I have the following code which runs on a simple main activity:
Executor executor = Executors.newFixedThreadPool(1);
executor.execute(new Runnable() {
public void run() {
try {
System.out.println("sleep");
Thread.sleep(5000);
} catch (InterruptedException e) {
System.out.println("Interrupted, so exiting.");
}
}
});
It looks like that when i run this code application doesnt terminate and nothing gets printed either (except the first sleep).
On the other hand when i run this:
Thread t = new Thread(new Runnable() {
public void run() {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
application terminates just fine. Why?
Executor interface would not allow you to shutdown the service. Preferred way is to use ExecutorService instead of Executor
ExecutorService executorService = Executors.newSingleThreadExecutor(factory);
executorService.execute(new Runnable() {
#Override
public void run() {
try {
doTask();
} catch (Exception e) {
logger.error("indexing failed", e);
}
}
});
executorService.shutdown();
Executor thread keeps running after main class is finished
Yep, it's designed to do that. The whole point of an ExecutorService is that there is a pool of threads. Even though the Runnable that you submitted has finished, the threads in the pool are waiting for other jobs to come along. You need to shutdown the pool to get your application to terminate.
It looks like that when i run this code the application doesn't terminate and nothing gets printed either (except the first sleep).
The right way to use the ExecutorService is something like the following:
ExecutorService threadPool = Executors.newFixedThreadPool(1);
threadPool.submit(new Runnable() ...);
// submit any other jobs to the pool here
...
// after last submit, you shutdown the pool, submitted jobs will continue to run
threadPool.shutdown();
// optionally wait for all jobs to finish in pool, similar to thread.join()
threadPool.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);
Once you shutdown the pool and the jobs that have submitted finish, the threads in the pool will terminate and, if there are no more non-daemon threads, your application will stop.
Thread t = new Thread(new Runnable() {
public void run() {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
With this, the thread will terminate after executing the run method.
Runnbale r = ...
Executor executor = Executors.newFixedThreadPool(1);
executor.execute(r);
With this, the executor will create a thread which works like below:
new Thread(new Runnable() {
#Override
public void run() {
while(notShutdown()) {
waitingForTask(); // may get blocked
runTask(); // execute Runnable user submits
}
}
});
This thread will not terminate after the first task, it will keep waiting for new tasks. You need call executor.shutdown() explicitly.
Cause
: You did not shutdown the executor .And When you do the shutdown be aware to do it after it's termination
Solution
: shut it down only after its termination using a simple code.
For example :
executor.shutdown();
while (!executor.isTerminated()) {}
Another solution is to use ExecutorCompletionService if you want to take tasks as they complete you need an ExecutorCompletionService. This acts as a BlockingQueue that will allow you to poll for tasks as and when they finish.
Related
final ExecutorService executor = Executors.newFixedThreadPool(1);
final Future<?> future = executor.submit(myRunnable);
executor.shutdown();
if(executor.awaitTermination(10, TimeUnit.SECONDS)) {
System.out.println("task completed");
}else{
System.out.println("Executor is shutdown now");
}
//MyRunnable method is defined as task which I want to execute in a different thread.
Here is run method of executor class:
public void run() {
try {
Thread.sleep(20 * 1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}}
Here it is waiting for 20 second but when i run the code it throws an exception:
java.lang.InterruptedException: sleep interrupted
at java.lang.Thread.sleep(Native Method)
I am not able to close the concurrent thread ruining in Java Executor class. Here is my Code flow:
Created a new Thread with Java executor class to run some task i.e written in MyRunnable
executor wait for 10 second to complete the tasks.
If the task has completed then runnable thread also got terminated.
If the task is not completed within 10 second then executor class should terminate the thread.
Everything works fine except the termination of tasks in the last scenario. How should I do it?
The shutDown() method simply prevents additional tasks from being scheduled. Instead, you could call shutDownNow() and check for thread interruption in your Runnable.
// in your Runnable...
if (Thread.interrupted()) {
// Executor has probably asked us to stop
}
An example, based on your code, might be:
final ExecutorService executor = Executors.newFixedThreadPool(1);
executor.submit(new Runnable() {
public void run() {
try {
Thread.sleep(20 * 1000);
} catch (InterruptedException e) {
System.out.println("Interrupted, so exiting.");
}
}
});
if (executor.awaitTermination(10, TimeUnit.SECONDS)) {
System.out.println("task completed");
} else {
System.out.println("Forcing shutdown...");
executor.shutdownNow();
}
It is generally a bad idea to terminate a running thread from the outside, because you don't know the state the thread is currently in. It's possible that it needs to do some cleanups, and it won't be able to do that when you forcefully shut it down. That's why all methods of Thread which do that are marked as deprecated.
It's much better to use one of the many techniques which are available for interprocess communication to signal the procedure running in the thread itself that it has to abort its work and exit normally. One way to do this is to add an abort() method to your runnable, which raises a flag declared as volatile. The inner loop of your Runnable checks that flag and exits (in a controlled fashion) when that flag is raised.
I am using Java executor in the following way, but not sure if every line is necessary and if this is the correct way to use it :
ExecutorService executor=Executors.newFixedThreadPool(30);
...
int N=200;
CountDownLatch doneSignal=new CountDownLatch(N);
for (int i=0;i<N;i++) executor.execute(new Test_Runner(doneSignal,...));
doneSignal.await();
executor.shutdown();
while (!executor.isTerminated()) { Thread.sleep(1000); }
// Blocks until all tasks have completed execution after a shutdown request
executor.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
...
class Test_Runner implements Runnable
{
private CountDownLatch doneSignal;
Thread Test_Runner_Thread;
public Tes_Runner(CountDownLatch doneSignal,...)
{
this.doneSignal=doneSignal;
}
// Define some methods
public void run()
{
try
{
// do some work
}
catch (Exception e)
{
e.printStackTrace();
}
doneSignal.countDown();
}
public void start()
{
if (Test_Runner_Thread==null)
{
Test_Runner_Thread=new Thread(this);
Test_Runner_Thread.setPriority(Thread.NORM_PRIORITY);
Test_Runner_Thread.start();
}
}
public void stop() { if (Test_Runner_Thread!=null) Test_Runner_Thread=null; }
}
Looks correct to me. In the past I have followed the suggested implementation from the Java 7 JavaDoc for ExecutorService for stopping it. You can get it fromt he Java 7 Javadoc but I provide it below for convenience. Edit it to fit your needs, for example you might want to pass the number of seconds to wait. The good thing about using a CountDownLatch is that by the time it is done waiting you know the ExecutorService will terminate right away. Also, you might want to add a timeout to your latch's await if needed in future real world cases. Also, put your latch.countDOwn() in a try's finally block when using in real world application.
void shutdownAndAwaitTermination(ExecutorService pool) {
pool.shutdown(); // Disable new tasks from being submitted
try {
// Wait a while for existing tasks to terminate
if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
pool.shutdownNow(); // Cancel currently executing tasks
// Wait a while for tasks to respond to being cancelled
if (!pool.awaitTermination(60, TimeUnit.SECONDS))
System.err.println("Pool did not terminate");
}
} catch (InterruptedException ie) {
// (Re-)Cancel if current thread also interrupted
pool.shutdownNow();
// Preserve interrupt status
Thread.currentThread().interrupt();
}
}
You can further simplify the code.
You can remove CountDownLatch.
Change Test_Runner to Callable task.
Create a ArrayList of Callable Tasks.
List<Test_Runner> callables = new ArrayList<Test_Runner>();
for (int i=0;i<N;i++) {
callables.add(new Test_Runner());
}
Use invokeAll() on executorService.
List<Future<String>> futures = executorService.invokeAll(callables);
From javadocs,
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException
Executes the given tasks, returning a list of Futures holding their status and results when all complete. Future.isDone() is true for each element of the returned list. Note that a completed task could have terminated either normally or by throwing an exception. The results of this method are undefined if the given collection is modified while this operation is in progress.
And you can shutdown executorService as proposed by Jose Martinez
Relate SE question : How to shutdown an ExecutorService?
in my app I am using ThreadPoolTaskExecutor. This executor contains method which return activeThread getActiveCount . Is there option how to stop this active threads ? I dont want to shutdown this executor I just want to stop some infinity process like
while(true){
}
PS: I have access just to this executor because I want to stop this thread through jmx
In my app I am add new threads like this:
taskExecutor.execute(new Runnable() {
#Override
public void run() {
doSomething()
}
});
You need to take a quick check as to how you are adding the tasks.
So long as you are using submit to add the tasks you can use the Future task which is returned (from this call) to cancel specific tasks. However if you use execute you are reliquishing control over the task.
try this :
public void run() {
while (true) {
doSomething();
try {
Thread.sleep(1000);
} catch (InterruptedException ex) {
//Very important.
Thread.currentThread().interrupt();
break;
}
}
System.out.println("Shutting down thread");
}
This is a good article javaspecialists.co.za/archive/Issue056.html
final ExecutorService executor = Executors.newFixedThreadPool(1);
final Future<?> future = executor.submit(myRunnable);
executor.shutdown();
if(executor.awaitTermination(10, TimeUnit.SECONDS)) {
System.out.println("task completed");
}else{
System.out.println("Executor is shutdown now");
}
//MyRunnable method is defined as task which I want to execute in a different thread.
Here is run method of executor class:
public void run() {
try {
Thread.sleep(20 * 1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}}
Here it is waiting for 20 second but when i run the code it throws an exception:
java.lang.InterruptedException: sleep interrupted
at java.lang.Thread.sleep(Native Method)
I am not able to close the concurrent thread ruining in Java Executor class. Here is my Code flow:
Created a new Thread with Java executor class to run some task i.e written in MyRunnable
executor wait for 10 second to complete the tasks.
If the task has completed then runnable thread also got terminated.
If the task is not completed within 10 second then executor class should terminate the thread.
Everything works fine except the termination of tasks in the last scenario. How should I do it?
The shutDown() method simply prevents additional tasks from being scheduled. Instead, you could call shutDownNow() and check for thread interruption in your Runnable.
// in your Runnable...
if (Thread.interrupted()) {
// Executor has probably asked us to stop
}
An example, based on your code, might be:
final ExecutorService executor = Executors.newFixedThreadPool(1);
executor.submit(new Runnable() {
public void run() {
try {
Thread.sleep(20 * 1000);
} catch (InterruptedException e) {
System.out.println("Interrupted, so exiting.");
}
}
});
if (executor.awaitTermination(10, TimeUnit.SECONDS)) {
System.out.println("task completed");
} else {
System.out.println("Forcing shutdown...");
executor.shutdownNow();
}
It is generally a bad idea to terminate a running thread from the outside, because you don't know the state the thread is currently in. It's possible that it needs to do some cleanups, and it won't be able to do that when you forcefully shut it down. That's why all methods of Thread which do that are marked as deprecated.
It's much better to use one of the many techniques which are available for interprocess communication to signal the procedure running in the thread itself that it has to abort its work and exit normally. One way to do this is to add an abort() method to your runnable, which raises a flag declared as volatile. The inner loop of your Runnable checks that flag and exits (in a controlled fashion) when that flag is raised.
Whenever I call shutdownNow() or shutdown() it doesn't shut down. I read of a few threads where it said that shutting down is not guaranteed - can someone provide me a good way of doing it?
The typical pattern is:
executorService.shutdownNow();
executorService.awaitTermination();
When calling shutdownNow, the executor will (generally) try to interrupt the threads that it manages. To make the shutdown graceful, you need to catch the interrupted exception in the threads or check the interrupted status. If you don't your threads will run forever and your executor will never be able to shutdown. This is because the interruption of threads in Java is a collaborative process (i.e. the interrupted code must do something when asked to stop, not the interrupting code).
For example, the following code prints Exiting normally.... But if you comment out the line if (Thread.currentThread().isInterrupted()) break;, it will print Still waiting... because the threads within the executor are still running.
public static void main(String args[]) throws InterruptedException {
ExecutorService executor = Executors.newFixedThreadPool(1);
executor.submit(new Runnable() {
#Override
public void run() {
while (true) {
if (Thread.currentThread().isInterrupted()) break;
}
}
});
executor.shutdownNow();
if (!executor.awaitTermination(100, TimeUnit.MICROSECONDS)) {
System.out.println("Still waiting...");
System.exit(0);
}
System.out.println("Exiting normally...");
}
Alternatively, it could be written with an InterruptedException like this:
public static void main(String args[]) throws InterruptedException {
ExecutorService executor = Executors.newFixedThreadPool(1);
executor.submit(new Runnable() {
#Override
public void run() {
try {
while (true) {Thread.sleep(10);}
} catch (InterruptedException e) {
//ok let's get out of here
}
}
});
executor.shutdownNow();
if (!executor.awaitTermination(100, TimeUnit.MICROSECONDS)) {
System.out.println("Still waiting...");
System.exit(0);
}
System.out.println("Exiting normally...");
}
The best way is what we actually have in the javadoc which is:
The following method shuts down an ExecutorService in two phases,
first by calling shutdown to reject incoming tasks, and then calling
shutdownNow, if necessary, to cancel any lingering tasks:
void shutdownAndAwaitTermination(ExecutorService pool) {
pool.shutdown(); // Disable new tasks from being submitted
try {
// Wait a while for existing tasks to terminate
if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
pool.shutdownNow(); // Cancel currently executing tasks
// Wait a while for tasks to respond to being cancelled
if (!pool.awaitTermination(60, TimeUnit.SECONDS))
System.err.println("Pool did not terminate");
}
} catch (InterruptedException ie) {
// (Re-)Cancel if current thread also interrupted
pool.shutdownNow();
// Preserve interrupt status
Thread.currentThread().interrupt();
}
}
Java 19 makes ExecutorService implement AutoCloseable, meaning it shuts down when exiting a try-with-resources block:
try (ExecutorService e = Executors.newFixedThreadPool(2)) {
e.submit(task1);
e.submit(task2);
} // blocks and waits
This is a structured concurrency approach developed as part of Project Loom, which is incubating in Java 19. As of July 2022, Java 19 is not officially released but early access builds are available.