Java中的Runnable Callable Future FutureTask的區別與示例
阿新 • • 發佈：2018-11-13
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Java中存在Runnable、Callable、Future、FutureTask這幾個與執行緒相關的類或者介面，在Java中也是比較重要的幾個概念，我們通過下面的簡單示例來了解一下它們的作用於區別。
Runnable
其中Runnable應該是我們最熟悉的介面，它只有一個run()函式，用於將耗時操作寫在其中，該函式沒有返回值。然後使用某個執行緒去執行該runnable即可實現多執行緒，Thread類在呼叫start()函式後就是執行的是Runnable的run()函式。Runnable的宣告如下 :
public interface Runnable { /** * 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> * * @see Callable
Callable與Runnable的功能大致相似，Callable中有一個call()函式，但是call()函式有返回值，而Runnable的run()函式不能將結果返回給客戶程式。Callable的宣告如下 :
public interface Callable<V> { /** * Computes a result, or throws an exception if unable to do so. * * @return Future
Executor就是Runnable和Callable的排程容器，Future就是對於具體的Runnable或者Callable任務的執行結果進行
取消、查詢是否完成、獲取結果、設定結果操作。get方法會阻塞，直到任務返回結果(Future簡介)。Future宣告如下 :
/*** @see FutureTask * @see Executor * @since 1.5 * @author Doug Lea * @param <V> The result type returned by this Future's <tt>get</tt> method */public interface Future<V> { /** * 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 <tt>cancel</tt> is called, * this task should never run. If the task has already started, * then the <tt>mayInterruptIfRunning</tt> parameter determines * whether the thread executing this task should be interrupted in * an attempt to stop the task. * */ boolean cancel(boolean mayInterruptIfRunning); /** * Returns <tt>true</tt> if this task was cancelled before it completed * normally. */ boolean isCancelled(); /** * Returns <tt>true</tt> if this task completed. * */ boolean isDone(); /** * Waits if necessary for the computation to complete, and then * retrieves its result. * * @return the computed result */ 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 */ V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;}FutureTask
FutureTask則是一個RunnableFuture<V>，而RunnableFuture實現了Runnbale又實現了Futrue<V>這兩個介面，
public class FutureTask<V> implements RunnableFuture<V>
public interface RunnableFuture<V> extends Runnable, Future<V> { /** * Sets this Future to the result of its computation * unless it has been cancelled. */ void run();}另外它還可以包裝Runnable和Callable<V>， 由建構函式注入依賴。
public FutureTask(Callable<V> callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; this.state = NEW; // ensure visibility of callable } public FutureTask(Runnable runnable, V result) { this.callable = Executors.callable(runnable, result); this.state = NEW; // ensure visibility of callable }
public static <T> Callable<T> callable(Runnable task, T result) { if (task == null) throw new NullPointerException(); return new RunnableAdapter<T>(task, result); }
/** * A callable that runs given task and returns given result */ static final class RunnableAdapter<T> implements Callable<T> { final Runnable task; final T result; RunnableAdapter(Runnable task, T result) { this.task = task; this.result = result; } public T call() { task.run(); return result; } }由於FutureTask實現了Runnable，因此它既可以通過Thread包裝來直接執行，也可以提交給ExecuteService來執行。
並且還可以直接通過get()函式獲取執行結果，該函式會阻塞，直到結果返回。因此FutureTask既是Future、
Runnable，又是包裝了Callable( 如果是Runnable最終也會被轉換為Callable )， 它是這兩者的合體。
簡單示例
package com.effective.java.concurrent.task;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;/** * * @author mrsimple * */public class RunnableFutureTask { /** * ExecutorService */ static ExecutorService mExecutor = Executors.newSingleThreadExecutor(); /** * * @param args */ public static void main(String[] args) { runnableDemo(); futureDemo(); } /** * runnable, 無返回值 */ static void runnableDemo() { new Thread(new Runnable() { @Override public void run() { System.out.println("runnable demo : " + fibc(20)); } }).start(); } /** * 其中Runnable實現的是void run()方法，無返回值；Callable實現的是 V * call()方法，並且可以返回執行結果。其中Runnable可以提交給Thread來包裝下 * ，直接啟動一個執行緒來執行，而Callable則一般都是提交給ExecuteService來執行。 */ static void futureDemo() { try { /** * 提交runnable則沒有返回值, future沒有資料 */ Future<?> result = mExecutor.submit(new Runnable() { @Override public void run() { fibc(20); } }); System.out.println("future result from runnable : " + result.get()); /** * 提交Callable, 有返回值, future中能夠獲取返回值 */ Future<Integer> result2 = mExecutor.submit(new Callable<Integer>() { @Override public Integer call() throws Exception { return fibc(20); } }); System.out .println("future result from callable : " + result2.get()); /** * FutureTask則是一個RunnableFuture<V>，即實現了Runnbale又實現了Futrue<V>這兩個介面， * 另外它還可以包裝Runnable(實際上會轉換為Callable)和Callable * <V>，所以一般來講是一個符合體了，它可以通過Thread包裝來直接執行，也可以提交給ExecuteService來執行 * ，並且還可以通過v get()返回執行結果，線上程體沒有執行完成的時候，主執行緒一直阻塞等待，執行完則直接返回結果。 */ FutureTask<Integer> futureTask = new FutureTask<Integer>( new Callable<Integer>() { @Override public Integer call() throws Exception { return fibc(20); } }); // 提交futureTask mExecutor.submit(futureTask) ; System.out.println("future result from futureTask : " + futureTask.get()); } catch (InterruptedException e) { e.printStackTrace(); } catch (ExecutionException e) { e.printStackTrace(); } } /** * 效率底下的斐波那契數列, 耗時的操作 * * @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); }}輸出結果
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