Excutor 源码解读：
/**
* An object that executes submitted {@link Runnable} tasks. This
* interface provides a way of decoupling task submission from the
* mechanics of how each task will be run, including details of thread
* use, scheduling, etc. An {@code Executor} is normally used
* instead of explicitly creating threads. For example, rather than
* invoking {@code new Thread(new(RunnableTask())).start()} for each
* of a set of tasks, you might use:
*
Excutor 是一个可以执行提交过来实现Runnable 接口的对象，这个接口提供了一种将任务的提交以及
这个任务如何去执行进行了解耦； 包含了线程线程的使用细节，调度等；
我们通常使用Excutor 而不是显式的创建线程，比如说，我们不会调用对每一个任务进行new Thread (Runnabe ()).start()
方法；而是使用如下：
* <pre>
* Executor executor = <em>anExecutor</em>;
* executor.execute(new RunnableTask1());
* executor.execute(new RunnableTask2());
* ...
* </pre>
创建一个执行器Excutor
调用执行器的execte方法
* However, the {@code Executor} interface does not strictly
* require that execution be asynchronous. In the simplest case, an
* executor can run the submitted task immediately in the caller's
* thread:
然而，Excutor执行器并不是严格的要求执行是异步的，一个简单的情况，
一个执行器在调用者线程中会立刻运行提交的任务，不理解看下面例子：
* <pre> {@code
* class DirectExecutor implements Executor {
* public void execute(Runnable r) {
* r.run();
* }
* }}</pre>
运行了.run 方法，并不会启用一个新的线程，.start 才会启动一个新的线程，就是这个意思
* More typically, tasks are executed in some thread other
* than the caller's thread. The executor below spawns a new thread
* for each task.
*
* <pre> {@code
* class ThreadPerTaskExecutor implements Executor {
* public void execute(Runnable r) {
* new Thread(r).start();
* }
* }}</pre>
*
* Many {@code Executor} implementations impose some sort of
* limitation on how and when tasks are scheduled. The executor below
* serializes the submission of tasks to a second executor,
* illustrating a composite executor.
*
* <pre> {@code
* class SerialExecutor implements Executor {
* final Queue<Runnable> tasks = new ArrayDeque<Runnable>();
* final Executor executor;
* Runnable active;
*
* SerialExecutor(Executor executor) {
* this.executor = executor;
* }
*
* public synchronized void execute(final Runnable r) {
* tasks.offer(new Runnable() {
* public void run() {
* try {
* r.run();
* } finally {
* scheduleNext();
* }
* }
* });
* if (active == null) {
* scheduleNext();
* }
* }
*
* protected synchronized void scheduleNext() {
* if ((active = tasks.poll()) != null) {
* executor.execute(active);
* }
* }
* }}</pre>
*
* The {@code Executor} implementations provided in this package
* implement {@link ExecutorService}, which is a more extensive
* interface. The {@link ThreadPoolExecutor} class provides an
* extensible thread pool implementation. The {@link Executors} class
* provides convenient factory methods for these Executors.
************很重要了***************
在这个package 中Excutor 的实现提供了 关于Excutor 接口的扩展ExcutorService
ThreadPoolExcutor 类提供了可扩展的线程池的实现， Excutors 提供了 方便的工厂方法（常用）
* <p>Memory consistency effects: Actions in a thread prior to
* submitting a {@code Runnable} object to an {@code Executor}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* its execution begins, perhaps in another thread.
*
ThreadFactory 源码解读“”
/**
* An object that creates new threads on demand. Using thread factories
* removes hardwiring of calls to {@link Thread#Thread(Runnable) new Thread},
* enabling applications to use special thread subclasses, priorities, etc.
ThreadFactory 是一个按需要创建线程的对象，使用线程工厂消除了使用
Thread(Runnable) new Thread 创建线程的硬连接 能够使应用程序使用使用特殊的线程子类，优先级等；下面是一个简单的例子：
* <p> * The simplest implementation of this interface is just: * <pre>
{@code * class SimpleThreadFactory implements ThreadFactory
{ * public Thread newThread(Runnable r) {
* return new Thread(r); * } * }}
</pre>