5个读线程，测试这些线程分别执行读、写操作时不同锁的特点。
其次，你的这个Foo类定义的write的finally分支错误了，应该是w.unlock()才对，稍微修正下你的Foo代码，输出当前线程的名称：

 import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class Foo {

    private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

    private final Lock r = rwl.readLock();

    private final Lock w = rwl.writeLock();

    // 读锁，允许同时N个线程进行读操作，不存在竞争
    public void read() {
        try {
            r.lock();
            Thread.sleep(10000);
            System.out.println(Thread.currentThread().getName() + " read...");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            r.unlock();
        }

    }

    //写锁，同时允许一个线程写，明显能看到互斥等待
    public void wirte() {

        try {
            w.lock();
            Thread.sleep(3000);
            System.out.println(Thread.currentThread().getName() + " writing...");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            w.unlock();
        }
    }
}

定义读任务：

 public class LockReadTask implements Runnable{

    private Foo source;

    public LockReadTask(Foo source){
        this.source = source;
    }

    @Override
    public void run() {
        source.read();
    }

}

定义写任务：

 public class LockWriteTask implements Runnable{
    private Foo source;

    public LockWriteTask(Foo source){
        this.source = source;
    }

    @Override
    public void run() {
        source.wirte();
    }
}

测试代码，定义2个写线程，看写锁的竞争状态；4个读线程，读锁步存在竞争。

 public class Main {
    public static void main(String[] args) {
        //定义共享数据源
        Foo source = new Foo();

        //开启2个写线程:能明显看到t1,t2写线程之间的互斥等待
        Thread t1 = new Thread(new LockWriteTask(source));
        t1.setName("write-Thread-1");
        Thread t2 = new Thread(new LockWriteTask(source));
        t2.setName("write-Thread-2");
        t1.start();
        t2.start();

        //开启5个读线程：读锁，允许同时N个线程进行操作，可以看到读打印操作同时秒出
        Thread rt1 = new Thread(new LockReadTask(source));
        rt1.setName("read-Thread-1");
        Thread rt2 = new Thread(new LockReadTask(source));
        rt2.setName("read-Thread-2");
        Thread rt3 = new Thread(new LockReadTask(source));
        rt3.setName("read-Thread-3");
        Thread rt4 = new Thread(new LockReadTask(source));
        rt4.setName("read-Thread-4");

        rt1.start();
        rt2.start();
        rt3.start();
        rt4.start();
    }
}   

可以看到测试结果：写线程之间有竞争，输出信息由先后；而都线程之间的读锁匙共享的，没有竞争，所以输出是同时秒出的。