Java中线程池的实现
当我们要使用线程时，就要去创建线程，这样看上去似乎很正常，但是频繁地创建线程、销毁线程会对系统资源造成不小地负担，大大降低了系统地效率。
Java中有一个很好地工具，可以事先创建好一些线程，当我们需要使用地时候就直接拿来使用就可以，这样就不会频繁地创建和销毁线程了，这个神奇地工具就是线程池。
线程池是通过Executor执行器来创建。
我们来看一下在java.util.concurrent包下面的Executor类都有哪些东西。
Executor提供的几种创建线程池的方法：
1.newFixedThreadPool
* 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>());
}简单翻译一下就是说这个类创建了些个固定数目的共享线程，即实现规定好创建的线程的数量，后面不能更改数量。
在任何一个时间点，活跃工作的线程数量都不能够超过这个最大的数量。
当所有的线程都在工作时，如果这时候有一个新的线程需要被创建，那么这个需要创建的线程需要就会被放进一个等待队列BlockingQueue，直到有空余的线程可以使用才执行。
如果有一个线程在执行期间由于发生了错误而终止运行，一个新的线程会接替这个终止的线程完成就下来的任务。
线程池中的线程们会一直存在直到被要求关闭。
初始化时指定nThreads线程池线程数量上限。
2.newWorkStealingPool：
/**
* Creates a thread pool that maintains enough threads to support
* the given parallelism level, and may use multiple queues to
* reduce contention. The parallelism level corresponds to the
* maximum number of threads actively engaged in, or available to
* engage in, task processing. The actual number of threads may
* grow and shrink dynamically. A work-stealing pool makes no
* guarantees about the order in which submitted tasks are
* executed.
*
* @param parallelism the targeted parallelism level
* @return the newly created thread pool
* @throws IllegalArgumentException if {@code parallelism <= 0}
* @since 1.8
*/
public static ExecutorService newWorkStealingPool(int parallelism) {
return new ForkJoinPool
(parallelism,
ForkJoinPool.defaultForkJoinWorkerThreadFactory,
null, true);
}parallelism为并行度，newWorkStealingPool也有一个不带参数的构造，默认并行度为cup的个数。
这个参数规定了同一时间同时执行任务的线程数量上限。平时线程的数量是会动态增加或减少的，并不像newFixedThreadPool那样是固定的线程数量。
同时也不保证按照任务提交的顺序来完成任务，它为了减少竞争，采取了多种的queue。
3.newCachedThreadPool
/**
* 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>());
}创建一个可以动态增长的线程池，当我们有新的任务提交时，但是当前线程池并没有空余的线程可以供我们使用，这时候就会新建一个线程，如果有以前创建的线程空闲下来，则不会创建新的线程而是使用以前已经创建的。
当一个线程空闲下来超过60秒，这个线程就会被从缓存线程池中移除。
这种缓存线程池在应对那些执行时间很短的异步任务时表现很好。
4.newScheduledThreadPool
/**
* 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
* @param threadFactory the factory to use when the executor
* creates a new thread
* @return a newly created scheduled thread pool
* @throws IllegalArgumentException if {@code corePoolSize < 0}
* @throws NullPointerException if threadFactory is null
*/
public static ScheduledExecutorService newScheduledThreadPool(
int corePoolSize, ThreadFactory threadFactory) {
return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
}定时周期性地执行任务，线程数量固定。
public class Main {
public static void main(String[] args) throws Exception {
// 指定大小为4
ScheduledExecutorService m = Executors.newScheduledThreadPool(4);
m.scheduleAtFixedRate(new Runnable() {
@Override
public void run() {
Date now = new Date();
System.out.println("线程" + Thread.currentThread() + "报时：" + now);
}
}, 1, 1, TimeUnit.SECONDS); // 延迟1s秒执行，每隔1s执行一次
}
}
5.newSingleThreadPool：
/**
* 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>()));
}顾名思义，这个线程池里只有一个线程，任何一个时刻，最多只有一个线程在工作。
每个被提交地任务按照先提交先执行的原则依次执行。
当原来的线程出现问题被终止了，会有一个新的线程被创建去执行后面的任务。
这个线程的阻塞队列采用了无边界的LinkedBlockingQueue。
还有一些扩展的比如
newSingleThreadScheduledExecutor
就是在上面一种的情况下加入了计时周期性执行。
这些就是Executor类提供给我们的创建线程池的方法，我们可以根据业务需要选择合适的方法。
这些线程池为了效率和应对特殊的情况，采用了不同的BlockingQueue，下面一篇文章我会介绍一下各种
BlockingQueue的实现类。