JAVA线程池: https://juejin.im/post/5a743c526fb9a063557d7eba
blockingqueue参考资料:https://blog.csdn.net/vernonzheng/article/details/8247564
一： ThreadPoolTaskExecutor是一个spring的线程池技术，查看代码可以看到这样一个字段：
private ThreadPoolExecutor threadPoolExecutor;
可以发现，spring的 ThreadPoolTaskExecutor是使用的jdk中的java.util.concurrent.ThreadPoolExecutor进行实现，
直接看代码：
@Override
protected ExecutorService initializeExecutor(
ThreadFactory threadFactory, RejectedExecutionHandler rejectedExecutionHandler) {
BlockingQueue<Runnable> queue = createQueue(this.queueCapacity);
ThreadPoolExecutor executor = new ThreadPoolExecutor(
this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
queue, threadFactory, rejectedExecutionHandler);
if (this.allowCoreThreadTimeOut) {
executor.allowCoreThreadTimeOut(true);
}
this.threadPoolExecutor = executor;
return executor;
}
这是ThreadPoolTaskExecutor用来初始化threadPoolExecutor的方法，BlockingQueue是一个阻塞队列，这个我们先不管。由于ThreadPoolTaskExecutor的实现方式完全是使用threadPoolExecutor进行实现，我们需要知道这个threadPoolExecutor的一些参数。
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;
}
这个是调用的构造函数：
int corePoolSize:线程池维护线程的最小数量.
int maximumPoolSize:线程池维护线程的最大数量.
long keepAliveTime:空闲线程的存活时间.
TimeUnit unit: 时间单位,现有纳秒,微秒,毫秒,秒枚举值.
BlockingQueue<Runnable> workQueue:持有等待执行的任务队列.
RejectedExecutionHandler handler:
用来拒绝一个任务的执行，有两种情况会发生这种情况。
一是在execute方法中若addIfUnderMaximumPoolSize(command)为false，即线程池已经饱和；
二是在execute方法中, 发现runState!=RUNNING || poolSize == 0,即已经shutdown,就调用ensureQueuedTaskHandled(Runnable command)，在该方法中有可能调用reject。
ThreadPoolExecutor池子的处理流程如下：
1）当池子大小小于corePoolSize就新建线程，并处理请求
2）当池子大小等于corePoolSize，把请求放入workQueue中，池子里的空闲线程就去从workQueue中取任务并处理
3）当workQueue放不下新入的任务时，新建线程入池，并处理请求，如果池子大小撑到了maximumPoolSize就用RejectedExecutionHandler来做拒绝处理
4）另外，当池子的线程数大于corePoolSize的时候，多余的线程会等待keepAliveTime长的时间，如果无请求可处理就自行销毁
其会优先创建 CorePoolSiz 线程， 当继续增加线程时，先放入Queue中，当 CorePoolSiz 和 Queue 都满的时候，就增加创建新线程，当线程达到MaxPoolSize的时候，就会抛出错 误 org.springframework.core.task.TaskRejectedException
另外MaxPoolSize的设定如果比系统支持的线程数还要大时，会抛出java.lang.OutOfMemoryError: unable to create new native thread 异常。
这个是ThreadPoolExecutor的运算流程，既然ThreadPoolTaskExecutor是直接使用ThreadPoolExecutor进行处理，所以运算规则肯定一样。
在spring中使用ThreadPoolTaskExecutor的配置：
<!-- 异步线程池 -->
<bean
class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor">
<!-- 核心线程数 -->
<property name="corePoolSize" value="3" />
<!-- 最大线程数 -->
<property name="maxPoolSize" value="10" />
<!-- 队列最大长度 >=mainExecutor.maxSize -->
<property name="queueCapacity" value="25" />
<!-- 线程池维护线程所允许的空闲时间 -->
<property name="keepAliveSeconds" value="300" />
<!-- 线程池对拒绝任务(无线程可用)的处理策略 ThreadPoolExecutor.CallerRunsPolicy策略 ,调用者的线程会执行该任务,如果执行器已关闭,则丢弃. -->
<property name="rejectedExecutionHandler">
<bean class="java.util.concurrent.ThreadPoolExecutor$CallerRunsPolicy" />
</property>
</bean>
Reject策略预定义有四种：
Reject 策略详解：https://blog.csdn.net/jgteng/article/details/54411423
(1)ThreadPoolExecutor.AbortPolicy策略，是默认的策略,处理程序遭到拒绝将抛出运行时 RejectedExecutionException。
(2)ThreadPoolExecutor.CallerRunsPolicy策略 ,调用者的线程会执行该任务,如果执行器已关闭,则丢弃.
(3)ThreadPoolExecutor.DiscardPolicy策略，不能执行的任务将被丢弃.
(4)ThreadPoolExecutor.DiscardOldestPolicy策略，如果执行程序尚未关闭，则位于工作队列头部的任务将被删除，然后重试执行程序（如果再次失败，则重复此过程）.
向spring容器中加入ThreadPoolTaskExecutor后，使用时只需要调用其的execute方法，其参数为一个Runnable。
threadPool.execute(new Runnable() {
@Override
public void run() {
System.out.println("=======");
}
});
ThreadPoolTaskExecutor有两个execute的重载，但翻看代码可以知道调用的是同一个方法，所以只调用execute就可以了
@Override
public void execute(Runnable task) {
Executor executor = getThreadPoolExecutor();
try {
executor.execute(task);
}
catch (RejectedExecutionException ex) {
throw new TaskRejectedException("Executor [" + executor + "] did not accept task: " + task, ex);
}
}
@Override
public void execute(Runnable task, long startTimeout) {
execute(task);
}
在execute中调用的是ThreadPoolExecutor中的execute方法，执行了上面的处理流程后执行任务。
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}