0Callables and Future
The FiutureTask is "A cancellable asynchronous computation. This class provides a base implementation of Future, with methods to start and cancel a computation, query to see if the computation is complete, and retrieve the result of the computation."
Callables are just like Runnables to define the smallest unit of work called tasks. The difference between Callable and Runnable is that Runnable cannot return any result whereas Callable can return result of type Future. Later we can get the data from this return value.
Once we submit the Callable task to the Executor, it does not block and returns immediately. We can determine if the task is finished or not by using the isDone api. Once isDone returns TRUE, we can access the result from the future which was returned from submitting the task to the executor using the future.get() API. However, we must remember that get API is blocking and hence if the task is not completed when the get has been called, it will block the thread.
ExecutorService
Actually FutureTask is designed to be used through the ExecutorService interface and the classes that implement it. It is those classes that use FutureTask and fork the threads and create non-blocking Asynchronous background task. Executors typically manage a pool of threads so that we don't have to create threads manually. All the threads in a thread-pool can be reused.
The source code mentioned below is a working example of the use of FutureTask alongwith Executor model of the Java Concurrency Framework. The basic motto of the Application is the following.
Suppose we need to do a very time consuming task at any time of an Application. Like reading a big chunk of data from a huge database. So the basic idea is that whenever the application starts we spawn a background thread through the executor framework and delegate the task of reading data to that background thread. While reading of the data is going on, we continue with our other task in the application. The background thread collects the data and keep it in the future variable which is returned when we submit the task to the executor service. Any time of the application lifecycle we can know if the task is completed or not by calling the api isDone() on the future returned from submitting the task. Then in later time we can access the already fetched data by using the get() api on the future variable which was returned when the task was submitted to the executor framework. Not only that, when the task is going on in the background we can cancel it at anytime we want.
Hope this article comes handy to the learners of Java who are ready to deep dive in the world of concurrent programming.
Class ProductInfo
package com.somitsolutions.training.java.ExperimentationWithFutureTask;
public class ProductInfo {
private String productName;
private float productPrice;
public ProductInfo(String productName, float productPrice){
this.productName = productName;
this.productPrice = productPrice;
}
public String getProductName() {
return productName;
}
public void setProductName(String productName) {
this.productName = productName;
}
public float getProductPrice() {
return productPrice;
}
public void setProductPrice(float productPrice) {
this.productPrice = productPrice;
}
}
Class Preloader
package com.somitsolutions.training.java.ExperimentationWithFutureTask;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.FutureTask;
import java.util.concurrent.Callable;
public class Preloader {
static ExecutorService executor = Executors.newFixedThreadPool(1);
List<ProductInfo> productInfo = new ArrayList<ProductInfo>();
//The difference between Callable & Runnable
//is that Callable can return a value (of type futuretask)
private FutureTask<List<ProductInfo>> future = null;
/*new FutureTask<List>(new LoadProductInfo());*/
public List<ProductInfo> get(){
//List retValue = null;
try {
//get is blocking
productInfo = future.get();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ExecutionException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return productInfo;
}
public boolean cancel(){
return future.cancel(true);
}
public boolean isDone(){
return future.isDone();
}
//private final Thread thread = new Thread(future);
public void start() {
System.out.println("The task is being submitted now...");
//submit will return immediately. So we can do the other work
//in the main thread. Later we can check if the task is
//finished or not using isDone method.
future = (FutureTask<List<ProductInfo>>) (executor.submit(new LoadProductInfo()));
}
//long running task
private List<ProductInfo> loadProductInfo(){
System.out.println(Thread.currentThread().getName());
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
//As if this data we have got from
//the database
for (int i = 0; i<100000; i++){
ProductInfo productI = new ProductInfo(Integer.toString(i), i);
productInfo.add(productI);
}
return productInfo;
}
//The difference between Callable & Runnable
//is that Callable can return a value (of type futuretask)
class LoadProductInfo implements Callable<List<ProductInfo>>{
@Override
public List<ProductInfo> call() throws Exception {
// TODO Auto-generated method stub
return loadProductInfo();
}
}
}
Class Main
package com.somitsolutions.training.java.ExperimentationWithFutureTask;
import java.util.List;
public 
