Callable与 Future | 易学教程

Callable与 Future 两功能是Java在后续版本中为了适应多并法才加入的，Callable是类似于Runnable的接口，实现Callable接口的类和实现Runnable的类都是可被其他线程执行的任务。

Callable的接口定义如下；

public interface Callable<V> {

V call() throws Exception;

}

Callable和Runnable的区别如下：

I Callable定义的方法是call，而Runnable定义的方法是run。

II Callable的call方法可以有返回值，而Runnable的run方法不能有返回值。

III Callable的call方法可抛出异常，而Runnable的run方法不能抛出异常。

Future 介绍

Future表示异步计算的结果，它提供了检查计算是否完成的方法，以等待计算的完成，并检索计算的结果。Future的cancel方法可以取消任务的执行，它有一布尔参数，参数为 true 表示立即中断任务的执行，参数为 false 表示允许正在运行的任务运行完成。Future的 get 方法等待计算完成，获取计算结果

例子：

  1 import java.io.*;
  2 import java.util.*;
  3 import java.util.concurrent.*;
  4 
  5 
  6 public class FutureTest
  7 {
  8    public static void main(String[] args) 
  9    {
 10       Scanner in = new Scanner(System.in);
 11       System.out.print("Enter base directory (e.g. /usr/local/jdk5.0/src): ");
 12       String directory = in.nextLine();
 13       System.out.print("Enter keyword (e.g. volatile): ");
 14       String keyword = in.nextLine();     
 15     
 16       MatchCounter counter = new MatchCounter(new File(directory), keyword);           
 17       FutureTask<Integer> task = new FutureTask<Integer>(counter);
 18       Thread t = new Thread(task);
 19       t.start();         
 20       try
 21       {
 22          System.out.println(task.get() + " matching files.");
 23       }
 24       catch (ExecutionException e)
 25       {
 26          e.printStackTrace();
 27       }
 28       catch (InterruptedException e) {}
 29    }
 30 }
 31 
 32 
 33 /**
 34    This task counts the files in a directory and its subdirectories that contain a given keyword.
 35 */
 36 class MatchCounter implements Callable<Integer>
 37 {
 38    /**
 39       Constructs a MatchCounter.
 40       @param directory the directory in which to start the search
 41       @param keyword the keyword to look for
 42    */
 43    public MatchCounter(File directory, String keyword)
 44    {
 45       this.directory = directory;      
 46       this.keyword = keyword;
 47    }
 48 
 49 
 50    public Integer call()
 51    {
 52       count = 0;
 53       try
 54       {
 55          File[] files = directory.listFiles();
 56          ArrayList<Future<Integer>> results = new ArrayList<Future<Integer>>();
 57 
 58 
 59          for (File file : files)      
 60             if (file.isDirectory()) 
 61             {               
 62                MatchCounter counter = new MatchCounter(file, keyword);           
 63                FutureTask<Integer> task = new FutureTask<Integer>(counter);
 64                results.add(task);
 65                Thread t = new Thread(task);
 66                t.start();         
 67             }
 68             else 
 69             {
 70                if (search(file)) count++;
 71             }
 72          
 73          for (Future<Integer> result : results)
 74             try
 75             {
 76                count += result.get();
 77             }
 78             catch (ExecutionException e)
 79             {
 80                e.printStackTrace();
 81             }
 82       }
 83       catch (InterruptedException e) {}
 84       return count;
 85    }
 86 
 87 
 88    /**
 89       Searches a file for a given keyword.
 90       @param file the file to search
 91       @return true if the keyword is contained in the file
 92    */
 93    public boolean search(File file)
 94    {     
 95       try
 96       {
 97          Scanner in = new Scanner(new FileInputStream(file));
 98          boolean found = false;
 99          while (!found && in.hasNextLine())
100          {
101             String line = in.nextLine();
102             if (line.contains(keyword)) found = true;
103          }
104          in.close();
105          return found;
106       }
107       catch (IOException e)
108       {
109          return false;
110       }
111    }
112 
113 
114    private File directory;
115    private String keyword;
116    private int count;
117 }
118 
119 
120

来源：https://www.cnblogs.com/djcsch2001/archive/2012/05/31/2529101.html

标签