Java中的Runnable、Callable、Future、FutureTask的区别

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 :

   @FunctionalInterfacepublicinterfaceRunnable{/**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     *@seejava.lang.Thread#run()
     */publicabstractvoidrun();
}

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 :

   @FunctionalInterfacepublic interface Callable<V> {/**
     * Computes a result, or throws an exception if unable to do so.
     *
     *@returncomputed result
     *@throwsException if unable to compute a result
     */V call()throwsException;
}

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:

   * @see FutureTask
 * @see Executor
 * @since1.5* @author Doug Lea
 * @param <V> Theresulttype returnedbythis Future's {@codeget} method
 */
public interface Future<V> {

    /**
     * Attemptstocancel executionofthis task.  This attempt will
     * failifthetask has already completed, has already been cancelled,
     *orcouldnotbe cancelledforsomeother reason. If successful,
     *andthis task hasnotstarted when {@code cancel}iscalled,
     * this task should neverrun.  Ifthetask has already started,
     *thenthe{@code mayInterruptIfRunning} parameter determines
     * whetherthethread executing this task should be interruptedin* an attempttostopthetask.
     *
     * <p>After this method returns, subsequent callsto{@link#isDone} will* alwaysreturn{@codetrue}.  Subsequent callsto{@link#isCancelled}* will alwaysreturn{@codetrue}ifthis method returned {@codetrue}.
     *
     * @param mayInterruptIfRunning {@codetrue}ifthethread executing this
     * task should be interrupted; otherwise,in-progress tasks are allowed
     *tocomplete
     * @return{@codefalse}ifthetask couldnotbe cancelled,
     * typically becauseithas already completed normally;
     * {@codetrue} otherwise
     */booleancancel(booleanmayInterruptIfRunning);

    /**
     * Returns {@codetrue}ifthis task was cancelledbeforeitcompleted
     * normally.
     *
     * @return{@codetrue}ifthis task was cancelledbeforeitcompleted
     */booleanisCancelled();

    /**
     * Returns {@codetrue}ifthis task completed.
     *
     * Completion may be duetonormal termination, an exception,or* cancellation-- in all of these cases, this method will return* {@codetrue}.
     *
     * @return{@codetrue}ifthis task completed
     */booleanisDone();

    /**
     * Waitsifnecessaryforthecomputationtocomplete,andthen* retrievesitsresult.
     *
     * @returnthecomputedresult* @throws CancellationExceptionifthecomputation was cancelled
     * @throws ExecutionExceptionifthecomputation threw an
     * exception
     * @throws InterruptedExceptionifthecurrent thread was interrupted
     *whilewaiting
     */
    Vget() throws InterruptedException, ExecutionException;

    /**
     * Waitsifnecessaryforatmostthegiventimeforthecomputation
     *tocomplete,andthenretrievesitsresult,ifavailable.
     *
     * @paramtimeoutthemaximumtimetowait
     * @param unitthetimeunitofthetimeoutargument
     * @returnthecomputedresult* @throws CancellationExceptionifthecomputation was cancelled
     * @throws ExecutionExceptionifthecomputation threw an
     * exception
     * @throws InterruptedExceptionifthecurrent thread was interrupted
     *whilewaiting
     * @throws TimeoutExceptionifthewait timed out
     */
    Vget(longtimeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

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：

   publicclassFutureTask<V>implementsRunnableFuture<V> {......
}

RunnableFuture

   /**
 * A {@linkFuture} that is {@linkRunnable}. Successful execution of
 * the {@coderun} method causes completion of the {@codeFuture}
 * and allows access to its results.
 *@seeFutureTask
 *@seeExecutor
 *@since1.6
 *@authorDoug Lea
 *@param<V> The result type returned by this Future's {@codeget} method
 */public interface RunnableFuture<V>extendsRunnable, Future<V> {/**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */void run();
}

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

   /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     *@paramcallable the callable task
     *@throwsNullPointerException if the callable is null
     */publicFutureTask(Callable<V> callable) {if(callable ==null)thrownewNullPointerException();this.callable = callable;this.state = NEW;// ensure visibility of callable}/**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Runnable}, and arrange that {@code get} will return the
     * given result on successful completion.
     *
     *@paramrunnable the runnable task
     *@paramresult the result to return on successful completion. If
     * you don't need a particular result, consider using
     * constructions of the form:
     * {@code Future<?> f = new FutureTask<Void>(runnable, null)}
     *@throwsNullPointerException if the runnable is null
     */publicFutureTask(Runnable runnable, V result) {this.callable = Executors.callable(runnable, result);this.state = NEW;// ensure visibility of callable}

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 ：

   /**
     * Returns a {@link Callable} objectthat, when
     * called, runsthegiventaskandreturnsthegivenresult.  This
     * can be useful when applying methods requiring a
     * {@code Callable}toan otherwise resultless action.
     * @param taskthetasktorun* @paramresulttheresulttoreturn* @param <T>thetypeoftheresult* @returna callable object
     * @throws NullPointerExceptioniftask null
     */
    public static <T> Callable<T> callable(Runnable task, Tresult) {if(task == null)
            throw new NullPointerException();returnnew RunnableAdapter<T>(task,result);
    }

RunnableAdapter适配器

   /**
     * A callable that runs given task and returns given result
     */staticfinalclassRunnableAdapter<T>implementsCallable<T> {finalRunnable task;finalT result;
        RunnableAdapter(Runnable task, T result) {this.task = task;this.result = result;
        }publicT call() {
            task.run();returnresult;
        }
    }

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。

完整示例：

   package com.stay4it.rx;

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;
import java.util.concurrent.FutureTask;publicclassFutureTest {publicstaticclassTask implements Runnable {

        @Overridepublicvoidrun() {// TODO Auto-generated method stubSystem.out.println("run");
        }

    }publicstaticclassTask2 implements Callable<Integer> {

        @OverridepublicIntegercall() throws Exception {
            System.out.println("call");returnfibc(30);
        }

    }/** 
     * runnable, 无返回值 
     */publicstaticvoidtestRunnable(){
        ExecutorService executorService = Executors.newCachedThreadPool();

        Future<String> future = (Future<String>) executorService.submit(newTask());try{
            System.out.println(future.get());
        }catch(InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();
        }catch(ExecutionException e) {// TODO Auto-generated catch blocke.printStackTrace();
        }

        executorService.shutdown();
    }/** 
     * Callable, 有返回值 
     */publicstaticvoidtestCallable(){
        ExecutorService executorService = Executors.newCachedThreadPool();

        Future<Integer> future = (Future<Integer>) executorService.submit(newTask2());try{
            System.out.println(future.get());
        }catch(InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();
        }catch(ExecutionException e) {// TODO Auto-generated catch blocke.printStackTrace();
        }

        executorService.shutdown();
    }/** 
     * FutureTask则是一个RunnableFuture<V>，即实现了Runnbale又实现了Futrue<V>这两个接口， 
     * 另外它还可以包装Runnable(实际上会转换为Callable)和Callable 
     * <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 
     * ，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。 
     */publicstaticvoidtestFutureTask(){
        ExecutorService executorService = Executors.newCachedThreadPool();
        FutureTask<Integer> futureTask =newFutureTask<Integer>(newTask2());

        executorService.submit(futureTask);try{
            System.out.println(futureTask.get());
        }catch(InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();
        }catch(ExecutionException e) {// TODO Auto-generated catch blocke.printStackTrace();
        }

        executorService.shutdown();
    }/** 
     * FutureTask则是一个RunnableFuture<V>，即实现了Runnbale又实现了Futrue<V>这两个接口， 
     * 另外它还可以包装Runnable(实际上会转换为Callable)和Callable 
     * <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 
     * ，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。 
     */publicstaticvoidtestFutureTask2(){
        ExecutorService executorService = Executors.newCachedThreadPool();
        FutureTask<Integer> futureTask =newFutureTask<Integer>(newRunnable() {

            @Overridepublicvoidrun() {// TODO Auto-generated method stubSystem.out.println("testFutureTask2 run");
            }
        },fibc(30));

        executorService.submit(futureTask);try{
            System.out.println(futureTask.get());
        }catch(InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();
        }catch(ExecutionException e) {// TODO Auto-generated catch blocke.printStackTrace();
        }

        executorService.shutdown();
    }publicstaticvoidmain(String[] args) {

        testCallable();

    }/** 
     * 效率低下的斐波那契数列, 耗时的操作 
     *  
     * @param num 
     * @return 
     */staticintfibc(intnum) {if(num ==0) {return0;  
        }if(num ==1) {return1;  
        }returnfibc(num -1) + fibc(num -2);  
    }  

}

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