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Shutdown executor without knowing when it will finish all the tasks

I have a task that should wait for a condition (OpenCms startup) and then notify some listeners.
to do this I used an ExecutorService:
public void check(final ExecutorService executorService) {
executorService.submit(() -> {
waitForInitialization();
notifyListeners();
});
}
private void waitForInitialization() {
while (OpenCms.getRunLevel() < OpenCms.RUNLEVEL_4_SERVLET_ACCESS) {
try {
TimeUnit.SECONDS.sleep(10);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}
}
the question is how to shutdown this executor. I could use awaitTermination but I should provide a timeout which I don't know exactly. it could vary from one environment to the other.

the question is how to shutdown this executor. I could use awaitTermination but I should provide a timeout which I don't know exactly. it could vary from one environment to the other.
The question I would ask is do you need a timeout at all? Often if you know that a particular job will finish at some point I just wait for a timeout Long.MAX_VALUE – effectively forever. Other times I'll do something like:
threadPool.shutdown();
threadPool.awaitTermination(...) of some small value (maybe 10 seconds)
threadPool.shutdownNow(); to interrupt the threads
threadPool.awaitTermination(...); of Long.MAX_VALUE because I know the jobs will finish eventually
it could vary from one environment to the other.
If it could vary then maybe you should be able to calculate what a proper timeout would be for each environment?
Lastly, don't be afraid of passing in a ThreadFactory that creates daemon threads. For some jobs I shutdown() the thread-pool but never wait for them to complete because I don't care about their status so I create the threads in the pool with daemon enabled maybe using something like the following thread-factory.
/** Thread factory which sets name and optionally daemon */
public class PoolNameThreadFactory implements ThreadFactory {
private final String poolName;
private final Boolean daemon;
private final AtomicInteger threadNum = new AtomicInteger(0);
public PoolNameThreadFactory(String poolName) {
this(poolName, null);
}
public PoolNameThreadFactory(String poolName, boolean daemon) {
this(poolName, (Boolean) daemon);
}
private PoolNameThreadFactory(String poolName, Boolean daemon) {
this.poolName = poolName;
this.daemon = daemon;
}
#Override
public Thread newThread(Runnable r) {
Thread thread = new Thread(r);
thread.setName(poolName + '-' + threadNum.incrementAndGet());
if (daemon != null) {
thread.setDaemon(daemon);
}
return thread;
}
}

Based on the provided information, I'd clearly recommend an event-based approach. Especially knowing that in your own code there is a call like notifyListeners(). In fact, that's the way to go.
In summary, once the precondition is met somewhere in your app, just notify the listeners of this event. In your example, the "OpenCms run level" change is typically an event. So, just go for an Observer pattern, or a pub-sub model to observe or monitor these changes.
If you modify your approach, you will not have to worry about the waiting time around the initialization, except if you wish to handle the absence of event specifically. That would be done again after some timeout, but with the advantage of not blocking an executor thread.

How to manage threads in Spring TaskExecutor framework

I have a BlockingQueue of Runnable - I can simply execute all tasks using one of TaskExecutor implementations, and all will be run in parallel.
However some Runnable depends on others, it means they need to wait when Runnable finish, then they can be executed.
Rule is quite simple: every Runnable has a code. Two Runnable with the same code cannot be run simultanously, but if the code differ they should be run in parallel.
In other words all running Runnable need to have different code, all "duplicates" should wait.
The problem is that there's no event/method/whatsoever when thread ends.
I can built such notification into every Runnable, but I don't like this approach, because it will be done just before thread ends, not after it's ended
java.util.concurrent.ThreadPoolExecutor has method afterExecute, but it needs to be implemented - Spring use only default implementation, and this method is ignored.
Even if I do that, it's getting complicated, because I need to track two additional collections: with Runnables already executing (no implementation gives access to this information) and with those postponed because they have duplicated code.
I like the BlockingQueue approach because there's no polling, thread simply activate when something new is in the queue. But maybe there's a better approach to manage such dependencies between Runnables, so I should give up with BlockingQueue and use different strategy?

If the number of different codes is not that large, the approach with a separate single thread executor for each possible code, offered by BarrySW19, is fine.
If the whole number of threads become unacceptable, then, instead of single thread executor, we can use an actor (from Akka or another similar library):
public class WorkerActor extends UntypedActor {
public void onReceive(Object message) {
if (message instanceof Runnable) {
Runnable work = (Runnable) message;
work.run();
} else {
// report an error
}
}
}
As in the original solution, ActorRefs for WorkerActors are collected in a HashMap. When an ActorRef workerActorRef corresponding to the given code is obtained (retrieved or created), the Runnable job is submitted to execution with workerActorRef.tell(job).
If you don't want to have a dependency to the actor library, you can program WorkerActor from scratch:
public class WorkerActor implements Runnable, Executor {
Executor executor=ForkJoinPool.commonPool(); // or can by assigned in constructor
LinkedBlockingQueue<Runnable> queue = new LinkedBlockingQueu<>();
boolean running = false;
public synchronized void execute(Runnable job) {
queue.put(job);
if (!running) {
executor.execute(this); // execute this worker, not job!
running=true;
}
public void run() {
for (;;) {
Runnable work=null;
synchronized (this) {
work = queue.poll();
if (work==null) {
running = false;
return;
}
}
work.run();
}
}
}
When a WorkerActor worker corresponding to the given code is obtained (retrieved or created), the Runnable job is submitted to execution with worker.execute(job).

One alternate strategy which springs to mind is to have a separate single thread executor for each possible code. Then, when you want to submit a new Runnable you simply lookup the correct executor to use for its code and submit the job.
This may, or may not be a good solution depending on how many different codes you have. The main thing to consider would be that the number of concurrent threads running could be as high as the number of different codes you have. If you have many different codes this could be a problem.
Of course, you could use a Semaphore to restrict the number of concurrently running jobs; you would still create one thread per code, but only a limited number could actually execute at the same time. For example, this would serialise jobs by code, allowing up to three different codes to run concurrently:
public class MultiPoolExecutor {
private final Semaphore semaphore = new Semaphore(3);
private final ConcurrentMap<String, ExecutorService> serviceMap
= new ConcurrentHashMap<>();
public void submit(String code, Runnable job) {
ExecutorService executorService = serviceMap.computeIfAbsent(
code, (k) -> Executors.newSingleThreadExecutor());
executorService.submit(() -> {
semaphore.acquireUninterruptibly();
try {
job.run();
} finally {
semaphore.release();
}
});
}
}
Another approach would be to modify the Runnable to release a lock and check for jobs which could be run upon completion (so avoiding polling) - something like this example, which keeps all the jobs in a list until they can be submitted. The boolean latch ensures only one job for each code has been submitted to the thread pool at any one time. Whenever a new job arrives or a running one completes the code checks again for new jobs which can be submitted (the CodedRunnable is simply an extension of Runnable which has a code property).
public class SubmissionService {
private final ExecutorService executorService = Executors.newFixedThreadPool(5);
private final ConcurrentMap<String, AtomicBoolean> locks = new ConcurrentHashMap<>();
private final List<CodedRunnable> jobs = new ArrayList<>();
public void submit(CodedRunnable codedRunnable) {
synchronized (jobs) {
jobs.add(codedRunnable);
}
submitWaitingJobs();
}
private void submitWaitingJobs() {
synchronized (jobs) {
for(Iterator<CodedRunnable> iter = jobs.iterator(); iter.hasNext(); ) {
CodedRunnable nextJob = iter.next();
AtomicBoolean latch = locks.computeIfAbsent(
nextJob.getCode(), (k) -> new AtomicBoolean(false));
if(latch.compareAndSet(false, true)) {
iter.remove();
executorService.submit(() -> {
try {
nextJob.run();
} finally {
latch.set(false);
submitWaitingJobs();
}
});
}
}
}
}
}
The downside of this approach is that the code needs to scan through the entire list of waiting jobs after each task completes. Of course, you could make this more efficient - a completing task would actually only need to check for other jobs with the same code, so the jobs could be stored in a Map<String, List<Runnable>> structure instead to allow for faster processing.

How to wait for completion of multiple tasks in Java?

What is the proper way to implement concurrency in Java applications? I know about Threads and stuff, of course, I have been programming for Java for 10 years now, but haven't had too much experience with concurrency.
For example, I have to asynchronously load a few resources, and only after all have been loaded, can I proceed and do more work. Needless to say, there is no order how they will finish. How do I do this?
In JavaScript, I like using the jQuery.deferred infrastructure, to say
$.when(deferred1,deferred2,deferred3...)
.done(
function(){//here everything is done
...
});
But what do I do in Java?

You can achieve it in multiple ways.
1.ExecutorService invokeAll() API
Executes the given tasks, returning a list of Futures holding their status and results when all complete.
2.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.
3.ForkJoinPool or newWorkStealingPool() in Executors is other way
Have a look at related SE questions:
How to wait for a thread that spawns it's own thread?
Executors: How to synchronously wait until all tasks have finished if tasks are created recursively?

I would use parallel stream.
Stream.of(runnable1, runnable2, runnable3).parallel().forEach(r -> r.run());
// do something after all these are done.
If you need this to be asynchronous, then you might use a pool or Thread.
I have to asynchronously load a few resources,
You could collect these resources like this.
List<String> urls = ....
Map<String, String> map = urls.parallelStream()
.collect(Collectors.toMap(u -> u, u -> download(u)));
This will give you a mapping of all the resources once they have been downloaded concurrently. The concurrency will be the number of CPUs you have by default.

If I'm not using parallel Streams or Spring MVC's TaskExecutor, I usually use CountDownLatch. Instantiate with # of tasks, reduce once for each thread that completes its task. CountDownLatch.await() waits until the latch is at 0. Really useful.
Read more here: JavaDocs

Personally, I would do something like this if I am using Java 8 or later.
// Retrieving instagram followers
CompletableFuture<Integer> instagramFollowers = CompletableFuture.supplyAsync(() -> {
// getInstaFollowers(userId);
return 0; // default value
});
// Retrieving twitter followers
CompletableFuture<Integer> twitterFollowers = CompletableFuture.supplyAsync(() -> {
// getTwFollowers(userId);
return 0; // default value
});
System.out.println("Calculating Total Followers...");
CompletableFuture<Integer> totalFollowers = instagramFollowers
.thenCombine(twitterFollowers, (instaFollowers, twFollowers) -> {
return instaFollowers + twFollowers; // can be replaced with method reference
});
System.out.println("Total followers: " + totalFollowers.get()); // blocks until both the above tasks are complete
I used supplyAsync() as I am returning some value (no. of followers in this case) from the tasks otherwise I could have used runAsync(). Both of these run the task in a separate thread.
Finally, I used thenCombine() to join both the CompletableFuture. You could also use thenCompose() to join two CompletableFuture if one depends on the other. But in this case, as both the tasks can be executed in parallel, I used thenCombine().
The methods getInstaFollowers(userId) and getTwFollowers(userId) are simple HTTP calls or something.

You can use a ThreadPool and Executors to do this.
https://docs.oracle.com/javase/tutorial/essential/concurrency/pools.html

This is an example I use Threads. Its a static executerService with a fixed size of 50 threads.
public class ThreadPoolExecutor {
private static final ExecutorService executorService = Executors.newFixedThreadPool(50,
new ThreadFactoryBuilder().setNameFormat("thread-%d").build());
private static ThreadPoolExecutor instance = new ThreadPoolExecutor();
public static ThreadPoolExecutor getInstance() {
return instance;
}
public <T> Future<? extends T> queueJob(Callable<? extends T> task) {
return executorService.submit(task);
}
public void shutdown() {
executorService.shutdown();
}
}
The business logic for the executer is used like this: (You can use Callable or Runnable. Callable can return something, Runnable not)
public class MultipleExecutor implements Callable<ReturnType> {//your code}
And the call of the executer:
ThreadPoolExecutor threadPoolExecutor = ThreadPoolExecutor.getInstance();
List<Future<? extends ReturnType>> results = new LinkedList<>();
for (Type Type : typeList) {
Future<? extends ReturnType> future = threadPoolExecutor.queueJob(
new MultipleExecutor(needed parameters));
results.add(future);
}
for (Future<? extends ReturnType> result : results) {
try {
if (result.get() != null) {
result.get(); // here you get the return of one thread
}
} catch (InterruptedException | ExecutionException e) {
logger.error(e, e);
}
}

The same behaviour as with $.Deferred in jQuery you can archive in Java 8 with a class called CompletableFuture. This class provides the API for working with Promises. In order to create async code you can use one of it's static creational methods like #runAsync, #supplyAsync. Then applying some computation of results with #thenApply.

I usually opt for an async notify-start, notify-progress, notify-end approach:
class Task extends Thread {
private ThreadLauncher parent;
public Task(ThreadLauncher parent) {
super();
this.parent = parent;
}
public void run() {
doStuff();
parent.notifyEnd(this);
}
public /*abstract*/ void doStuff() {
// ...
}
}
class ThreadLauncher {
public void stuff() {
for (int i=0; i<10; i++)
new Task(this).start();
}
public void notifyEnd(Task who) {
// ...
}
}

Using concurrent classes to process files in a directory in parallel

I am trying to figure out how to use the types from the java.util.concurrent package to parallelize processing of all the files in a directory.
I am familiar with the multiprocessing package in Python, which is very simple to use, so ideally I am looking for something similar:
public interface FictionalFunctor<T>{
void handle(T arg);
}
public class FictionalThreadPool {
public FictionalThreadPool(int threadCount){
...
}
public <T> FictionalThreadPoolMapResult<T> map(FictionalFunctor<T> functor, List<T> args){
// Executes the given functor on each and every arg from args in parallel. Returns, when
// all the parallel branches return.
// FictionalThreadPoolMapResult allows to abort the whole mapping process, at the least.
}
}
dir = getDirectoryToProcess();
pool = new FictionalThreadPool(10); // 10 threads in the pool
pool.map(new FictionalFunctor<File>(){
#Override
public void handle(File file){
// process the file
}
}, dir.listFiles());
I have a feeling that the types in java.util.concurrent allow me to do something similar, but I have absolutely no idea where to start.
Any ideas?
Thanks.
EDIT 1
Following the advices given in the answers, I have written something like this:
public void processAllFiles() throws IOException {
ExecutorService exec = Executors.newFixedThreadPool(6);
BlockingQueue<Runnable> tasks = new LinkedBlockingQueue<Runnable>(5); // Figured we can keep the contents of 6 files simultaneously.
exec.submit(new MyCoordinator(exec, tasks));
for (File file : dir.listFiles(getMyFilter()) {
try {
tasks.add(new MyTask(file));
} catch (IOException exc) {
System.err.println(String.format("Failed to read %s - %s", file.getName(), exc.getMessage()));
}
}
}
public class MyTask implements Runnable {
private final byte[] m_buffer;
private final String m_name;
public MyTask(File file) throws IOException {
m_name = file.getName();
m_buffer = Files.toByteArray(file);
}
#Override
public void run() {
// Process the file contents
}
}
private class MyCoordinator implements Runnable {
private final ExecutorService m_exec;
private final BlockingQueue<Runnable> m_tasks;
public MyCoordinator(ExecutorService exec, BlockingQueue<Runnable> tasks) {
m_exec = exec;
m_tasks = tasks;
}
#Override
public void run() {
while (true) {
Runnable task = m_tasks.remove();
m_exec.submit(task);
}
}
}
How I thought the code works is:
The files are read one after another.
A file contents are saved in a dedicated MyTask instance.
A blocking queue with the capacity of 5 to hold the tasks. I count on the ability of the server to keep the contents of at most 6 files at one time - 5 in the queue and another fully initialized task waiting to enter the queue.
A special MyCoordinator task fetches the file tasks from the queue and dispatches them to the same pool.
OK, so there is a bug - more than 6 tasks can be created. Some will be submitted, even though all the pool threads are busy. I've planned to solve it later.
The problem is that it does not work at all. The MyCoordinator thread blocks on the first remove - this is fine. But it never unblocks, even though new tasks were placed in the queue. Can anyone tell me what am I doing wrong?

The thread pool you are looking for is the ExecutorService class. You can create a fixed-size thread pool using newFixedThreadPool. This allows you to easily implement a producer-consumer pattern, with the pool encapsulating all the queue and worker functionality for you:
ExecutorService exec = Executors.newFixedThreadPool(10);
You can then submit tasks in the form of objects whose type implements Runnable (or Callable if you want to also get a result):
class ThreadTask implements Runnable {
public void run() {
// task code
}
}
...
exec.submit(new ThreadTask());
// alternatively, using an anonymous type
exec.submit(new Runnable() {
public void run() {
// task code
}
});
A big word of advice on processing multiple files in parallel: if you have a single mechanical disk holding the files it's wise to use a single thread to read them one-by-one and submit each file to a thread pool task as above, for processing. Do not do the actual reading in parallel as it will degrade performance.

A simpler solution than using ExecuterService is to implement your own producer-consumer scheme. Have a thread that create tasks and submits to a LinkedBlockingQueue or ArrayBlockingQueue and have worker threads that check this queue to retrieve the tasks and do them. You may need a special kind of tasks name ExitTask that forces the workers to exit. So at the end of the jobs if you have n workers you need to add n ExitTasks into the queue.

Basically, what #Tudor said, use an ExecutorService, but I wanted to expand on his code and I always feel strange editing other people's posts. Here's a sksleton of what you would submit to the ExecutorService:
public class MyFileTask implements Runnable {
final File fileToProcess;
public MyFileTask(File file) {
fileToProcess = file;
}
public void run() {
// your code goes here, e.g.
handle(fileToProcess);
// if you prefer, implement Callable instead
}
}
See also my blog post here for some more details if you get stuck
Since processing Files often leads to IOExceptions, I'd prefer a Callable (which can throw a checked Exception) to a Runnable, but YMMV.

Are there any executor in java concurrent package which guarantee that all tasks will be done in order they were submitted?

A code sample for demonstration of the idea from the title:
executor.submit(runnable1);
executor.submit(runnable2);
I need to be sure that runnable1 will finish before runnable2 start and I haven't found any proofs of such behavior in the executors documentation.
About the problem I'm solving:
I need write lots of logs to a file. Each log requires much precomputing (formatting and some other stuff). So, I want to put each logging task to a kind of queue and process these tasks in a separate thread. And, of course, it's important to keep logs ordering.

A single threaded executor will perform all tasks in the order submitted. You would only use a thread pool with multiple threads if you wanted the tasks to be perform concurrently.
Adding tasks to a queue can be expensive in itself. You can use an Exchanger like this
http://vanillajava.blogspot.com/2011/09/exchange-and-gc-less-java.html?z#!/2011/09/exchange-and-gc-less-java.html
This avoid using a queue or creating object.
An alternative which is faster is to use a memory mapped file which doesn't require a background thread (actually the OS is working in the background) This is much faster again. It supports sub-microsecond latencies and millions of messages per second.
https://github.com/peter-lawrey/Java-Chronicle

You could create a simple wrapper like the one below so that all your Runnables are executed in the same thread (i.e. sequentially), and submit that wrapper to the executor instead. That does not address the logging issue.
class MyRunnable implements Runnable {
private List<Runnable> runnables = new ArrayList<>();
public void add(Runnable r) {
runnables.add(r);
}
#Override
public void run() {
for (Runnable r : runnables) {
r.run();
}
}
}
//......
MyRunnable r = new MyRunnable();
r.add(runnable1);
r.add(runnable2);
executor.submit(r);

Presumably you are doing some post-analysis of the logfile? Have you considered not caring about the order they're written and re-ordering offline later. You could allocate a unique id at submit time using, a timestamp or AtomicLong?
a code sketch (untested) would look like this:
import java.util.concurrent.atomic.AtomicLong;
class MyProcessor {
public void work()
for (Object data: allData) {
executor.submit(new MySequencedRunnable(data);
}
}
}
class MySequencedRunnable implements Runnable {
private static final AtomicLong LOG_SEQUENCE_ID = new AtomicLong(0);
private final Object data;
MySequencedRunnable(Object data) {
this.data = data;
}
public void run() {
LOGGER.log(LOG_SEQUENCE_ID.incrementAndGet(), data);
}
}
Also consider, if you're using something like log4j, using NDC or MDC to assist with the re-ordering.