JAVA中ThreadPoolExecutor线程池的submit方法详解

2020年1月29日01:37:28
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下面小编就为大家带来一篇简单谈谈ThreadPoolExecutor线程池之submit方法。小编觉得挺不错的,现在就分享给大家,也给大家做个参考。一起跟随小编过来看看吧

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在上一篇《ThreadPoolExecutor线程池原理及其execute方法》中提到了线程池ThreadPoolExecutor的原理以及它的execute方法。本文解析ThreadPoolExecutor#submit。

对于一个任务的执行有时我们不需要它返回结果,但是有我们需要它的返回执行结果。对于线程来讲,如果不需要它返回结果则实现Runnable,而如果需要执行结果的话则可以实现Callable。在线程池同样execute提供一个不需要返回结果的任务执行,而对于需要结果返回的则可调用其submit方法。

回顾ThreadPoolExecutor的继承关系。

JAVA中ThreadPoolExecutor线程池的submit方法详解

在Executor接口中只定义了execute方法,而submit方法则是在ExecutorService接口中定义的。

//ExecutorService
public interface ExecutorService extends Executor {
  ...
  <T> Future<T> submit(Callable<T> task);
  <T> Future<T> submit(Runnable task, T result);
  <T> Future<T> submit(Runnable task);
  ...
}

而在其子类AbstractExecutorService实现了submit方法。

//AbstractExecutorService
public abstract class AbstractExecutorService implements ExecutorService {
  ...
  public <T> Future<T> submit(Callable<T> task) {
    if (task == null) throw new NullPointerException();
    RunnableFuture<T> ftask = newTaskFor(task);
    execute(ftask);
    return ftask;
  }
  public <T> Future<T> submit(Runnable task, T result) {
    if (task == null) throw new NullPointerException();
    RunnableFuture<T> ftask = newTaskFor(task);
    execute(ftask);
    return ftask;
  }
  public Future<?> submit(Runnable task) {
    if (task == null) throw new NullPointerExeption();
    RunnableFuture<Void> ftask = newTaskFor(task, null);
    execute(ftask);
    return ftask; 
  }
  ...
}

在AbstractExecutorService实现的submit方法实际上是一个模板方法,定义了submit方法的算法骨架,其execute交给了子类。(可以看到在很多源码中,模板方法模式被大量运用,有关模板方法模式可参考《模板方法模式》)

尽管submit方法能提供线程执行的返回值,但只有实现了Callable才会有返回值,而实现Runnable的线程则是没有返回值的,也就是说在上面的3个方法中,submit(Callable<T> task)能获取到它的返回值,submit(Runnable task, T result)能通过传入的载体result间接获得线程的返回值或者准确来说交给线程处理一下,而最后一个方法submit(Runnable task)则是没有返回值的,就算获取它的返回值也是null。

下面给出3个例子,来感受下submit方法。

submit(Callable<T> task)

package com.threadpoolexecutor;

import java.util.concurrent.*;

/**
 * ThreadPoolExecutor#sumit(Callable<T> task)
 * Created by yulinfeng on 6/17/17.
 */
public class Sumit1 {

 public static void main(String[] args) throws ExecutionException, InterruptedException {
 Callable<String> callable = new Callable<String>() {
 public String call() throws Exception {
 System.out.println("This is ThreadPoolExetor#submit(Callable<T> task) method.");
 return "result";
 }
 };

 ExecutorService executor = Executors.newSingleThreadExecutor();
 Future<String> future = executor.submit(callable);
 System.out.println(future.get());
 }
}

submit(Runnable task, T result)

package com.threadpoolexecutor;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

/**
 * ThreadPoolExecutor#submit(Runnable task, T result)
 * Created by yulinfeng on 6/17/17.
 */
public class Submit2 {

 public static void main(String[] args) throws ExecutionException, InterruptedException {

 ExecutorService executor = Executors.newSingleThreadExecutor();
 Data data = new Data();
 Future<Data> future = executor.submit(new Task(data), data);
 System.out.println(future.get().getName());
 }
}

class Data {
 String name;

 public String getName() {
 return name;
 }

 public void setName(String name) {
 this.name = name;
 }
}

class Task implements Runnable {
 Data data;

 public Task(Data data) {
 this.data = data;
 }
 public void run() {
 System.out.println("This is ThreadPoolExetor#submit(Runnable task, T result) method.");
 data.setName("kevin");
 }
}

submit(Runnable task)

package com.threadpoolexecutor;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

/**
 * ThreadPoolExecutor#sumit(Runnable runnables)
 * Created by yulinfeng on 6/17/17.
 */
public class Submit {

 public static void main(String[] args) throws ExecutionException, InterruptedException {
 Runnable runnable = new Runnable() {
 public void run() {
 System.out.println("This is ThreadPoolExetor#submit(Runnable runnable) method.");
 }
 };

 ExecutorService executor = Executors.newSingleThreadExecutor();
 Future future = executor.submit(runnable);
 System.out.println(future.get());
 }
}

通过上面的实例可以看到在调用submit(Runnable runnable)的时候是不需要其定义类型的,也就是说虽然在ExecutorService中对其定义的是泛型方法,而在AbstractExecutorService中则不是泛型方法,因为它没有返回值。(有关Object、T、?这三者的区别,可参考《Java中的Object、T(泛型)、?区别》)。

从上面的源码可以看到,这三者方法几乎是一样的,关键就在于:

RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);

它是如何将一个任务作为参数传递给了newTaskFor,然后调用execute方法,最后进而返回ftask的呢?

//AbstractExecutorService#newTaskFor
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
  return new FutureTask<T>(callable);
}
  protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
  return new FutureTask<T>(runnable, value);
}

看来是返回了一个FutureTask实例,FutureTask实现了Future和Runnable接口。Future接口是Java线程Future模式的实现,可用用来异步计算,实现Runnable接口表示可以作为一个线程执行。FutureTask实现了这两个接口意味着它代表异步计算的结果,同时可以作为一个线程交给Executor来执行。有关FutureTask放到下章来单独解析。所以本文对于线程池ThreadPoolExecutor线程池的submit方法解析并不完整,必须得了解Java线程的Future模式——《老生常谈Java中的Future模式》。

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