I want to have a class that changes its own private variables every 2 seconds. I know that if I do something like
import java.util.Timer;
//...
Timer timer;
//...
timer.schedule(new ChangeSomething(), 2000);
It will execute ChangeSomething() after 2 seconds, is there a way to tell it to do something every 2 seconds, or, If I put inside ChangeSomething()
timer.schedule(new ChangeSomething(), 2000);
will it work?
On a side-note, what does timer.cancel() exactly do?
Use timer.scheduleAtFixedRate() to schedule it to recur every two seconds:
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.
From the javadoc for Timer.cancel():
Terminates this timer, discarding any currently scheduled tasks. Does not interfere with a currently executing task (if it exists). Once a timer has been terminated, its execution thread terminates gracefully, and no more tasks may be scheduled on it.
EDIT:
Relating to internal execution thread for a Timer that executes a single task once:
After the last live reference to a Timer object goes away and all outstanding tasks have completed execution, the timer's task execution thread terminates gracefully (and becomes subject to garbage collection). However, this can take arbitrarily long to occur. By default, the task execution thread does not run as a daemon thread, so it is capable of keeping an application from terminating. If a caller wants to terminate a timer's task execution thread rapidly, the caller should invoke the timer's cancel method.
You will need to call to a different scheduling method of Timer, called scheduleAtFixedRate(...) which can get 3 parameters. The first 2 are identical to those of schedule you've used, while The third parameter indicates a period time in milliseconds between successive task executions.
import java.util.Timer;
//...
Timer timer;
//...
timer.scheduleAtFixedRate(new ChangeSomething(), 2000, 2000);
You can check the java pai doc for this method here: http://docs.oracle.com/javase/6/docs/api/java/util/Timer.html#scheduleAtFixedRate(java.util.TimerTask, java.util.Date, long)
Here is an example
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
public class Test extends TimerTask {
private int age;
public Test() {
Timer timer = new Timer();
timer.scheduleAtFixedRate(this, new Date(), 2000);
}
/**
* Implements TimerTask's abstract run method.
*/
public void run(){
//toy implementation
System.out.print("Changing Data ... before change age is "+age+" ");
changeAge();
System.out.println("after change age is "+age);
}
private void changeAge() {
age = (int)Math.round(Math.random()*1000);
}
public static void main(String[] args) {
new Test();
}
}
To be more precise here: ChangeSomething() is the constructor of your ChangeSomething class. The constructor will be executed immediately when you pass the ChangeSomething instace object to the timer, not after 2 seconds. It's that object's run() method will be triggered after 2 seconds.
To execute that run() method repeatedly all 2 seconds, use schedule(TimerTask task, long delay, long period)
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 this simple code using util.Timer
public class CheckDBAccessListOpenAccesses extends TimerTask {
public static void main(String[] args) {
CheckDBAccessListOpenAccesses object = new CheckDBAccessListOpenAccesses();
}
private CheckDBAccessListOpenAccesses() {
System.out.println("Start");
Timer whatAccessAreOpen = new Timer();
whatAccessAreOpen.scheduleAtFixedRate(this, TimeUnit.MINUTES.toMillis(1), TimeUnit.MINUTES.toMillis(1));
Thread.sleep(100*60*1000);
System.out.println("finish");
}
#Override
public void run() {
System.out.println("one minute");
}
}
When I run the code, the process runs forever.
I want the process to stop when the main Thread ends. Why is the Timer keeps the process alive? is there a way to prevent it?
Java has two types of Threads: user threads and daemon threads. User threads prevent the JVM from terminating, daemon threads do not. By default, a Timer is backed by a user thread, unless it is specified to run as a daemon in the constructor.
To prevent the Timer from keeping the process alive, instantiate the Timer with:
new Timer (true)
See: https://docs.oracle.com/javase/8/docs/api/java/util/Timer.html#Timer-boolean-
Well, that's in the Timer's API.
The Timer#scheduleAtFixedRate method:
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.
[...]
You may want to use Timer#schedule for one-shot operations, instead.
Otherwise, and what you likely really want, is to have the Timer object visible from your main thread, so you can just invoke Timer#cancel on it when you wish to cancel the scheduled operation, e.g. when your main thread terminates.
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);
I tried everything. This one too How to stop the task scheduled in Java.util.Timer class
I have one task that implements java.util.TimerTask
I call that task in 2 ways:
I schedule Timer like this:
timer.schedule(timerTask, 60 * 1000);
sometimes I need that work to start immediately and it has to cancel timerTask if there is any that is working
cancelCurrentWork();
timer.schedule(timerTask, 0);
This implementation doesn't stop current work:
(documentation says: If the task is running when this call occurs, the task will run to completion, but will never run again)
But I need it to stop.
public static void cancelCurrentwork() {
if (timerTask!= null) {
timerTask.cancel();
}
}
This implementation just cancels the timer but leaves currently doing task to be finished.
public static void cancelCurrentwork() {
if (timer!= null) {
timer.cancel();
}
}
Is there a way in timer to STOP current executing taks, something like Thread.kill() or something? When I need that task to stop I want it to loose all its data.
There is no way for the Timer to stop the task in its tracks.
You will need to have a separate mechanism in the running task itself, that checks if it should keep running. You could for instance have an AtomicBoolean keepRunning variable which you set to false when you want the task to terminate.
if your timer is using some sort of file/socket etc, you can close that object from outside the timer, and the timer task will throw an exception, and you can use it to stop the timer.
but in general you need some sort of poison pill to successfully stop a separate thread/timer.