承接Spring源码情操陶冶-自定义节点的解析。线程池是jdk的一个很重要的概念，在很多的场景都会应用到，多用于处理多任务的并发处理，此处借由spring整合jdk的cocurrent包的方式来进行深入的分析

spring配置文件样例

配置简单线程池

<task-executor keep-alive="60" queue-capacity="20" pool-size="5" rejection-policy="DISCARD">
</task-executor>

以上属性是spring基于task命令空间提供的对外属性，一般是线程池的基础属性，我们可以剖析下相应的解析类具体了解下spring是如何整合线程池的

ExecutorBeanDefinitionParser-解析task-executor节点

1. 我们可以直接看下解析的具体方法doParse()，代码如下

   @Override
   protected void doParse(Element element, ParserContext parserContext, BeanDefinitionBuilder builder) {
       String keepAliveSeconds = element.getAttribute("keep-alive");
       if (StringUtils.hasText(keepAliveSeconds)) {
           builder.addPropertyValue("keepAliveSeconds", keepAliveSeconds);
       }
       String queueCapacity = element.getAttribute("queue-capacity");
       if (StringUtils.hasText(queueCapacity)) {
           builder.addPropertyValue("queueCapacity", queueCapacity);
       }
       configureRejectionPolicy(element, builder);
       String poolSize = element.getAttribute("pool-size");
       if (StringUtils.hasText(poolSize)) {
           builder.addPropertyValue("poolSize", poolSize);
       }
   }

   很简单就是读取我们第一部分所罗列的属性值，并塞入至BeanDefinitionBuilder对象的属性集合中。借此
   对任务的拒绝策略属性解析方法configureRejectionPolicy()我们也可以简单的看下

   private void configureRejectionPolicy(Element element, BeanDefinitionBuilder builder) {
       String rejectionPolicy = element.getAttribute("rejection-policy");
       if (!StringUtils.hasText(rejectionPolicy)) {
           return;
       }
       String prefix = "java.util.concurrent.ThreadPoolExecutor.";
       if (builder.getRawBeanDefinition().getBeanClassName().contains("backport")) {
           prefix = "edu.emory.mathcs.backport." + prefix;
       }
       String policyClassName;
       if (rejectionPolicy.equals("ABORT")) {
           policyClassName = prefix + "AbortPolicy";
       }
       else if (rejectionPolicy.equals("CALLER_RUNS")) {
           policyClassName = prefix + "CallerRunsPolicy";
       }
       else if (rejectionPolicy.equals("DISCARD")) {
           policyClassName = prefix + "DiscardPolicy";
       }
       else if (rejectionPolicy.equals("DISCARD_OLDEST")) {
           policyClassName = prefix + "DiscardOldestPolicy";
       }
       else {
           policyClassName = rejectionPolicy;
       }
       builder.addPropertyValue("rejectedExecutionHandler", new RootBeanDefinition(policyClassName));
   }

   由此可看出拒绝策略采取的基本上是java.util.concurrent.ThreadPoolExecutor工具包下的静态内部类，同时也支持用户自定义的拒绝策略。对拒绝策略此处作下小总结

   rejection-policy(Spring)	jdk对应类	含义
   ABORT	java.util.concurrent.ThreadPoolExecutor.AbortPolicy	抛出拒绝的异常信息
   CALLER_RUNS	java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy	直接执行对应的任务(线程池不关闭)
   DISCARD	java.util.concurrent.ThreadPoolExecutor.DiscardPolicy	直接丢弃任务
   DISCARD_OLDEST	java.util.concurrent.ThreadPoolExecutor.DiscardOldestPolicy	直接丢弃队列最老的任务(最靠头部)，塞入此任务到队列尾部

2. bean实体类
   那么我们肯定要知道是spring的哪个bean来实例化我们的线程池配置呢?答案就在getBeanClassName()方法

   @Override
   protected String getBeanClassName(Element element) {
       return "org.springframework.scheduling.config.TaskExecutorFactoryBean";
   }

   直接通过TaskExecutorFactoryBeanbean工厂来实例化线程池，很有意思，我们也别忘了其获取实例化对象其实是调用了其内部的getObject()方法。我们继续跟踪把

TaskExecutorFactoryBean-线程池bean工厂类

看继承结构，我们细心的发现其实现了InitializingBean接口，那我们直接关注afterPropertiesSet()方法

    @Override
    public void afterPropertiesSet() throws Exception {
        // 实例化的bean类型为ThreadPoolTaskExecutor
        BeanWrapper bw = new BeanWrapperImpl(ThreadPoolTaskExecutor.class);
        determinePoolSizeRange(bw);
        // 基本属性保存
        if (this.queueCapacity != null) {
            bw.setPropertyValue("queueCapacity", this.queueCapacity);
        }
        if (this.keepAliveSeconds != null) {
            bw.setPropertyValue("keepAliveSeconds", this.keepAliveSeconds);
        }
        if (this.rejectedExecutionHandler != null) {
            bw.setPropertyValue("rejectedExecutionHandler", this.rejectedExecutionHandler);
        }
        if (this.beanName != null) {
            bw.setPropertyValue("threadNamePrefix", this.beanName + "-");
        }
        // 实例化ThreadPoolTaskExecutor对象
        this.target = (TaskExecutor) bw.getWrappedInstance();
        // 并执行相应的afterPropertiesSet方法
        if (this.target instanceof InitializingBean) {
            ((InitializingBean) this.target).afterPropertiesSet();
        }
    }

1. 真正实例化的对象为ThreadPoolTaskExecutor.class

2. task-executor指定的pool-size支持-作为分隔符，比如2-4，表示线程池核心线程数为2个，最大线程数为4；如果没有分隔符，则最大线程数等同于核心线程数

3. task-executor如果指定pool-size为0-4且queue-capacity为null，则会指定线程池的allowCoreThreadTimeout为true，表明支持核心线程超时释放，默认不支持

4. ThreadPoolTaskExecutor.class也是InitializingBean的实现类，也会被调用afterPropertiesSet()方法

ThreadPoolTaskExecutor#afterPropertiesSet()-实例化

直接查看initializeExecutor()初始化jdk对应的线程池

    // 默认的rejectedExecutionHandler为AbortPolicy策略 
    @Override
    protected ExecutorService initializeExecutor(
            ThreadFactory threadFactory, RejectedExecutionHandler rejectedExecutionHandler) {
        // 先创建阻塞的队列
        BlockingQueue<Runnable> queue = createQueue(this.queueCapacity);
        // 创建线程池
        ThreadPoolExecutor executor  = new ThreadPoolExecutor(
                this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
                queue, threadFactory, rejectedExecutionHandler);
        // 设置是否允许核心线程超时被收回。默认不允许
        if (this.allowCoreThreadTimeOut) {
            executor.allowCoreThreadTimeOut(true);
        }

        this.threadPoolExecutor = executor;
        return executor;
    }

我们按照上述的注释，按照两步走进行解析

1. 阻塞队列的创建

   protected BlockingQueue<Runnable> createQueue(int queueCapacity) {
       if (queueCapacity > 0) {
           return new LinkedBlockingQueue<Runnable>(queueCapacity);
       }
       else {
           return new SynchronousQueue<Runnable>();
       }
   }

   默认情况下是创建LinkedBlockingQueue链式队列，因为默认的queueCapacity大小为Integer.MAX_VALUE。而queueCapacity为0的情况下则采取SynchronousQueue同步队列，其约定塞入一个元素必须等待另外的线程消费其内部的一个元素，其内部最多指定一个元素用于被消费

2. 线程池创建
   用到最基本的构造方法ThreadPoolExecutor()

   /**
    * 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.corePoolSize = corePoolSize;
       this.maximumPoolSize = maximumPoolSize;
       this.workQueue = workQueue;
       this.keepAliveTime = unit.toNanos(keepAliveTime);
       this.threadFactory = threadFactory;
       this.handler = handler;
   }

   篇幅限于过长，具体就不解释了直接看注释就明白了

TaskExecutorFactoryBean#getObject()-获取实体类

public TaskExecutor getObject() {
        return this.target;
    }

由上述可知this.target对应的class类为ThreadPoolTaskExecutor，其内部已实例化了ThreadPoolExecutor线程池对象

小结

1. Spring创建线程池本质上是通过ThreadPoolExecutor的构造方法来进行创建的。由此可知jdk的concurrent工具包很有参考价值

2. Spring默认指定的线程池队列为LinkedBlockingQueue链式队列，默认支持无线的任务添加，用户也可以指定queue-capacity来指定队列接受的最多任务数；并采用AborPolicy策略来拒绝多余的任务

3. Spring指定的pool-size支持-分隔符，具体解释见上文

4. Spring的task-executor多与task-scheduler和task-scheduled-tasks搭配使用，具体见后文分析