在 2 个线程的帮助下打印自然序列（1 个打印偶数，2'nd 打印奇数）

Question

I have tired this question, and i ended up with some doubts. Please help me out

Doubt : If any thread is in wait state , and no other thread is notifying that one , so will it never come to and end ? Even after using wait(long milliseconds).

For Code : What my requirement is from the code(Please Refer My Code) :

a : Should print "Even Thread Finish " and "Odd Thread Finish" (Order is not imp , but must print both)

b: Also in main function should print " Exit Main Thread"

What is actually happening : After lot of runs , in some cases , it prints "Even Thread Finish" then hangs here or vice-versa. In some cases it prints both.

Also it never prints "Exit Main Thread".

So How to modify code , so it must print all 3 statement .(Of Course "Exit Main.. " in last , as i am using join for main.)

In brief : Main start-> t1 start -> t2 start ,, then i need t2/t1 finish -> main finish.

Please help me out for this problem

Here is my code :

import javax.sql.CommonDataSource;

public class ThreadTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Share commonObj = new Share();

        Thread even = new Thread(new EvenThread(commonObj));

        Thread odd = new Thread(new OddThread(commonObj));

        even.start();

        odd.start();

        try {
            Thread.currentThread().join();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("Exit Main Thread");

    }

}

class EvenThread implements Runnable {

    private Share commShare;
    public EvenThread(Share obj) {
        // TODO Auto-generated constructor stub
        this.commShare = obj;
    }

    private int number = 2;

    public void run() {
        System.out.println("Even Thread start");
        while (number <= 50) {
            if (commShare.flag == true) {
                System.out.println("Even Thread" + number);
                number += 2;
                commShare.flag = false;
                synchronized(commShare) {
                    try {
                        commShare.notify();
                        commShare.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    commShare.notify();
                }

            } else {
                synchronized(commShare) {
                    try {
                        commShare.notify();
                        commShare.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    commShare.notify();
                }
            }

        }

        System.out.println("Even Thread Finish");
    }
}


class OddThread implements Runnable {

    private int number = 1;
    private Share commShare;


    public OddThread(Share obj) {
        // TODO Auto-generated constructor stub
        this.commShare = obj;
    }



    public void run() {
        System.out.println("Odd Thread start");
        while (number <= 50) {
            if (commShare.flag == false) {
                System.out.println("Odd Thread :" + number);
                number += 2;
                commShare.flag = true;
                synchronized(commShare) {
                    try {
                        commShare.notify();
                        commShare.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    commShare.notify();
                }
            }
        }
        System.out.println("Odd Thread Finish");
    }
}

class Share {

    Share sharedObj;
    public boolean flag = false;
}

Answer 1

Although this is not the exact answer of your question, but this implementation is an alternative of your problem .

public class EvenOddThreads {
    public static void main(String[] args) {
        Thread odd = new Thread(new OddThread(), "oddThread");

        Thread even = new Thread(new EvenThread(), "Even Thread");

        odd.start();
        even.start();
        try {
            odd.join();
            even.join();
            System.out.println("Main thread exited");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}
class OddThread implements Runnable{
    public void run() {
        synchronized (CommonUtil.mLock) {
            System.out.println(Thread.currentThread().getName()+"---> job starting");
            int i = 1;
            while(i<50){
                System.out.print(i + "\t");
                i = i + 2;
                CommonUtil.mLock.notify();
                try {
                    CommonUtil.mLock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("OddThread---> job completed");
            CommonUtil.mLock.notify();

        }
    }
}

class EvenThread implements Runnable{
    @Override
    public void run() {
        synchronized (CommonUtil.mLock) {
            System.out.println(Thread.currentThread().getName()+"---> job started");
            int i =2;
            while(i<50){
                System.out.print(i + "\t");
                i = i+2;
                CommonUtil.mLock.notify();
                try {
                    CommonUtil.mLock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("EvenThread---> job completed");
            CommonUtil.mLock.notify();
        }
    }
}

class CommonUtil{
    static final Object mLock= new Object();
}

Output:

oddThread---> job starting
1   Even Thread---> job started
2   3   4   5   6   7   8   9   10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  EvenThread---> job completed
OddThread---> job completed
Main thread exited

Answer 2

Well, I have spent last three hours reading a Java sychronization tutorial (a very good one) followed by more info about wait, notify and notifyAll , and i ended up with program that uses N threads to count from A to B, set N to 2 and you have odd and even.

pastebin

Also, my program has no comments whatsoever, so make sure you read the tutorial(s) before you try understand this code.

Answer 3

This could be an exercise on threads and lock monitors, but there is nothing to do in parallel that give you advantages.

In your code when a thread 1 (OddThread or EvenThread) ends his work and prints out "Odd Thread Finish" (or "Even Thread Finish") the other thread 2 is waiting a notify() or a notifyAll() that never will happen because the first is over.

You have to change EvenThread and OddThread adding a synchronized block with a notify call on commShare just after the while cycle. I removed the second if-branch because in this way you don't continue to check the while condition but get a wait on commShare soon.

class EvenThread implements Runnable {
    private Share commShare;
    private int number = 2;

    public EvenThread(Share obj) {
        this.commShare = obj;
    }
    public void run() {
        System.out.println("Even Thread start");
        while (number <= 50) {
            synchronized (commShare) {
                if (commShare.flag) {
                    System.out.println("Even Thread:" + number);
                    number += 2;
                    commShare.flag = false;
                }
                commShare.notify();
                try {
                    commShare.wait();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
        synchronized (commShare) {
            commShare.notify();
            System.out.println("Even Thread Finish");
        }
    }
}

class OddThread implements Runnable {
    private int number = 1;
    private Share commShare;

    public OddThread(Share obj) {
        this.commShare = obj;
    }
    public void run() {
        System.out.println("Odd Thread start");
        while (number <= 50) {
            synchronized (commShare) {
                if (!commShare.flag) {
                    System.out.println("Odd Thread: " + number);
                    number += 2;
                    commShare.flag = true;
                }
                commShare.notify();
                try {
                    commShare.wait();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

            }
        }
        synchronized (commShare) {
            commShare.notify();
            System.out.println("Odd Thread Finish");
        }
    }

Finally, in the main you have to join for each thread you started. It's sure that Thread.currentThread() returns just one of yours threads? We have started two threads and those threads we should join.

try {
      even.join();
      odd.join();
} catch (InterruptedException e) {
         // TODO Auto-generated catch block
         e.printStackTrace();
}

Answer 4

Also it never prints "Exit Main Thread".

That is because maybe because your threads are waiting on the lock for someone to notify() but due to missed signal or no one signalling them, they never get out of waiting state. For that the best solution is to use:

public final void wait(long timeout)
                throws InterruptedException

Causes the current thread to wait until either another thread invokes the notify() method or the notifyAll() method for this object, or a specified amount of time has elapsed.

This overloaded method will wait for other thread to notify for specific amount of time and then return if timeout occurs. So in case of a missed signal the thread will still resume its work.

NOTE: After returning from wait state always check for PRE-CONDITION again, as it can be a Spurious Wakeup .

Here is my flavor of program that I coded some time back for the same.

import java.util.concurrent.atomic.AtomicInteger;


public class Main {

    private static int range = 10;
    private static volatile AtomicInteger present = new AtomicInteger(0);
    private static Object lock = new Object();

    public static void main(String[] args) {
        new Thread(new OddRunnable()).start();
        new Thread(new EvenRunnable()).start();
    }

    static class OddRunnable implements Runnable{

        @Override
        public void run() {
            while(present.get() <= range){
                if((present.get() % 2) != 0){
                    System.out.println(present.get());
                    present.incrementAndGet();
                    synchronized (lock) {
                        lock.notifyAll();
                    }
                }else{
                    synchronized (lock) {
                        try {
                            lock.wait(1000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                            break;
                        }
                    }
                }
            }
        }
    }

    static class EvenRunnable implements Runnable{

        @Override
        public void run() {
            while(present.get() <= range){
                if((present.get() % 2) == 0){
                    System.out.println(present.get());
                    present.incrementAndGet();
                    synchronized (lock) {
                        lock.notifyAll();
                    }
                }else{
                    synchronized (lock) {
                        try {
                            lock.wait(1000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                            break;
                        }
                    }
                }
            }
        }
    }
}

See the solution, I have kept a lock that works for notifying the chance of even or odd thread. If even thread finds that the present number is not even it waits on the lock and hopes that odd thread will notify it when it prints that odd number. And similarly it works for odd thread too.

I am not suggesting that this is the best solution but this is something that came out in the first try, some other options are also possible.

Also I would like to point out that this question though as a practice is good, but do keep in mind that you are not doing anything parallel there.

Answer 5

I will not vote for using wait() and notify(). The things that you can do with wait and notify can be done through more sophisticated tools like semaphore , countDownLatch , CyclicBarrier . You can find this advice in the famous book Effective java in item number 69 prefer concurrency utilities to wait and notify .

Even in this case we don't need this things at all, we can achieve this functionality by a simple volatile boolean variable. And for stopping a thread the best possible way is to use interrupt . After certain amount of time or some predefined condition we can interrupt threads. Please find my implementation attached:

Thread 1 for printing even numbers:

public class MyRunnable1 implements Runnable
{
    public static volatile boolean isRun = false;
    private int k = 0 ;
    @Override
    public void run() {
        while(!Thread.currentThread().isInterrupted()){
            if(isRun){
                System.out.println(k);
                k+=2;
                isRun=false;
                MyRunnable2.isRun=true;
            }
        }
    }
}

Thread 2 for printing even numbers:

public class MyRunnable2 implements Runnable{
    public static volatile boolean isRun = false;
    private int k = 1 ;
    @Override
    public void run() {
        while(!Thread.currentThread().isInterrupted()){
            if(isRun){
                System.out.println(k);
                k+=2;
                isRun=false;
                MyRunnable1.isRun=true;
            }
        }
    }
}

Now main method which drives the above threads

public class MyMain{
    public static void main(String[] args) throws InterruptedException{
        Thread t1 = new Thread(new MyRunnable1());
        Thread t2 = new Thread(new MyRunnable2());
        MyRunnable1.isRun=true;
        t1.start();
        t2.start();
        Thread.currentThread().sleep(1000);
        t1.interrupt();
        t2.interrupt();
    }
}

There may be some places you need to change a bit this is just a skeletal implementation. Hope it helps and please let me know if you need something else.

Answer 6

public class PrintNumbers {

    public static class Condition {
        private boolean start = false;
        public boolean getStart() {
            return start;
        }

        public void setStart(boolean start) {
            this.start = start;
        }
    }

    public static void main(String[] args) {

        final Object lock = new Object();
        // condition used to start the odd number thread first
        final Condition condition = new Condition();

        Thread oddThread = new Thread(new Runnable() {
            public void run() {
                synchronized (lock) {
                    for (int i = 1; i <= 10; i = i + 2) { //For simplicity assume  only printing till 10;
                        System.out.println(i);
                        //update condition value to signify that odd number thread has printed first
                        if (condition.getStart() == false) {
                            condition.setStart(true);
                        }
                        lock.notify();
                        try {
                            if (i + 2 <= 10) { 
                                lock.wait(); //if more numbers to print, wait;
                            }
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }

        });

        Thread evenThread = new Thread(new Runnable() {
            public void run() {
                synchronized (lock) {
                    for (int i = 2; i <= 10; i = i + 2) { //For simplicity assume only printing till 10;
                        // if thread with odd number has not printed first, then wait
                        while (condition.getStart() == false) {
                            try {
                                lock.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        System.out.println(i);
                        lock.notify();
                        try {
                            if (i + 2 <= 10) { //if more numbers to print, wait;
                                lock.wait();
                            }
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }

        });

        oddThread.start();
        evenThread.start();

    }

}

Answer 7

I did it using ReentrantLock with 25 threads . One thread Print One number and it will notify to other .

public class ReentrantLockHolder 
{
    private Lock lock;

    private Condition condition;

    public ReentrantLockHolder(Lock lock )
    {
        this.lock=lock;
        this.condition=this.lock.newCondition();
    }

    public Lock getLock() {
        return lock;
    }

    public void setLock(Lock lock) {
        this.lock = lock;
    }

    public Condition getCondition() {
        return condition;
    }

    public void setCondition(Condition condition) {
        this.condition = condition;
    }
}
public class PrintThreadUsingReentrantLock implements Runnable
{
    private ReentrantLockHolder currHolder;

    private ReentrantLockHolder nextHolder;

    private PrintWriter writer;

    private static int i=0;

    public PrintThreadUsingReentrantLock(ReentrantLockHolder currHolder, ReentrantLockHolder nextHolder ,PrintWriter writer)
    {
        this.currHolder=currHolder;
        this.nextHolder=nextHolder;
        this.writer=writer;
    }

    @Override
    public void run() 
    {
        while (true) 
        {
            writer.println(Thread.currentThread().getName()+ " "+ ++i);

            try{
                nextHolder.getLock().lock();
                nextHolder.getCondition().signal();
            }finally{
                nextHolder.getLock().unlock();  
            }

            try {
                currHolder.getLock().lock();
                currHolder.getCondition().await();
            }catch (InterruptedException e) 
            {
                e.printStackTrace();
            }
            finally{
                currHolder.getLock().unlock();
            }
        }
    }
}
public static void main(String[] args) 
    {
        PrintWriter printWriter =null;
        try {
                printWriter=new PrintWriter(new FileOutputStream(new File("D://myFile.txt")));
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        ReentrantLockHolder obj[]=new ReentrantLockHolder[25];
        for(int i=0;i<25;i++)
        {
            obj[i]=new ReentrantLockHolder(new ReentrantLock());
        }

        for(int i=0;i<25;i++)
        {
            Thread t1=new Thread(new PrintThreadUsingReentrantLock(obj[i], obj[i+1 == 25 ? 0 : i+1],printWriter ),"T"+i );
            t1.start();
        }
    }

Answer 8

I tried the similar stuff where Thread 1 prints Odd numbers and Thread 2 prints even numbers in a correct order and also when the printing is over, the desired messages as you had suggested will be printed. Please have a look at this code

package practice;


class Test {

  private static boolean oddFlag = true;
  int count = 1;

  private void oddPrinter() {
    synchronized (this) {
      while(true) {
        try {
          if(count < 10) {
            if(oddFlag) {
              Thread.sleep(500);
              System.out.println(Thread.currentThread().getName() + ": " + count++);
              oddFlag = !oddFlag;
              notifyAll();
            }
            else {
              wait();
            }
          }
          else {
            System.out.println("Odd Thread finished");
            notify();
            break;
          }
        }
        catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
    }
  }

  private void evenPrinter() {
    synchronized (this) {
      while (true) {
        try {
          if(count < 10) {
            if(!oddFlag) {
              Thread.sleep(500);
              System.out.println(Thread.currentThread().getName() + ": " + count++);
              oddFlag = !oddFlag;
              notify();
            }
            else {
              wait();
            }
          }
          else {
            System.out.println("Even Thread finished");
            notify();
            break;
          }
        }
        catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
    }
  }


  public static void main(String[] args) throws InterruptedException{
    final Test test = new Test();

    Thread t1 = new Thread(new Runnable() {
      public void run() {
        test.oddPrinter();
      }
    }, "Thread 1");

    Thread t2 = new Thread(new Runnable() {
      public void run() {
        test.evenPrinter();
      }
    }, "Thread 2");

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

    t1.join();
    t2.join();

    System.out.println("Main thread finished");
  }
}

Answer 9

package test;

public class Interview2 {

public static void main(String[] args) {
    Obj obj = new Obj();

    Runnable evenThread = ()-> {
        synchronized (obj) {
            for(int i=2;i<=50;i+=2) {
                while(!obj.printEven) {
                    try {
                        obj.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }                   
                }
                System.out.println(i);
                obj.printEven = false;
                obj.notify();
            }
        }           
    };
    Runnable oddThread = ()-> {
        synchronized (obj) {
            for(int i=1;i<=49;i+=2) {
                while(obj.printEven) {
                    try {
                        obj.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }                   
                }
                System.out.println(i);
                obj.printEven = true;
                obj.notify();
            }
        }
    };      
    new Thread(evenThread).start();
    new Thread(oddThread).start();
   }
}
class Obj {
  boolean printEven;
}

Answer 10

This is very generic solution. It uses semaphores to do signaling among threads. This is general solution where N threads prints M natural numbers in sequence turn by turn. that is if we have 3 threads and we want to print 7 natural numbers, output would be:

Thread 1 : 1

Thread 2 : 2

Thread 3 : 3

Thread 1 : 4

Thread 2 : 5

Thread 3 : 6

Thread 1 : 7

import java.util.concurrent.Semaphore;

/*
 * Logic is based on simple idea
 * each thread should wait for previous thread and then notify next thread in circular fashion
* There is no locking required
* Semaphores will do the signaling work among threads.
*/

public class NThreadsMNaturalNumbers {

private static volatile int nextNumberToPrint = 1;
private static int MaxNumberToPrint;

public static void main(String[] args) {

    int numberOfThreads = 2;
    MaxNumberToPrint = 50;

    Semaphore s[] = new Semaphore[numberOfThreads];

    // initialize Semaphores
    for (int i = 0; i < numberOfThreads; i++) {
        s[i] = new Semaphore(0);
    }

    // Create threads and initialize which thread they wait for and notify to
    for (int i = 1; i <= numberOfThreads; i++) {
        new Thread(new NumberPrinter("Thread " + i, s[i - 1], s[i % numberOfThreads])).start();
    }
    s[0].release();// So that First Thread can start Processing
}

private static class NumberPrinter implements Runnable {

    private final Semaphore waitFor;
    private final Semaphore notifyTo;
    private final String name;

    public NumberPrinter(String name, Semaphore waitFor, Semaphore notifyTo) {
        this.waitFor = waitFor;
        this.notifyTo = notifyTo;
        this.name = name;
    }

    @Override
    public void run() {

        while (NThreadsMNaturalNumbers.nextNumberToPrint <= NThreadsMNaturalNumbers.MaxNumberToPrint) {
            waitFor.acquireUninterruptibly();
            if (NThreadsMNaturalNumbers.nextNumberToPrint <= NThreadsMNaturalNumbers.MaxNumberToPrint) {
                System.out.println(name + " : " + NThreadsMNaturalNumbers.nextNumberToPrint++);
                notifyTo.release();
            }

        }
        notifyTo.release();
    }

}

}

Answer 11

This Class prints Even Number:

public class EvenThreadDetails extends Thread{

    int countNumber;
     public EvenThreadDetails(int countNumber) {
        this.countNumber=countNumber;
    }
    @Override
    public void run()
    {
        for (int i = 0; i < countNumber; i++) {
            if(i%2==0)
            {
                System.out.println("Even Number :"+i);
            }
            try {
                Thread.sleep(2);
            } catch (InterruptedException ex) {
                // code to resume or terminate...
            }
        }
    }
}

    

This Class prints Odd Numbers:

public class OddThreadDetails extends Thread {

    int countNumber;
     public OddThreadDetails(int countNumber) {
        this.countNumber=countNumber;
    }
    @Override
    public void run()
    {
        for (int i = 0; i < countNumber; i++) {
            if(i%2!=0)
            {
                System.out.println("Odd Number :"+i);
            }
            try {
                Thread.sleep(2);
            } catch (InterruptedException ex) {
                // code to resume or terminate...
            }       
        }
    }
}

This is Main class:

public class EvenOddDemo {

    public static void main(String[] args) throws InterruptedException
    {
        Thread eventhread= new EvenThreadDetails(100);
        Thread oddhread=new OddThreadDetails(100);
        eventhread.start();
        oddhread.start();   
    }
}

Answer 12

I have done it this way and its working...

class Printoddeven{

public synchronized void print(String msg){
    try {
        if(msg.equals("Even"))
        {
        for(int i=0;i<=10;i+=2){
            System.out.println(msg+" "+i);
            Thread.sleep(2000);
            notify();
            wait();
        }
        }

        else{
            for(int i=1;i<=10;i+=2){
                System.out.println(msg+" "+i);
                Thread.sleep(2000);
                notify();
                wait();
            }
        }

    } catch (Exception e) {

        e.printStackTrace();
    }
}

}

class PrintOdd extends Thread{
Printoddeven oddeven;
public PrintOdd(Printoddeven oddeven){
    this.oddeven=oddeven;
}

public void run(){
    oddeven.print("ODD");

}

}

class PrintEven extends Thread{
Printoddeven oddeven;
public PrintEven(Printoddeven oddeven){
    this.oddeven=oddeven;
}

public void run(){
    oddeven.print("Even");

}

}



public class mainclass 
{

public static void main(String[] args) 
{

    Printoddeven obj = new Printoddeven();//only one object  
    PrintEven t1=new PrintEven(obj);  
    PrintOdd t2=new PrintOdd(obj);  
    t1.start();  
    t2.start();  

}
}

Answer 13

public class Driver {
    static Object lock = new Object();

    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            public void run() {

                for (int itr = 1; itr < 51; itr = itr + 2) {
                    synchronized (lock) {
                        System.out.print(" " + itr);
                        try {
                            lock.notify();
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
                System.out.println("\nEven Thread Finish ");
            }
        });
        Thread t2 = new Thread(new Runnable() {
            public void run() {

                for (int itr = 2; itr < 51; itr = itr + 2) {
                    synchronized (lock) {
                        System.out.print(" " + itr);
                        try {
                            lock.notify();
                            if(itr==50)
                                break;
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
                System.out.println("\nOdd Thread Finish ");
            }
        });
        try {
            t1.start();
            t2.start();
            t1.join();
            t2.join();
            System.out.println("Exit Main Thread");
        } catch (Exception e) {

        }
    }
}