Java Executors callable() Method with Examples

by Online Tutorials Library July 22, 2022

Java Executors Callable() Method

The Callable() method of Executors class returns a Callable object that, when called, runs the given task and returns null.

Syntax

  public static Callable<Object> callable(Runnable task)  public static <T> Callable<T> callable(Runnable task, T result)  public static Callable<Object> callable(PrivilegeAction<?> action)  public static Callable<Object> callable(PrivilegedExceptionAction<?> action)  

Parameters

task – the task to run

result – the result to return

action – the privileged action to run

Return

a callable object

Throw

NullPointerException

Example 1

      import java.util.ArrayList;        import java.util.Date;        import java.util.List;        import java.util.concurrent.Callable;        import java.util.concurrent.ExecutionException;        import java.util.concurrent.ExecutorService;        import java.util.concurrent.Executors;        import java.util.concurrent.Future;                public class ExecutorCallableExample1 implements Callable {                    @Override            public String call() throws Exception {                Thread.sleep(1000);                return Thread.currentThread().getName();            }                        public static void main(String args[]){                        ExecutorService executor = Executors.newFixedThreadPool(20);                List> list = new ArrayList>();                Callable callable = new ExecutorCallableExample1();                        for(int i=0; i< 10; i++){                    Future ftr = executor.submit(callable);                    list.add(ftr);                }                for(Future fut : list){                    try {                        System.out.println(new Date()+ “::”+fut.get());                            } catch (InterruptedException | ExecutionException e) {                        e.printStackTrace();                    }                }                executor.shutdown();            }                }   

Test it Now

Output:

Sun Mar 17 02:29:46 IST 2019::pool-1-thread-1  Sun Mar 17 02:29:48 IST 2019::pool-1-thread-2  Sun Mar 17 02:29:48 IST 2019::pool-1-thread-3  Sun Mar 17 02:29:48 IST 2019::pool-1-thread-4  Sun Mar 17 02:29:48 IST 2019::pool-1-thread-5  Sun Mar 17 02:29:48 IST 2019::pool-1-thread-6  Sun Mar 17 02:29:48 IST 2019::pool-1-thread-7  Sun Mar 17 02:29:48 IST 2019::pool-1-thread-8  Sun Mar 17 02:29:48 IST 2019::pool-1-thread-9  Sun Mar 17 02:29:48 IST 2019::pool-1-thread-10

Example 2

      import java.util.ArrayList;        import java.util.Date;        import java.util.List;        import java.util.concurrent.Callable;        import java.util.concurrent.ExecutionException;        import java.util.concurrent.ExecutorService;        import java.util.concurrent.Executors;        import java.util.concurrent.Future;                public class ExecutorCallableExample2 implements Callable {                    @Override            public String call() throws Exception {                Thread.sleep(500);                return Thread.currentThread().getName();            }               public static void main(String args[]){                        ExecutorService executor = Executors.newFixedThreadPool(20);                List> lst = new ArrayList>();                Callable clbl = new ExecutorCallableExample1();                   int i =0;              while( i < 5){                    Future ftr = executor.submit(clbl);                    lst.add(ftr);                  i++;              }                for(Future fut : lst){                    try {                        System.out.println( “::”+fut.get());                            } catch (InterruptedException | ExecutionException e) {                        e.printStackTrace();                    }                }                executor.shutdown();            }                }   

Test it Now

Output:

::pool-1-thread-1  ::pool-1-thread-2  ::pool-1-thread-3  ::pool-1-thread-4  ::pool-1-thread-5

Next TopicJava Executors

callable()defaultThreadFactory()Executors java newCachedThreadPool()newScheduledThreadPool()newSingleThreadExecutor()privilegedThreadFactory()