Develop Reference

Custom thread pool implementation in java

For leaning purpose i am trying to implement my own thread pool in java. Below is what i have implemented. I have few questions about this implementation:
Although i am using BlockingQueue like built in java Executors expect us to provide Runnable objects (through execute method). But in my case i feel like i can create any object instead of Runnable. So then why does Java executors expect Runnable, i tried looking into the source code but could not figure it out yet.
Is there anything else wrong with this primitive implementation ?
Please find the code.
public class CustomThreadPool {
private final BlockingQueue<Runnable> blockingQueue;
private final Worker[] workers;
public CustomThreadPool(final int numOfThreads) {
blockingQueue = new LinkedBlockingQueue<>();
workers = new Worker[numOfThreads];
for (int i = 0; i < numOfThreads; i++) {
workers[i] = new Worker();
workers[i].start();
}
}
public void execute(final Runnable task) {
blockingQueue.add(task);
}
public void shutdownImmediately() {
for (int i = 0; i < workers.length; i++) {
workers[i].shutdownSignal = true;
workers[i] = null;
}
}
private class Worker extends Thread {
private Runnable taskToPerform = null;
boolean shutdownSignal = false;
#Override
public void run() {
while(true && !shutdownSignal) {
taskToPerform = blockingQueue.poll();
if (taskToPerform != null) {
taskToPerform.run();
}
if(shutdownSignal) {
break;
}
}
}
}
public static void main(String[] args) throws Exception {
final CustomThreadPool threadPool = new CustomThreadPool(5);
for (int i = 0; i < 20; i++) {
threadPool.execute(() -> System.out.println(Thread.currentThread().getName()));
}
Thread.sleep(1*1000);
threadPool.shutdownImmediately();
}
}

Executor expects Runnable or Callable because it will call run or call method of these interfaces when it is running the tasks you submitted.
In Your implementation You don't use blocking aspects of BlockingQueue. Your thread pool threads will spin constantly(take cpu time) on Your while(true && !shutdownSignal) loop when there is no task exists in Your queue. Because poll() method is not blocking. And this not something You would want when implementing a thread pool.
You should use one of the blocking methods instead of poll().
You can use poll(long timeout,TimeUnit unit) method which takes time out parameter. In this case if You call Your shutdownImmediately method while any thread pool thread waiting on this call. They will wait for the time out duration and poll will return null to them They will see shutdownSignal is being set and get out of the loop. You can also call interrupt method If You don't want them to wait for the timeout.
// in run method
try {
taskToPerform = blockingQueue.poll(5, TimeUnit.SECONDS);
} catch (InterruptedException e) {
break; // this thread is interrupted. Time to leave this loop
}
// in shutdownImmediately method
workers[i].shutdownSignal = true;
// If You don't call interrupt, Threads will wait at max 5 sec for this case.
workers[i].interrupt();
workers[i] = null;
Or You can use take() method which blocks when there is nothing on the queue. But in this case You have to interrupt the threads that might be waiting on the take method call when You call Your shutdownImmediately method. Otherwise they will stuck at the take() call.
// in run method
try {
taskToPerform = blockingQueue.take();
} catch (InterruptedException e) {
break; // this thread is interrupted. Time to leave this loop
}
// in shutdownImmediately method
workers[i].shutdownSignal = true;
workers[i].interrupt(); // this is crucial for this case
workers[i] = null;

Parallel Processing or Multithreading inside a for loop Java 7

I need to achieve the following functionality using multithreading/parallel processing using Java 7
List<String> permissionsList = new ArrayList<String>();
for (Integer site : getSites()) {
String perm = authenticationService.getUserPermissionsForSite(site);
permissionsList.add(perm);
}
The call getUserPermissionsForSite is a web service call. After the end of the for loop, the permissionsList should be fully populated with all the values.
What would be the best way to achieve it?

This is how it would typically be done in Java 7. A CountDownLatch is used for waiting till all the sites are called.
int numOfThreads = 5;
List<String> permissionsList = Collections.synchronizedList(new ArrayList<>());
List<Integer> sites = getSites();
CountDownLatch countDownLatch = new CountDownLatch(sites.size());
ExecutorService es = Executors.newFixedThreadPool(numOfThreads);
for (Integer site : sites) {
es.submit(new Runnable() {
#Override
public void run() {
try {
String perm = authenticationService.getUserPermissionsForSite(site);
permissionsList.add(perm);
} finally {
countDownLatch.countDown();
}
}
});
}
countDownLatch.await();

You can use ExecutorService.invokeAll() to schedule all your requests at once. You can specify a timeout to avoid waiting indefinitely. Upon return, all requests will have completed, faulted, or timed out. Iterate over the tasks and fetch the results, and handle timeouts and partial failures however you like. Note that if you add a timeout, you may want to scale it by the number of requests.
You'll need to decide what degree of parallelism you want. My example will create up to as many threads as there are processors available to run them, but the thread count won't exceed the number of requests. That's pretty aggressive; you could probably get away with fewer threads.
List<String> getPermissions(final List<Integer> sites) throws InterruptedException {
if (sites.isEmpty()) {
return Collections.emptyList();
}
final List<String> permissions = new ArrayList<>();
final List<Callable<String>> tasks = new ArrayList<>();
final int threadCount = Math.min(
Runtime.getRuntime().availableProcessors(),
sites.size()
);
final ExecutorService executor = Executors.newFixedThreadPool(threadCount);
try {
for (final Integer site : sites) {
tasks.add(
new Callable<String>() {
#Override
public String call() throws Exception {
return authenticationService.getUserPermissionsForSite(site);
}
}
);
}
for (final Future<String> f : executor.invokeAll(tasks/*, timeout? */)) {
try {
// If you added a timeout, you can check f.isCancelled().
// If timed out, get() will throw a CancellationException.
permissions.add(f.get());
}
catch (final ExecutionException e) {
// This request failed. Handle it however you like.
}
}
}
finally {
executor.shutdown();
}
return permissions;
}
Like most Java operations that perform a wait, invokeAll may throw an InterruptedException. It's up to you whether you want to catch it or let it propagate.

Servlet "has started a thread but failed to stop it" - memory leak in Tomcat

Apache Tomcat says many times:
The web application [/MyServlet] appears to have started a thread named [pool-61-thread-2] but has failed to stop it. This is very likely to create a memory leak.
Is this dangerous? The servlet should be able to handle 10.000requests/day.
How to close the threads when they have finished?
class Worker {
private final CountDownLatch startSignal;
private final CountDownLatch doneSignal;
private final int threadNumber;
Worker(
CountDownLatch startSignal,
CountDownLatch doneSignal,
int threadNumber
){
this.startSignal = startSignal;
this.doneSignal = doneSignal;
this.threadNumber = threadNumber;
}
public String[][] getSomeStrArrArr() {
String[][] isRs = new String[8][20];
String[][] inRs = new String[8][20];
String[][] iwRs = new String[8][20];
try {
startSignal.await();
if (threadNumber == 1) {
// get String[][] result for thread number 1
isRs = getIS(erg1, erg2, request);
}
if (threadNumber == 2) {
// get String[][] result for thread number 2
inRs = getIN(search_plz, request);
}
if (threadNumber == 3) {
// get String[][] result for thread number 3
iwRs = getIW(erg1, erg2, request);
}
doneSignal.countDown();
} catch (InterruptedException ex) {
System.out.println(
"Thread number "+threadNumber+" has been interrupted."
);
}
if (threadNumber == 1) {
return isRs;
}
if (threadNumber == 2) {
return inRs;
}
if (threadNumber == 3) {
return iwRs;
}
return null;
}
public Callable<String[][]> getSomeCallableStrArrArr(){
return new Callable<String[][]>() {
public String[][] call() throws Exception {
return getSomeStrArrArr();
}
};
}
}
ExecutorService pool = Executors.newFixedThreadPool(3);
Set<Future<String[][]>> set = new HashSet<Future<String[][]>>();
CountDownLatch startSignal = new CountDownLatch(1);
CountDownLatch doneSignal = new CountDownLatch(3);
for (int i=1;i<=3;i++) {
Worker worker = new Worker(startSignal,doneSignal,i);
Callable<String[][]> callable =
worker.getSomeCallableStrArrArr();
Future<String[][]> future = pool.submit(callable);
set.add(future);
}
startSignal.countDown();
try {
doneSignal.await();

Yes, it's a problem. If your code starts non-daemon threads then those threads will continue working until they exit their run method. Even if everything else finishes, the old JVM will hang around while those threads continue on. If you start up a new instance then you can have a situation where the old threads are still working alongside the ones created by the new instance.
The tasks need to be designed so that they will be responsive to interruption (as opposed to eating the exception and going on, which is what your example shows). That means checking the interrupted flag on the current thread, and catching InterruptedException in a helpful way that allows the task to break its work off and also resets the interrupted flag if needed. ExecutorService implementations have a shutdownNow method that will interrupt the current tasks.
Here's an example of how to stop a thread using interruption.
Make sure the executor gets shut down, you can handle this in a ServletContextListener.

Parallel execution and termination of multiple threads

I have a simple application in which I create 3 threads inside a class to ping 3 different websites and note the time taken to do so.
I wish to enhance it by seeing which thread out of the 3 executes successfully first and terminating the other two .
Which class of the JDK would be helpful in doing so ? and how ?
Sample code to ping websites :
public static boolean pingUrl(final String address) {
try {
final URL url = new URL("http://" + address);
final HttpURLConnection urlConn = (HttpURLConnection) url.openConnection();
urlConn.setConnectTimeout(1000 * 10); // mTimeout is in seconds
final long startTime = System.currentTimeMillis();
urlConn.connect();
final long endTime = System.currentTimeMillis();
if (urlConn.getResponseCode() == HttpURLConnection.HTTP_OK) {
System.out.println("Time (ms) : " + (endTime - startTime));
System.out.println("Ping to "+address +" was success");
return true;
}
} catch (final MalformedURLException e1) {
e1.printStackTrace();
} catch (final IOException e) {
e.printStackTrace();
}
return false;
}

I wish to enhance it by seeing which thread out of the 3 executes successfully first and terminating the other two .
I would use an ExecutorService combined with a ExecutorCompletionService. Then, when the first Future is returned from the completion service when the first task completes, you would call shutdownNow() on the ExecutorService.
The javadocs for ExecutorCompletionService are pretty good and show how to use it.
// maybe you want 10 threads working on your tasks
ExecutorService threadPool = Executors.newFixedThreadPool(10);
CompletionService<Result> ecs
= new ExecutorCompletionService<Result>(threadPool);
for (Callable<Result> task : tasks) {
// submit your tasks to the completion service, they run in the thread-pool
ecs.submit(task);
}
// once you get one result
Future<Result> future = ecs.take();
// kill the rest of the tasks
threadPool.shutdownNow();
Result result = future.get();
// probably will need to close the thread connections, see below
// maybe call threadPool.awaitShutdown(...) here to wait for the others to die
The only problem with this mechanism is that this will only interrupt the threads. In your case they are going to be stuck in urlConn.connect(); which is not interruptible. Once the ecs.take() returns, you are going to have to run back over your tasks and call disconnect() on the the HttpURLConnection that are still in progress. Even then I'm not sure if it will stop a connection that is currently underway. If that doesn't work then you may need to switch to using Apache HttpClient or some other class that you can close to stop the threads from waiting longer.
for (Callable<Result> task : tasks) {
// you'll need to do something like this
task.closeConnection();
}
In your case, your task might look something like:
public class MyPingTask implements Callable<Boolean> {
private String address;
public MyPingTask(String address) {
this.address = address;
}
public Boolean call() throws Exception {
// obviously the pingUrl code could go right here
return pingUrl(address);
}
}
Here is the Java tutorial on ExecutorService and related classes.

I suppose BlockingQueue may be useful. The main idea that spawned thread writes some value to BlockingQueue when finished and gracefully closes on InterruptedException
For example:
public void runPing(List<String> urls) {
Collection<Thread> runningThreads = new ArrayList<>(urls.size());
final BlockingQueue<Integer> queue = new ArrayBlockingQueue<>(urls.size());
for (int i = 0; i < 3; i++) {
final String url = urls.get(i);
Thread t = new Thread(new Runnable() {
public void run() {
pingUrl(url);
queue.add(1);
}
});
runningThreads.add(t);
}
try {
queue.poll(1, TimeUnit.HOURS);
interruptChilds(runningThreads);
} catch (Exception e) {
interruptChilds(runningThreads);
}
}
private void interruptChilds(Collection<Thread> runningThreads) {
for (Thread t : runningThreads) {
t.interrupt();
}
}
Please note that in there are no handling of InterruptedException. It should be added in your method

How to wait for all threads to finish, using ExecutorService?

I need to execute some amount of tasks 4 at a time, something like this:
ExecutorService taskExecutor = Executors.newFixedThreadPool(4);
while(...) {
taskExecutor.execute(new MyTask());
}
//...wait for completion somehow
How can I get notified once all of them are complete? For now I can't think about anything better than setting some global task counter and decrease it at the end of every task, then monitor in infinite loop this counter to become 0; or get a list of Futures and in infinite loop monitor isDone for all of them. What are better solutions not involving infinite loops?
Thanks.

Basically on an ExecutorService you call shutdown() and then awaitTermination():
ExecutorService taskExecutor = Executors.newFixedThreadPool(4);
while(...) {
taskExecutor.execute(new MyTask());
}
taskExecutor.shutdown();
try {
taskExecutor.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
} catch (InterruptedException e) {
...
}

Use a CountDownLatch:
CountDownLatch latch = new CountDownLatch(totalNumberOfTasks);
ExecutorService taskExecutor = Executors.newFixedThreadPool(4);
while(...) {
taskExecutor.execute(new MyTask());
}
try {
latch.await();
} catch (InterruptedException E) {
// handle
}
and within your task (enclose in try / finally)
latch.countDown();

ExecutorService.invokeAll() does it for you.
ExecutorService taskExecutor = Executors.newFixedThreadPool(4);
List<Callable<?>> tasks; // your tasks
// invokeAll() returns when all tasks are complete
List<Future<?>> futures = taskExecutor.invokeAll(tasks);

You can use Lists of Futures, as well:
List<Future> futures = new ArrayList<Future>();
// now add to it:
futures.add(executorInstance.submit(new Callable<Void>() {
public Void call() throws IOException {
// do something
return null;
}
}));
then when you want to join on all of them, its essentially the equivalent of joining on each, (with the added benefit that it re-raises exceptions from child threads to the main):
for(Future f: this.futures) { f.get(); }
Basically the trick is to call .get() on each Future one at a time, instead of infinite looping calling isDone() on (all or each). So you're guaranteed to "move on" through and past this block as soon as the last thread finishes. The caveat is that since the .get() call re-raises exceptions, if one of the threads dies, you would raise from this possibly before the other threads have finished to completion [to avoid this, you could add a catch ExecutionException around the get call]. The other caveat is it keeps a reference to all threads so if they have thread local variables they won't get collected till after you get past this block (though you might be able to get around this, if it became a problem, by removing Future's off the ArrayList). If you wanted to know which Future "finishes first" you could use some something like https://stackoverflow.com/a/31885029/32453

In Java8 you can do it with CompletableFuture:
ExecutorService es = Executors.newFixedThreadPool(4);
List<Runnable> tasks = getTasks();
CompletableFuture<?>[] futures = tasks.stream()
.map(task -> CompletableFuture.runAsync(task, es))
.toArray(CompletableFuture[]::new);
CompletableFuture.allOf(futures).join();
es.shutdown();

Just my two cents.
To overcome the requirement of CountDownLatch to know the number of tasks beforehand, you could do it the old fashion way by using a simple Semaphore.
ExecutorService taskExecutor = Executors.newFixedThreadPool(4);
int numberOfTasks=0;
Semaphore s=new Semaphore(0);
while(...) {
taskExecutor.execute(new MyTask());
numberOfTasks++;
}
try {
s.aquire(numberOfTasks);
...
In your task just call s.release() as you would latch.countDown();

A bit late to the game but for the sake of completion...
Instead of 'waiting' for all tasks to finish, you can think in terms of the Hollywood principle, "don't call me, I'll call you" - when I'm finished.
I think the resulting code is more elegant...
Guava offers some interesting tools to accomplish this.
An example:
Wrap an ExecutorService into a ListeningExecutorService:
ListeningExecutorService service = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(10));
Submit a collection of callables for execution ::
for (Callable<Integer> callable : callables) {
ListenableFuture<Integer> lf = service.submit(callable);
// listenableFutures is a collection
listenableFutures.add(lf)
});
Now the essential part:
ListenableFuture<List<Integer>> lf = Futures.successfulAsList(listenableFutures);
Attach a callback to the ListenableFuture, that you can use to be notified when all futures complete:
Futures.addCallback(lf, new FutureCallback<List<Integer>> () {
#Override
public void onSuccess(List<Integer> result) {
// do something with all the results
}
#Override
public void onFailure(Throwable t) {
// log failure
}
});
This also offers the advantage that you can collect all the results in one place once the processing is finished...
More information here

The CyclicBarrier class in Java 5 and later is designed for this sort of thing.

here is two options , just bit confuse which one is best to go.
Option 1:
ExecutorService es = Executors.newFixedThreadPool(4);
List<Runnable> tasks = getTasks();
CompletableFuture<?>[] futures = tasks.stream()
.map(task -> CompletableFuture.runAsync(task, es))
.toArray(CompletableFuture[]::new);
CompletableFuture.allOf(futures).join();
es.shutdown();
Option 2:
ExecutorService es = Executors.newFixedThreadPool(4);
List< Future<?>> futures = new ArrayList<>();
for(Runnable task : taskList) {
futures.add(es.submit(task));
}
for(Future<?> future : futures) {
try {
future.get();
}catch(Exception e){
// do logging and nothing else
}
}
es.shutdown();
Here putting future.get(); in try catch is good idea right?

Follow one of below approaches.
Iterate through all Future tasks, returned from submit on ExecutorService and check the status with blocking call get() on Future object as suggested by Kiran
Use invokeAll() on ExecutorService
CountDownLatch
ForkJoinPool or Executors.html#newWorkStealingPool
Use shutdown, awaitTermination, shutdownNow APIs of ThreadPoolExecutor in proper sequence
Related SE questions:
How is CountDownLatch used in Java Multithreading?
How to properly shutdown java ExecutorService

You could wrap your tasks in another runnable, that will send notifications:
taskExecutor.execute(new Runnable() {
public void run() {
taskStartedNotification();
new MyTask().run();
taskFinishedNotification();
}
});

Clean way with ExecutorService
List<Future<Void>> results = null;
try {
List<Callable<Void>> tasks = new ArrayList<>();
ExecutorService executorService = Executors.newFixedThreadPool(4);
results = executorService.invokeAll(tasks);
} catch (InterruptedException ex) {
...
} catch (Exception ex) {
...
}

I've just written a sample program that solves your problem. There was no concise implementation given, so I'll add one. While you can use executor.shutdown() and executor.awaitTermination(), it is not the best practice as the time taken by different threads would be unpredictable.
ExecutorService es = Executors.newCachedThreadPool();
List<Callable<Integer>> tasks = new ArrayList<>();
for (int j = 1; j <= 10; j++) {
tasks.add(new Callable<Integer>() {
#Override
public Integer call() throws Exception {
int sum = 0;
System.out.println("Starting Thread "
+ Thread.currentThread().getId());
for (int i = 0; i < 1000000; i++) {
sum += i;
}
System.out.println("Stopping Thread "
+ Thread.currentThread().getId());
return sum;
}
});
}
try {
List<Future<Integer>> futures = es.invokeAll(tasks);
int flag = 0;
for (Future<Integer> f : futures) {
Integer res = f.get();
System.out.println("Sum: " + res);
if (!f.isDone())
flag = 1;
}
if (flag == 0)
System.out.println("SUCCESS");
else
System.out.println("FAILED");
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}

Just to provide more alternatives here different to use latch/barriers.
You can also get the partial results until all of them finish using CompletionService.
From Java Concurrency in practice:
"If you have a batch of computations to submit to an Executor and you want to retrieve their results as they become
available, you could retain the Future associated with each task and repeatedly poll for completion by calling get with a
timeout of zero. This is possible, but tedious. Fortunately there is a better way: a completion service."
Here the implementation
public class TaskSubmiter {
private final ExecutorService executor;
TaskSubmiter(ExecutorService executor) { this.executor = executor; }
void doSomethingLarge(AnySourceClass source) {
final List<InterestedResult> info = doPartialAsyncProcess(source);
CompletionService<PartialResult> completionService = new ExecutorCompletionService<PartialResult>(executor);
for (final InterestedResult interestedResultItem : info)
completionService.submit(new Callable<PartialResult>() {
public PartialResult call() {
return InterestedResult.doAnOperationToGetPartialResult();
}
});
try {
for (int t = 0, n = info.size(); t < n; t++) {
Future<PartialResult> f = completionService.take();
PartialResult PartialResult = f.get();
processThisSegment(PartialResult);
}
}
catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
catch (ExecutionException e) {
throw somethinghrowable(e.getCause());
}
}
}

This is my solution, based in "AdamSkywalker" tip, and it works
package frss.main;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class TestHilos {
void procesar() {
ExecutorService es = Executors.newFixedThreadPool(4);
List<Runnable> tasks = getTasks();
CompletableFuture<?>[] futures = tasks.stream().map(task -> CompletableFuture.runAsync(task, es)).toArray(CompletableFuture[]::new);
CompletableFuture.allOf(futures).join();
es.shutdown();
System.out.println("FIN DEL PROCESO DE HILOS");
}
private List<Runnable> getTasks() {
List<Runnable> tasks = new ArrayList<Runnable>();
Hilo01 task1 = new Hilo01();
tasks.add(task1);
Hilo02 task2 = new Hilo02();
tasks.add(task2);
return tasks;
}
private class Hilo01 extends Thread {
#Override
public void run() {
System.out.println("HILO 1");
}
}
private class Hilo02 extends Thread {
#Override
public void run() {
try {
sleep(2000);
}
catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("HILO 2");
}
}
public static void main(String[] args) {
TestHilos test = new TestHilos();
test.procesar();
}
}

You could use this code:
public class MyTask implements Runnable {
private CountDownLatch countDownLatch;
public MyTask(CountDownLatch countDownLatch {
this.countDownLatch = countDownLatch;
}
#Override
public void run() {
try {
//Do somethings
//
this.countDownLatch.countDown();//important
} catch (InterruptedException ex) {
Thread.currentThread().interrupt();
}
}
}
CountDownLatch countDownLatch = new CountDownLatch(NUMBER_OF_TASKS);
ExecutorService taskExecutor = Executors.newFixedThreadPool(4);
for (int i = 0; i < NUMBER_OF_TASKS; i++){
taskExecutor.execute(new MyTask(countDownLatch));
}
countDownLatch.await();
System.out.println("Finish tasks");

So I post my answer from linked question here, incase someone want a simpler way to do this
ExecutorService executor = Executors.newFixedThreadPool(10);
CompletableFuture[] futures = new CompletableFuture[10];
int i = 0;
while (...) {
futures[i++] = CompletableFuture.runAsync(runner, executor);
}
CompletableFuture.allOf(futures).join(); // THis will wait until all future ready.

I created the following working example. The idea is to have a way to process a pool of tasks (I am using a queue as example) with many Threads (determined programmatically by the numberOfTasks/threshold), and wait until all Threads are completed to continue with some other processing.
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/** Testing CountDownLatch and ExecutorService to manage scenario where
* multiple Threads work together to complete tasks from a single
* resource provider, so the processing can be faster. */
public class ThreadCountDown {
private CountDownLatch threadsCountdown = null;
private static Queue<Integer> tasks = new PriorityQueue<>();
public static void main(String[] args) {
// Create a queue with "Tasks"
int numberOfTasks = 2000;
while(numberOfTasks-- > 0) {
tasks.add(numberOfTasks);
}
// Initiate Processing of Tasks
ThreadCountDown main = new ThreadCountDown();
main.process(tasks);
}
/* Receiving the Tasks to process, and creating multiple Threads
* to process in parallel. */
private void process(Queue<Integer> tasks) {
int numberOfThreads = getNumberOfThreadsRequired(tasks.size());
threadsCountdown = new CountDownLatch(numberOfThreads);
ExecutorService threadExecutor = Executors.newFixedThreadPool(numberOfThreads);
//Initialize each Thread
while(numberOfThreads-- > 0) {
System.out.println("Initializing Thread: "+numberOfThreads);
threadExecutor.execute(new MyThread("Thread "+numberOfThreads));
}
try {
//Shutdown the Executor, so it cannot receive more Threads.
threadExecutor.shutdown();
threadsCountdown.await();
System.out.println("ALL THREADS COMPLETED!");
//continue With Some Other Process Here
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
/* Determine the number of Threads to create */
private int getNumberOfThreadsRequired(int size) {
int threshold = 100;
int threads = size / threshold;
if( size > (threads*threshold) ){
threads++;
}
return threads;
}
/* Task Provider. All Threads will get their task from here */
private synchronized static Integer getTask(){
return tasks.poll();
}
/* The Threads will get Tasks and process them, while still available.
* When no more tasks available, the thread will complete and reduce the threadsCountdown */
private class MyThread implements Runnable {
private String threadName;
protected MyThread(String threadName) {
super();
this.threadName = threadName;
}
#Override
public void run() {
Integer task;
try{
//Check in the Task pool if anything pending to process
while( (task = getTask()) != null ){
processTask(task);
}
}catch (Exception ex){
ex.printStackTrace();
}finally {
/*Reduce count when no more tasks to process. Eventually all
Threads will end-up here, reducing the count to 0, allowing
the flow to continue after threadsCountdown.await(); */
threadsCountdown.countDown();
}
}
private void processTask(Integer task){
try{
System.out.println(this.threadName+" is Working on Task: "+ task);
}catch (Exception ex){
ex.printStackTrace();
}
}
}
}
Hope it helps!

You could use your own subclass of ExecutorCompletionService to wrap taskExecutor, and your own implementation of BlockingQueue to get informed when each task completes and perform whatever callback or other action you desire when the number of completed tasks reaches your desired goal.

you should use executorService.shutdown() and executorService.awaitTermination method.
An example as follows :
public class ScheduledThreadPoolExample {
public static void main(String[] args) throws InterruptedException {
ScheduledExecutorService executorService = Executors.newScheduledThreadPool(5);
executorService.scheduleAtFixedRate(() -> System.out.println("process task."),
0, 1, TimeUnit.SECONDS);
TimeUnit.SECONDS.sleep(10);
executorService.shutdown();
executorService.awaitTermination(1, TimeUnit.DAYS);
}
}

if you use more thread ExecutionServices SEQUENTIALLY and want to wait EACH EXECUTIONSERVICE to be finished. The best way is like below;
ExecutorService executer1 = Executors.newFixedThreadPool(THREAD_SIZE1);
for (<loop>) {
executer1.execute(new Runnable() {
#Override
public void run() {
...
}
});
}
executer1.shutdown();
try{
executer1.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
ExecutorService executer2 = Executors.newFixedThreadPool(THREAD_SIZE2);
for (true) {
executer2.execute(new Runnable() {
#Override
public void run() {
...
}
});
}
executer2.shutdown();
} catch (Exception e){
...
}

Try-with-Resources syntax on AutoCloseable executor service with Project Loom
Project Loom seeks to add new features to the concurrency abilities in Java.
One of those features is making the ExecutorService AutoCloseable. This means every ExecutorService implementation will offer a close method. And it means we can use try-with-resources syntax to automatically close an ExecutorService object.
The ExecutorService#close method blocks until all submitted tasks are completed. Using close takes the place of calling shutdown & awaitTermination.
Being AutoCloseable contributes to Project Loom’s attempt to bring “structured concurrency” to Java.
try (
ExecutorService executorService = Executors.… ;
) {
// Submit your `Runnable`/`Callable` tasks to the executor service.
…
}
// At this point, flow-of-control blocks until all submitted tasks are done/canceled/failed.
// After this point, the executor service will have been automatically shutdown, wia `close` method called by try-with-resources syntax.
For more information on Project Loom, search for talks and interviews given by Ron Pressler and others on the Project Loom team. Focus on the more recent, as Project Loom has evolved.
Experimental builds of Project Loom technology are available now, based on early-access Java 18.

Java 8 - We can use stream API to process stream. Please see snippet below
final List<Runnable> tasks = ...; //or any other functional interface
tasks.stream().parallel().forEach(Runnable::run) // Uses default pool
//alternatively to specify parallelism
new ForkJoinPool(15).submit(
() -> tasks.stream().parallel().forEach(Runnable::run)
).get();

ExecutorService WORKER_THREAD_POOL
= Executors.newFixedThreadPool(10);
CountDownLatch latch = new CountDownLatch(2);
for (int i = 0; i < 2; i++) {
WORKER_THREAD_POOL.submit(() -> {
try {
// doSomething();
latch.countDown();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
});
}
// wait for the latch to be decremented by the two remaining threads
latch.await();
If doSomething() throw some other exceptions, the latch.countDown() seems will not execute, so what should I do?

This might help
Log.i(LOG_TAG, "shutting down executor...");
executor.shutdown();
while (true) {
try {
Log.i(LOG_TAG, "Waiting for executor to terminate...");
if (executor.isTerminated())
break;
if (executor.awaitTermination(5000, TimeUnit.MILLISECONDS)) {
break;
}
} catch (InterruptedException ignored) {}
}

You could call waitTillDone() on this Runner class:
Runner runner = Runner.runner(4); // create pool with 4 threads in thread pool
while(...) {
runner.run(new MyTask()); // here you submit your task
}
runner.waitTillDone(); // and this blocks until all tasks are finished (or failed)
runner.shutdown(); // once you done you can shutdown the runner
You can reuse this class and call waitTillDone() as many times as you want to before calling shutdown(), plus your code is extremly simple. Also you don't have to know the number of tasks upfront.
To use it just add this gradle/maven compile 'com.github.matejtymes:javafixes:1.3.1' dependency to your project.
More details can be found here:
https://github.com/MatejTymes/JavaFixes

There is a method in executor getActiveCount() - that gives the count of active threads.
After spanning the thread, we can check if the activeCount() value is 0. Once the value is zero, it is meant that there are no active threads currently running which means task is finished:
while (true) {
if (executor.getActiveCount() == 0) {
//ur own piece of code
break;
}
}