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分布式定时任务系列1：XXL-job安装

分布式定时任务系列2：XXL-job使用

分布式定时任务系列3：任务执行引擎设计

分布式定时任务系列4：任务执行引擎设计续

 Java并发编程实战1：java中的阻塞队列

引子

 这篇文章的主要目不是讨论XXL-job的使用，而是要通过它的任务线程实现机制来分析java中阻塞队列的应用！

而这一切要从上周某天，公司一个普通的下午说起。

当时一个同事要添加任务，就随口问了一句：“阻塞处理策略选啥？”，我心里一惊，以前没注意过这个地方，每次都是默认的，也就是单机串行。

就是下图这个：

 这里面有3个选项：

 众人先是侃侃讨论了半天，后面再去翻了源码来查看，发现了下面的blockingQueue

阻塞队列

什么是阻塞队列

关于java阻塞队列，可以看看java中的阻塞队列。里面例子用到的是有界队列ArrayBlockingQueue，XXL-job里面用的是无界队列LinkedBlockingQueue。不论它们的相似及区别点，最重要的都是拥有阻塞特性。

要分析XXL-job是怎么使用阻塞队列的，就从XXL-job的调度触发机制入手！

什么是XXL-job任务

前面在分布式定时任务系列2：XXL-job使用里面介绍过XXL-job的使用，注册一个任务也很简单，只要在业务代码方法中打上@XxlJob注解即可！下面是官方提供的demo：

/*** 1、简单任务示例（Bean模式）*/@XxlJob("demoJobHandler")public void demoJobHandler() throws Exception {XxlJobHelper.log("XXL-JOB, Hello World.");for (int i = 0; i < 5; i++) {XxlJobHelper.log("beat at:" + i);TimeUnit.SECONDS.sleep(2);}// default success}

开发步骤

1、任务开发：在Spring Bean实例中，开发Job方法；

2、注解配置：为Job方法添加注解 "@XxlJob(value="自定义jobhandler名称", init = "JobHandler初始化方法", destroy = "JobHandler销毁方法")"，注解value值对应的是调度中心新建任务的JobHandler属性的值。

3、执行日志：需要通过 "XxlJobHelper.log" 打印执行日志；

4、任务结果：默认任务结果为 "成功" 状态，不需要主动设置；如有诉求，比如设置任务结果为失败，可以通过 "XxlJobHelper.handleFail/handleSuccess" 自主设置任务结果； 

任务注册

对于XXL-job来说，是怎么注册并管理这些任务的呢？

在程序启动的时候，xxl-job-core包中的XxlJobSpringExecutor类由于实现了接口SmartInitializingSingleton，在Bean实例化完成时会调用afterSingletonsInstantiated()方法：

@Overridepublic void afterSingletonsInstantiated() {// init JobHandler Repository (for method)initJobHandlerMethodRepository(applicationContext);// refresh GlueFactoryGlueFactory.refreshInstance(1);// super starttry {super.start();} catch (Exception e) {throw new RuntimeException(e);}}

在initJobHandlerMethodRepository(applicationContext)方法中，现在看一下这个方法的代码：该方法会提取所有SpringBean的实例中方法上添加了XxlJob注解的方法，并进行注册！

private void initJobHandlerMethodRepository(ApplicationContext applicationContext) {if (applicationContext == null) {return;}// init job handler from methodString[] beanDefinitionNames = applicationContext.getBeanNamesForType(Object.class, false, true);for (String beanDefinitionName : beanDefinitionNames) {Object bean = applicationContext.getBean(beanDefinitionName);Map<Method, XxlJob> annotatedMethods = null;   // referred to ：org.springframework.context.event.EventListenerMethodProcessor.processBeantry {annotatedMethods = MethodIntrospector.selectMethods(bean.getClass(),new MethodIntrospector.MetadataLookup<XxlJob>() {@Overridepublic XxlJob inspect(Method method) {return AnnotatedElementUtils.findMergedAnnotation(method, XxlJob.class);}});} catch (Throwable ex) {logger.error("xxl-job method-jobhandler resolve error for bean[" + beanDefinitionName + "].", ex);}if (annotatedMethods==null || annotatedMethods.isEmpty()) {continue;}for (Map.Entry<Method, XxlJob> methodXxlJobEntry : annotatedMethods.entrySet()) {Method executeMethod = methodXxlJobEntry.getKey();XxlJob xxlJob = methodXxlJobEntry.getValue();// registregistJobHandler(xxlJob, bean, executeMethod);}}// ---------------------- job handler repository ----------------------private static ConcurrentMap<String, IJobHandler> jobHandlerRepository = new ConcurrentHashMap<String, IJobHandler>();public static IJobHandler loadJobHandler(String name){return jobHandlerRepository.get(name);}public static IJobHandler registJobHandler(String name, IJobHandler jobHandler){logger.info(">>>>>>>>>>> xxl-job register jobhandler success, name:{}, jobHandler:{}", name, jobHandler);return jobHandlerRepository.put(name, jobHandler);}

注册的方法就是registJobHandler(name, new MethodJobHandler(bean, executeMethod, initMethod, destroyMethod))，会将XxlJob注解标记的方法，提取出对应的方法名，通过反射得到Method信息并实例为对应的MethodJobHandler，最后通过ConcurrentMap来管理，其中key是注任务名称，value为JobHandler实例。

 看到这里，不知你有没有一个疑问：这里的JobHandler里面并没有阻塞队列BlockingQueue，它们是怎么关联起来的呢？所以这里就要讨论一下XXL-job的任务执行!

XXL-job的任务执行

在后台管理手动触发一下测试任务，并把代码打上debug断点：

 首先会发送后台请求：

一路跟踪下去，会进入到XxlJobExecutor.runExecutor(TriggerParam triggerParam, String address)方法，最终是到ExecutorBizImpl.run(TriggerParam triggerParam)方法：

public ReturnT<String> run(TriggerParam triggerParam) {// load old：jobHandler + jobThreadJobThread jobThread = XxlJobExecutor.loadJobThread(triggerParam.getJobId());IJobHandler jobHandler = jobThread!=null?jobThread.getHandler():null;String removeOldReason = null;// valid：jobHandler + jobThreadGlueTypeEnum glueTypeEnum = GlueTypeEnum.match(triggerParam.getGlueType());if (GlueTypeEnum.BEAN == glueTypeEnum) {// new jobhandlerIJobHandler newJobHandler = XxlJobExecutor.loadJobHandler(triggerParam.getExecutorHandler());// valid old jobThreadif (jobThread!=null && jobHandler != newJobHandler) {// change handler, need kill old threadremoveOldReason = "change jobhandler or glue type, and terminate the old job thread.";jobThread = null;jobHandler = null;}// valid handlerif (jobHandler == null) {jobHandler = newJobHandler;if (jobHandler == null) {return new ReturnT<String>(ReturnT.FAIL_CODE, "job handler [" + triggerParam.getExecutorHandler() + "] not found.");}}} else if (GlueTypeEnum.GLUE_GROOVY == glueTypeEnum) {// valid old jobThreadif (jobThread != null &&!(jobThread.getHandler() instanceof GlueJobHandler&& ((GlueJobHandler) jobThread.getHandler()).getGlueUpdatetime()==triggerParam.getGlueUpdatetime() )) {// change handler or gluesource updated, need kill old threadremoveOldReason = "change job source or glue type, and terminate the old job thread.";jobThread = null;jobHandler = null;}// valid handlerif (jobHandler == null) {try {IJobHandler originJobHandler = GlueFactory.getInstance().loadNewInstance(triggerParam.getGlueSource());jobHandler = new GlueJobHandler(originJobHandler, triggerParam.getGlueUpdatetime());} catch (Exception e) {logger.error(e.getMessage(), e);return new ReturnT<String>(ReturnT.FAIL_CODE, e.getMessage());}}} else if (glueTypeEnum!=null && glueTypeEnum.isScript()) {// valid old jobThreadif (jobThread != null &&!(jobThread.getHandler() instanceof ScriptJobHandler&& ((ScriptJobHandler) jobThread.getHandler()).getGlueUpdatetime()==triggerParam.getGlueUpdatetime() )) {// change script or gluesource updated, need kill old threadremoveOldReason = "change job source or glue type, and terminate the old job thread.";jobThread = null;jobHandler = null;}// valid handlerif (jobHandler == null) {jobHandler = new ScriptJobHandler(triggerParam.getJobId(), triggerParam.getGlueUpdatetime(), triggerParam.getGlueSource(), GlueTypeEnum.match(triggerParam.getGlueType()));}} else {return new ReturnT<String>(ReturnT.FAIL_CODE, "glueType[" + triggerParam.getGlueType() + "] is not valid.");}// executor block strategyif (jobThread != null) {ExecutorBlockStrategyEnum blockStrategy = ExecutorBlockStrategyEnum.match(triggerParam.getExecutorBlockStrategy(), null);if (ExecutorBlockStrategyEnum.DISCARD_LATER == blockStrategy) {// discard when runningif (jobThread.isRunningOrHasQueue()) {return new ReturnT<String>(ReturnT.FAIL_CODE, "block strategy effect："+ExecutorBlockStrategyEnum.DISCARD_LATER.getTitle());}} else if (ExecutorBlockStrategyEnum.COVER_EARLY == blockStrategy) {// kill running jobThreadif (jobThread.isRunningOrHasQueue()) {removeOldReason = "block strategy effect：" + ExecutorBlockStrategyEnum.COVER_EARLY.getTitle();jobThread = null;}} else {// just queue trigger}}// replace thread (new or exists invalid)if (jobThread == null) {jobThread = XxlJobExecutor.registJobThread(triggerParam.getJobId(), jobHandler, removeOldReason);}// push data to queueReturnT<String> pushResult = jobThread.pushTriggerQueue(triggerParam);return pushResult;}

 任务执行过程中，如果发现XXL-job对应的执行线程不存在，会创建一个新new Thread实例，并绑定前面的JobHandler实例，这样对于每一个任务都有一个单独的线程，也就将执行线程JobThread跟任务关联起来了：

public static JobThread registJobThread(int jobId, IJobHandler handler, String removeOldReason){JobThread newJobThread = new JobThread(jobId, handler);newJobThread.start();logger.info(">>>>>>>>>>> xxl-job regist JobThread success, jobId:{}, handler:{}", new Object[]{jobId, handler});JobThread oldJobThread = jobThreadRepository.put(jobId, newJobThread);	// putIfAbsent | oh my god, map's put method return the old value!!!if (oldJobThread != null) {oldJobThread.toStop(removeOldReason);oldJobThread.interrupt();}return newJobThread;}public JobThread(int jobId, IJobHandler handler) {this.jobId = jobId;this.handler = handler;// 初始化阻塞队列this.triggerQueue = new LinkedBlockingQueue<TriggerParam>();this.triggerLogIdSet = Collections.synchronizedSet(new HashSet<Long>());// assign job thread namethis.setName("xxl-job, JobThread-"+jobId+"-"+System.currentTimeMillis());}

而执行任务的最后一步，就是将当前任务入队到刚才的执行线程的BlockingQueue中：

// push data to queueReturnT<String> pushResult = jobThread.pushTriggerQueue(triggerParam);

 而这也就跟文章最前面的代码关联起来了：

/*** new trigger to queue** @param triggerParam* @return*/public ReturnT<String> pushTriggerQueue(TriggerParam triggerParam) {// avoid repeatif (triggerLogIdSet.contains(triggerParam.getLogId())) {logger.info(">>>>>>>>>>> repeate trigger job, logId:{}", triggerParam.getLogId());return new ReturnT<String>(ReturnT.FAIL_CODE, "repeate trigger job, logId:" + triggerParam.getLogId());}triggerLogIdSet.add(triggerParam.getLogId());triggerQueue.add(triggerParam);return ReturnT.SUCCESS;}

BlockingQueue的应用

消息入队

至此，就可以看出任务触发的大致逻辑了，用户在admin手工触发任务之后最终会放入到任务对应的队列之中：通过add方法将当前执行参数入队（FIFO），不阻塞队列，如果满了就抛出异常（这也是一个比较大的隐患，如果触了过于频繁，可能会导致OOM，而这也就是为什么要设计不同阻塞策略的原因）。

消息消费

而这些触发的任务怎么消费的呢？其实也是在上面的提到的执行线程JobThread：

• 当实例化一个线程的时候（com.xxl.job.core.executor.XxlJobExecutor类），会立即启动：调用线程的start()方法

• 启动之后会调用线程的run()方法（com.xxl.job.core.thread.JobThread类）

• 在执行线程中，会写一个while的无限循环来不停的从阻塞队列中取出任务

 

• 循环结束的条件就是上面的阻塞队列：如果是覆盖之前调度将会结束循环（com.xxl.job.core.biz.impl.ExecutorBizImpl类）