I'm having troubles trying to stop my program that has multiple threads running, all threads running are trying to find the same solution but once one thread finds the solution all other threads are to stop.
In the main method I have created a thread group and add threads to it using a for loop and start them
ThreadGroup tg = new ThreadGroup("thread group");
Thread th;
for(int i = 0; i<4; i++){
th = new Thread(tg, new Runnable(), "Thread " + i)
th.start();
}
in the class that implements Runnable I am having troubles trying to figure out how to make it so that once one of the thread finds a solution all the threads will stop. What ends up happening is that either the other threads keep running and sometimes the threads will interupt each other and write over each other.
You have to interrupt those thread (and handle interruption in the runnable). I also not sure if you should use ThreadGroup - I remember seeing a Sonar warning about them.
You would perhaps better have to an ExecutorService and do that using a CountDownLatch (that's one way to do that):
ExecutorService es = Executors.newFixedThreadPool(100);
CountDownLatch cdl = new CountDownLatch(1);
for (int i = 0; i < 100; ++i) {
es.submit(() -> {
Thread.sleep(TimeUnit.SECONDS.toMillis(30)); // + exception handling
cdl.countDown();
});
}
cdl.await(); // or await(5, TimeUnit.MINUTES);
es.shutdownNow();
The trick is:
You create an ExecutorService with a pool of 100 threads.
You create a CoundDownLatch - a barrier - with a count of 1.
You submit your task which, when their job is done, invoke cdl.countDown(); reducing the counter from 1 to 0.
The parent thread wait for the CountDownLatch to reduce to 0 - you should probably use the second version (to block until 5 minutes for example).
If all Runnable fails, you won't have a result: either use a maximum await time, either you could add another CountDownLatch, this time with a count of 100 (the number of threads), countDown() in a try/finally, and in another thread, interrupt the one awaiting on the cdl. You could also do that in a loop:
CountDownLatch allCdl = new CountDownLatch(100);
for (;allCdl.getCount() != 0;) {
if (!cdl.await(60, TimeUnit.SECONDS)) {
if (allCdl.getCount() == 0) {
break;
}
}
}
However, the javadoc of getCount() mention that This method is typically used for debugging and testing purposes. (see CyclicBarrier). Not sure if this is the correct usage.
Upon realizing that a solution has been found, the victorious thread should signal the parent – which then signals all other children to stop, or simply kills them.
ThreadGroup tg = new ThreadGroup("thread group");
CountDownLatch latch = new CountDownLatch(1);
AtomicInteger result = new AtomicInteger();
Random random = new Random();
for (int i = 0; i < 4; i++) {
Thread th = new Thread(tg, () -> {
try {
Thread.sleep(random.nextInt(10000));
result.set(42);
latch.countDown();
System.out.println(Thread.currentThread().getName() + " completed task first");
} catch (InterruptedException e) {
System.out.println(Thread.currentThread().getName() + " was interrupted before it could finish the task");
}
}, "Thread " + i);
th.start();
}
while (latch.getCount() > 0) {
try {
latch.await();
} catch (InterruptedException ignored) {
}
}
tg.interrupt();
System.out.println("The result is " + result.get());
This example shows how to wait until a thread finishes.
You need to make sure your action is interruptible. Thread.sleep as shown in this example is interrubtible by default. See oracle docs for more info.
Also note that it is impossible to guarantee that all other threads will be interrupted before they complete. If you need to make sure to handle only one result, synchronize the access to your result variable and discard any changes beyond the first.
The routine myProcessToRun() needs to be executed 100 times but there needs to be about a second delay between each execution.
The following FOR Loop is used in conjunction with the ScheduledThreadPoolExecutor object.
for (int n=0; n<100; n++)
{
final ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(1);
executor.schedule(new Runnable() {
#Override
public void run() {
myProcessToRun();
}
}, (n+2), TimeUnit.SECONDS);
}
This actually works fine but the threads still remain.
Using JVisualVM the number of Threads increases by 100 threads when the routine is executed. When the routine finishes, the 100 threads still remain.
Clicking the "Perform GC" button doesn't clean them up so Java still believe they should exist.
How do these threads get cleaned up using an example like above?
---Edited---
I noticed the ScheduledThreadPoolExecutor was being instantiated within the Loop which was a terrible idea. After moving it outside the LOOP the threads created weren't so bad.
After attempting to implement the solution there was unexpected behavior.
final ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(10);
for (int n=0; n<100; n++)
{
//final ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(2);
executor.schedule(new Runnable() {
#Override
public void run() {
doAddNewCondSet();
}
}, (n+2), TimeUnit.SECONDS);
}
try
{
executor.shutdown();
if (!executor.awaitTermination(400, TimeUnit.SECONDS))
executor.shutdownNow();
} catch (InterruptedException e1)
{
e1.printStackTrace();
}
With the modified code, it would immediate stop all the processes with the shutdown and nothing was executed.
With the executor.shutdown(); commented out and just using the awaitTermination(), the program just hung and after a few minutes, all the processes kicked off at the same time without delay which resulted in errors.
I suspect my implementation was wrong.
There are a number of ways that you can do this. You can view some of them here:
https://www.baeldung.com/java-executor-wait-for-threads
My personal favorite is the CountDownLatch:
Next, let’s look at another approach to solving this problem – using a
CountDownLatch to signal the completion of a task.
We can initialize it with a value that represents the number of times
it can be decremented before all threads, that have called the await()
method, are notified.
For example, if we need the current thread to wait for another N
threads to finish their execution, we can initialize the latch using
N:
ExecutorService WORKER_THREAD_POOL
= Executors.newFixedThreadPool(10);
CountDownLatch latch = new CountDownLatch(2);
for (int i = 0; i < 2; i++) {
WORKER_THREAD_POOL.submit(() -> {
try {
// ...
latch.countDown();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
});
}
// wait for the latch to be decremented by the two remaining threads
latch.await();
Imagine I have following code:
final ExecutorService threadPool = Executors.newFixedThreadPool(
NUMBER_OF_WORKERS);
for (int i=0; i < NUMBER_OF_WORKERS; i++)
{
final Worker worker = new BirthWorker(...);
threadPool.execute(worker);
}
Now I need a piece of code, which waits, until all workers have completed their work.
Options I'm aware of:
while (!threadPool.isTerminated()) {}
Modify the code like that:
final List futures = new ArrayList(NUMBER_OF_WORKERS);
final ExecutorService threadPool = Executors.newFixedThreadPool(NUMBER_OF_WORKERS);
for (int i=0; i < NUMBER_OF_WORKERS; i++)
{
final Worker worker = new Worker(...);
futures.add(threadPool.submit(worker));
}
for (final Future future : futures) {
future.get();
}
// When we arrive here, all workers are guaranteed to have completed their work.
What is the best practice to wait for the completion of all workers?
I would suggest you use CountDownLatch (assuming this is one time activity) where in your constructor, you can specify how many threads you want to wait for and you share that instance accross the threads and you wait on all the threads to complete using await api (using timeout or complete blocking) and your thread's calling countdown api when they are done.
Another option would be, to call join method in thread to wait for their completion if you have access to each and every thread that you wish to complete.
I would use ThreadPoolExecutor.invokeAll(Collection<? extends Callable<T>> tasks)
API: Executes the given tasks, returning a list of Futures holding their status and results when all complete
CountDownLatch,as stated above, would do the work well, just keep in mind that you want to shut down the executur after your done:
final ExecutorService threadPool = Executors.newFixedThreadPool(
NUMBER_OF_WORKERS);
for (int i=0; i < NUMBER_OF_WORKERS; i++)
{
final Worker worker = new BirthWorker(...);
threadPool.execute(worker);
}
threadPool.shutdown();
unless you shut it down, threadPool.isTerminated will stay false, even when all the workers are done.
I am writing a thread pool utility in my multithreading program. i just need to validate the following methods are correct and are they return the right values for me. i am using a LinkedBlockingQueue with size of 1. and also I refer to the java doc and it always says 'method will return approximate' number phrase. so i doubt weather following conditions are correct.
public boolean isPoolIdle() {
return myThreadPool.getActiveCount() == 0;
}
public int getAcceptableTaskCount() {
//initially poolSize is 0 ( after pool executes something it started to change )
if (myThreadPool.getPoolSize() == 0) {
return myThreadPool.getCorePoolSize() - myThreadPool.getActiveCount();
}
return myThreadPool.getPoolSize() - myThreadPool.getActiveCount();
}
public boolean isPoolReadyToAcceptTasks(){
return myThreadPool.getActiveCount()<myThreadPool.getCorePoolSize();
}
Please let me know your thoughts and suggestions.
UPDATE
interesting thing was if pool returns me there are 3 threads available from the getAcceptableTaskCount method and when i pass 3 tasks to the pool some times one task got rejected and it is handle by RejectedExecutionHandler. some times pool will handle all the tasks i passed. i am wondering why pool is rejected the tasks since i am passing tasks according to the available thread count.
--------- implementation of the answer of gray---
class MyTask implements Runnable {
#Override
public void run() {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("exec");
}
}
#Test
public void testTPool(){
ExecutorService pool = Executors.newFixedThreadPool(5);
List<Future<MyTask>> list = new ArrayList<Future<MyTask>>();
for (int i = 0; i < 5; i++) {
MyTask t = new MyTask();
list.add(pool.submit(t, t));
}
for (int i = 0; i < list.size(); i++) {
Future<MyTask> t = list.get(i);
System.out.println("Result -"+t.isDone());
MyTask m = new MyTask();
list.add(pool.submit(m,m));
}
}
This will print Result -false in the console meaning that task is not complete.
From your comments:
i need to know that if pool is idle or pool can accept the tasks. if pool can accept, i need to know how much free threads in the pool. if it is 5 i will send 5 tasks to the pool to do the processing.
I don't think that you should be doing the pool accounting yourself. For your thread pool if you use Executors.newFixedThreadPool(5) then you can submit as many tasks as you want and it will only run them in 5 threads.
so i get the first most 5 tasks from the vector and assign them to the pool.ignore the other tasks in the vector since they may be update / remove from a separate cycle
Ok, I see. So you want to maximize parallelization while at the same time not pre-loading jobs? I would think that something like the following pseudo code would work:
int numThreads = 5;
ExecutorService threadPool = Executors.newFixedThreadPool(numThreads);
List<Future<MyJob>> futures = new ArrayList<Future<MyJob>>();
// submit the initial jobs
for (int i = 0; i < numThreads; i++) {
MyJob myJob = getNextBestJob();
futures.add(threadPool.submit(myJob, myJob));
}
// the list is growing so we use for i
for (int i = 0; i < futures.size(); i++) {
// wait for a job to finish
MyJob myJob = futures.get(i);
// process the job somehow
// get the next best job now that the previous one finished
MyJob nextJob = getNextBestJob();
if (nextJob != null) {
// submit the next job unless we are done
futures.add(threadPool.submit(myJob, myJob));
}
}
However, I don't quite understand how the thread count would change however. If you edit your question with some more details I can tweak my response.
I'm writing an application that has 5 threads that get some information from web simultaneously and fill 5 different fields in a buffer class.
I need to validate buffer data and store it in a database when all threads finished their job.
How can I do this (get alerted when all threads finished their work) ?
The approach I take is to use an ExecutorService to manage pools of threads.
ExecutorService es = Executors.newCachedThreadPool();
for(int i=0;i<5;i++)
es.execute(new Runnable() { /* your task */ });
es.shutdown();
boolean finished = es.awaitTermination(1, TimeUnit.MINUTES);
// all tasks have finished or the time has been reached.
You can join to the threads. The join blocks until the thread completes.
for (Thread thread : threads) {
thread.join();
}
Note that join throws an InterruptedException. You'll have to decide what to do if that happens (e.g. try to cancel the other threads to prevent unnecessary work being done).
Have a look at various solutions.
join() API has been introduced in early versions of Java. Some good alternatives are available with this concurrent package since the JDK 1.5 release.
ExecutorService#invokeAll()
Executes the given tasks, returning a list of Futures holding their status and results when everything is completed.
Refer to this related SE question for code example:
How to use invokeAll() to let all thread pool do their task?
CountDownLatch
A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
A CountDownLatch is initialized with a given count. The await methods block until the current count reaches zero due to invocations of the countDown() method, after which all waiting threads are released and any subsequent invocations of await return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using a CyclicBarrier.
Refer to this question for usage of CountDownLatch
How to wait for a thread that spawns it's own thread?
ForkJoinPool or newWorkStealingPool() in Executors
Iterate through all Future objects created after submitting to ExecutorService
Wait/block the Thread Main until some other threads complete their work.
As #Ravindra babu said it can be achieved in various ways, but showing with examples.
java.lang.Thread.join() Since:1.0
public static void joiningThreads() throws InterruptedException {
Thread t1 = new Thread( new LatchTask(1, null), "T1" );
Thread t2 = new Thread( new LatchTask(7, null), "T2" );
Thread t3 = new Thread( new LatchTask(5, null), "T3" );
Thread t4 = new Thread( new LatchTask(2, null), "T4" );
// Start all the threads
t1.start();
t2.start();
t3.start();
t4.start();
// Wait till all threads completes
t1.join();
t2.join();
t3.join();
t4.join();
}
java.util.concurrent.CountDownLatch Since:1.5
.countDown() « Decrements the count of the latch group.
.await() « The await methods block until the current count reaches zero.
If you created latchGroupCount = 4 then countDown() should be called 4 times to make count 0. So, that await() will release the blocking threads.
public static void latchThreads() throws InterruptedException {
int latchGroupCount = 4;
CountDownLatch latch = new CountDownLatch(latchGroupCount);
Thread t1 = new Thread( new LatchTask(1, latch), "T1" );
Thread t2 = new Thread( new LatchTask(7, latch), "T2" );
Thread t3 = new Thread( new LatchTask(5, latch), "T3" );
Thread t4 = new Thread( new LatchTask(2, latch), "T4" );
t1.start();
t2.start();
t3.start();
t4.start();
//latch.countDown();
latch.await(); // block until latchGroupCount is 0.
}
Example code of Threaded class LatchTask. To test the approach use joiningThreads();
and latchThreads(); from main method.
class LatchTask extends Thread {
CountDownLatch latch;
int iterations = 10;
public LatchTask(int iterations, CountDownLatch latch) {
this.iterations = iterations;
this.latch = latch;
}
#Override
public void run() {
String threadName = Thread.currentThread().getName();
System.out.println(threadName + " : Started Task...");
for (int i = 0; i < iterations; i++) {
System.out.println(threadName + " : " + i);
MainThread_Wait_TillWorkerThreadsComplete.sleep(1);
}
System.out.println(threadName + " : Completed Task");
// countDown() « Decrements the count of the latch group.
if(latch != null)
latch.countDown();
}
}
CyclicBarriers A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.CyclicBarriers are useful in programs involving a fixed sized party of threads that must occasionally wait for each other. The barrier is called cyclic because it can be re-used after the waiting threads are released.
CyclicBarrier barrier = new CyclicBarrier(3);
barrier.await();
For example refer this Concurrent_ParallelNotifyies class.
Executer framework: we can use ExecutorService to create a thread pool, and tracks the progress of the asynchronous tasks with Future.
submit(Runnable), submit(Callable) which return Future Object. By using future.get() function we can block the main thread till the working threads completes its work.
invokeAll(...) - returns a list of Future objects via which you can obtain the results of the executions of each Callable.
Find example of using Interfaces Runnable, Callable with Executor framework.
#See also
Find out thread is still alive?
Apart from Thread.join() suggested by others, java 5 introduced the executor framework. There you don't work with Thread objects. Instead, you submit your Callable or Runnable objects to an executor. There's a special executor that is meant to execute multiple tasks and return their results out of order. That's the ExecutorCompletionService:
ExecutorCompletionService executor;
for (..) {
executor.submit(Executors.callable(yourRunnable));
}
Then you can repeatedly call take() until there are no more Future<?> objects to return, which means all of them are completed.
Another thing that may be relevant, depending on your scenario is CyclicBarrier.
A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point. CyclicBarriers are useful in programs involving a fixed sized party of threads that must occasionally wait for each other. The barrier is called cyclic because it can be re-used after the waiting threads are released.
Another possibility is the CountDownLatch object, which is useful for simple situations : since you know in advance the number of threads, you initialize it with the relevant count, and pass the reference of the object to each thread.
Upon completion of its task, each thread calls CountDownLatch.countDown() which decrements the internal counter. The main thread, after starting all others, should do the CountDownLatch.await() blocking call. It will be released as soon as the internal counter has reached 0.
Pay attention that with this object, an InterruptedException can be thrown as well.
You do
for (Thread t : new Thread[] { th1, th2, th3, th4, th5 })
t.join()
After this for loop, you can be sure all threads have finished their jobs.
Store the Thread-objects into some collection (like a List or a Set), then loop through the collection once the threads are started and call join() on the Threads.
You can use Threadf#join method for this purpose.
Although not relevant to OP's problem, if you are interested in synchronization (more precisely, a rendez-vous) with exactly one thread, you may use an Exchanger
In my case, I needed to pause the parent thread until the child thread did something, e.g. completed its initialization. A CountDownLatch also works well.
I created a small helper method to wait for a few Threads to finish:
public static void waitForThreadsToFinish(Thread... threads) {
try {
for (Thread thread : threads) {
thread.join();
}
}
catch (InterruptedException e) {
e.printStackTrace();
}
}
An executor service can be used to manage multiple threads including status and completion. See http://programmingexamples.wikidot.com/executorservice
try this, will work.
Thread[] threads = new Thread[10];
List<Thread> allThreads = new ArrayList<Thread>();
for(Thread thread : threads){
if(null != thread){
if(thread.isAlive()){
allThreads.add(thread);
}
}
}
while(!allThreads.isEmpty()){
Iterator<Thread> ite = allThreads.iterator();
while(ite.hasNext()){
Thread thread = ite.next();
if(!thread.isAlive()){
ite.remove();
}
}
}
I had a similar problem and ended up using Java 8 parallelStream.
requestList.parallelStream().forEach(req -> makeRequest(req));
It's super simple and readable.
Behind the scenes it is using default JVM’s fork join pool which means that it will wait for all the threads to finish before continuing. For my case it was a neat solution, because it was the only parallelStream in my application. If you have more than one parallelStream running simultaneously, please read the link below.
More information about parallel streams here.
The existing answers said could join() each thread.
But there are several ways to get the thread array / list:
Add the Thread into a list on creation.
Use ThreadGroup to manage the threads.
Following code will use the ThreadGruop approach. It create a group first, then when create each thread specify the group in constructor, later could get the thread array via ThreadGroup.enumerate()
Code
SyncBlockLearn.java
import org.testng.Assert;
import org.testng.annotations.Test;
/**
* synchronized block - learn,
*
* #author eric
* #date Apr 20, 2015 1:37:11 PM
*/
public class SyncBlockLearn {
private static final int TD_COUNT = 5; // thread count
private static final int ROUND_PER_THREAD = 100; // round for each thread,
private static final long INC_DELAY = 10; // delay of each increase,
// sync block test,
#Test
public void syncBlockTest() throws InterruptedException {
Counter ct = new Counter();
ThreadGroup tg = new ThreadGroup("runner");
for (int i = 0; i < TD_COUNT; i++) {
new Thread(tg, ct, "t-" + i).start();
}
Thread[] tArr = new Thread[TD_COUNT];
tg.enumerate(tArr); // get threads,
// wait all runner to finish,
for (Thread t : tArr) {
t.join();
}
System.out.printf("\nfinal count: %d\n", ct.getCount());
Assert.assertEquals(ct.getCount(), TD_COUNT * ROUND_PER_THREAD);
}
static class Counter implements Runnable {
private final Object lkOn = new Object(); // the object to lock on,
private int count = 0;
#Override
public void run() {
System.out.printf("[%s] begin\n", Thread.currentThread().getName());
for (int i = 0; i < ROUND_PER_THREAD; i++) {
synchronized (lkOn) {
System.out.printf("[%s] [%d] inc to: %d\n", Thread.currentThread().getName(), i, ++count);
}
try {
Thread.sleep(INC_DELAY); // wait a while,
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.printf("[%s] end\n", Thread.currentThread().getName());
}
public int getCount() {
return count;
}
}
}
The main thread will wait for all threads in the group to finish.
I had similar situation , where i had to wait till all child threads complete its execution then only i could get the status result for each of them .. hence i needed to wait till all child thread completed.
below is my code where i did multi-threading using
public static void main(String[] args) {
List<RunnerPojo> testList = ExcelObject.getTestStepsList();//.parallelStream().collect(Collectors.toList());
int threadCount = ConfigFileReader.getInstance().readConfig().getParallelThreadCount();
System.out.println("Thread count is : ========= " + threadCount); // 5
ExecutorService threadExecutor = new DriverScript().threadExecutor(testList, threadCount);
boolean isProcessCompleted = waitUntilCondition(() -> threadExecutor.isTerminated()); // Here i used waitUntil condition
if (isProcessCompleted) {
testList.forEach(x -> {
System.out.println("Test Name: " + x.getTestCaseId());
System.out.println("Test Status : " + x.getStatus());
System.out.println("======= Test Steps ===== ");
x.getTestStepsList().forEach(y -> {
System.out.println("Step Name: " + y.getDescription());
System.out.println("Test caseId : " + y.getTestCaseId());
System.out.println("Step Status: " + y.getResult());
System.out.println("\n ============ ==========");
});
});
}
Below method is for distribution of list with parallel proccessing
// This method will split my list and run in a parallel process with mutliple threads
private ExecutorService threadExecutor(List<RunnerPojo> testList, int threadSize) {
ExecutorService exec = Executors.newFixedThreadPool(threadSize);
testList.forEach(tests -> {
exec.submit(() -> {
driverScript(tests);
});
});
exec.shutdown();
return exec;
}
This is my wait until method: here you can wait till your condition satisfies within do while loop . in my case i waited for some max timeout .
this will keep checking until your threadExecutor.isTerminated() is true with polling period of 5 sec.
static boolean waitUntilCondition(Supplier<Boolean> function) {
Double timer = 0.0;
Double maxTimeOut = 20.0;
boolean isFound;
do {
isFound = function.get();
if (isFound) {
break;
} else {
try {
Thread.sleep(5000); // Sleeping for 5 sec (main thread will sleep for 5 sec)
} catch (InterruptedException e) {
e.printStackTrace();
}
timer++;
System.out.println("Waiting for condition to be true .. waited .." + timer * 5 + " sec.");
}
} while (timer < maxTimeOut + 1.0);
return isFound;
}
Use this in your main thread: while(!executor.isTerminated());
Put this line of code after starting all the threads from executor service. This will only start the main thread after all the threads started by executors are finished. Make sure to call executor.shutdown(); before the above loop.