Callable interface use Generic to define the return type of Object. Executors class provide useful methods to execute Callable in a thread pool. Since callable tasks run in parallel, we have to wait for the returned Object. Callable tasks return java.util.concurrent.Future object. Using Future we can find out the status of the Callable task and get the returned Object. It provides get() method that can wait for the Callable to finish and then return the result.
Future provides cancel() method to cancel the associated Callable task. There is an overloaded version of get() method where we can specify the time to wait for the result, it’s useful to avoid current thread getting blocked for longer time. There are isDone() and isCancelled() methods to find out the current status of associated Callable task.
Here is a simple example of Callable task that returns the name of thread executing the task after one second. We are using Executor framework to execute 100 tasks in parallel and use Future to get the result of the submitted tasks.
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 MyCallable implements Callable<String> {
@Override
public String call() throws Exception {
Thread.sleep(1000);
//return the thread name executing this callable task
return Thread.currentThread().getName();
}
public static void main(String args[]){
//Get ExecutorService from Executors utility class, thread pool size is 10
ExecutorService executor = Executors.newFixedThreadPool(10);
//create a list to hold the Future object associated with Callable
List<Future<String>> list = new ArrayList<Future<String>>();
//Create MyCallable instance
Callable<String> callable = new MyCallable();
for(int i=0; i< 100; i++){
//submit Callable tasks to be executed by thread pool
Future<String> future = executor.submit(callable);
//add Future to the list, we can get return value using Future
list.add(future);
}
for(Future<String> fut : list){
try {
//print the return value of Future, notice the output delay in console
// because Future.get() waits for task to get completed
System.out.println(new Date()+ "::"+fut.get());
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
}
//shut down the executor service now
executor.shutdown();
}
}
Once we execute the above program, you will notice the delay in output because Future get() method waits for the callable task to complete. Also notice that there are only 10 threads executing these tasks.
Here is snippet of the output of above program.
Mon Dec 31 20:40:15 PST 2012::pool-1-thread-1 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-2 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-3 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-4 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-5 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-6 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-7 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-8 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-9 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-10 Mon Dec 31 20:40:16 PST 2012::pool-1-thread-2
