CountDownLatch导致的线程阻塞问题及线程池的使用
CountDownLatch是什么
countdownlatch 是一个同步工具类，它允许一个或多个线程一直等待，直到其他线程的操作执行完毕再执行。从命名可以解读到countdown 是倒数的意思，类似于我们倒计时的概念。countdownlatch 提供了两个方法，一个是 countDown，一个是 await，countdownlatch 初始化的时候需要传入一个整数，在这个整数倒数到 0 之前，调用了 await 方法的程序都必须要等待，然后通过countDown 来倒数。
项目中的应用
我们项目中有查询业绩功能,sql查询次数较多,于是使用多线程查询,但是查询结果需要统一封装返回,于是使用多线程查询的同时辅以CountDownLatch进行线程的约束
问题记录
查询响应较慢,每次响应时间大约都是10秒左右,经查看countDownLatch.await(10,TimeUnit.SECONDS)设定的阻塞时间为10秒,深入代码走查,发现新开的线程中有异常抛出而未捕获,导致线程计数器未按照理想状态下进行countDowm,最终过10秒线程主动向下执行导致的。
ThreadFactory threadFactory=Executors.defaultThreadFactory();CountDownLatch countDownLatch=newCountDownLatch(4);
threadFactory.newThread(()->{Resps[0]=getDistributionPerformanceOfcurDay(req);
countDownLatch.countDown();}).start();
threadFactory.newThread(()->{Resps[1]=getDistributionPerformanceOfWeek(req);
countDownLatch.countDown();}).start();
threadFactory.newThread(()->{Resps[2]=getDistributionPerformanceOfMonth(req);
countDownLatch.countDown();}).start();
threadFactory.newThread(()->{Resps[3]=getDistributionPerformanceOfPreMonth(req);
countDownLatch.countDown();}).start();上述每个线程中都是查询语句，分别查询当天、当周、当月以及上月的业绩。
在每个线程中都加上try-catch，finally中执行countDownLatch.countDown();解决！
学习下CountDownLatch以及线程池
jdk封装的线程池问题
- FixedThreadPool 和 SingleThreadPool：
允许的请求队列长度为 Integer.MAX_VALUE ，会堆积大量请求OOM - CachedThreadPool 和 ScheduledThreadPool：
允许的创建线程数量为 Integer.MAX_VALUE，可能会创建大量线程OOM
创建线程池
publicThreadPoolExecutor(int corePoolSize,int maximumPoolSize,long keepAliveTime,TimeUnit unit,BlockingQueue<Runnable> workQueue,RejectedExecutionHandler handler)虽然可以这么写，但是我可能需要在项目中使用很多个线程池，如果用的时候就新建，那岂不是跟new Thread的做法一样了？
所以自定义一个线程池，每次用这个线程池的时候保证只有一个，也就是搞成单例模式的，就是代码中可以随时取用，但是返回的永远是同一个。
publicclassMyThreadPool{privatestaticvolatileThreadPoolExecutor threadPoolExecutor=null;publicstaticThreadPoolExecutorgetThreadPool(){if(threadPoolExecutor!=null){return threadPoolExecutor;}synchronized(MyThreadPool.class){if(threadPoolExecutor==null){
threadPoolExecutor=newThreadPoolExecutor(10,10,300L,TimeUnit.SECONDS,newLinkedBlockingQueue<Runnable>(100),newThreadPoolRejectHandler());
threadPoolExecutor.allowCoreThreadTimeOut(true);//任务执行完之后退出线程池}}return threadPoolExecutor;}publicstaticclassThreadPoolRejectHandlerimplementsRejectedExecutionHandler{@OverridepublicvoidrejectedExecution(Runnable r,ThreadPoolExecutor executor){// 可以直接抛异常,记录日志// 可以另外起一个临时线程处理这个被决绝的任务}}}- 线程池的线程数量长期维持在 corePoolSize 个（核心线程数量）
- 线程池的线程数量最大可以扩展到 maximumPoolSize 个
- 在 corePoolSize ~ maximumPoolSize 这个区间的线程，一旦空闲超过keepAliveTime时间，就会被杀掉（时间单位）
- 送来工作的线程数量超过最大数以后，送到 workQueue 里面等号
- 等号的队列也满了，就按照事先约定的策略 RejectedExecutionHandler 给拒绝掉
- 业务逻辑中调用时直接MyThreadPool.getThreadPool()即可
线程池的拒绝策略-默认4种
源码如下（加了注释），列出四种默认的拒绝策略
自己也可以定义个拒绝策略，实现RejectedExecutionHandler即可，然后在新建的线程池中加进去就行。我上面贴出来的自定义的线程池就是
publicstaticclassCallerRunsPolicyimplementsRejectedExecutionHandler{/**
* 在调用者线程中（也就是说是谁把任务放进线程池）运行当前被丢弃的任务。
* 只会用调用者所在线程来运行任务，也就是说任务不会进入线程池。
* 如果线程池已经被关闭，则直接丢弃该任务。
*/publicCallerRunsPolicy(){}/**
* 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
*/publicvoidrejectedExecution(Runnable r,ThreadPoolExecutor e){if(!e.isShutdown()){
r.run();}}}/**
* A handler for rejected tasks that throws a
* {@link RejectedExecutionException}.
* 创建线程池时如若不指定拒绝策略，默认的就是这个
* 直接抛出拒绝异常，会中断调用者的处理过程
* This is the default handler for {@link ThreadPoolExecutor} and
* {@link ScheduledThreadPoolExecutor}.
*/publicstaticclassAbortPolicyimplementsRejectedExecutionHandler{/**
* Creates an {@code AbortPolicy}.
*/publicAbortPolicy(){}/**
* Always throws RejectedExecutionException.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
* @throws RejectedExecutionException always
*/publicvoidrejectedExecution(Runnable r,ThreadPoolExecutor e){thrownewRejectedExecutionException("Task "+ r.toString()+" rejected from "+
e.toString());}}/**
* A handler for rejected tasks that silently discards the
* rejected task.
* 默默的丢弃这个被拒绝的任务。英文底子好真棒！
*/publicstaticclassDiscardPolicyimplementsRejectedExecutionHandler{/**
* Creates a {@code DiscardPolicy}.
*/publicDiscardPolicy(){}/**
* 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
*/publicvoidrejectedExecution(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.
* 丢弃队列中最老的，然后再次尝试提交新任务
*/publicstaticclassDiscardOldestPolicyimplementsRejectedExecutionHandler{/**
* Creates a {@code DiscardOldestPolicy} for the given executor.
*/publicDiscardOldestPolicy(){}/**
* 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
*/publicvoidrejectedExecution(Runnable r,ThreadPoolExecutor e){if(!e.isShutdown()){
e.getQueue().poll();
e.execute(r);}}}