Concurrency issue with threads

Concurrency issue with threads - java

I have a simple code, consisting of 4 threads (2 modify the data and 2 read the data). I just wrote this sample code to play around with Semaphor and I am not sure why I get ava.base/java.util.ArrayList$Itr.checkForComodification exception? Here are the source code and thanks for any insights.
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Semaphore;
public class SemaphoreExample {
public static class InnerWriterSemaphoreThread implements Runnable {
private final List<String> fList;
private final Semaphore fWriteSem;
InnerWriterSemaphoreThread(List<String> list, Semaphore w) {
fList = list;
fWriteSem = w;
}
private void prune() {
System.out.println(Thread.currentThread().getName()+" in prune()..");
for (String s : fList) {
fList.remove(s);
}
}
#Override
public void run() {
String name = Thread.currentThread().getName();
String text;
while (true) {
text = RandomTextGenerator.getRandomSNumbertring();
try {
while(!fWriteSem.tryAcquire()){
System.out.println(name+" waiting to accquire semaphore to write..");
Thread.sleep(0L,4);
}
if (fList.size() > 10) {
prune();
}
fList.add(text);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
fWriteSem.release();// notify readers that write has completed
System.out.println(name+" finished writing, releasing semaphore..");
}
}//while()
}//run()
}//WriterSemaphoreThread
public static class InnerReaderSemaphoreThread implements Runnable {
private final List<String> fList;
private final Semaphore fWriteSem;
InnerReaderSemaphoreThread(List<String> list,Semaphore w) {
fList = list;
fWriteSem = w;
}
private void sleep(){
try{
Thread.sleep(0L, 4);
}catch(InterruptedException e){
e.printStackTrace();
}
}
#Override
public void run() {
String name = Thread.currentThread().getName();
while (true) {
System.out.println(name + " in run()..");
try {
while(fList.isEmpty()){
System.out.println(name+" list is empty, going to sleep..");
sleep();
}
while(!fWriteSem.tryAcquire()){
System.out.println(name+" waiting to accquire semaphor to read..");
Thread.sleep(0l,4);
}
for (String text : fList) {
System.out.println(name + " reading from list " + text);
}
}catch (InterruptedException e) {
e.printStackTrace();
}
finally{
fWriteSem.release(); //Notify threads who want to write to the list
System.out.println(name+" finished reading, releasing semaphore and going to sleep..");
sleep();
}
}
}
}//ReaderSemaphoreThread
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
List<String> list = new ArrayList<>();
Semaphore r = new Semaphore(1);
Thread th1 = new Thread(new InnerWriterSemaphoreThread(list, r), "Thread 1");
Thread th2 = new Thread(new InnerReaderSemaphoreThread(list, r), "Thread 2");
Thread th3 = new Thread(new InnerWriterSemaphoreThread(list, r), "Thread 3");
Thread th4 = new Thread(new InnerReaderSemaphoreThread(list, r), "Thread 4");
th2.start();
th4.start();
th1.start();
th3.start();
}
}
Above is the sample source code

As #assylias mentioned in comment it happens when you remove elements from list in foreach loop. Just replace
for (String s : fList) {
fList.remove(s);
}
with
fList.clear();

Related

shutdownNow method in ExecutorService

In the following code, I wrote producer-consumer with executor but I do not understand why when I call shutDownNow() the System.out.println(item) statement is executed twice. in fact, tryLock () returns the wrong value first, and then for the second time the item is printed and then it acquires the lock, and then the interrupt occurs.
public class Main {
public static void main(String[] args) {
Deque<String> queue = new ArrayDeque<>();
ReentrantLock bufferLock = new ReentrantLock();
Condition fullBuffer = bufferLock.newCondition();
MyProducer p = new MyProducer(queue, ThreadColor.ANSI_YELLOW, bufferLock, fullBuffer);
MyConsumer c1 = new MyConsumer(queue, ThreadColor.ANSI_BLUE, bufferLock, fullBuffer);
MyConsumer c2 = new MyConsumer(queue, ThreadColor.ANSI_BLUE, bufferLock, fullBuffer);
ExecutorService executorService = Executors.newFixedThreadPool(3);
executorService.execute(p);
executorService.shutdownNow();
try {
executorService.execute(c1);
executorService.execute(c2);
} catch (RejectedExecutionException e) {
System.out.println("no new task has not accepted");
}
}
}
part of producer code:
public class MyProducer implements Runnable {
private final Deque<String> buffer;
private final String color;
private final ReentrantLock bufferLock;
private final Condition fullBuffer;
public MyProducer(Deque<String> buffer, String color, ReentrantLock bufferLock, Condition fullBuffer) {
this.buffer = buffer;
this.color = color;
this.bufferLock = bufferLock;
this.fullBuffer = fullBuffer;
}
#Override
public void run() {
String[] items = {"apple", "ball", "laptop", "mouse", "cup", "pc", "pencil", "pen"};
long start = System.currentTimeMillis();
for (String item : items) {
System.out.println(item);
try {
if (bufferLock.tryLock(1, TimeUnit.SECONDS)) {
try {
while (items.length == buffer.size()) {
try {
fullBuffer.await();
} catch (InterruptedException e) {
System.out.println(color + "Producer was interrupted");
}
}
System.out.println(color + "Adding:" + item);
try {
Thread.sleep(250);
} catch (InterruptedException e) {
System.out.println(color + "failed to add");
}
buffer.add(item);
fullBuffer.signalAll();
} finally {
bufferLock.unlock();
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
bufferLock.lock();
try {
buffer.add("EOF");
} finally {
bufferLock.unlock();
}
System.out.println(Thread.currentThread().getName() + ":" + (System.currentTimeMillis() - start) + " ms");
}
}
output image

Interrupt a thread from another thread in java?

I have to create a program that simulate a bomb... The user has 5 seconds to digit the right code, if he can't, the bomb explodes.
class Codice implements Runnable{
String code;
#Override
public void run() {
code = JOptionPane.showInputDialog("Inserire codice disinnesco:");
if(code.equals(Bomba.check)) {
Bomba.s = "true";
JOptionPane.showMessageDialog(null, "Bomba disinnescata");
}
System.out.println(Bomba.s);
}
}
class Esplosione implements Runnable{
#Override
public void run() {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if(!Bomba.s.equals("true")) {
JOptionPane.showMessageDialog(null, "BOOM!");
}
}
}
public class Bomba {
static String s = "false";
static String check = "123456";
public static void main(String[] args) throws IOException {
Codice c = new Codice();
Esplosione ex = new Esplosione();
Thread t1 = new Thread(c);
Thread t2 = new Thread(ex);
t1.start();
t2.start();
}
}
With this code i can insert the code, and if it's right the bomb doesn't explodes and the program finish. If 5 seconds passes, the message "boom" appears but the first thread doeasn't stop... How can i do?

There are many ways to coordinate between threads; interrupting is one way. Here is an example using Thread.interrupt. It dispenses with your state variable s, which becomes unnecessary:
import javax.swing.*;
import java.io.*;
class Codice implements Runnable {
String code;
Thread other;
Codice(Thread other) {
this.other = other;
}
#Override
public void run() {
code = JOptionPane.showInputDialog("Inserire codice disinnesco:");
if(code.equals(Bomba.check)) {
other.interrupt();
JOptionPane.showMessageDialog(null, "Bomba disinnescata");
}
}
}
class Esplosione implements Runnable {
#Override
public void run() {
try {
Thread.sleep(5000);
JOptionPane.showMessageDialog(null, "BOOM!");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public class Bomba {
static String check = "123456";
public static void main(String[] args) throws IOException {
Esplosione ex = new Esplosione();
Thread t2 = new Thread(ex);
Codice c = new Codice(t2);
Thread t1 = new Thread(c);
t1.start();
t2.start();
}
}

Synchronized Block locked on class [duplicate]

This question already has an answer here:
Why Java throw java.lang.IllegalMonitorStateException when I invoke wait() in static way synchronized block?
(1 answer)
Closed 2 years ago.
In the below code for producer and consumer, I thought that the produce() and consume() methods are synchronized on Class Lock (Processor.class), but i am getting an exception stating IllegalMonitorStateException, which occurs for objects on which we don't acquire lock but we notify on that objects.
Can anyone tell me where i have gone wrong in the program.
package ProducerConsumer;
public class Main {
public static void main(String[] args) {
Processor processor = new Processor();
Thread producer = new Thread(new Runnable() {
public void run() {
try {
processor.produce();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
Thread consumer = new Thread(new Runnable() {
public void run() {
try {
processor.consume();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
System.out.println("\t\t\tStarting both producer and consumer Threads.");
producer.start();
consumer.start();
try {
producer.join();
consumer.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("\t\t\tEnding all the Threads.");
}
}
import java.util.List;
import java.util.ArrayList;
public class Processor {
private List<Integer> list = new ArrayList<>();
private int value = 0;
private final int LIMIT = 5;
public void produce() throws InterruptedException
{
synchronized(Processor.class){
while(true)
{
if(list.size() == LIMIT){
System.out.println("Waiting for consumer to consume resources");
wait();
}
else{
value++;
System.out.println("The produced resource is : "+value);
list.add(value);
notify();
}
}
}
}
public void consume() throws InterruptedException
{
synchronized(Processor.class){
while(true)
{
if(list.isEmpty()){
System.out.println("Waiting for producer to produce the resources");
wait();
}
else{
System.out.println("The consumer Consumed Resource is : "+list.remove(0));
notify();
}
}
}
}
}

Your wait() & notify() are invoked on this i.e. Processor processor = new Processor(); but your are locking/synchronizing on Processor.class object. You can fix your code to work as below.
class Processor {
private List<Integer> list = new ArrayList<>();
private int value = 0;
private final int LIMIT = 5;
public void produce() throws InterruptedException
{
synchronized(Processor.class){
while(true)
{
if(list.size() == LIMIT){
System.out.println("Waiting for consumer to consume resources");
Processor.class.wait();
}
else{
value++;
System.out.println("The produced resource is : "+value);
list.add(value);
Processor.class.notify();
}
}
}
}
public void consume() throws InterruptedException
{
synchronized(Processor.class){
while(true)
{
if(list.isEmpty()){
System.out.println("Waiting for producer to produce the resources");
Processor.class.wait();
}
else{
System.out.println("The consumer Consumed Resource is : "+list.remove(0));
Processor.class.notifyAll();
}
}
}
}
}

Multithreaded Web Crawler in Java

I am trying to write a Multithreaded Web Crawler in Java using Jsoup.I have a Java Class "Master" which creates 6 threads(5 for crawling and 1 for maintenance of queues) ,and 3 queues namely "to_do","to_do_next"(to be done in next iteration) and "done"(final links).
I am using sunchronized locks on shared queues.The idea is as soon as all the 5 threads find the "to_do" queue empty they notify a maintenance thread which does some work and notify these threads back.But the problem is the program is getting blocked sometimes (so i assume there is some race condition I am not able to take care of)....also upon checking I found that not all threads are getting notified by maintenace thread.so is it possible that some notify signals might be lost??
Code for Master class
private Queue<String> to_do = new LinkedList<String>();
private Queue<String> done= new LinkedList<String>();
private Queue<String> to_do_next = new LinkedList<String>();
private int level = 1;
private Object lock1 = new Object();
private Object lock2 = new Object();
private Object lock3 = new Object();
private static Thread maintenance;
public static Master mref;
public static Object wait1 = new Object();
public static Object wait2 = new Object();
public static Object wait3 = new Object();
public static int flag = 5;
public static int missedSignals = -1;
public boolean checkToDoEmpty(){
return to_do.isEmpty();
}
public int getLevel() {
return level;
}
public void incLevel() {
this.level++;
}
public static void interrupt() {
maintenance.interrupt();
}
public void transfer() {
to_do = to_do_next;
}
public String accessToDo() {
synchronized(lock1){
String tmp = to_do.peek();
if(tmp != null)
tmp = to_do.remove();
return tmp;
}
}
public void addToDoNext(String url){
synchronized(lock2){
to_do_next.add(url);
}
}
public void addDone(String string) {
synchronized(lock3){
done.add(string);
}
}
public static void main(String[] args){
Master m = new Master();
mref = m;
URL startUrl = null;
try {
startUrl = new URL("http://cse.iitkgp.ac.in");
}catch (MalformedURLException e1) {
e1.printStackTrace();
}
Thread t1 = new Thread(new Worker(1));
Thread t2 = new Thread(new Worker(2));
Thread t3 = new Thread(new Worker(3));
Thread t4 = new Thread(new Worker(4));
Thread t5 = new Thread(new Worker(5));
maintenance = new Thread(new MaintenanceThread());
m.to_do.add(startUrl.toString());
maintenance.start();
t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
try {
t1.join();
t2.join();
t3.join();
t4.join();
t5.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
/*for(String s:m.done)
System.out.println(s);
for(String s:m.to_do)
System.out.println(s);*/
}
Code for Worker threads
public void run() {
while(Master.mref.getLevel() != 3){
if(!Master.mref.checkToDoEmpty()){
String url = Master.mref.accessToDo();
if(url != null && url.contains("iitkgp") && url.contains("http://")){
try {
Document doc = Jsoup.connect(url).get();
org.jsoup.select.Elements links = doc.select("a[href]");
for(org.jsoup.nodes.Element l: links){
Master.mref.addToDoNext(l.attr("abs:href").toString());
}
Master.mref.addDone(url);
} catch (IOException e) {
System.out.println(url);
e.printStackTrace();
}
continue;
}
}
//System.out.println("thread " + id + " about to notify on wait1");
synchronized(Master.wait1){
Master.wait1.notify();
Master.missedSignals++;
}
synchronized(Master.wait2){
try {
Master.wait2.wait();
System.out.println("thread " + id + " coming out of wait2");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
System.out.println("Terminating " + id + " thread");
Master.flag--;
if(Master.flag == 0)
Master.interrupt();
}
Code for Maintenace thread
while(Master.flag != 0){
try {
synchronized(Master.wait1){
if(Master.missedSignals != -1){
count += Master.missedSignals;
Master.missedSignals = -1;
}
while(count != 5){
Master.wait1.wait();
if(Master.missedSignals != -1)
count += Master.missedSignals;
Master.missedSignals = -1;
count++;
}
count = 0;
}
//System.out.println("in between");
Master.mref.incLevel();
Master.mref.transfer();
synchronized(Master.wait2){
Master.wait2.notifyAll();
}
} catch (InterruptedException e) {
break;
}
}
System.out.println("Mainta thread gone");

Your design is way too complicated
i suggest using for your to_do queue the following: LinkedBlockingQueue
This is a blocking queue, which means that your threads will ask for an object from the queue and only when one will appear they will get the object, till then they will stay blocking.
Just use the following methods to put and take objects in the queue: put() & take()
Please look at the following two links for more explanations on this special queue:
http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/LinkedBlockingQueue.html
http://tutorials.jenkov.com/java-util-concurrent/linkedblockingqueue.html
Now, your only concern is killing the threads when they are finished with their work, for that I suggest the following:
boolean someThreadStillAlive = true;
while (someThreadStillAlive) {
someThreadStillAlive = false;
Thread.sleep(200);
for (Thread t : fetchAndParseThreads) {
someThreadStillAlive = someThreadStillAlive || t.isAlive();
}
}
This will occur in your main code block, where it will loop & sleep till all threads are finished.
Ohh, instead of take(), you can use poll(int timeout...) where it will wait for the timeout to finish and if no new object is inserted into the queue it will kill the thread.
All of the above, were used successfully in my own crawler.

Print Natural Sequence with help of 2 threads(1 is printing even and 2'nd is printing odd)

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) {
}
}
}

We Keep Coding

Java is a programming language and computing platform first released by Sun Microsystems in 1995.

Concurrency issue with threads - java

As #assylias mentioned in comment it happens when you remove elements from list in foreach loop. Just replace for (String s : fList) { fList.remove(s); } with fList.clear();

Related

shutdownNow method in ExecutorService

Interrupt a thread from another thread in java?

Synchronized Block locked on class [duplicate]

Multithreaded Web Crawler in Java

Print Natural Sequence with help of 2 threads(1 is printing even and 2'nd is printing odd)

Categories

Resources