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Java中Runnable和Callable有何不同?针对这个问题,这篇文章详细介绍了相对应的分析和解答,希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。
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Java中的Runnable,Callable,Future,FutureTask的比较
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 :
@FunctionalInterface public interface Runnable { /** * When an object implementing interfaceRunnable
is used * to create a thread, starting the thread causes the object's *run
method to be called in that separately executing * thread. ** The general contract of the method
run
is that it may * take any action whatsoever. * * @see java.lang.Thread#run() */ public abstract void run(); }
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 :
@FunctionalInterface public interface Callable{ /** * Computes a result, or throws an exception if unable to do so. * * @return computed result * @throws Exception if unable to compute a result */ V call() throws Exception; }
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 * @since 1.5 * @author Doug Lea * @paramThe result type returned by this Future's {@code get} method */ public interface Future { /** * Attempts to cancel execution of this task. This attempt will * fail if the task has already completed, has already been cancelled, * or could not be cancelled for some other reason. If successful, * and this task has not started when {@code cancel} is called, * this task should never run. If the task has already started, * then the {@code mayInterruptIfRunning} parameter determines * whether the thread executing this task should be interrupted in * an attempt to stop the task. * * After this method returns, subsequent calls to {@link #isDone} will * always return {@code true}. Subsequent calls to {@link #isCancelled} * will always return {@code true} if this method returned {@code true}. * * @param mayInterruptIfRunning {@code true} if the thread executing this * task should be interrupted; otherwise, in-progress tasks are allowed * to complete * @return {@code false} if the task could not be cancelled, * typically because it has already completed normally; * {@code true} otherwise */ boolean cancel(boolean mayInterruptIfRunning); /** * Returns {@code true} if this task was cancelled before it completed * normally. * * @return {@code true} if this task was cancelled before it completed */ boolean isCancelled(); /** * Returns {@code true} if this task completed. * * Completion may be due to normal termination, an exception, or * cancellation -- in all of these cases, this method will return * {@code true}. * * @return {@code true} if this task completed */ boolean isDone(); /** * Waits if necessary for the computation to complete, and then * retrieves its result. * * @return the computed result * @throws CancellationException if the computation was cancelled * @throws ExecutionException if the computation threw an * exception * @throws InterruptedException if the current thread was interrupted * while waiting */ V get() throws InterruptedException, ExecutionException; /** * Waits if necessary for at most the given time for the computation * to complete, and then retrieves its result, if available. * * @param timeout the maximum time to wait * @param unit the time unit of the timeout argument * @return the computed result * @throws CancellationException if the computation was cancelled * @throws ExecutionException if the computation threw an * exception * @throws InterruptedException if the current thread was interrupted * while waiting * @throws TimeoutException if the wait timed out */ V get(long timeout, 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:
public class FutureTaskimplements RunnableFuture { ...... }
RunnableFuture
/** * A {@link Future} that is {@link Runnable}. Successful execution of * the {@code run} method causes completion of the {@code Future} * and allows access to its results. * @see FutureTask * @see Executor * @since 1.6 * @author Doug Lea * @paramThe result type returned by this Future's {@code get} method */ public interface RunnableFuture extends Runnable, Future { /** * 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}. * * @param callable the callable task * @throws NullPointerException if the callable is null */ public FutureTask(Callablecallable) { if (callable == null) throw new NullPointerException(); 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. * * @param runnable the runnable task * @param result 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 (runnable, null)} * @throws NullPointerException if the runnable is null */ public FutureTask(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} object that, when * called, runs the given task and returns the given result. This * can be useful when applying methods requiring a * {@code Callable} to an otherwise resultless action. * @param task the task to run * @param result the result to return * @paramthe type of the result * @return a callable object * @throws NullPointerException if task null */ public static Callable callable(Runnable task, T result) { if (task == null) throw new NullPointerException(); return new RunnableAdapter (task, result); }
RunnableAdapter适配器
/** * A callable that runs given task and returns given result */ static final class RunnableAdapterimplements Callable { final Runnable task; final T result; RunnableAdapter(Runnable task, T result) { this.task = task; this.result = result; } public T call() { task.run(); return result; } }
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; public class FutureTest { public static class Task implements Runnable { @Override public void run() { // TODO Auto-generated method stub System.out.println("run"); } } public static class Task2 implements Callable{ @Override public Integer call() throws Exception { System.out.println("call"); return fibc(30); } } /** * runnable, 无返回值 */ public static void testRunnable(){ ExecutorService executorService = Executors.newCachedThreadPool(); Future future = (Future ) executorService.submit(new Task()); try { System.out.println(future.get()); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ExecutionException e) { // TODO Auto-generated catch block e.printStackTrace(); } executorService.shutdown(); } /** * Callable, 有返回值 */ public static void testCallable(){ ExecutorService executorService = Executors.newCachedThreadPool(); Future future = (Future ) executorService.submit(new Task2()); try { System.out.println(future.get()); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ExecutionException e) { // TODO Auto-generated catch block e.printStackTrace(); } executorService.shutdown(); } /** * FutureTask则是一个RunnableFuture ,即实现了Runnbale又实现了Futrue 这两个接口, * 另外它还可以包装Runnable(实际上会转换为Callable)和Callable * ,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。 */ public static void testFutureTask(){ ExecutorService executorService = Executors.newCachedThreadPool(); FutureTask futureTask = new FutureTask (new Task2()); executorService.submit(futureTask); try { System.out.println(futureTask.get()); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ExecutionException e) { // TODO Auto-generated catch block e.printStackTrace(); } executorService.shutdown(); } /** * FutureTask则是一个RunnableFuture ,即实现了Runnbale又实现了Futrue 这两个接口, * 另外它还可以包装Runnable(实际上会转换为Callable)和Callable * ,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。 */ public static void testFutureTask2(){ ExecutorService executorService = Executors.newCachedThreadPool(); FutureTask futureTask = new FutureTask (new Runnable() { @Override public void run() { // TODO Auto-generated method stub System.out.println("testFutureTask2 run"); } },fibc(30)); executorService.submit(futureTask); try { System.out.println(futureTask.get()); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ExecutionException e) { // TODO Auto-generated catch block e.printStackTrace(); } executorService.shutdown(); } public static void main(String[] args) { testCallable(); } /** * 效率低下的斐波那契数列, 耗时的操作 * * @param num * @return */ static int fibc(int num) { if (num == 0) { return 0; } if (num == 1) { return 1; } return fibc(num - 1) + fibc(num - 2); } }
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