Java中存在Runnable、Callable、Future、FutureTask這幾個與線程相關的類或者接口，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于區別。

Runnable

其中Runnable應該是我們最熟悉的接口，它只有一個run()函數，用于將耗時操作寫在其中，該函數沒有返回值。然后使用某個線程去執行該runnable即可實現多線程，Thread類在調用start()函數后就是執行的是Runnable的run()函數。Runnable的聲明如下 :

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 java.lang.Thread#run() */ public abstract void run(); } 復制代碼

Callable

Callable與Runnable的功能大致相似，Callable中有一個call()函數，但是call()函數有返回值，而Runnable的run()函數不能將結果返回給客戶程序。Callable的聲明如下 :

public interface Callable<V> { /** * 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; } 復制代碼

可以看到，這是一個泛型接口，call()函數返回的類型就是客戶程序傳遞進來的V類型。

Future

Executor是Runnable和Callable的調度容器，而Future就是對于具體的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、設置結果操作。get方法會阻塞，直到任務返回結果(Future簡介)。Future聲明如下:

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; } 復制代碼

FutureTask

FutureTask則是一個RunnableFuture，而RunnableFuture實現了Runnbale又實現了Futrue這兩個接口，

public class FutureTask<V> implements RunnableFuture<V> { } 復制代碼

RunnableFuture

public interface RunnableFuture<V> extends Runnable, Future<V> { /** * Sets this Future to the result of its computation * unless it has been cancelled. */ void run(); } 復制代碼

另外它還可以包裝Runnable和Callable， 由構造函數注入依賴。

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 } 復制代碼

可以看到，Runnable注入會被Executors.callable()函數轉換為Callable類型，即FutureTask最終都是執行Callable類型的任務。該適配函數的實現如下 ：

public static <T> Callable<T> callable(Runnable task, T result) { if (task == null) throw new NullPointerException(); return new RunnableAdapter<T>(task, result); } 復制代碼

RunnableAdapter適配器

/** * 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;} } 復制代碼

由于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。

并且還可以直接通過get()函數獲取執行結果，該函數會阻塞，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

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，并且可以返回執行結果。其中Runnable可以提交給Thread來包裝下 * ，直接啟動一個線程來執行，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沒有數據 */ 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>這兩個接口， * 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 * ，并且還可以通過v get()返回執行結果，在線程體沒有執行完成的時候，主線程一直阻塞等待，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); } } 復制代碼

來源：blog.csdn.net/bboyfeiyu/a…

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