Java 併發 - Future 和 Callabler

java.util.concurrent.Callable 物件可以返回由執行緒進行的計算結果，與只會執行執行緒的 runnable 介面相反。Callable 物件返回 Future 物件，其提供了用於監視由執行緒執行的任務進度的多個方法。Future 物件可用於檢查 Callable 的狀態，然後線上程完成後從 Callable 中檢索結果。它還提供了超時功能。

語法

//submit the callable using ThreadExecutor
//and get the result as a Future object
Future<Long> result10 = executor.submit(new FactorialService(10));
 
//get the result using get method of the Future object
//get method waits till the thread execution and then return the result of the execution.
Long factorial10 = result10.get();

示例

下面的 TestThread 程式顯示了在基於執行緒的環境中使用 Future 和 Callable。

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 TestThread {

   public static void main(final String[] arguments) throws InterruptedException,
      ExecutionException {

      ExecutorService executor = Executors.newSingleThreadExecutor();

      System.out.println("Factorial Service called for 10!");
      Future<Long> result10 = executor.submit(new FactorialService(10));

      System.out.println("Factorial Service called for 20!");
      Future<Long> result20 = executor.submit(new FactorialService(20));

      Long factorial10 = result10.get();
      System.out.println("10! = " + factorial10);

      Long factorial20 = result20.get();
      System.out.println("20! = " + factorial20);

      executor.shutdown();
   }  

   static class FactorialService implements Callable<Long> {
      private int number;

      public FactorialService(int number) {
         this.number = number;
      }

      @Override
      public Long call() throws Exception {
         return factorial();
      }

      private Long factorial() throws InterruptedException {
         long result = 1; 
         
         while (number != 0) { 
            result = number * result; 
            number--; 
            Thread.sleep(100); 
         }
         return result;	
      }
   }
}

這將產生以下結果。

輸出

Factorial Service called for 10!
Factorial Service called for 20!
10! = 3628800
20! = 2432902008176640000