前一段时间，在看Google官方的MVP架构蓝图最基础的没有用RxJava的时候看到，Google工程师把操作数据请求网路，读取数据库，查找磁盘缓存都用的线程池来管理这些线程了，最近，经理让我看萤石云抓图（其实不是抓图应该只是截图，哈哈，反正我不管我最牛批，有图就好），我一看每一步都要线程，so我很快就想到这个线程池，用都用了还不把他研究一下，系不系对不起Google工程师，不管你说系不系，反正我说系。
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定义
线程：进程中负责程序执行的执行单元。一个进程中至少有一个线程。
多线程：解决多任务同时执行的需求，合理使用CPU资源。多线程的运行是根据CPU切换完成，如何切换由CPU决定，因此多线程运行具有不确定性。
线程池：基本思想还是一种对象池的思想，开辟一块内存空间，里面存放了众多(未死亡)的线程，池中线程执行调度由池管理器来处理。当有线程任务时，从池中取一个，执行完成后线程对象归池，这样可以避免反复创建线程对象所带来的性能开销，节省了系统的资源。
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线程的创建分类
1、继承Thread类
2、实现Runnable接口
虽说这两种方式都可以创建出一个线程，不过它们之间还是有一点区别的，主要区别在于在多线程访问同一资源的情况下，用Runnable接口创建的线程可以处理同一资源，而用Thread类创建的线程则各自独立处理，各自拥有自己的资源。
所以，在Java中大多数多线程程序都是通过实现Runnable来完成的，而对于Android来说也不例外，当涉及到需要开启线程去完成某件事时，我们都会这样写：
new Thread(new Runnable() {
@Override
public void run() {
//do sth .
}
}).start();
如果有很多地方需要开启大量线程来处理任务，还是用上述的方式去创建线程处理的话，那么将导致系统的性能表现的非常糟糕，重用已有的线程，从而减少线程的创建，所以这就涉及到线程池（ExecutorService）的概念了，线程池的基本作用就是进行线程的复用。
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线程池的介绍
在java和Android线程池框架的结构主要为
1.任务：包括被执行任务需要实现的接口类：Runnable 或 Callable。
2.任务的执行器：包括任务执行机制的核心接口类Executor，以及继承自Executor的EexcutorService接口。
3.执行器的创建者，工厂类Executors。
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Executor、ExecutorService和Executors工厂类
Executor只是一个接口，它是Java／Android线程池框架的基础，它将任务的提交与任务的执行分离开来。
ExecutorService继承自Executor，有两个关键类实现了ExecutorService接口：ThreadPoolExecutor和ScheduledThreadPoolExecutor。
- ThreadPoolExecutor 是线程池的核心实现类，用来执行被提交的任务。
- ScheduledThreadPoolExecutor 也是一个实现类，可以在给定的延迟后运行命令，或者定期执行命令。它比Timer更灵活，功能更强大。
ThreadPoolExecutor 可以创建一个线程池，而如果这样创建线程池的话，我们需要配置一堆东西，非常麻烦，所以，官方也不推荐使用这种方法来创建线程池，而是推荐使用Executors的工厂方法来创建线程池。
/**
*
* corePoolSize：线程池中的核心线程数量
* maximumPoolSize：线程池中的最大线程数量
* keepAliveTime：这个就是上面说到的“保持活动时间“，它起作用必须在一个前提下，就是当线程池中的线程数量超过
了corePoolSize时，它表示多余的空闲线程的存活时间，即：多余的空闲线
程在超过keepAliveTime时间内没有任务的话则被销毁。而这个主要应用在缓存线程池中
* unit：它是一个枚举类型，表示keepAliveTime的单位，常用的如：TimeUnit.SECONDS（秒）、
TimeUnit.MILLISECONDS（毫秒）
* workQueue：任务队列，主要用来存储已经提交但未被执行的任务，不同的线程池采用的排队策略不一样，稍后再讲
* threadFactory：线程工厂，用来创建线程池中的线程，通常用默认的即可
* handler：通常叫做拒绝策略，1、在线程池已经关闭的情况下 2、任务太多导致最大线程数和任务队列已经饱和，无
法再接收新的任务 。在上面两种情况下，只要满足其中一种时，在使用execute()来提交新的任务时将会拒绝，而默
认的拒绝策略是抛一个RejectedExecutionException异常
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
Executors是一个工厂类，它不继承任何其它类，它通过ThreadPoolExecutor、ScheduledThreadPoolExecutor创建出五种不同的线程池
- newCachedThreadPool 创建一个可缓存线程池，线程池的最大长度无限制，但如果线程池长度超过处理需要，可灵活回收空闲线程，若无可回收，则新建线程。
/**
* Creates a thread pool that creates new threads as needed, but
* will reuse previously constructed threads when they are
* available. These pools will typically improve the performance
* of programs that execute many short-lived asynchronous tasks.
* Calls to {@code execute} will reuse previously constructed
* threads if available. If no existing thread is available, a new
* thread will be created and added to the pool. Threads that have
* not been used for sixty seconds are terminated and removed from
* the cache. Thus, a pool that remains idle for long enough will
* not consume any resources. Note that pools with similar
* properties but different details (for example, timeout parameters)
* may be created using {@link ThreadPoolExecutor} constructors.
*
* @return the newly created thread pool
*/
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
/**
* Creates a thread pool that creates new threads as needed, but
* will reuse previously constructed threads when they are
* available, and uses the provided
* ThreadFactory to create new threads when needed.
* @param threadFactory the factory to use when creating new threads
* @return the newly created thread pool
* @throws NullPointerException if threadFactory is null
*/
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>(),
threadFactory);
}
- newFixedThreadPool 创建一个定长线程池，可控制线程最大并发数，超出的线程会在队列中等待。
/**
* Creates a thread pool that reuses a fixed number of threads
* operating off a shared unbounded queue. At any point, at most
* {@code nThreads} threads will be active processing tasks.
* If additional tasks are submitted when all threads are active,
* they will wait in the queue until a thread is available.
* If any thread terminates due to a failure during execution
* prior to shutdown, a new one will take its place if needed to
* execute subsequent tasks. The threads in the pool will exist
* until it is explicitly {@link ExecutorService#shutdown shutdown}.
*
* @param nThreads the number of threads in the pool
* @return the newly created thread pool
* @throws IllegalArgumentException if {@code nThreads <= 0}
*/
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
- newScheduledThreadPool 创建一个定长线程池，支持定时及周期性任务执行。
/**
* Creates a thread pool that can schedule commands to run after a
* given delay, or to execute periodically.
* @param corePoolSize the number of threads to keep in the pool,
* even if they are idle
* @return a newly created scheduled thread pool
* @throws IllegalArgumentException if {@code corePoolSize < 0}
*/
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
- newSingleThreadExecutor 创建一个单线程化的线程池，它只会用唯一的工作线程来执行任务，保证所有任务按照指定顺序(FIFO, LIFO, 优先级)执行。
/**
* Creates an Executor that uses a single worker thread operating
* off an unbounded queue. (Note however that if this single
* thread terminates due to a failure during execution prior to
* shutdown, a new one will take its place if needed to execute
* subsequent tasks.) Tasks are guaranteed to execute
* sequentially, and no more than one task will be active at any
* given time. Unlike the otherwise equivalent
* {@code newFixedThreadPool(1)} the returned executor is
* guaranteed not to be reconfigurable to use additional threads.
*
* @return the newly created single-threaded Executor
*/
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
5.newSingleThreadScheduledExecutor 该方法返回一个可以控制线程池内线程定时或周期性执行某任务的线程池。只不过和上面的区别是该线程池大小为1，而上面的可以指定线程池的大小
/**
* Creates a single-threaded executor that can schedule commands
* to run after a given delay, or to execute periodically.
* (Note however that if this single
* thread terminates due to a failure during execution prior to
* shutdown, a new one will take its place if needed to execute
* subsequent tasks.) Tasks are guaranteed to execute
* sequentially, and no more than one task will be active at any
* given time. Unlike the otherwise equivalent
* {@code newScheduledThreadPool(1)} the returned executor is
* guaranteed not to be reconfigurable to use additional threads.
* @return the newly created scheduled executor
*/
public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
return new DelegatedScheduledExecutorService
(new ScheduledThreadPoolExecutor(1));
}
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