I know this may be completely obvious for more experenced programmers but I can't find anything on it.
I need to make a thread that "ticks" at a constant amount of times per second no matter how long the task it has to execute is. A task that takes longer than each tick would obviously not be possible and slow the number of ticks per second.
Thanks.
Use java.util.Timer.scheduleAtFixedRate:
public void scheduleAtFixedRate(TimerTask task,
Date firstTime,
long period)
Schedules the specified task for repeated fixed-rate execution, beginning at the specified time. Subsequent executions take place at approximately regular intervals, separated by the specified period.
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." In the long run, the frequency of execution will be exactly the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate). As a consequence of the above, if the scheduled first time is in the past, then any "missed" executions will be scheduled for immediate "catch up" execution.
Fixed-rate execution is appropriate for recurring activities that are sensitive to absolute time, such as ringing a chime every hour on the hour, or running scheduled maintenance every day at a particular time. It is also appropriate for recurring activities where the total time to perform a fixed number of executions is important, such as a countdown timer that ticks once every second for ten seconds. Finally, fixed-rate execution is appropriate for scheduling multiple repeating timer tasks that must remain synchronized with respect to one another.
Here is a Thread version. A Timer would be the way to go but if you really want to use your own thread.
new Thread(new Runnable() {
#Override
public void run() {
try {
long before, sleepDuration, operationTime;
for(int i=0;i<100;i++) {
before = System.currentTimeMillis();
// do your operations
operationTime = (long)(1500*Math.random());
System.out.print("Doing operations for "+operationTime+"ms\t");
Thread.sleep(operationTime);
// sleep for up to 1000ms
sleepDuration = Math.min(1000, Math.max(1000 - (System.currentTimeMillis() - before), 0));
Thread.sleep(sleepDuration);
System.out.println("wait\t"+sleepDuration+"ms =\telapsed " + (operationTime+sleepDuration) + (operationTime > 1000 ? "<" : ""));
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}).start();
The Timer and TimerTask classes are perfect for your use.
Take a look at the docs http://docs.oracle.com/javase/6/docs/api/index.html?java/util/Timer.html or follow this lesson http://enos.itcollege.ee/~jpoial/docs/tutorial/essential/threads/timer.html
Let me know if I misunderstood your question.
Related
I was trying some codes to implement a scheduled task and came up with these codes .
import java.util.*;
class Task extends TimerTask {
int count = 1;
// run is a abstract method that defines task performed at scheduled time.
public void run() {
System.out.println(count+" : Mahendra Singh");
count++;
}
}
class TaskScheduling {
public static void main(String[] args) {
Timer timer = new Timer();
// Schedule to run after every 3 second(3000 millisecond)
timer.schedule( new Task(), 3000);
}
}
My output :
1 : Mahendra Singh
I expected the compiler to print a series of Mahendra Singh at periodic interval of 3 s but despite waiting for around 15 minutes, I get only one output...How do I solve this out?
Advantage of ScheduledExecutorService over Timer
I wish to offer you an alternative to Timer using - ScheduledThreadPoolExecutor, an implementation of the ScheduledExecutorService interface. It has some advantages over the Timer class, according to "Java in Concurrency":
A Timer creates only a single thread for executing timer tasks. If a
timer task takes too long to run, the timing accuracy of other
TimerTask can suffer. If a recurring TimerTask is scheduled to run
every 10 ms and another Timer-Task takes 40 ms to run, the recurring
task either (depending on whether it was scheduled at fixed rate or
fixed delay) gets called four times in rapid succession after the
long-running task completes, or "misses" four invocations completely.
Scheduled thread pools address this limitation by letting you provide
multiple threads for executing deferred and periodic tasks.
Another problem with Timer is that it behaves poorly if a TimerTask throws an unchecked exception. Also, called "thread leakage"
The Timer thread doesn't catch the exception, so an unchecked
exception thrown from a TimerTask terminates the timer thread. Timer
also doesn't resurrect the thread in this situation; instead, it
erroneously assumes the entire Timer was cancelled. In this case,
TimerTasks that are already scheduled but not yet executed are never
run, and new tasks cannot be scheduled.
And another recommendation if you need to build your own scheduling service, you may still be able to take advantage of the library by using a DelayQueue, a BlockingQueue implementation that provides the scheduling functionality of ScheduledThreadPoolExecutor. A DelayQueue manages a collection of Delayed objects. A Delayed has a delay time associated with it: DelayQueue lets you take an element only if its delay has expired. Objects are returned from a DelayQueue ordered by the time associated with their delay.
Use timer.scheduleAtFixedRate
public void scheduleAtFixedRate(TimerTask task,
long delay,
long period)
Schedules the specified task for repeated fixed-rate execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals, separated by the specified period.
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." In the long run, the frequency of execution will be exactly the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).
Fixed-rate execution is appropriate for recurring activities that are sensitive to absolute time, such as ringing a chime every hour on the hour, or running scheduled maintenance every day at a particular time. It is also appropriate for recurring activities where the total time to perform a fixed number of executions is important, such as a countdown timer that ticks once every second for ten seconds. Finally, fixed-rate execution is appropriate for scheduling multiple repeating timer tasks that must remain synchronized with respect to one another.
Parameters:
task - task to be scheduled.
delay - delay in milliseconds before task is to be executed.
period - time in milliseconds between successive task executions.
Throws:
IllegalArgumentException - if delay is negative, or delay + System.currentTimeMillis() is negative.
IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.
public void schedule(TimerTask task,long delay)
Schedules the specified task for execution after the specified delay.
you want:
public void schedule(TimerTask task, long delay, long period)
Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals separated by the specified period.
timer.scheduleAtFixedRate( new Task(), 1000,3000);
I have the following code:
public Move chooseMove(Board board) {
// start parallel thread
this.tn = new TreeNode(this.myboard, this, null);
tn.stop = false;
this.curT = (new Thread(tn));
this.curT.start();
try {
long startTime = System.currentTimeMillis();
Thread.sleep(4000 );
System.out.println(System.currentTimeMillis() - startTime);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
tn.stop=true;
return getBestMove();
}
}
The output is sometimes a value that is way greater than 4000ms like 5400ms which means that the thread is sleeping more than it should. Any help?
Thanks.
EDIT:
I understand that there is NO guarantee of Thread#sleep stoping precisely after the specified delay. However, an additional 1400ms is a long delay. Basically,I am implementing a game player agent and I need a way to run a task and then return a value to the server after 5s (or else the server ends the game). The server is using java.util.Timer.schedule(TimerTask task, long delay). There is only one thread running concurent to the main thread which is this.curT in the code above, so there is ins't really heavy multithreading.
From docs.oracle.com:
Two overloaded versions of sleep are provided: one that specifies the sleep time to the millisecond and one that specifies the sleep time to the nanosecond. However, these sleep times are not guaranteed to be precise, because they are limited by the facilities provided by the underlying OS. Also, the sleep period can be terminated by interrupts, as we'll see in a later section. In any case, you cannot assume that invoking sleep will suspend the thread for precisely the time period specified.
This is common behavior and it is described in Thread#sleep javadoc:
Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers.
Based on this, there's no guarantee of Thread#sleep stoping the work of the thread by the amount of milliseconds stated in the parameter.
Thread#sleep will make your thread sleep for exactly 4 seconds and then wake up.
Once your thread has woken up the OS scheduler places it in the runnable queue.
When it is next picked by the scheduler it will become a running thread i.e. will occupy the CPU.
So there is an additional delay overhead due to the OS scheduler which can vary per OS, system load etc. This is unrelated to Java
I use the timer schedule method to send location information to the db every 5 min.
Here is the code:
public void onStart(Intent intent, int startId)
{
this.timer.schedule(new Send(), new Date(), TEN_SECONDS*6*5); //ten_seconds = 10000
}
class Send extends TimerTask
{
public void run()
{
String address = LocationService.this.address;
new SendLocation(LocationService.this.id,address); // a thread that sends the info to the db
LocationService.this.gpsLocation = null;
LocationService.this.networkLocation = null;
}
}
But how come that my db has locations with 7/6 minute diffrence?
The sendLocation checks if the location that i'm going to send to the db is the same as the last one, if true ignore the location else send it.
Which means that the diffrence between each location in my db should be in jump of 5 minute.
Watch this link about Timer schedule Method. It clearly says that :
Schedules the specified task for repeated fixed-delay execution,
beginning at the specified time. Subsequent executions take place at
approximately regular intervals, separated by the specified period.
In fixed-delay execution, each execution is scheduled relative to the
actual execution time of the previous execution. If an execution is
delayed for any reason (such as garbage collection or other background
activity), subsequent executions will be delayed as well. In the long
run, the frequency of execution will generally be slightly lower than
the reciprocal of the specified period (assuming the system clock
underlying Object.wait(long) is accurate).
In your code It might be possible that the run method of Send is taking more than 5 minutes to complete its task because of some heavy task to be performed by SendLocation Thread. So Your db has locations with 7/6 minute difference.
There is no guarantee your TimerTasks will be executed exactly when you request. From the Timer docs --
Each timer has one thread on which tasks are executed sequentially.
When this thread is busy running a task, runnable tasks may be subject
to delays.
and
This class does not offer guarantees about the real-time nature of
task scheduling
Granted, two minutes is a long time, so there might be delays elsewhere.
I was trying some codes to implement a scheduled task and came up with these codes .
import java.util.*;
class Task extends TimerTask {
int count = 1;
// run is a abstract method that defines task performed at scheduled time.
public void run() {
System.out.println(count+" : Mahendra Singh");
count++;
}
}
class TaskScheduling {
public static void main(String[] args) {
Timer timer = new Timer();
// Schedule to run after every 3 second(3000 millisecond)
timer.schedule( new Task(), 3000);
}
}
My output :
1 : Mahendra Singh
I expected the compiler to print a series of Mahendra Singh at periodic interval of 3 s but despite waiting for around 15 minutes, I get only one output...How do I solve this out?
Advantage of ScheduledExecutorService over Timer
I wish to offer you an alternative to Timer using - ScheduledThreadPoolExecutor, an implementation of the ScheduledExecutorService interface. It has some advantages over the Timer class, according to "Java in Concurrency":
A Timer creates only a single thread for executing timer tasks. If a
timer task takes too long to run, the timing accuracy of other
TimerTask can suffer. If a recurring TimerTask is scheduled to run
every 10 ms and another Timer-Task takes 40 ms to run, the recurring
task either (depending on whether it was scheduled at fixed rate or
fixed delay) gets called four times in rapid succession after the
long-running task completes, or "misses" four invocations completely.
Scheduled thread pools address this limitation by letting you provide
multiple threads for executing deferred and periodic tasks.
Another problem with Timer is that it behaves poorly if a TimerTask throws an unchecked exception. Also, called "thread leakage"
The Timer thread doesn't catch the exception, so an unchecked
exception thrown from a TimerTask terminates the timer thread. Timer
also doesn't resurrect the thread in this situation; instead, it
erroneously assumes the entire Timer was cancelled. In this case,
TimerTasks that are already scheduled but not yet executed are never
run, and new tasks cannot be scheduled.
And another recommendation if you need to build your own scheduling service, you may still be able to take advantage of the library by using a DelayQueue, a BlockingQueue implementation that provides the scheduling functionality of ScheduledThreadPoolExecutor. A DelayQueue manages a collection of Delayed objects. A Delayed has a delay time associated with it: DelayQueue lets you take an element only if its delay has expired. Objects are returned from a DelayQueue ordered by the time associated with their delay.
Use timer.scheduleAtFixedRate
public void scheduleAtFixedRate(TimerTask task,
long delay,
long period)
Schedules the specified task for repeated fixed-rate execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals, separated by the specified period.
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." In the long run, the frequency of execution will be exactly the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).
Fixed-rate execution is appropriate for recurring activities that are sensitive to absolute time, such as ringing a chime every hour on the hour, or running scheduled maintenance every day at a particular time. It is also appropriate for recurring activities where the total time to perform a fixed number of executions is important, such as a countdown timer that ticks once every second for ten seconds. Finally, fixed-rate execution is appropriate for scheduling multiple repeating timer tasks that must remain synchronized with respect to one another.
Parameters:
task - task to be scheduled.
delay - delay in milliseconds before task is to be executed.
period - time in milliseconds between successive task executions.
Throws:
IllegalArgumentException - if delay is negative, or delay + System.currentTimeMillis() is negative.
IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.
public void schedule(TimerTask task,long delay)
Schedules the specified task for execution after the specified delay.
you want:
public void schedule(TimerTask task, long delay, long period)
Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals separated by the specified period.
timer.scheduleAtFixedRate( new Task(), 1000,3000);
I was trying some codes to implement a scheduled task and came up with these codes .
import java.util.*;
class Task extends TimerTask {
int count = 1;
// run is a abstract method that defines task performed at scheduled time.
public void run() {
System.out.println(count+" : Mahendra Singh");
count++;
}
}
class TaskScheduling {
public static void main(String[] args) {
Timer timer = new Timer();
// Schedule to run after every 3 second(3000 millisecond)
timer.schedule( new Task(), 3000);
}
}
My output :
1 : Mahendra Singh
I expected the compiler to print a series of Mahendra Singh at periodic interval of 3 s but despite waiting for around 15 minutes, I get only one output...How do I solve this out?
Advantage of ScheduledExecutorService over Timer
I wish to offer you an alternative to Timer using - ScheduledThreadPoolExecutor, an implementation of the ScheduledExecutorService interface. It has some advantages over the Timer class, according to "Java in Concurrency":
A Timer creates only a single thread for executing timer tasks. If a
timer task takes too long to run, the timing accuracy of other
TimerTask can suffer. If a recurring TimerTask is scheduled to run
every 10 ms and another Timer-Task takes 40 ms to run, the recurring
task either (depending on whether it was scheduled at fixed rate or
fixed delay) gets called four times in rapid succession after the
long-running task completes, or "misses" four invocations completely.
Scheduled thread pools address this limitation by letting you provide
multiple threads for executing deferred and periodic tasks.
Another problem with Timer is that it behaves poorly if a TimerTask throws an unchecked exception. Also, called "thread leakage"
The Timer thread doesn't catch the exception, so an unchecked
exception thrown from a TimerTask terminates the timer thread. Timer
also doesn't resurrect the thread in this situation; instead, it
erroneously assumes the entire Timer was cancelled. In this case,
TimerTasks that are already scheduled but not yet executed are never
run, and new tasks cannot be scheduled.
And another recommendation if you need to build your own scheduling service, you may still be able to take advantage of the library by using a DelayQueue, a BlockingQueue implementation that provides the scheduling functionality of ScheduledThreadPoolExecutor. A DelayQueue manages a collection of Delayed objects. A Delayed has a delay time associated with it: DelayQueue lets you take an element only if its delay has expired. Objects are returned from a DelayQueue ordered by the time associated with their delay.
Use timer.scheduleAtFixedRate
public void scheduleAtFixedRate(TimerTask task,
long delay,
long period)
Schedules the specified task for repeated fixed-rate execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals, separated by the specified period.
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." In the long run, the frequency of execution will be exactly the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).
Fixed-rate execution is appropriate for recurring activities that are sensitive to absolute time, such as ringing a chime every hour on the hour, or running scheduled maintenance every day at a particular time. It is also appropriate for recurring activities where the total time to perform a fixed number of executions is important, such as a countdown timer that ticks once every second for ten seconds. Finally, fixed-rate execution is appropriate for scheduling multiple repeating timer tasks that must remain synchronized with respect to one another.
Parameters:
task - task to be scheduled.
delay - delay in milliseconds before task is to be executed.
period - time in milliseconds between successive task executions.
Throws:
IllegalArgumentException - if delay is negative, or delay + System.currentTimeMillis() is negative.
IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.
public void schedule(TimerTask task,long delay)
Schedules the specified task for execution after the specified delay.
you want:
public void schedule(TimerTask task, long delay, long period)
Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals separated by the specified period.
timer.scheduleAtFixedRate( new Task(), 1000,3000);