为什么须要调用 ThreadPoolExecutor 构造方法去创立线程池，因为阿里巴巴开发手册如是说：
那咱们先理解下 ThreadPoolExecutor 构造方法的七个参数代表什么。
ThreadPoolExecutor 类的源码如下：
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @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.acc = System.getSecurityManager() == null ?
null :
AccessController.getContext();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}其实源码中办法上的正文曾经很分明了（浏览源码正文是个很好的习惯），咱们这里简略翻译下：
corePoolSize：外围线程数，即始终保留在线程池当中的线程数量，即便没有工作须要解决。
maximumPoolSize：最大线程数，以后线程池中容许创立的最大线程数。
keepAliveTime：存活工夫，大于外围线程数的线程期待新工作的最长工夫，这个工夫内没有新工作须要解决，则销毁这个线程
unit：存活工夫的单位，TumeUnit 类，有 NANOSECONDS，MICROSECONDS，MILLISECONDS，SECONDS，MINUTES，HOURS，DAYS 选项
workQueue：工作队列。用于在执行工作之前保留工作的队列。此队列将仅保留由 execute 办法提交的 Runnable 工作。当有新工作来时，首先判断沉闷的线程是否小于外围线程数，如果小于，则新增线程直到等于外围线程数，再后续则判断当前任务队列是否曾经满员，如果未满，退出队列等待执行，如果已满则开启新的线程。如果线程数达到最大线程数，则执行回绝策略(第七个参数)
threadFactory：执行器创立新线程时应用的工厂，默认 DefaultThreadFactory
handler：回绝策略 RejectedExecutionHandler。默认 AbortPolicy
咱们能够看到实现 RejectedExecutionHandler 的实现类有四个：
见名知意咱们简略了解下：
AbortPolicy：停止策略，针对满员后的新工作间接抛出 RejectedExecutionException 异样
CallerRunsPolicy: 由以后线程次所在的线程去执行被回绝的新工作。
DiscardPolicy：抛弃策略，空办法什么都不干，等于间接疏忽了新工作，不执行然而也不抛出异样。
DiscardOldestPolicy：抛弃最早的策略。当工作被回绝是，会摈弃工作队列中最旧的工作也就是最先退出队列的，再把这个新工作增加进去。e.getQueue().poll();
/* Predefined RejectedExecutionHandlers */
/**
* A handler for rejected tasks that runs the rejected task
* directly in the calling thread of the {@code execute} method,
* unless the executor has been shut down, in which case the task
* is discarded.
*/
public static class CallerRunsPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code CallerRunsPolicy}.
*/
public CallerRunsPolicy() {}
/**
* Executes task r in the caller's thread, unless the executor
* has been shut down, in which case the task is discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {if (!e.isShutdown()) {r.run();
}
}
}
/**
* A handler for rejected tasks that throws a
* {@code RejectedExecutionException}.
*/
public static class AbortPolicy implements RejectedExecutionHandler {
/**
* Creates an {@code AbortPolicy}.
*/
public AbortPolicy() {}
/**
* Always throws RejectedExecutionException.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
* @throws RejectedExecutionException always
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {throw new RejectedExecutionException("Task" + r.toString() +
"rejected from" +
e.toString());
}
}
/**
* A handler for rejected tasks that silently discards the
* rejected task.
*/
public static class DiscardPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardPolicy}.
*/
public DiscardPolicy() {}
/**
* Does nothing, which has the effect of discarding task r.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {}}
/**
* A handler for rejected tasks that discards the oldest unhandled
* request and then retries {@code execute}, unless the executor
* is shut down, in which case the task is discarded.
*/
public static class DiscardOldestPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardOldestPolicy} for the given executor.
*/
public DiscardOldestPolicy() {}
/**
* Obtains and ignores the next task that the executor
* would otherwise execute, if one is immediately available,
* and then retries execution of task r, unless the executor
* is shut down, in which case task r is instead discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {if (!e.isShutdown()) {e.getQueue().poll();
e.execute(r);
}
}
}理解了上述七个参数后，咱们再看看阿里云标准中不提倡的线程池对象：
FixedThreadPool
/**
* FixedThreadPool 是外围线程和最大线程一样，然而队列没指定容量，默认是 MAX_VALUE。所有能够始终创立新工作，导致 OOM
*/
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public LinkedBlockingQueue() {this(Integer.MAX_VALUE);
}
SingleThreadExecutor
/**
* SingleThreadExecutor 是单个线程，然而队列没指定容量，默认是 MAX_VALUE。所有能够始终创立新工作，导致 OOM
*/
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}CachedThreadPool
/**
* CachedThreadPool 最大线程是是 MAX_VALUE，默认的队列是 SynchronousQueue，是阻塞的。所以有新工作来就会创立新的线程，导致 OOM
*/
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}ScheduledThreadPool
/**
* CachedThreadPool 最大线程是是 MAX_VALUE，队列是 DelayedWorkQueue 默认长度为 16。所以能够始终创立新的线程，导致 OOM
*/
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}