So, lets say I have a ScheduledExecutorService with 10 threads, generated as such, and I start a periodic task to run once a minute.
svc = Executors.newScheduledThreadPool(10);
svc.scheduleAtFixedRate(myRunnable, 0, 1, TimeUnit.MINUTES);
This is a perfect way to make myRunnable run once a minute. Now let's say the execution of myRunnable takes longer than a minute (therefore the task duration now exceeds the task period). The current behavior that I have noticed is that the thread pool will wait until myRunnable completes execution before running again. It looks like what I am saying with my code is to schedule the task but only occupy one thread in the pool with myRunnable.
Is there an option/another API/additional method call where I can make this thread pool to just grab the next available thread and run if the first thread hasn't completed execution in one minute? (Or am I doing this wrong?)
Why not schedule your Runnable for one-time-only execution (after a delay), then at the beginning of your Runnable schedule it again?
Related
I created a Timer object scheduled to run every 1 second and the run method takes 20 seconds to complete. The
Timer.schedule method works as expected: it starts the task immediately after the first task is completed in 20 seconds.
But the Timer.scheduleAtFixedRate method also behaves in the same way. This is what is in the documentation:
In fixed-rate execution, each execution is scheduled relative to the scheduled execution time of the initial execution. If an execution is delayed for any reason (such as garbage collection or other background activity), two or more executions will occur in rapid succession to "catch up.".
I expect that multiple threads will be spun to catch up, but this is not happening.
How can this be explained? What is a good example to demonstrate the difference between these methods?
Java documentation for the Timer class:
Corresponding to each Timer object is a single background thread that is used to execute all of the timer's tasks, sequentially. Timer tasks should complete quickly. If a timer task takes excessive time to complete, it "hogs" the timer's task execution thread. This can, in turn, delay the execution of subsequent tasks, which may "bunch up" and execute in rapid succession when (and if) the offending task finally completes.
The expectation that additional threads will be created to catch up is incorrect. According to the documentation, Timer tasks should complete quickly. A Timer task should not take 20 seconds to complete. An alternative is the ScheduledThreadPoolExecutor class:
A ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically. This class is preferable to Timer when multiple worker threads are needed, or when the additional flexibility or capabilities of ThreadPoolExecutor (which this class extends) are required.
To answer the second question: The difference is that the schedule method "schedules the specified task for repeated fixed-delay execution" and the
scheduleAtFixedRate method "schedules the specified task for repeated fixed-rate execution". This answer explains this difference well.
yes,Java Timer object can be created to run the associated tasks as a daemon thread.
https://www.geeksforgeeks.org/java-util-timer-class-java/
I am writing a piece of code where I need to periodically schedule a Runnable. I am using ScheduledExecutorService.scheduleWithFixedDelay with initial delay as 0 and delay as 2 minutes. There is only task in this SchedulerService. I am expecting that 2nd execution of the task will start 2 minutes after the completion of 1st execution of the task. To validate this, I started my app in debug mode and placed a breakpoint at the beginning of the Runnable Task. 1st execution halts at the breakpoint and thread executing the task goes in suspended state. To my utter surprise, after 2 minutes of the start of 1st execution of the task, SchedulerService starts new thread to execute 2nd execution of the task. I was expecting that unless 1st execution completes, ScheduledExecutorService.scheduleWithFixedDelay will not start 2nd execution. Please help me understand this behavior as this contradicts the Javadoc explanation of this method.
The number of threads are controllable. Single threaded executor can be initiated.
ScheduledExecutorService executorService = Executors.newSingleThreadScheduledExecutor();
public class MyTimerTask extends TimerTask{
#Override
public void run() {
int i = 0;
try {
Thread.sleep(100000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Run Me ~" + ++i);
System.out.println("Test");
}
}
Case 1 :-
TimerTask task = new MyTimerTask();
Timer timer = new Timer();
timer.schedule(task, 1000,6000); // line 1
System.out.println("End"); // here is bebug point.
My Expectation of schedule() method (as per my understanding given in javadocs where each execution is scheduled once previous task execution is completed)
that two threads should be
created after line 1.
One for timer which spawns another thread for tasks. Once first task thread dies
another will be created and son on. But at debug point , i just see one thread corresponding to Timer. Why
not thread for tasks which implement Runnable?
Case 2 :-
TimerTask task = new MyTimerTask();
Timer timer = new Timer();
timer.scheduleAtFixedRate(task, 1000,6000); // line 1
System.out.println("End"); // here is bebug point.
My Expectation of scheduleAtFixedRate() method(as per my understanding given in javadocs where each execution is scheduled relative to the scheduled
execution time of the initial execution) that around 17 threads(dont pay much attention
to 17. It can be more or less to that. But it should be greater than 2 ) should be
created after line 1.
One for timer which should spawn 16 other thread corresponding two each task. At first task sleeps
for 100 second, Timer should create another thread corresponding to next task and similarly for other task.
But at debug point , i just see one thread corresponding to Timer. Here also i can see sequential execution of task. Why not 17 threads?
UPDATE :- As per ScheduleAtFixedRate javadocs , each execution is scheduled relative to the scheduled execution time of the initial execution. If an execution is delayed for any reason (such as garbage collection or other background activity), two or more executions will occur in rapid succession to "catch up. what does that mean? To me it gives impression, if second task is due even first task is not completed, then timer will create new thread for due task. Is n't it?
Timer uses the Active Object pattern under the hood, so there is only ever a single thread being used and scheduling a new task on the timer adds that task to the thread's tasks queue.
The timer thread keeps track of all the tasks in it's queue and sleeps until the next task is scheduled. Then, it wakes up and executes the task itself by invoking task.run() directly, meaning that it does not spawn another thread to execute the code.
This also means that if you schedule two tasks to execute at the same time then, true to the Active Object pattern, they will be executed sequentially (one after another) on the same thread of control. This means the second task will execute after it's scheduled time (but probably not by much).
Now, to unequivocally answer your question, here is the scheduling logic from Timer.class that schedules the next time that the task should be run again (from lines 262-272 here):
// set when the next task should be launched
if (task.fixedRate) {
// task is scheduled at fixed rate
task.when = task.when + task.period;
} else {
// task is scheduled at fixed delay
task.when = System.currentTimeMillis()
+ task.period;
}
// insert this task into queue
insertTask(task);
task.fixedRate is set to true if you use one of the timer.scheduleAtFixedRate() methods and is set to false if you use one of the timer.schedule() methods.
task.when is the "time" (ticks) that the task was scheduled to run.
task.period is the interval you passed to the timer.schedule*() method.
So, from the code we can see that if you use a fixed rate then a repeating task will be scheduled to run relative to when it was first started. If you don't use a fixed rate, then it is scheduled to run relative to when it was last run (which will drift relative to a fixed rate, unless your task is never delayed and takes less than one tick to execute).
This also means that if a task falls behind and it is on a fixed rate, then Timer will keep rescheduling the task for immediate execution until it catches up to the total number of times it should have ran over a given period.
So if you have a task, say a ping() that you schedule to run at a fixed rate every 10ms and there is temporary blocking in the ping() method to where it takes 20ms to execute, then the Timer will call ping() again immediately after the previous call finished, and it will keep doing so until the given rate is achieved.
The javadoc for Timer says
Corresponding to each Timer object is a single background thread that
is used to execute all of the timer's tasks, sequentially.
Basically it holds a queue of tasks to which it adds when you schedule them. It uses one thread to iterate over the queue and execute the tasks.
The timer class creates one thread per instance of the timer class and this thread do all tasks scheduled Timer#schedule or Timer#scheduleAtFixRate.
So, as you ovserved, the timer creates only one thread.
A task would have came start time before the preciding task has finished, then the follwing task has waited until the preciding task has finished.
So, Timer "never" create another thread although the preciding task hasn't finished and the time the following task has to start has come.
So, I advise you that:
if you want to schedule tasks and do the tasks on time whether a preciding task has finished or not, use ScheduledThreadPoolExecutor instead of Timer.
And though if you do not want, it's prefer to use ScheduledThreadPoolExecutor than Timer because for one thing, tasks scheduled by Timer would never have done if a task would have threw RuntimeException or Error.
Schedule will not execute the missed task if the start time is in the past.
scheduleAtFixedRate will execute the missed tasks if the start time is in the past.For the missed tasks, the start time will be calculated based last task's end time. When missed tasks are executed fully, the new normal tasks' start time will be calculated based on last task's start time.
BR Sanchez
I'm interested in using ScheduledExecutorService to spawn multiple threads for tasks if task before did not yet finish. For example I need to process a file every 0.5s. First task starts processing file, after 0.5s if first thread is not finished second thread is spawned and starts processing second file and so on. This can be done with something like this:
ScheduledExecutorService executor = Executors.newScheduledThreadPool(4)
while (!executor.isShutdown()) {
executor.execute(task);
try {
Thread.sleep(500);
} catch (InterruptedException e) {
// handle
}
}
Now my question: Why I can't do it with executor.scheduleAtFixedRate?
What I get is if the first task takes longer, the second task is started as soon as first finished, but no new thread is started even if executor has pool of threads. executor.scheduleWithFixedDelay is clear - it executes tasks with same time span between them and it doesn't matter how long it takes to complete the task. So probably I misunderstood ScheduledExecutorService purpose.
Maybe I should look at another kind of executor? Or just use code which I posted here? Any thoughts?
I've solved the problem by launching a nested anonymous runnable in each scheduled execution:
final ScheduledExecutorService service = Executors.newScheduledThreadPool(POOL_SIZE);
final Runnable command = new SlowRunnable();
service.scheduleAtFixedRate(
new Runnable() {
#Override
public void run() {
service.execute(command);
}
}, 0, 1, TimeUnit.SECONDS);
With this example there will be 1 thread executing at every interval a fast instruction, so it will be surely be finished when the next interval is expired. The remaining POOL_SIZE-1 threads will be executing the SlowRunnable's run() in parallel, which may take longer time than the duration of the single interval.
Please note that while I like this solution as it minimize the code and reuse the same ScheduledExecutorService, it must be sized correctly and may not be usable in every context: if the SlowRunnable is so slow that up to POOL_SIZE jobs get executed together, there will be no threads to run the the scheduled task in time.
Also, if you set the interval at 1 TimeUnit.NANOSECONDS it will probably became too slow also the execution of the main runnable.
One of the scheduleAtFixedRate methods is what you're looking for. It starts a task in a thread from the pool at the given interval, even if previous tasks haven't finished. If you're running out of threads to do the processing, adjust the pool size constraints as detailed in the ThreadPoolExecutor docs.
This question is a followup on this one.
Essentially what I am doing is declaring a ThreadPoolExecutor with just one thread. I am overriding the beforeExecute() method to put a sleep so that each of my tasks are executed with some delay among themselves. This is basically to give away the CPU to other threads since my thread is kind of thrashing.
So the expected behavior is:
For each new task in the ThreadPoolExecutor, it calls the before execute function before executing the task and hence it sleeps for say 20s before it executes the task.
However this is what I see:
For each new task submitted:
It executes the task
Calls the beforeExecute method
sleeps for say 20s
RE-EXECUTE the task!
The order of 1. & 2. is not the same all the time.
Here are my questions:
It is appearing that a new thread comes in after/during sleeping and goes ahead and executes my task right away while the actual thread is sleeping.
So does the ThreadPoolExecutor spawn a new thread as soon as an existing thread sleeps [thinking that the thread is terminated]??
I tried to put the keepAliveTime > sleeptime ..so that in case the above assertion is true .. it atleast waits for more than sleep time to spawn a new thread ...[hoping in the mean time the sleeping thread would be awake and the ThreadPoolExecutor would dump the idea of spawning a new thread
Even if it does spawn a new thread and execute my task right away, why would the task be re-executed after the sleeping thread wakes up !! Shouldn't the task be taken out of task Queue before that ??
Am I missing something here ? Any other way to debug this scenario ?
=> An alternative method I was thinking to do the desired task [and not solve the peoblem] was to wrap the runnable with one more runnable and sleep the outer runnable before calling the inner one.
I think what you're looking for is a ScheduledExecutorService
From what I understand of your question, scheduleAtFixedRate(...) should do the deal:
scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit)
Creates and executes a periodic action
that becomes enabled first after the
given initial delay, and subsequently
with the given period; that is
executions will commence after
initialDelay then initialDelay+period,
then initialDelay + 2 * period, and so
on.
No, that is not how it works. The ThreadPoolExecutor knows it has a worker thread, even if that worker is RUNNABLE, WAITING, BLOCKED, or any other state.
The task is removed from the BlockingQueue long before the beforeExecute method is invoked.
You can look at the code for the API yourself and determine what it is doing. Every Java JDK installation includes a "src.zip" file which contains the entire Java Library. If yu haven't already, you can attach this source in eclipse and then while debugging in eclipse diving into a library method will show you source instead of just the class file.