java并发之Future与Callable使用

这篇文章需要大家知道线程、线程池的知识，尤其是线程池。
有的时候我们要获取线程的执行结果，这个时候就需要用到Callable、Future、FutureTask了
先看下Future、Callable接口、RunnableFuture、FutureTask。

Future

Future是一个接口，能够取消任务、获取任务取消状态、获取结果。

package java.util.concurrent;
public interface Future<V> {boolean cancel(boolean mayInterruptIfRunning);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;V get(long timeout, TimeUnit unit)throws InterruptedException, ExecutionException, TimeoutException;
}

Callable

Callable只有一个功能就是获取结果的V 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;
}

FutureTask

FutureTask是一个实现类，它继承了RunnableFuture，而接口RunnableFuture继承了接口Runable,Future。
因此FutureTask可以通过new Thread(FutureTask task)这样的方式来创建线程。

public class FutureTask<V> implements RunnableFuture<V>

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();
}

通过Future创建线程

获取线程的执行结果，最重要就是FutureTask调用。下面的例子是通过线程池来执行线程，并获取结果 。
1.创建实现Callable接口的类Task，就是返回数字和
2.创建线程池ThreadPoolExecutor，新建Task实例
3.线程池ThreadPoolExecutor执行pool.submit(task)
4.返回了Future result
5.调用Future的get()获取结果
难点是在pool.submit(task)。

package com.java.javabase.thread.future;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;/*** @author*/
@Slf4j
public class FutureTest {public static void main(String[] args) {//ExecutorService pool = Executors.newCachedThreadPool();ThreadPoolExecutor pool =new ThreadPoolExecutor(1,1,0, TimeUnit.SECONDS,new ArrayBlockingQueue<Runnable>(1));Task task =new Task();Future<Integer> result=pool.submit(task);/*public <T> Future<T> submit(Callable<T> task) {if (task == null) throw new NullPointerException();RunnableFuture<T> ftask = newTaskFor(task);execute(ftask);return ftask;}*/pool.shutdown();try {log.info("sum {}",result.get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}static class Task implements Callable<Integer> {@Overridepublic Integer call() throws Exception {int sum = 0;for (int i = 0; i < 100; i++) {sum += i;}return sum;}}}

线程池获取结果流程说明

ThreadPoolExecutor.submit方法中执行void execute(Runnable command);
如果了解线程池就应该知道这个方法是能够让command的run方法执行的。实现接口 RunnableFuture的
FutureTask的run方法最后返回线程结果的关键

    public <T> Future<T> submit(Callable<T> task) {if (task == null) throw new NullPointerException();RunnableFuture<T> ftask = newTaskFor(task);execute(ftask);return ftask;}

ThreadPoolExecutor.execute方法

    public void execute(Runnable command) {if (command == null)throw new NullPointerException();int c = ctl.get();if (workerCountOf(c) < corePoolSize) {if (addWorker(command, true))return;c = ctl.get();}if (isRunning(c) && workQueue.offer(command)) {int recheck = ctl.get();if (! isRunning(recheck) && remove(command))reject(command);else if (workerCountOf(recheck) == 0)addWorker(null, false);}else if (!addWorker(command, false))reject(command);}

FutureTask构造器

 public FutureTask(Callable<V> callable) {if (callable == null)throw new NullPointerException();this.callable = callable;this.state = NEW;       // ensure visibility of callable}

FutureTask的run方法
我们创建FutureTask的时候，传入了实现Callable接口的实现类Task,而run()方法中调用了result = c.call();
也就是说FutureTask调用了Callable接口的实现类Task的call方法获取结果，并保存，然后通过对外的get()方法返回。

    public void run() {if (state != NEW ||!UNSAFE.compareAndSwapObject(this, runnerOffset,null, Thread.currentThread()))return;try {Callable<V> c = callable;if (c != null && state == NEW) {V result;boolean ran;try {result = c.call();ran = true;} catch (Throwable ex) {result = null;ran = false;setException(ex);}if (ran)set(result);}} finally {// runner must be non-null until state is settled to// prevent concurrent calls to run()runner = null;// state must be re-read after nulling runner to prevent// leaked interruptsint s = state;if (s >= INTERRUPTING)handlePossibleCancellationInterrupt(s);}}

通过FutureTask创建线程

值得注意的就是获取结果的方式，创建FutureTask，也只能通过FutureTask对象get到结果。
pool.submit(futureTask)返回的Future调用get()返回的是null!

package com.java.javabase.thread.future;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;/*** @author*/
@Slf4j
public class FutureTaskTest {public static void main(String[] args) {//ExecutorService pool = Executors.newCachedThreadPool();ThreadPoolExecutor pool =new ThreadPoolExecutor(1,1,0, TimeUnit.SECONDS,new ArrayBlockingQueue<Runnable>(1));Task task =new Task();FutureTask<Integer> futureTask=new FutureTask<Integer>(task);//Future<?> result =pool.submit(futureTask);pool.submit(futureTask);pool.shutdown();try {log.info("sum {}",futureTask.get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}Task t2=new Task();FutureTask<Integer> futureTask2=new FutureTask<Integer>(t2);new Thread(futureTask2).start();try {log.info("sum 2 {}",futureTask2.get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}static class Task implements Callable<Integer> {@Overridepublic Integer call() throws Exception {int sum = 0;for (int i = 0; i < 100; i++) {sum += i;}return sum;}}
}

只能通过FutureTask对象get到结果的原因

我就简单强调一点下面的：ftask与task的区别就是获取结果不一致的原因
ftask的run方法调用了对象task的run方法，ftask的get()返回的是null，而实际我们看到自己创建的task的run方法才能返回结果

    public Future<?> submit(Runnable task) {if (task == null) throw new NullPointerException();RunnableFuture<Void> ftask = newTaskFor(task, null);execute(ftask);return ftask;}