I have a question, I want to know if it's possible to check continuously for adds to a list.
I have a node with a file to transfer, I connect it to a directory.
I have another node without the file and requires a download, I also connect it to the directory.
What is currently happening is:
Node 1 Port 40
And after I add the second node, only then does the SECOND node know there's him and another node:
Node 1 Port 40
Node 2 Port 60
But the first one, he still only sees himself. So my question is, how can I make the first node, or all the nodes, continuously check if there are new ones added to the list.
public void addExistingNodes(String sta) {
if (sta.equals("end")) return;
if (!(checkNodesList.contains(sta))) {
checkNodesList.add(sta);
nodes.add(new Nodes(checkNodesList.get(checkNodesList.size() - 1)));
}
}
This code is used to avoid, the following:
Node 1 Port 40
Node 1 Port 40
Node 2 Port 60
In case you stop the process without closing the directory.
A REALLY basic idea of a "observable list". Please note, I consider this a "basic" implementation, I only support add and remove, and while I've taken some steps to ensure thread safety, there is still more which could/should be done to prevent possible multi-thread mutations of some of the states
The demonstrates the basic concept of produce/consumer and observable pattern
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.logging.Level;
import java.util.logging.Logger;
public class Test {
public static void main(String[] args) {
new Test();
}
public Test() {
ObservableList<String> list = new ObservableList<>(new ArrayList<String>(8));
Thread consumer = new Thread(new Runnable() {
private ReentrantLock lock = new ReentrantLock();
private Condition con = lock.newCondition();
#Override
public void run() {
list.addObserver(new ObservableList.ObservableListListener<String>() {
#Override
public void listDidAddValue(ObservableList<String> source, String value) {
System.out.println("Did add " + value);
lock.lock();
try {
con.signal();
} finally {
lock.unlock();
}
}
#Override
public void listDidRemoveValue(ObservableList<String> source, String value) {
System.out.println("Did remove " + value);
lock.lock();
try {
con.signal();
} finally {
lock.unlock();
}
}
});
try {
while (true) {
lock.lock();
try {
System.out.println("Waiting for something to happen");
con.await();
} finally {
lock.unlock();
}
System.out.println("Something happended, process it...");
}
} catch (InterruptedException ex) {
Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
}
}
});
consumer.setDaemon(true);
consumer.start();
try {
// Give the consumer timer to spin up ;)
Thread.sleep(1000);
} catch (InterruptedException ex) {
Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
}
Thread produce = new Thread(new Runnable() {
#Override
public void run() {
try {
String text = "Hello world";
for (int index = 0; index < text.length(); index++) {
String value = text.substring(index, index + 1);
list.add(value);
Thread.sleep(500);
}
for (int index = 0; index < text.length(); index++) {
String value = text.substring(index, index + 1);
list.remove(value);
Thread.sleep(500);
}
} catch (InterruptedException ex) {
Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
}
System.out.println("All done here, leaving now");
}
});
produce.start();
}
public class ObservableList<E> {
public interface ObservableListListener<E> {
public void listDidAddValue(ObservableList<E> source, E value);
public void listDidRemoveValue(ObservableList<E> source, E value);
}
private List<E> backingStore;
private List<ObservableListListener<E>> observers;
public ObservableList(List<E> list) {
this.backingStore = Collections.synchronizedList(list);
observers = Collections.synchronizedList(new ArrayList<>(8));
}
public void add(E value) {
backingStore.add(value);
fireDidAddValue(value);
}
public void remove(E value) {
boolean removed = backingStore.remove(value);
fireDidRemoveValue(value);
}
public int size() {
return backingStore.size();
}
public void addObserver(ObservableListListener<E> observer) {
observers.add(observer);
}
public void removeObserver(ObservableListListener<E> observer) {
observers.remove(observer);
}
protected void fireDidAddValue(E value) {
if (observers.isEmpty()) {
return;
}
for (ObservableListListener<E> observer : observers) {
observer.listDidAddValue(this, value);
}
}
protected void fireDidRemoveValue(E value) {
if (observers.isEmpty()) {
return;
}
for (ObservableListListener<E> observer : observers) {
observer.listDidRemoveValue(this, value);
}
}
}
}
I would also encourage having a look at Java 9 Reactive Streams which would seem to fulfil your overall needs
I am learning multithreading. I am implementing producer and consumer problem. I am stuck on scenario where i want that when I press anything apart from integer from keyboard, all my threads should die and there is no memory in use by threads. Please have your valuable inputs to help me achieve it. Below is all the code I am using.
package com.java.concurrency;
public class ThreadSignaling {
private int i = -1;
private boolean valueSet = false;
private boolean stopFlag = false;
public void put(int value) {
synchronized (this) {
while (valueSet) {
if (stopFlag) {
System.out.println("Byeeeeeeeeeeeee");
break;
}
try {
this.wait();
} catch (InterruptedException e) {
System.out.println("InterruptedException while waiting in put() : " + e);
}
}
this.i = value;
this.valueSet = true;
System.out.println("Value put : " + this.i);
this.notify();
}
}
public void get() {
synchronized (this) {
while (!valueSet) {
if (stopFlag) {
System.out.println("Byeeeeeeeeeeeee");
break;
}
try {
this.wait();
} catch (InterruptedException e) {
System.out.println("InterruptedException while waiting in get() : " + e);
}
}
System.out.println("Value get : " + this.i);
valueSet = false;
this.notify();
}
}
public void finish() {
synchronized (this) {
stopFlag = true;
this.notifyAll();
}
}
}
public class Producer implements Runnable {
private ThreadSignaling sharedObj = null;
private final Scanner input = new Scanner(System.in);
public Producer(ThreadSignaling obj) {
this.sharedObj = obj;
}
#Override
public void run() {
int value = -1;
System.out.println("Press Ctrl-c to stop... ");
while (true) {
System.out.println("Enter any integer value : ");
if (input.hasNextInt()) {
value = input.nextInt();
} else {
this.sharedObj.finish();
return;
}
this.sharedObj.put(value);
try {
Thread.sleep(500);
} catch (InterruptedException e) {
System.out.println("InterruptedException while sleeping" + e);
}
}
}
}
public class Consumer implements Runnable {
private ThreadSignaling sharedObj = null;
public Consumer(ThreadSignaling obj) {
this.sharedObj = obj;
}
#Override
public void run() {
while (true) {
this.sharedObj.get();
}
}
}
public class MainThread {
public static void main(String[] args) {
ThreadSignaling sharedObj = new ThreadSignaling();
Producer in = new Producer(sharedObj);
Consumer out = new Consumer(sharedObj);
Thread t1 = new Thread(in);
Thread t2 = new Thread(out);
t1.start();
t2.start();
}
} enter code here
The problem with your code is that you do not have an exit condition for the Consumer. The run() method of the Consumer will run forever, and while doing repeated get calls on the shared object.
What you need to do is to make aware the Consumer that the Producer has set the stopFlag in the shared object. And if that stopFlag is true then the loop in the Consumer should also finish. There are several ways you can do that:
redefine get method to return the value of stopFlag;
define a new method to return just the value of stopFlag;
In either cases, make a test in the Consumer.run() and if the value is true, just do a return so the infinite loop ends.
I'm trying to make a program run two threads through a semaphore. The semaphore must be started to 0.
At the end should display a message on screen:
System.out.println ("string number" + i);
where 'i' is the number of the process
My code is:
public class Orden extends Thread {
int id;
int num;
static Semaphore semaphore = new Semaphore (0);
public Orden(int id)
{
this.id= id;
}
#Override
public synchronized void run()
{
try
{
if(semaphore.availablePermits() == 0){
semaphore.release(1);
System.out.println(semaphore.availablePermits());
}
else{
System.out.println("Thread: " + this.id);
}
}
catch (Exception e)
{
System.out.println(e.toString());
}
}
public static void main(String[] args)
{
Orden o1 = new Orden(1);
Orden o2 = new Orden(2);
o1.start();
o2.start();
}
}
Where am I wrong? What am I doing wrong?
Thanks
Your run is synchronized but you are synchronized on two different objects o1 and o2 so it is effectively not synchronized. You could use a synchronized block on a static object such as semaphore so they would both be synchronized on the same object.
#Override
public void run() {
try {
synchronized (semaphore) {
if (semaphore.availablePermits() == 0) {
semaphore.release(1);
System.out.println(semaphore.availablePermits());
} else {
System.out.println("Thread: " + this.id);
}
}
} catch (Exception e) {
System.out.println(e.toString());
}
}
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;
}
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
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.
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.
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();
}
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.
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();
}
}
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();
}
}
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");
}
}
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;
}
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();
}
}
}
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();
}
}
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();
}
}
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) {
}
}
}
I am trying to implement Bully Algorithm in Java using threads.
Here is the code which I have written.
package newbully;
public class NewBully {
public static void main(String[] args) {
int total_processes = 4;
Thread1[] t = new Thread1[total_processes];
for (int i = 0; i < total_processes; i++) {
t[i] = new Thread1(new Process(i+1, i+1), total_processes);
}
try {
Election.initialElection(t);
} catch (Exception e) {
System.out.println("Possibly you are using null references in array");
}
for (int i = 0; i < total_processes; i++) {
new Thread(t[i]).start();
}
}
}
package newbully;
public class Election {
private static boolean pingFlag = false;
private static boolean electionFlag = false;
private static boolean messageFlag = false;
public static boolean isMessageFlag() {
return messageFlag;
}
public static void setMessageFlag(boolean messageFlag) {
Election.messageFlag = messageFlag;
}
public static boolean isPingFlag() {
return pingFlag;
}
public static void setPingFlag(boolean pingFlag) {
Election.pingFlag = pingFlag;
}
public static boolean isElectionFlag() {
return electionFlag;
}
public static void setElectionFlag(boolean electionFlag) {
Election.electionFlag = electionFlag;
}
public static void initialElection(Thread1[] t) {
Process temp = new Process(-1, -1);
for (int i = 0; i < t.length; i++) {
if (temp.getPriority() < t[i].getProcess().getPriority()) {
temp = t[i].getProcess();
}
}
t[temp.pid - 1].getProcess().CoOrdinatorFlag = true;
}
}
package newbully;
public class Process {
int pid;
boolean downflag,CoOrdinatorFlag;
public boolean isCoOrdinatorFlag() {
return CoOrdinatorFlag;
}
public void setCoOrdinatorFlag(boolean isCoOrdinator) {
this.CoOrdinatorFlag = isCoOrdinator;
}
int priority;
public boolean isDownflag() {
return downflag;
}
public void setDownflag(boolean downflag) {
this.downflag = downflag;
}
public int getPid() {
return pid;
}
public void setPid(int pid) {
this.pid = pid;
}
public int getPriority() {
return priority;
}
public void setPriority(int priority) {
this.priority = priority;
}
public Process() {
}
public Process(int pid, int priority) {
this.pid = pid;
this.downflag = false;
this.priority = priority;
this.CoOrdinatorFlag = false;
}
}
package newbully;
import java.util.*;
import java.io.*;
import java.net.*;
public class Thread1 implements Runnable {
private Process process;
private int total_processes;
ServerSocket[] sock;
Random r;
public Process getProcess() {
return process;
}
public void setProcess(Process process) {
this.process = process;
}
public Thread1(Process process, int total_processes) {
this.process = process;
this.total_processes = total_processes;
this.r = new Random();
this.sock = new ServerSocket[total_processes];
}
private void recovery() {
}
synchronized private void pingCoOrdinator() {
try {
if (Election.isPingFlag()) {
wait();
}
if (!Election.isElectionFlag()) {
Election.setPingFlag(true);
System.out.println("Process[" + this.process.getPid() + "]: Are you alive?");
Socket outgoing = new Socket(InetAddress.getLocalHost(), 12345);
outgoing.close();
Election.setPingFlag(false);
notifyAll();
}
} catch (Exception ex) {
//Initiate Election
System.out.println("process[" + this.process.getPid() + "]: -> Co-Ordinator is down\nInitiating Election");
Election.setElectionFlag(true);
Election.setPingFlag(false);
notifyAll();
}
}
synchronized private void executeJob() {
int temp = r.nextInt(20);
for (int i = 0; i <= temp; i++) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
System.out.println("Error Executing Thread:" + process.getPid());
System.out.println(e.getMessage());
}
}
}
synchronized private boolean sendMessage() {
boolean response = false;
int i = 0;
try {
if (Election.isMessageFlag()) {
wait();
}
Election.setMessageFlag(true);
for (i = this.process.getPid() + 1; i <= this.total_processes; i++) {
try {
Socket electionMessage = new Socket(InetAddress.getLocalHost(), 10000 + i);
System.out.println("Process[" + this.process.getPid() + "] -> Process[" + i + "] responded to election message successfully");
electionMessage.close();
response = true;
} catch (Exception ex) {
System.out.println("Process[" + this.process.getPid() + "] -> Process[" + i + "] did not respond to election message");
}
}
Election.setMessageFlag(false);
notifyAll();
} catch (Exception ex1) {
System.out.println(ex1.getMessage());
}
return response;
}
synchronized private void serve() {
try {
//service counter
ServerSocket s = new ServerSocket(12345);
for (int counter = 0; counter <= 10; counter++) {
Socket incoming = s.accept();
System.out.println("Process[" + this.process.getPid() + "]:Yes");
Scanner scan = new Scanner(incoming.getInputStream());
PrintWriter out = new PrintWriter(incoming.getOutputStream(), true);
if (scan.hasNextLine()) {
if (scan.nextLine().equals("Who is the co-ordinator?")) {
System.out.print("Process[" + this.process.getPid() + "]:");
out.println(this.process);
}
}
if (counter == 10) {//after serving 10 requests go down
this.process.setCoOrdinatorFlag(false);
this.process.setDownflag(true);
try {
incoming.close();
s.close();
sock[this.process.getPid() - 1].close();
Thread.sleep((this.r.nextInt(10) + 1) * 50000);//going down
recovery();
} catch (InterruptedException e) {
System.out.println(e.getMessage());
}
}
}
} catch (IOException ex) {
System.out.println(ex.getMessage());
}
}
#Override
public void run() {
try {
sock[this.process.getPid() - 1] = new ServerSocket(10000 + this.process.getPid());
} catch (IOException ex) {
System.out.println(ex.getMessage());
}
while (true) {
if (process.isCoOrdinatorFlag()) {
//serve other processes
serve();
} else {
while (true) {
//Execute some task
executeJob();
//Ping the co-ordinator
pingCoOrdinator();
if (Election.isElectionFlag()) {
if (!sendMessage()) {//elect self as co-ordinator
System.out.println("New Co-Ordinator: Process[" + this.process.getPid() + "]");
this.process.setCoOrdinatorFlag(true);
Election.setElectionFlag(false);
break;
}
}
}
}
}
}
}
When I am trying to execute the code out of the 4 threads which I have created some threads are waiting premanently using wait() call. They are not being notified by notifyAll(). Can anyone suggest why this is happening?
Each thread is calling wait() on itself (on its own Thread1 instance). That means that when you call notifyAll() on that same Thread1 instance, only the single Thread1 that is waiting it will be notified, and not all the other threads.
What you have to do is make all your Thread1 objects call wait() on a single, common object, and also call notifyAll() on that same object.
Ofcourse you have to synchronize on the common object when you call wait() or notifyAll() on it; if you don't do that, you'll get an IllegalMonitorStateException.
// Object to be used as a lock; pass this to all Thread1 instances
Object lock = new Object();
// Somewhere else in your code
synchronized (lock) {
lock.wait();
}
// Where you want to notify
synchronized (lock) {
lock.notifyAll();
}
Both notify() (or notifyAll()) and wait() must be written into synchronized block on the same monitor.
For example:
synchronized(myLock) {
wait();
}
..................
synchronized(myLock) {
notifyAll();
}