Execute a task repeatedly in Java using Timer

Timer in Java

In order to execute a task repeatedly in Java, you can try to use java.util.Timer class. This is the description of official documentation saying about Timer.

A facility for threads to schedule tasks for future execution in a background thread. Tasks may be scheduled for one-time execution, or for repeated execution at regular intervals.
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.

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.

If the timer's task execution thread terminates unexpectedly, for example, because its stop method is invoked, any further attempt to schedule a task on the timer will result in an IllegalStateException, as if the timer's cancel method had been invoked.

This class is thread-safe: multiple threads can share a single Timer object without the need for external synchronization.

This class does not offer real-time guarantees: it schedules tasks using the Object.wait(long) method.

Java 5.0 introduced the java.util.concurrent package and one of the concurrency utilities therein is the ScheduledThreadPoolExecutor which is a thread pool for repeatedly executing tasks at a given rate or delay. It is effectively a more versatile replacement for the Timer/TimerTask combination, as it allows multiple service threads, accepts various time units, and doesn't require subclassing TimerTask (just implement Runnable). Configuring ScheduledThreadPoolExecutor with one thread makes it equivalent to Timer.

Implementation note: This class scales to large numbers of concurrently scheduled tasks (thousands should present no problem). Internally, it uses a binary heap to represent its task queue, so the cost to schedule a task is O(log n), where n is the number of concurrently scheduled tasks.

Implementation note: All constructors start a timer thread.

Task that is to be executed repeatedly should extends from the java.util.TimerTask class. Below is a simple example of using it.

    public static void main(String... args) {
        Timer timer = new Timer();

        timer.schedule(new TimerTask() {
            public void run() {
                System.out.println("Task started at: " + new Date());
        }, 5000, 1000);

The above example uses method schedule() to execute task, that means after 5 seconds delay of calling method, task will be executed, and after that one second, task is going to be repeated again and again.

Have a deeper look at the documentation, you will see another method scheduleAtFixedRate(). What is the difference between the two methods?

It’s very simple. When using schedule(), Timer object will guarantee that next task will trigger only when current one is finished; however, scheduleAtFixedRate() does not care about current task completion, it just executes next repetition after time period is over. So there is a chance that there will be multiple tasks executed simultaneously with scheduleAtFixedRate.