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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?

Ahmed Kaddoura
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Panayappan Swaminathan
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  • Well, have a look at the `Timer` source as well as the [JavaDoc](https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/util/Timer.html). There it states: "Corresponding to each Timer object is a single backgroundthread that is used to execute all of the timer's tasks, sequentially." - so yes, each `Timer` has its own thread but if you schedule 2 tasks on the same timer they will be executed sequentially. – Thomas Oct 14 '19 at 12:40
  • Also see this answer : https://stackoverflow.com/a/52306459/5612858 – Arnaud Oct 14 '19 at 12:42
  • Thanks Thomas. So the behaviour is same with Schedule and ScheduleatFixedRate (Can you please tell me what does this mean exactly - two or more executions will occur in rapid succession to "catch up.".) – Panayappan Swaminathan Oct 14 '19 at 12:44
  • Nowadays you will probably prefer to use [`ScheduledExecutorService`](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/ScheduledExecutorService.html) over a `Timer`. – Ole V.V. Mar 26 '22 at 12:35

2 Answers2

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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.

Ahmed Kaddoura
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-1

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/

RamiReddy P
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