Java中的Runnable、Callable、Future、FutureTask的区别与示例
  fztgkkRjHIsV 2023年11月09日 19 0


Java中存在Runnable、Callable、Future、FutureTask这几个与线程相关的类或者接口,在Java中也是比较重要的几个概念,我们通过下面的简单示例来了解一下它们的作用于区别。

Runnable

其中Runnable应该是我们最熟悉的接口,它只有一个run()函数,用于将耗时操作写在其中,该函数没有返回值。然后使用某个线程去执行该runnable即可实现多线程,Thread类在调用start()函数后就是执行的是Runnable的run()函数。Runnable的声明如下 : 


 
 
1. public interface Runnable {  
2. /**
3.      * When an object implementing interface <code>Runnable</code> is used
4.      * to create a thread, starting the thread causes the object's
5.      * <code>run</code> method to be called in that separately executing
6.      * thread.
7.      * <p>
8.      *
9.      * @see     java.lang.Thread#run()
10.      */  
11. public abstract void run();  
12. }

Callable

Callable与Runnable的功能大致相似,Callable中有一个call()函数,但是call()函数有返回值,而Runnable的run()函数不能将结果返回给客户程序。Callable的声明如下 :


1. public interface Callable<V> {  
2. /**
3.      * Computes a result, or throws an exception if unable to do so.
4.      *
5.      * @return computed result
6.      * @throws Exception if unable to compute a result
7.      */  
8. throws Exception;  
9. }

可以看到,这是一个泛型接口,call()函数返回的类型就是客户程序传递进来的V类型。

Future

Executor就是Runnable和Callable的调度容器,Future就是对于具体的Runnable或者Callable任务的执行结果进行

取消、查询是否完成、获取结果、设置结果操作。get方法会阻塞,直到任务返回结果(Future简介)。Future声明如下 :


 
 
1. /**
2. * @see FutureTask
3.  * @see Executor
4.  * @since 1.5
5.  * @author Doug Lea
6.  * @param <V> The result type returned by this Future's <tt>get</tt> method
7.  */  
8. public interface Future<V> {  
9.   
10. /**
11.      * Attempts to cancel execution of this task.  This attempt will
12.      * fail if the task has already completed, has already been cancelled,
13.      * or could not be cancelled for some other reason. If successful,
14.      * and this task has not started when <tt>cancel</tt> is called,
15.      * this task should never run.  If the task has already started,
16.      * then the <tt>mayInterruptIfRunning</tt> parameter determines
17.      * whether the thread executing this task should be interrupted in
18.      * an attempt to stop the task.     *
19.      */  
20. boolean cancel(boolean mayInterruptIfRunning);  
21.   
22. /**
23.      * Returns <tt>true</tt> if this task was cancelled before it completed
24.      * normally.
25.      */  
26. boolean isCancelled();  
27.   
28. /**
29.      * Returns <tt>true</tt> if this task completed.
30.      *
31.      */  
32. boolean isDone();  
33.   
34. /**
35.      * Waits if necessary for the computation to complete, and then
36.      * retrieves its result.
37.      *
38.      * @return the computed result
39.      */  
40. throws InterruptedException, ExecutionException;  
41.   
42. /**
43.      * Waits if necessary for at most the given time for the computation
44.      * to complete, and then retrieves its result, if available.
45.      *
46.      * @param timeout the maximum time to wait
47.      * @param unit the time unit of the timeout argument
48.      * @return the computed result
49.      */  
50. long timeout, TimeUnit unit)  
51. throws InterruptedException, ExecutionException, TimeoutException;  
52. }


FutureTask

FutureTask则是一个RunnableFuture<V>,而RunnableFuture实现了Runnbale又实现了Futrue<V>这两个接口,


1. public class FutureTask<V> implements RunnableFuture<V>

RunnableFuture


1. public interface RunnableFuture<V> extends Runnable, Future<V> {  
2. /**
3.      * Sets this Future to the result of its computation
4.      * unless it has been cancelled.
5.      */  
6. void run();  
7. }

另外它还可以包装Runnable和Callable<V>, 由构造函数注入依赖。


1. public FutureTask(Callable<V> callable) {  
2. if (callable == null)  
3. throw new NullPointerException();  
4. this.callable = callable;  
5. this.state = NEW;       // ensure visibility of callable  
6. }  
7.   
8. public FutureTask(Runnable runnable, V result) {  
9. this.callable = Executors.callable(runnable, result);  
10. this.state = NEW;       // ensure visibility of callable  
11. }

可以看到,Runnable注入会被Executors.callable()函数转换为Callable类型,即FutureTask最终都是执行Callable类型的任务。该适配函数的实现如下 :


1. public static <T> Callable<T> callable(Runnable task, T result) {  
2. if (task == null)  
3. throw new NullPointerException();  
4. return new RunnableAdapter<T>(task, result);  
5. }

RunnableAdapter适配器



1. /**
2.  * A callable that runs given task and returns given result
3.  */  
4. static final class RunnableAdapter<T> implements Callable<T> {  
5. final Runnable task;  
6. final T result;  
7.     RunnableAdapter(Runnable task, T result) {  
8. this.task = task;  
9. this.result = result;  
10.     }  
11. public T call() {  
12.         task.run();  
13. return result;  
14.     }  
15. }

由于FutureTask实现了Runnable,因此它既可以通过Thread包装来直接执行,也可以提交给ExecuteService来执行。

并且还可以直接通过get()函数获取执行结果,该函数会阻塞,直到结果返回。因此FutureTask既是Future、

Runnable,又是包装了Callable( 如果是Runnable最终也会被转换为Callable ), 它是这两者的合体。


简单示例


1.  package com.effective.java.concurrent.task;  
2.   
3. import java.util.concurrent.Callable;  
4. import java.util.concurrent.ExecutionException;  
5. import java.util.concurrent.ExecutorService;  
6. import java.util.concurrent.Executors;  
7. import java.util.concurrent.Future;  
8. import java.util.concurrent.FutureTask;  
9.   
10. /**
11.  * 
12.  * @author mrsimple
13.  *
14.  */  
15. public class RunnableFutureTask {  
16.   
17. /**
18.      * ExecutorService
19.      */  
20. static ExecutorService mExecutor = Executors.newSingleThreadExecutor();  
21.   
22. /**
23.      * 
24.      * @param args
25.      */  
26. public static void main(String[] args) {  
27.         runnableDemo();  
28.         futureDemo();  
29.     }  
30.   
31. /**
32.      * runnable, 无返回值
33.      */  
34. static void runnableDemo() {  
35.   
36. new Thread(new Runnable() {  
37.   
38. @Override  
39. public void run() {  
40. "runnable demo : " + fibc(20));  
41.             }  
42.         }).start();  
43.     }  
44.   
45. /**
46.      * 其中Runnable实现的是void run()方法,无返回值;Callable实现的是 V
47.      * call()方法,并且可以返回执行结果。其中Runnable可以提交给Thread来包装下
48.      * ,直接启动一个线程来执行,而Callable则一般都是提交给ExecuteService来执行。
49.      */  
50. static void futureDemo() {  
51. try {  
52. /**
53.              * 提交runnable则没有返回值, future没有数据
54.              */  
55. new Runnable() {  
56.   
57. @Override  
58. public void run() {  
59. 20);  
60.                 }  
61.             });  
62.   
63. "future result from runnable : " + result.get());  
64.   
65. /**
66.              * 提交Callable, 有返回值, future中能够获取返回值
67.              */  
68. new Callable<Integer>() {  
69. @Override  
70. public Integer call() throws Exception {  
71. return fibc(20);  
72.                 }  
73.             });  
74.   
75.             System.out  
76. "future result from callable : " + result2.get());  
77.   
78. /**
79.              * FutureTask则是一个RunnableFuture<V>,即实现了Runnbale又实现了Futrue<V>这两个接口,
80.              * 另外它还可以包装Runnable(实际上会转换为Callable)和Callable
81.              * <V>,所以一般来讲是一个符合体了,它可以通过Thread包装来直接执行,也可以提交给ExecuteService来执行
82.              * ,并且还可以通过v get()返回执行结果,在线程体没有执行完成的时候,主线程一直阻塞等待,执行完则直接返回结果。
83.              */  
84. new FutureTask<Integer>(  
85. new Callable<Integer>() {  
86. @Override  
87. public Integer call() throws Exception {  
88. return fibc(20);  
89.                         }  
90.                     });  
91. // 提交futureTask  
92.             mExecutor.submit(futureTask) ;  
93. "future result from futureTask : "  
94.                     + futureTask.get());  
95.   
96. catch (InterruptedException e) {  
97.             e.printStackTrace();  
98. catch (ExecutionException e) {  
99.             e.printStackTrace();  
100.         }  
101.     }  
102.   
103. /**
104.      * 效率底下的斐波那契数列, 耗时的操作
105.      * 
106.      * @param num
107.      * @return
108.      */  
109. static int fibc(int num) {  
110. if (num == 0) {  
111. return 0;  
112.         }  
113. if (num == 1) {  
114. return 1;  
115.         }  
116. return fibc(num - 1) + fibc(num - 2);  
117.     }  
118.   
119. }

输出结果




Java中的Runnable、Callable、Future、FutureTask的区别与示例_ide


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