The ExecutorService interface extends Executor by adding methods that help manage and control the execution of threads. It is defined in java.util.concurrent package. It defines methods that execute the threads that return results, a set of threads that determine the shutdown status. The ExecutorService interface is implemented in a utility class called Executors. It defines methods that provide an implementation of the ExecutorService interface and many other interfaces, with some default settings.
The class hierarchy is as follows:
--> java.util.concurrent Package
--> Interface ExecutorService Class
Note: ScheduledExecutorService is Implementing Sub-Interfaces and classes implemented are as follows:
- AbstractExecutorService
- ForkJoinPool
- ScheduledThreadPoolExecutor
- ThreadPoolExecutor
Methods in Executor Interface
| Method | Action Performed |
|---|---|
| awaitTermination() | Waits for all the tasks to complete their execution after a shutdown request is found. It waits for the time specified by the timelimit argument |
| invokeAll() | Executes all the tasks contained in the collection. The list of Future objects is returned which contains the status and return values of the various tasks |
| invokeAny() | Executes all the tasks contained in the collection. On completion of any single task it returns its result and all the other tasks are canceled. |
| isShutdown() | Tells whether the invoking executor is shut down or not. Returns true if shutdown otherwise returns false |
| isTerminated() | Checks if all the tasks have been completed post-shutdown. Return true if completed, otherwise returns false. |
| shutdown() | Causes all the currently executing tasks to terminate after completion in the order in which they were started and rejects any new incoming tasks. |
| shutdownNow() | Forcefully terminates all the tasks, regardless of their current state i.e running, waiting, or ready. The lists of tasks were in a ready state in return. |
| submit() | Adds a task that returns a result to the list of executing tasks for execution. It returns a Future object which returns the result of the task after completion |
Implementation: Executors
Java
// Java Program to Demonstrate ExecutorService Interface// Importing required classesimport java.util.concurrent.*;// Class// Main classpublic class SimpleExecutor { // Main driver method public static void main(String[] args) { // Creating objects of CountDownLatch class CountDownLatch cd1 = new CountDownLatch(5); CountDownLatch cd2 = new CountDownLatch(5); CountDownLatch cd3 = new CountDownLatch(5); CountDownLatch cd4 = new CountDownLatch(5); // Creating objects of ExecutorService class ExecutorService es = Executors.newFixedThreadPool(2); // Display message only for better readability System.out.println("Starting"); // Executing the tasks es.execute(new MyThread(cd1, "A")); es.execute(new MyThread(cd2, "B")); es.execute(new MyThread(cd3, "C")); es.execute(new MyThread(cd4, "D")); // Try block to check for exceptions try { // Waiting for tasks to complete cd1.await(); cd2.await(); cd3.await(); cd4.await(); } // Catch block to handle exceptions catch (InterruptedException e) { System.out.println(e); } // Making all current executing threads to terminate es.shutdown(); // Display message only for better readability System.out.println("Done"); }}// Class 2// Helper classclass MyThread implements Runnable { // Class data members String name; CountDownLatch latch; // Constructor MyThread(CountDownLatch latch, String name) { // this keyword refers to current instance itself this.name = name; this.latch = latch; new Thread(this); } // Method // Called automatically when thread is started public void run() { for (int i = 0; i < 5; i++) { System.out.println(name + ": " + i); latch.countDown(); } }} |
Output:
