Callable & Future

There are two wats of creating threads – one by extending the Thread class and other by creating a thread with a Runnable. However, one feature lacking in Runnable is that we cannot make a thread return result when it terminates, i.e. when run() completes. For supporting this feature, the Callable interface is present in Java.

Ref: Callable and Future in Java - GeeksforGeeks

有兩種創建線程的方法-一種是通過擴展Thread類，另一種是通過使用Runnable創建線程。 但是，Runnable缺少的一項功能是，當線程終止（即run()完成）時，我們無法使線程返回結果。 為了支持此功能，Java中提供了Callable接口。

Runnable JDK

其中Runnable應該是我們最熟悉的接口，它只有一個run()方法，用於執行線程任務，該函數沒有返回值。

Thread類在呼叫start()函數後，執行的是Runnablerun()函數。

@FunctionalInterface
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>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

Callable JDK

Callable interface has the call() method. In this method, we have to implement the logic of a task. The Callable interface is a parameterized interface, meaning we have to indicate the type of data the call() method will return.

Callable接口定義了call()方法。在這個方法中，我們西部實現任務的邏輯。該Callable接口是一個參數化接口，這意味著我們必須指示該call()方法將返回的數據類型。


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使用者程序。

用簡單話來說，Callable就是Runnable具有返回值的擴展。

這是一個泛型接口，call()返回的型別就是使用者傳遞來的V型別。

package java.util.concurrent;

/**
 * A task that returns a result and may throw an exception.
 * Implementors define a single method with no arguments called
 * {@code call}.
 *
 * <p>The {@code Callable} interface is similar to {@link
 * java.lang.Runnable}, in that both are designed for classes whose
 * instances are potentially executed by another thread.  A
 * {@code Runnable}, however, does not return a result and cannot
 * throw a checked exception.
 *
 * <p>The {@link Executors} class contains utility methods to
 * convert from other common forms to {@code Callable} classes.
 *
 * @see Executor
 * @since 1.5
 * @author Doug Lea
 * @param <V> the result type of method {@code call}
 */
@FunctionalInterface
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;
}

Future JDK

Future interface has methods to obtain the result generated by a Callable object and to manage its state.

Future接口具有獲取對像生成的結果Callable並管理其狀態的方法。


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。

主要作用：獲取任務執行結果，中斷任務等（代表的是任务的生命周期）。

在Future中有三個比較重要的方法：

  • public boolean cancel(boolean mayInterruptIfRunning): 用於停止任務。
    • 如果尚未啟動，它將停止執行任務。
    • 如果已經啟動，則僅在mayInterruptIfRunning為true時才會中斷任務。
  • public Object get() throws InterruptedException, ExecutionException: 用於獲取任務的結果。
    • 如果任務完成，它將立即返回結果，
    • 否則將等待任務完成，然後返回結果。
  • public boolean isDone(): 如果任務完成，則返回true，否則返回false。
package java.util.concurrent;

/**
 * A {@code Future} represents the result of an asynchronous
 * computation.  Methods are provided to check if the computation is
 * complete, to wait for its completion, and to retrieve the result of
 * the computation.  The result can only be retrieved using method
 * {@code get} when the computation has completed, blocking if
 * necessary until it is ready.  Cancellation is performed by the
 * {@code cancel} method.  Additional methods are provided to
 * determine if the task completed normally or was cancelled. Once a
 * computation has completed, the computation cannot be cancelled.
 * If you would like to use a {@code Future} for the sake
 * of cancellability but not provide a usable result, you can
 * declare types of the form {@code Future<?>} and
 * return {@code null} as a result of the underlying task.
 *
 * <p>
 * <b>Sample Usage</b> (Note that the following classes are all
 * made-up.)
 * <pre> {@code
 * interface ArchiveSearcher { String search(String target); }
 * class App {
 *   ExecutorService executor = ...
 *   ArchiveSearcher searcher = ...
 *   void showSearch(final String target)
 *       throws InterruptedException {
 *     Future<String> future
 *       = executor.submit(new Callable<String>() {
 *         public String call() {
 *             return searcher.search(target);
 *         }});
 *     displayOtherThings(); // do other things while searching
 *     try {
 *       displayText(future.get()); // use future
 *     } catch (ExecutionException ex) { cleanup(); return; }
 *   }
 * }}</pre>
 *
 * The {@link FutureTask} class is an implementation of {@code Future} that
 * implements {@code Runnable}, and so may be executed by an {@code Executor}.
 * For example, the above construction with {@code submit} could be replaced by:
 *  <pre> {@code
 * FutureTask<String> future =
 *   new FutureTask<String>(new Callable<String>() {
 *     public String call() {
 *       return searcher.search(target);
 *   }});
 * executor.execute(future);}</pre>
 *
 * <p>Memory consistency effects: Actions taken by the asynchronous computation
 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
 * actions following the corresponding {@code Future.get()} in another thread.
 *
 * @see FutureTask
 * @see Executor
 * @since 1.5
 * @author Doug Lea
 * @param <V> The result type returned by this Future's {@code get} 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 {@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.
     *
     * <p>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;
}

Example

這是一個Java Callable任務的簡單示例，該示例返回一秒鐘後執行該任務的線程的名稱。我們使用Executor框架並行執行100個任務， 並使用Java Future獲取提交的任務的結果。

示例出處：Java Callable Future Example - JournalDev

import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;

public class MyCallable implements Callable<String> {

    @Override
    public String call() throws Exception {
        Thread.sleep(1000);

        // return the thread name executing this callable task
        return Thread.currentThread().getName();
    }

    public static void main(String[] args) {
        // Get ExecutorService from Executors utility class, thread pool size is 10
        ExecutorService executor = Executors.newFixedThreadPool(10);

        // create a list to hold the Future object associated with Callable
        List<Future<String>> list = new ArrayList<Future<String>>();

        // Create MyCallable instance
        Callable<String> callable = new MyCallable();

        for (int i = 0; i < 100; i++) {
            // submit Callable tasks to be executed by thread pool
            Future<String> future = executor.submit(callable);

            // add Future to the list, we can get return value using Future
            list.add(future);
        }
        for (Future<String> fut : list) {
            try {
                // print the return value of Future, notice the output delay in console 
                // because Future.get() waits for task to get completed
                System.out.println(new Date() + "::" + fut.get());
            } catch (InterruptedException | ExecutionException e) {
                e.printStackTrace();
            }
        }
        // shut down the executor service now
        executor.shutdown();
    }

}

Once we execute the above program, you will notice the delay in output because java Future get() method waits for the java callable task to complete. Also notice that there are only 10 threads executing these tasks.

一旦執行了上述程序，您將注意到輸出的延遲，因為java Future get()方法等待java可調用的任務完成。另請注意，只有10個線程執行這些任務。

console:

Mon Dec 31 20:40:15 PST 2012::pool-1-thread-1
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-2
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-3
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-4
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-5
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-6
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-7
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-8
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-9
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-10
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-2
...

Reference