Java多个线程顺序打印数字

要求

启动N个线程, 这N个线程要不间断按顺序打印数字1-N. 将问题简化为3个线程无限循环打印1到3

方法一: 使用synchronized

三个线程无序竞争同步锁, 如果遇上的是自己的数字, 就打印. 这种方式会浪费大量的循环

public class TestSequential1 {
    private volatile int pos = 1;
    private volatile int count = 0;
 
    public void one(int i) {
        synchronized (this) {
            if (pos == i) {
                System.out.println("T-" + i + " " + count);
                pos = i % 3 + 1;
                count = 0;
            } else {
                count++;
            }
        }
    }
 
    public static void main(String[] args) {
        TestSequential1 demo = new TestSequential1();
        for (int i = 1; i <=3; i++) {
            int j = i;
            new Thread(()->{
                while(true) {
                    demo.one(j);
                }
            }).start();
        }
    }
}

输出

T-1 0
T-2 5793
T-3 5285
T-1 2616
T-2 33
T-3 28
T-1 22
T-2 44
T-3 6
T-1 881
T-2 118358
T-3 247380
T-1 30803
T-2 29627
T-3 52044
...

方法二: 使用synchronized配合wait()和notifyAll()

竞争同步锁时使用wait()和notifyAll(), 可以避免浪费循环

public class TestSequential4 {
    private volatile int pos = 1;
    private volatile int count = 0;
    private final Object obj = new Object();
 
    public void one(int i) {
        System.out.println(i + " try");
        synchronized (obj) {
            System.out.println(i + " in");
            try {
                while (pos != i) {
                    count++;
                    System.out.println(i + " wait");
                    obj.wait();
                }
                System.out.println("T-" + i + " " + count);
                pos = i % 3 + 1;
                count = 0;
                obj.notifyAll();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
 
    public static void main(String[] args) {
        TestSequential4 demo = new TestSequential4();
        for (int i = 3; i >=1; i--) {
            int j = i;
            new Thread(()->{
                while(true) {
                    demo.one(j);
                }
            }).start();
        }
    }
}

输出

3 try
3 in
3 wait
2 try
2 in
2 wait
1 try
1 in
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
...

.

方法三: 使用可重入锁

用Lock做, 非公平锁, 三个线程竞争, 如果遇上的是自己的数字, 就打印. 这种方式也会浪费大量的循环

public class TestSequential2 {
    private final Lock lock = new ReentrantLock();
    private volatile int pos = 1;
    private volatile int count = 0;
 
    public void one(int i) {
        lock.lock();
        if (pos == i) {
            System.out.println("T-" + i + " " + count);
            pos = i % 3 + 1;
            count = 0;
        } else {
            count++;
        }
        lock.unlock();
    }
 
    public static void main(String[] args) {
        TestSequential2 demo = new TestSequential2();
        for (int i = 1; i <=3; i++) {
            int j = i;
            new Thread(()->{
                while(true) {
                    demo.one(j);
                }
            }).start();
        }
    }
}

输出

T-1 0
T-2 0
T-3 323
T-1 54
T-2 68964
T-3 97642
T-1 6504
T-2 100603
T-3 6989
T-1 1313
T-2 0
T-3 183741
T-1 233
T-2 5081
T-3 164367
..

.

方法四: 使用可重入锁, 启用公平锁

和3一样, 但是使用公平锁, 这种情况下基本上可以做到顺序执行, 偶尔会产生多一次循环

private final Lock lock = new ReentrantLock(true);

输出

T-1 0
T-2 0
T-3 0
T-1 0
T-2 0
T-3 0
T-1 0
T-2 0
T-3 0
T-1 0
T-2 0
T-3 1
T-1 1
T-2 1
T-3 1
...

.

方法五: 使用Condition

每个线程如果看到不是自己的计数, 就await(), 如果是自己的计数, 就完成打印动作, 再signalAll()所有其他线程去继续运行, 自己在下一个循环后, 即使又继续执行, 也会因为计数已经变了而await.

如果ReentrantLock构造参数使用true, 可以基本消除 ~await 这一步的输出.

public class ReentrantLockCondition2 {
    private static Lock lock = new ReentrantLock();
    private static Condition condition = lock.newCondition();
    private volatile int state = 1;
 
    private void handle(int state) {
        lock.lock();
        try {
            while(true) {
                while(this.state != state) {
                    System.out.println(state + " ~await");
                    condition.await();
                }
                System.out.println(state);
                this.state = state % 3 + 1;
                condition.signalAll();
                System.out.println(state + " await");
                condition.await();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
 
    public static void main(String[] args) {
        ReentrantLockCondition2 rlc = new ReentrantLockCondition2();
        new Thread(()->rlc.handle(1)).start();
        new Thread(()->rlc.handle(2)).start();
        new Thread(()->rlc.handle(3)).start();
    }
}

.

方法六: 使用多个Condition

给每个线程不同的condition. 这个和4的区别是, 可以用condition.signal()精确地通知对应的线程继续执行(在对应的condition上await的线程, 可能是多个). 这种情况下是可以多个线程都不unlock锁的情况下进行协作的. 注意下面的while(true)循环是在lock.lock()方法内部的.

public class ReentrantLockCondition {
    private static Lock lock = new ReentrantLock();
    private static Condition[] conditions = {lock.newCondition(), lock.newCondition(), lock.newCondition()};
    private volatile int state = 1;
 
    private void handle(int state) {
        lock.lock();
        try {
            while(true) {
                while(this.state != state) {
                    conditions[state - 1].await();
                }
                System.out.println(state);
                this.state = state % 3 + 1;
                conditions[this.state - 1].signal();
                conditions[state - 1].await();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
 
    public static void main(String[] args) {
        ReentrantLockCondition rlc = new ReentrantLockCondition();
        new Thread(()->rlc.handle(1)).start();
        new Thread(()->rlc.handle(2)).start();
        new Thread(()->rlc.handle(3)).start();
    }
}

.