带有多线程Java的Hello World

Question

I'm tying to understand how to use the keywords: wait, notify/All, synchronized, so I decided to try with a simple example. Basically what I'm trying to do is to create two threads that are going to print a string. The first thread has the string "Hello", while the second thread has the String "World".

The output I'd like to reach is the following: Hello World Hello World Hello World ...

This is the code I wrote so far, but the output right now is: Hello Hello Hello ... World World World ...

Where is/are the mistake/s? Thank you. :)

Here's the code:

class MyThread implements Runnable {
    private SimpleSyncThread sync;
    private String s;

    public MyThread(SimpleSyncThread sync, String s) {
        this.sync = sync;
        this.s = s;
    }

    public static void pause(long time) {
        try {Thread.sleep(time); }
        catch (InterruptedException e) {Thread.currentThread().interrupt();}
    }

    @Override
    public void run() {
        synchronized (sync) {
            for (int i = 0; i < 10; i++) {
                sync.print(s);
            }
        }
    }
}

public class SimpleSyncThread {

    public void print(String s) {
        System.out.println(s);
        MyThread.pause(200);
    }

    public static void main(String[] args) {
        SimpleSyncThread sync = new SimpleSyncThread();
        MyThread st1 = new MyThread(sync, "Hello");
        MyThread st2 = new MyThread(sync, "World");

        Thread t1 = new Thread(st1);
        Thread t2 = new Thread(st2);

        t1.start();
        t2.start();
    }
}

Answer 1

You are holding the lock here so only one process can print at a time

   synchronized (sync) {
        for (int i = 0; i < 10; i++) {
            sync.print(s);
        }
    }

Instead of doing this you can release the lock temporarily with

   synchronized (sync) {
        for (int i = 0; i < 10; i++) {
            sync.print(s);
            // I have done my bit, wake other threads.
            sync.notifyAll();
            try {
                // give up the lock and let another thread run.
                sync.wait(10);
            } catch(InterruptedException ie) {
                throw new AssertionError(ie);
            }
        }
    }

---

What you might have had in mind is what I call a Ping Pong test. You wouldn't do this in a real program but this pattern makes a useful micro-benchmark.

public class PingPongMain {
    public static void main(String[] args) throws InterruptedException {
        boolean[] next = {false};
        AtomicInteger count = new AtomicInteger();
        Thread t1 = new Thread(() -> {
            try {
                synchronized (next) {
                    for(;;) {
                        // handle spurious wake ups.
                        while (next[0])
                            next.wait();

                        System.out.println("ping");

                        // state change before notify
                        next[0] = true;
                        next.notifyAll();
                    }
                }
            } catch (InterruptedException e) {
                // expected
            }
        });

        Thread t2 = new Thread(() -> {
            try {
                synchronized (next) {
                    for(;;) {
                        // handle spurious wake ups.
                        while (!next[0])
                            next.wait();

                        System.out.println("pong");

                        // state change before notify
                        next[0] = false;
                        next.notifyAll();

                        count.incrementAndGet();
                    }
                }
            } catch (InterruptedException e) {
                // expected
            }
        });

        t1.start();
        t2.start();

        Thread.sleep(5000);
        t1.interrupt();
        t2.interrupt();
        System.out.println("Ping ponged " + count + " times in 5 seconds");

    }
}

prints

ping
pong
ping
pong
.. deleted ...
Ping ponged 323596 times in 5 seconds

Answer 2

I guess you want to simulate a job with two independent thread accessing the same resource: eg the System.out, but the sleep parts can probably run concurrently.

In your simulation, you shouldn't put the pause inside the synchronized block:

public class SimpleSyncThread {

    public void print(String s) {
        synchronized(this){
           System.out.println(s);
        }
        MyThread.pause(200);
    }

In the run function, you don't need the synchronized anymore:

public void run() {
        for (int i = 0; i < 10; i++) {
            sync.print(s);
        }
}

Now, you'll get 'Hello World Hello World', or maybe 'Hello World World Hello'.

Answer 3

You have gone on an example with a non small amount of difficulty: The basic synchronization in java through wait and notify is aimed to synchronize a consumer-producer paradigm: There are some producer threads and some consumer threads. Each producer does its job without waiting, and then wakes up (notify) a consumer thread, which is waiting for a notification. This scheme would serve even if the runnable class is simultaneosly consumer and producer. But the communication is always unidirectional :

   producer -> consumer

Instead, what you try to do is bidirectional alternative thread communication:

   producer -> consumer -> producer -> consumer ...

I think you need a more complicated way to communicate your threads: A token manager , a class that contains an integer token from 0 to N, and rotates it:

public class TokenManager
{
    private final int maxTokens;

    private int token;

    public TokenManager(int maxTokens, int initialToken)
    {
        super();
        this.maxTokens=maxTokens;
        this.token=initialToken % this.maxTokens;
    }

    public int getToken()
    {
        return this.token;
    }

    public void nextToken()
    {
        this.token=++this.token % this.maxTokens;
    }
}

And then a runnable class that receives a TokenManager and uses it for synchronization:

public class MyRunnable implements Runnable
{
    private final String text;

    private final TokenManager token;

    // Identifier token value for this object.
    private final int id;

    public MyRunnable(int id, String text, TokenManager token)
    {
        super();
        this.id=id;
        this.text=text;
        this.token=token;
    }

    @Override
    public void run()
    {
        try
        {
            for (int i=0; i < 10; i++)
            {
                synchronized (this.token)
                {
                    // Wait until the TokenManager token is equal to the id of this object:
                    while (this.token.getToken() != this.id)
                    {
                        this.token.wait();
                    }

                    // Now it's our turn to print:
                    System.out.printf("%d: %s\n", i, this.text);
                    this.token.nextToken();

                    // Ask the TokenManager to progress to the next token:
                    this.token.notifyAll();
                }
            }
        }
        catch (InterruptedException e)
        {
            throw new Error(e);
        }
    }
}

public static void main(String[] args)
{
    // Instantiate the TokenManager for a specified number of threads:
    TokenManager token=new TokenManager(2, 0);

    // Instantiate and start the thread with id=0:
    new Thread(new MyRunnable(0, "Hello", token)).start();
    // Instantiate and start the thread with id=1:
    new Thread(new MyRunnable(1, "World", token)).start();
}

In this way, it is the main method who decides what will be the sequence of activation, by assigning IDs to the instantiated threads (in ascending order). And, if you want a new type of thread, you'll have just to pass 3 to the TokenManager (instead of 2 ) and start a new Thread with a proper ID:

    new Thread(new MyRunnable(2, "I'm here", token)).start();

Notes (Thanks to Andy Brown ):

• The token IDs must be given sequentially with no gaps.
• There could be more than one Thread with the same Token ID. In this case, they would be executed randomly within their turn.