We Keep Coding

Java multi threading - run threads run method only once in sequence

In my applications there are an n number of actions that must happen, one after the other in sequence, for the whole life of the program. Instead of creating methods which implement those actions and calling them in order in a while(true) loop, I decided to create one thread for each action, and make them execute their run method once, then wait until all the other threads have done the same, wait for its turn, and re-execute again, and so on...
To implement this mechanism I created a class called StatusHolder, which has a single field called threadTurn (which signifies which thread should execute), a method to read this value, and one for updating it. (Note, this class uses the Singleton design pattern)
package Test;
public class StatusHolder
{
private static volatile StatusHolder statusHolderInstance = null;
public static volatile int threadTurn = 0;
public synchronized static int getTurn()
{
return threadTurn;
}
public synchronized static void nextTurn()
{
System.out.print("Thread turn: " + threadTurn + " --> ");
if (threadTurn == 1)
{
threadTurn = 0;
}
else
{
threadTurn++;
}
System.out.println(threadTurn);
//Wake up all Threads waiting on this obj for the right turn to come
synchronized (getStatusHolder())
{
getStatusHolder().notifyAll();
}
}
public static synchronized StatusHolder getStatusHolder()
{//Returns reference to this object
if (statusHolderInstance == null)
{
statusHolderInstance = new StatusHolder();
}
return statusHolderInstance;
}
}
Then I have, let's say, two threads which must be execute in the way explained above, t1 and t2.
T1 class looks like this:
package Test;
public class ThreadOne implements Runnable
{
#Override
public void run()
{
while (true)
{
ThreadUtils.waitForTurn(0);
//Execute job, code's not here for simplicity
System.out.println("T1 executed");
StatusHolder.nextTurn();
}
}
}
And T2 its the same, just change 0 to 1 in waitForTurn(0) and T1 to T2 in the print statement.
And my main is the following:
package Test;
public class Main
{
public static void main(String[] args) throws InterruptedException
{
Thread t1 = new Thread(new ThreadOne());
Thread t2 = new Thread(new ThreadTwo());
t1.start();
t2.start();
}
}
So the run method goes like this:
At the start of the loop the thread looks if it can act by checking the turn value with the waitForTurn() call:
package Test;
public class ThreadUtils
{
public static void waitForTurn(int codeNumber)
{ //Wait until turn value is equal to the given number
synchronized (StatusHolder.getStatusHolder())
{
while (StatusHolder.getTurn() != codeNumber)
{
try
{
StatusHolder.getStatusHolder().wait();
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
}
}
If the two values are equal, the thread executes, otherwise it waits on the StatusHolder object to be awaken from the nextTurn() call, because when the turn value changes all the threads are awaken so that they can check if the new turn value is the one they are waiting for so they can run.
Note thatnextTurn() cycles between 0 and 1: that is because in this scenario I just have two threads, the first executes when the turn flag is 0, and the second when its 1, and then 0 again and so on. I can easily change the number of turns by changing this value.
The problem: If I run it, all goes well and seems to work, but suddenly the output console stops flowing, even if the program doesn't crash at all. I tried to put a t1.join() and then a print in the main but that print never executes, this means that the threads never stop/dies, but instead they remain locked sometimes.
This looks to be even more evident if I put three threads: it stops even sooner than with two threads.
I'm relatively new to threads, so I might be missing something really stupid here...
EDIT: I'd prefer not to delete a thread and create a new one every time: creating and deleting thousands of objs every second seems a big work load for the garbage collector.
The reason why I'm using threads and not functions is because in my real application (this code is just simplified) at a certain turn there actually are multiple threads that must run (in parallel), for example: turn 1 one thread, turn 2 one thread, turn 3 30 threads, repeat. So I thought why not creating threads also for the single functions and make the whole think sequential.

This is a bad approach. Multiple threads allow you to execute tasks concurrently. Executing actions "one after the other in sequence" is a job for a single thread.
Just do something like this:
List<Runnable> tasks = new ArrayList<>();
tasks.add(new ThreadOne()); /* Pick better names for tasks */
tasks.add(new ThreadTwo());
...
ExecutorService worker = Executors.newSingleThreadExecutor();
worker.submit(() -> {
while (!Thread.interrupted())
tasks.forEach(Runnable::run);
});
worker.shutdown();
Call worker.shutdownNow() when your application is cleanly exiting to stop these tasks at the end of their cycle.

you can use Semaphore class it's more simple
class t1 :
public class t1 implements Runnable{
private Semaphore s2;
private Semaphore s1;
public t1(Semaphore s1,Semaphore s2){
this.s1=s1;
this.s2=s2;
}
public void run()
{
while (true)
{
try {
s1.acquire();
} catch (InterruptedException ex) {
Logger.getLogger(t1.class.getName()).log(Level.SEVERE, null, ex);
}
//Execute job, code's not here for simplicity
System.out.println("T1 executed");
s2.release();
}
}
}
class t2:
public class t2 implements Runnable{
private Semaphore s2;
private Semaphore s1;
public t2(Semaphore s1,Semaphore s2){
this.s1=s1;
this.s2=s2;
}
public void run()
{
while (true)
{
try {
s2.acquire();
} catch (InterruptedException ex) {
Logger.getLogger(t2.class.getName()).log(Level.SEVERE, null, ex);
}
//Execute job, code's not here for simplicity
System.out.println("T2 executed");
s1.release();
}
}
}
class main:
public class Testing {
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
Semaphore s2=new Semaphore(0);
Semaphore s1=new Semaphore(1);
Thread th1 = new Thread(new t1(s1,s2));
Thread th2 = new Thread(new t2(s1,s2));
th1.start();
th2.start();
}}

Java: notify main class when all threads in threadpool are finished / same instance of object in different threads

How do I notify my main class which instantiates a ThreadPoolExecutor when all threads within the ThreadPoolExecutor are completed?
ThreadPoolExecutor threadPool = null;
ThreadClass threadclass1;
ThreadClass threadclass2;
final ArrayBlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(maxPoolSize);
puclic MyClass(){
threadPool = new ThreadPoolExecutor(poolSize, maxPoolSize, keepAliveTime, TimeUnit.SECONDS, queue);
threadClass1 = new ThreadClass;
threadClass2 = new ThreadClass;
threadPool.execute(threadClass1);
threadPool.execute(threadClass2);
//Now I would like to do something until the threadPool is done working
//The threads fill a ConcurrentLinkedQueueand I would like to poll
//the queue as it gets filled by the threads and output
//it to XML via JAX-RS
}
EDIT 1
Wile my threads fetch data from somewhere and fill this information into a ConcurrentLinkedQueue I basically would like to perform some action in MyClass to update the XML output with the results. When all threads are terminated I would like to return true to the JAX-RS webservice which instantiated MyClass so the webservice knows all data has been fetched and it can now display the final XML file
EDIT 2
I am passing a Queue to threads so they can add items to the queue. When one driver is done adding items to the articleQueue I want to perform an action within my main class, polling the entity from the Queue and handing it over to the response object to display it in some way.
When I pass the queue to the threads, are they working with the same object or with a "copy" of the object so that changes within the thread do not effect the main object? That is not the behavior I want. When I check the size of the articleQueue within the Driver it is 18, the size of the articleQueue in the DriverController is 0.
Is there a nicer way to react when a thread has added something to the queue other than my while loop? How do I have to modify my code to acces the same object within different classes?
DriverController
public class DriverController {
Queue<Article> articleQueue;
ThreadPoolExecutor threadPool = null;
final ArrayBlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(
maxPoolSize);
public DriverController(Response response) {
articleQueue = new ConcurrentLinkedQueue<Article>();
threadPool = new ThreadPoolExecutor();
Driver driver = new Driver(this.articleQueue);
threadPool.execute(driver);
// More drivers would be executed here which add to the queue
while (threadPool.getActiveCount() > 0) {
// this.articleQueue.size() gives back 0 here ... why?
if(articleQueue.size()>0){
response.addArticle(articleQueue.poll());
}
}
}
}
Driver
public class Driver implements Runnable{
private Queue<Article> articleQueue;
public DriverAlliedElectronics(Queue articleQueue) {
this.articleQueue = articleQueue;
}
public boolean getData() {
// Here would be the code where the article is created ...
this.articleQueue.offer(article);
return true;
}
public void run() {
this.getData();
// this.articleQueue.size() gives back 18 here ...
}
}

You should try to use following snippet
//Now I would like to wait until the threadPool is done working
threadPool.shutdown();
while (!threadPool.isTerminated()) {
try {
threadPool.awaitTermination(10, TimeUnit.MILLISECONDS);
} catch (InterruptedException e) {
e.printStackTrace();
}
}

Maybe a ExecutorCompletionService might be the right thing for you:
http://download.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorCompletionService.html
Example from the link above:
void solve(Executor e, Collection<Callable<Result>> solvers)
throws InterruptedException, ExecutionException {
CompletionService<Result> ecs = new ExecutorCompletionService<Result>(e);
for (Callable<Result> s : solvers)
ecs.submit(s);
int n = solvers.size();
for (int i = 0; i < n; ++i) {
Result r = ecs.take().get();
if (r != null)
use(r);
}
}

Instead of using execute you should use submit. This will return a Future instance on which you can wait for the task(s) to complete. That way you don't need polling or shutting down the pool.

I don't think there's a way to do this explicitly. You could poll the getCompletedTaskCount() to wait for that to become zero.
Why not collect the Future objects returned upon submission and check for all of those being completed ? Simply call get() on each one in turn. Since that call blocks you'll simply wait for each in turn and gradually fall through the set until you've waited on each on.
Alternatively you could submit the threads, and call shutdown() on the executor. That way, the submitted tasks will be executed, and then the terminated() method is called. If you override this then you'll get a callback once all tasks are completed (you couldn't use that executor again, obviously).

Judging from the reference documentation you have a few options:
ThreadPoolExecutor threadPool = null;
ThreadClass threadclass1;
ThreadClass threadclass2;
final ArrayBlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(maxPoolSize);
puclic MyClass(){
threadPool = new ThreadPoolExecutor(poolSize, maxPoolSize, keepAliveTime, TimeUnit.SECONDS, queue);
threadClass1 = new ThreadClass;
threadClass2 = new ThreadClass;
threadPool.execute(threadClass1);
threadPool.execute(threadClass2);
//Now I would like to wait until the threadPool is done working
//Option 1: shutdown() and awaitTermination()
threadPool.shutDown();
try {
threadPool.awaitTermination(Long.MAX_VALUE, TimeUnit.SECONDS)
}
catch (InterruptedException e) {
e.printStackTrace();
}
//Option 2: getActiveCount()
while (threadPool.getActiveCount() > 0) {
try {
Thread.sleep(1000);
}
catch (InterruptedException ignored) {}
}
//Option 3: getCompletedTaskCount()
while (threadPool.getCompletedTaskCount() < totalNumTasks) {
try {
Thread.sleep(1000);
}
catch (InterruptedException ignored) {}
}
}
All things considered, I think shutdown() and awaitTermination() is the best option of the three.

I think you're overengineering things a bit. You don't really care about the threads or the thread pool, and rightly so. Java provides nice abstractions so that you don't have to. You just need to know when your tasks are complete, and methods exist for that. Just submit your jobs, and wait for the futures to say they're done. If you really want to know as soon as a single task completes, you can watch all the futures and take action as soon as any one is finished. If not and you only care that everything is finished, you can remove some complexity from the code I'm about to post. Try this on for size (note MultithreadedJaxrsResource is executable):
import javax.ws.rs.*;
import javax.ws.rs.core.MediaType;
import java.util.*;
import java.util.concurrent.*;
#Path("foo")
public class MultithreadedJaxrsResource {
private ExecutorService executorService;
public MultithreadedJaxrsResource(ExecutorService executorService) {
this.executorService = executorService;
}
#GET
#Produces(MediaType.APPLICATION_XML)
public AllMyArticles getStuff() {
List<Future<Article>> futures = new ArrayList<Future<Article>>();
// Submit all the tasks to run
for (int i = 0; i < 10; i++) {
futures.add(executorService.submit(new Driver(i + 1)));
}
AllMyArticles articles = new AllMyArticles();
// Wait for all tasks to finish
// If you only care that everything is done and not about seeing
// when each one finishes, this outer do/while can go away, and
// you only need a single for loop to wait on each future.
boolean allDone;
do {
allDone = true;
Iterator<Future<Article>> futureIterator = futures.iterator();
while (futureIterator.hasNext()) {
Future<Article> future = futureIterator.next();
if (future.isDone()) {
try {
articles.articles.add(future.get());
futureIterator.remove();
} catch (InterruptedException e) {
// thread was interrupted. don't do that.
throw new IllegalStateException("broken", e);
} catch (ExecutionException e) {
// execution of the Callable failed with an
// exception. check it out.
throw new IllegalStateException("broken", e);
}
} else {
allDone = false;
}
}
} while (!allDone);
return articles;
}
public static void main(String[] args) {
ExecutorService executorService = Executors.newFixedThreadPool(10);
AllMyArticles stuff =
new MultithreadedJaxrsResource(executorService).getStuff();
System.out.println(stuff.articles);
executorService.shutdown();
}
}
class Driver implements Callable<Article> {
private int i; // Just to differentiate the instances
public Driver(int i) {
this.i = i;
}
public Article call() {
// Simulate taking some time for each call
try {
Thread.sleep(1000 / i);
} catch (InterruptedException e) {
System.err.println("oops");
}
return new Article(i);
}
}
class AllMyArticles {
public final List<Article> articles = new ArrayList<Article>();
}
class Article {
public final int i;
public Article(int i) {
this.i = i;
}
#Override
public String toString() {
return "Article{" +
"i=" + i +
'}';
}
}
Done that way, you can plainly see that the tasks are returned in the order they complete, as the last task finishes first thanks to sleeping the shortest time. If you don't care about completion order and just want to wait for all to finish, the loop becomes much simpler:
for (Future<Article> future : futures) {
try {
articles.articles.add(future.get());
} catch (InterruptedException e) {
// thread was interrupted. don't do that.
throw new IllegalStateException("broken", e);
} catch (ExecutionException e) {
// execution of the Callable failed with an exception. check it out.
throw new IllegalStateException("broken", e);
}
}

Why does ExecutorService keep executing when threads are blocking?

I am trying to write a part of a multithreaded program where each thread from a fixed thread pool tries to fetch an object from a Queue and if the Queue is empty the thread waits.
The problem I am experiencing is that the memory used by the program keeps increasing.
public class Ex3 {
public static LinkedBlockingQueue<Integer> myLBQ = new LinkedBlockingQueue<Integer>(10);
public static void main(String argc[]) throws Exception {
ExecutorService executor = Executors.newFixedThreadPool(3);
myLBQ.add(new Integer(1));
for (;;) {
executor.execute(new MyHandler(myLBQ));
}
}
}
class MyHandler implements Runnable {
LinkedBlockingQueue<Integer> myLBQ;
MyHandler(LinkedBlockingQueue<Integer> myLBQ) {
this.myLBQ = myLBQ;
}
public void run() {
try {
myLBQ.take();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
I don't understand why the executor.execute keeps firing when the threads should be waiting for an item to be added to the Queue. How do I modify my code to reflect this?

This adds tasks to the executor as fast as it can.
for (;;) {
executor.execute(new MyHandler(myLBQ));
}
This will consume about 200 MB per second. It doesn't have anything to do with whether there are tasks to perform or not.
If you don't want to do this I suggest you move the loop to the runnable and add only one. This will cause it to wait for tasks forever.
A better approach is to use the ExecutorService's builtin queue to queue tasks.
ExecutorService executor = Executors.newFixedThreadPool(3);
final int taskId = 1;
executor.submit(new Runnable() {
#Override
public void run() {
doSomething(taskId);
}
});
executor.shutdown();
This does the same thing, but is much simpler IMHO.

it's because you're creating a gazillion instances of MyHandler and inserting them in the internal queue of the executor.
That infinite for loop is quite mean.

Run 100 threads in parallel and run missing threads if some previous are finished

For example I need to always run 100 threads to do some action.
I have class which called ThreadsWorker which looks for threads count and runs missing threads if some previous are finished.
So, this is the table which describes situation:
1 second: 100 threads
2 second: 92 threads (ThreadsWorker generates new 8 threads)
3 second: 100 theads
4 second: 72 threads (ThreadsWorker generates 28 threads)
And so on.
My threads are anonymous calls (just new Thread(new Runnable(...)).start()) because I don't know how to correctly save them to Threads[] array because, while ThreadsWorker will save threads[i] = new Threads(), some threads may be finished and then there will be some collision with array indexes.
Because of anonymous calls I use threadsCount variable now and increment it in threads body beginning and decrements in threads body end (using synchronized). Okay, it works correctly and my single way is to use while() loop which checks if threadsCount == 0 when the progress is complete.
I think that this is C-style but not Java-way :) So, can you help me to do it in Java-way?

If your goal is simply to have 100 threads actively processing, I suggest looking at Java thread pools (and Executors more generally).
I'm unclear as to whether you want to keep all 100 threads going or wait for them all to finish. Your question references both (ThreadsWorker spawning 28 new threads, threadsCount==0) and they seem contradictory.

Put all the threads into an array or collection.
Then loop through the collection calling Thread.join() on each. When this loop completes, all threads are done.
ArrayList threads = new ArrayList();
for (int i = 0; i < 5; i++) {
Thread t = new AweseomeThread();
t.start();
threads.add(t);
}
for (Thread t : threads) {
t.join();
}
You'll need some exception handling too (such as InterruptedException). But, I'll leave that as an exercise for the reader... :)

http://download.oracle.com/javase/6/docs/api/java/util/concurrent/CountDownLatch.html
You can try class CountDownLatch jdk api
private CountDownLatch latch;
private static class SimpleThread extends Thread {
public void run() {
latch.countDown();
}
}
public static void main(String[] args) {
int threadcount = 10;
latch = new CountDownLatch(threadcount);
for (int i = 0; i < 10; i++) {
Thread t = new SimpleThread();
t.start();
}
// waiting threads all finished
latch.await();
}
Fetch the thread count from attibute latch of the Main class

I believe you are trying to have ThreadWorker submit new threads for all thread that have been completed.
I'd use a BlockingQueue that threads (Your Runnable(s)) add to when complete. ThreadWorker will wait until a thread completes and then will start a new thread.
public class YourRunnable implements Runnable {
private final BlockingQueue<YourRunnable> queue;
public YourRunnable(BlockingQueue<YourRunnable> queue){
this.queue = queue;
}
public void run{
// Your Code...
// Finished Processing
queue.add(this);
}
}
public class ThreadWorkder implements Runnable {
private final BlockingQueue<YourRunnable> queue;
ThreadWorker(BlockingQueue<YourRunnable> queue){
this.queue = queue;
}
public void run{
while(queue.take()){
(new Thread(new YourRunnable(queue))).start();
}
}
// general main method
public static void main(String [] args){
BlockingQueue<YourRunnable> queue = new LinkedBlockingQueue<YourRunnable>();
ThreadWorker worker = new ThreadWorker(queue);
Thread(worker).start();
for (int i = 0; i < 100; i++){
(new Thread(new YourRunnable(queue))).start();
}
}
}

Use a collection instead of an array. As the threads are completed, have them remove themselves from the array. Something like this:
public class Foo {
Vector<Thread> threads = new Vector<Thread>(); //Vector is threadsafe
public ensureThreadCount(int count) {
while (threads.size() < count) {
Thread t = new AweseomeThread(threads);
threads.add(t);
t.start();
}
}
}
public class AwesomeThread {
Collection threads;
public AwesomeThread(Collection threads) {
this.threads = threads;
}
public void run() {
try {
// do stuff
} catch (Throwable t) {
} finally {
threads.remove(this);
}
}
}
Then, have your worker just call Foo.ensureThreadCount().

Java: Threads, how to make them all do something

I am trying to implement nodes talking to each other in Java. I am doing this by creating a new thread for every node that wants to talk to the server.
When the given number of nodes, i.e. that many threads have been created, have connected to the server I want each thread to execute their next bit of code after adding to the "sharedCounter".
I think I need to use 'locks' on the shared variable, and something like signalAll() or notifyAll() to get all the threads going, but I can't seem to make clear sense of exactly how this works or to implement it.
Any help explaining these Java concepts would be greatly appreciated :D
Below is roughly the structure of my code:
import java.net.*;
import java.io.*;
public class Node {
public static void main(String[] args) {
...
// Chooses server or client launchers depend on parameters.
...
}
}
class sharedResource {
private int sharedCounter;
public sharedResource(int i) {
sharedCounter = i;
}
public synchronized void incSharedCounter() {
sharedCounter--;
if (sharedCounter == 0)
// Get all threads to do something
}
}
class Server {
...
for (int i = 0; i < numberOfThreads; i++) {
new serverThread(serverSocket.accept()).start();
}
...
sharedResource threadCount = new sharedResource(numberOfThreads);
...
}
class serverThread extends Thread {
...
//some code
Server.threadCount.incSharedCounter();
// Some more code to run when sharedCounte == 0
...
}
class Client {
...
}

　　　　　// Get all threads to do something
Threads (or rather Runnables, which you should implement rather than extending Thread) have a run method that contains the code they are expected to execute.
Once you call Thread#start (which in turn calls Runnable#run), the thread will start doing exactly that.
Since you seem to be new to multi-threading in Java, I recommend that you read an introduction to the Concurrency Utility package, that has been introduced in Java5 to make it easier to implement concurrent operations.
Specifically what you seem to be looking for is a way to "pause" the operation until a condition is met (in your case a counter having reached zero). For this, you should look at a CountDownLatch.

Indeed, the subject is broad, but I'll try to explain the basics. More details can be read from various blogs and articles. One of which is the Java trail.
It is best to see each thread as being runners (physical persons) that run alongside each other in a race. Each runner may perform any task while running. For example, take a cup of water from a table at a given moment in the race. Physically, they cannot both drink from the same cup at once, but in the virtual world, it is possible (this is where the line is drawn).
For example, take again two runners; each of them has to run back and forth a track, and push a button (shared by the runners) at each end for 1'000'000 times, the button is simply incrementing a counter by one each time. When they completed their run, what would be the value of the counter? In the physical world, it would be 2'000'000 because the runners cannot push the button at the same time, they would wait for the first one to leave first... that is unless they fight over it... Well, this is exactly what two threads would do. Consider this code :
public class ThreadTest extends Thread {
static public final int TOTAL_INC = 1000000;
static public int counter = 0;
#Override
public void run() {
for (int i=0; i<TOTAL_INC; i++) {
counter++;
}
System.out.println("Thread stopped incrementing counter " + TOTAL_INC + " times");
}
public static void main(String[] args) throws InterruptedException {
Thread t1 = new ThreadTest();
Thread t2 = new ThreadTest();
t1.start();
t2.start();
t1.join(); // wait for each thread to stop on their own...
t2.join(); //
System.out.println("Final counter is : " + counter + " which should be equal to " + TOTAL_INC * 2);
}
}
An output could be something like
Thread stopped incrementing counter 1000000 times
Thread stopped incrementing counter 1000000 times
Final counter is : 1143470 which should be equal to 2000000
Once in a while, the two thread would just increment the same value twice; this is called a race condition.
Synchronizing the run method will not work, and you'd have to use some locking mechanism to prevent this from happening. Consider the following changes in the run method :
static private Object lock = new Object();
#Override
public void run() {
for (int i=0; i<TOTAL_INC; i++) {
synchronized(lock) {
counter++;
}
}
System.out.println("Thread stopped incrementing counter " + TOTAL_INC + " times");
}
Now the expected output is
...
Final counter is : 2000000 which should be equal to 2000000
We have synchronized our counter with a shared object. This is like putting a queue line before only one runner can access the button at once.
NOTE : this locking mechanism is called a mutex. If a resource can be accessed by n threads at once, you might consider using a semaphore.
Multithreading is also associated with deadlocking. A deadlock is when two threads mutually waits for the other to free some synchronized resource to continue. For example :
Thread 1 starts
Thread 2 starts
Thread 1 acquire synchronized object1
Thread 2 acquire synchronized object2
Thread 2 needs to acquire object2 for continuing (locked by Thread 1)
Thread 1 needs to acquire object1 for continuing (locked by Thread 2)
Program hangs in deadlock
While there are many ways to prevent this from happening (it depends on what your threads are doing, and how they are implemented...) You should read about that particularly.
NOTE : the methods wait, notify and notifyAll can only be called when an object is synchronized. For example :
static public final int TOTAL_INC = 10;
static private int counter = 0;
static private Object lock = new Object();
static class Thread1 extends Thread {
#Override
public void run() {
synchronized (lock) {
for (int i=0; i<TOTAL_INC; i++) {
try {
lock.wait();
counter++;
lock.notify();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
static class Thread2 extends Thread {
#Override
public void run() {
synchronized (lock) {
for (int i=0; i<TOTAL_INC; i++) {
try {
lock.notify();
counter--;
lock.wait();
} catch (InterruptedException e) {
/* ignored */
}
}
}
}
}
Notice that both threads are running their for...loop blocks within the synchronized block. (The result of counter == 0 when both threads end.) This can be achieved because they "let each other" access the synchronized resource via the resource's wait and notify methods. Without using those two methods, both threads would simply run sequentially and not concurrently (or more precisely, alternately).
I hope this shed some light about threads (in Java).
** UPDATE **
Here is a little proof of concept of everything discussed above, using the CountDownLatch class suggested by Thilo earlier :
static class Server {
static public final int NODE_COUNT = 5;
private List<RunnableNode> nodes;
private CountDownLatch startSignal;
private Object lock = new Object();
public Server() {
nodes = Collections.synchronizedList(new ArrayList<RunnableNode>());
startSignal = new CountDownLatch(Server.NODE_COUNT);
}
public Object getLock() {
return lock;
}
public synchronized void connect(RunnableNode node) {
if (startSignal.getCount() > 0) {
startSignal.countDown();
nodes.add(node);
System.out.println("Received connection from node " + node.getId() + " (" + startSignal.getCount() + " remaining...)");
} else {
System.out.println("Client overflow! Refusing connection from node " + node.getId());
throw new IllegalStateException("Too many nodes connected");
}
}
public void shutdown() {
for (RunnableNode node : nodes) {
node.shutdown();
}
}
public void awaitAllConnections() {
try {
startSignal.await();
synchronized (lock) {
lock.notifyAll(); // awake all nodes
}
} catch (InterruptedException e) {
/* ignore */
shutdown(); // properly close any connected node now
}
}
}
static class RunnableNode implements Runnable {
private Server server;
private int id;
private boolean working;
public RunnableNode(int id, Server server) {
this.id = id;
this.server = server;
this.working = true;
}
public int getId() {
return id;
}
public void run() {
try {
Thread.sleep((long) (Math.random() * 5) * 1000); // just wait randomly from 0 to 5 seconds....
synchronized (server.getLock()) {
server.connect(this);
server.getLock().wait();
}
if (!Thread.currentThread().isAlive()) {
throw new InterruptedException();
} else {
System.out.println("Node " + id + " started successfully!");
while (working) {
Thread.yield();
}
}
} catch (InterruptedException e1) {
System.out.print("Ooop! ...");
} catch (IllegalStateException e2) {
System.out.print("Awwww! Too late! ...");
}
System.out.println("Node " + id + " is shutting down");
}
public void shutdown() {
working = false; // shutdown node here...
}
}
static public void main(String...args) throws InterruptedException {
Server server = new Server();
for (int i=0; i<Server.NODE_COUNT + 4; i++) { // create 4 more nodes than needed...
new Thread(new RunnableNode(i, server)).start();
}
server.awaitAllConnections();
System.out.println("All connection received! Server started!");
Thread.sleep(6000);
server.shutdown();
}

This is a broad topic. You might try reading through the official guides for concurrency (i.e. threading, more or less) in Java. This isn't something with cut-and-dried solutions; you have to design something.