jdk1.7.0_79
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。
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。
//ExecutorServicepublic 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。
//AbstractExecutorServicepublic 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
下面给出3个例子，来感受下submit方法。
submit(Callable
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#newTaskForprotected <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——《14.Java中的Future模式》。
