Java线程同步的Monitor机制(Lock配合Condition)

Monitor模式是一种常见的并行开发机制, 一个Monitor实例可以被多个线程安全使用, 所有的monitor下面的方法在运行时是互斥的, 这种互斥机制机制可以用于一些特性, 例如让线程等待某种条件, 在等待时线程会将CPU时间交出去, 但是在条件满足时确保重新获得CPU时间. 在条件达成时, 你可以同时通知一个或多个线程. 这样做有以下的优点:

1. 所有的同步代码都集中在一起, 用户不需要知道这是如何实现的
2. 代码不依赖于线程数量, 线程数量只取决于业务需要
3. 不需要对某个互斥对象做释放, 不存在忘记的风险

一个Monitor的结构是这样的

public class SimpleMonitor {
    public method void testA(){
        //Some code
    }
 
    public method int testB(){
        return 1;
    }
}

使用Java代码不能直接创建一个Monitor, 要实现Monitor, 需要使用Lock和Condition类. 一般使用的Lock是ReentrantLock, 例如

public class SimpleMonitor {
    private final Lock lock = new ReentrantLock();
 
    public void testA() {
        lock.lock();
 
        try {
            //Some code
        } finally {
            lock.unlock();
        }
    }
 
    public int testB() {
        lock.lock();
 
        try {
            return 1;
        } finally {
            lock.unlock();
        }
    }
}

如果不需要判断条件, 那么用synchronized就可以了. 在需要判断条件的情况下, 使用Lock的newCondition()方法创建Condition, 可以通过Condition的await方法, 让当前线程wait, 放弃cpu时间. 然后用signal或者signalAll方法让线程重新获得CPU时间. signalAll方法会唤起所有wait在当前condition的线程. 下面是一个例子, 一个需要被多个线程使用的容量固定的buffer.

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
 
public class BoundedBuffer {
    private final String[] buffer;
    private final int capacity;
 
    private int front;
    private int rear;
    private int count;
 
    private final Lock lock = new ReentrantLock();
 
    private final Condition notFull = lock.newCondition();
    private final Condition notEmpty = lock.newCondition();
 
    public BoundedBuffer(int capacity) {
        super();
 
        this.capacity = capacity;
 
        buffer = new String[capacity];
    }
 
    public void deposit(String data) throws InterruptedException {
        lock.lock();
 
        try {
            while (count == capacity) {
                notFull.await();
            }
 
            buffer[rear] = data;
            rear = (rear + 1) % capacity;
            count++;
 
            notEmpty.signal();
        } finally {
            lock.unlock();
        }
    }
 
    public String fetch() throws InterruptedException {
        lock.lock();
 
        try {
            while (count == 0) {
                notEmpty.await();
            }
 
            String result = buffer[front];
            front = (front + 1) % capacity;
            count--;
 
            notFull.signal();
 
            return result;
        } finally {
            lock.unlock();
        }
    }
}

代码说明

1. 这两个方法通过lock互斥
2. 然后通过两个condition变量, 一个用于在buffer非空时等待, 一个用于buffer未满时等待
3. 上面使用while循环将await包围, 这是为了防止在使用Signal&Condition时产生signal stealers问题.
4. 以上方法可以安全地在多个线程中被调用

还有一个例子, 用于协调多个线程按固定顺序进行输出

public class TestSequentialThreads {
    private final Lock lock = new ReentrantLock();
    private final Condition[] conditions = {lock.newCondition(), lock.newCondition(), lock.newCondition()};
    private int count = 0;
 
    public void action(int i) {
        while (true) {
            print(i + " wait lock");
            lock.lock();
            print(i + " has lock");
            try {
                while (count != i) {
                    print(i + " await");
                    conditions[i].await();
                }
                print("===== " + i + " =====");
                Thread.sleep(500);
                count = (count + 1) % 3;
                int j = (i + 1) % 3;
                print(i + " signal " + j);
                conditions[j].signal();
            } catch (InterruptedException e) {
                print(i + " InterruptedException");
            } finally {
                print(i + " unlock");
                lock.unlock();
            }
        }
    }
 
    public static void main(String[] args) {
        TestSequentialThreads ts = new TestSequentialThreads();
        new Thread(()->ts.action(0)).start();
        new Thread(()->ts.action(2)).start();
        new Thread(()->ts.action(1)).start();
        new Thread(()->ts.action(1)).start();
        new Thread(()->ts.action(0)).start();
        new Thread(()->ts.action(2)).start();
    }
 
    public static void print(String str) {
        System.out.println(str);
    }
}

　　

如果是使用wait()和notify()的话, 就要写成这样, 这种情况下, 运行时notify()随机通知的线程, 是有可能不满足而跳过的.

public class DemoThreadWait2 {
    private Object obj = 0;
    private int pos = 1;
 
    public void one(int i) {
        synchronized (obj) {
            if (pos == i) {
                System.out.println("T-" + i);
                pos = i % 3 + 1;
            } else {
                // System.out.println(".");
            }
            obj.notify();
            try {
                obj.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
 
    public static void main(String[] args) {
        DemoThreadWait2 demo = new DemoThreadWait2();
        new Thread(()->{
            while(true) {
                demo.one(1);
            }
        }).start();
 
        new Thread(()->{
            while(true) {
                demo.one(2);
            }
        }).start();
 
        new Thread(()->{
            while(true) {
                demo.one(3);
            }
        }).start();
    }
}