Quartz源码分析（一）------ 以线程等待的方式实现按时间调度

Quartz是运用最广的任务调度框架，它最核心的组成部分是Scheduler、Trigger、JobDetail，然后给Scheduler配置个线程QuartzSchedulerThread，此线程在Scheduler初始化时启动，等待Scheduler start，然后从JobStore里拿到最近要触发的Trigger，以线程等待的方式等到trigger触发时间点，之后就是执行trigger所关联的JobDetail，最后清扫战场。Scheduler初始化、start和trigger执行的时序图如下所示：

其中，最核心的地方是QuartzSchedulerThread运行机制。下面解析一下它的run方法：

 public void run() {
         boolean lastAcquireFailed = false;

         while (!halted) {
             try {
                 // check if we're supposed to pause...
                 synchronized (pauseLock) {
                     while (paused && !halted) {
                         try {
                             // wait until togglePause(false) is called...
                             pauseLock.wait(100L);
                         } catch (InterruptedException ignore) {
                         }
                     }

                     if (halted) {
                         break;
                     }
                 }
            ......
       }
 }

 以上是run的最开头的一段，不难看出这是在等待scheduler的start，实际上Quartz就是通过线程的wait或sleep来实现时间调度。继续看代码：

 Trigger trigger = null;
 long now = System.currentTimeMillis();
 signaled = false;
 try {
     trigger = qsRsrcs.getJobStore().acquireNextTrigger(
             ctxt, now + idleWaitTime);
     lastAcquireFailed = false;
 } catch (JobPersistenceException jpe) {
     if(!lastAcquireFailed) {
         qs.notifySchedulerListenersError(
             "An error occured while scanning for the next trigger to fire.",
             jpe);
     }
     lastAcquireFailed = true;
 } catch (RuntimeException e) {
     if(!lastAcquireFailed) {
         getLog().error("quartzSchedulerThreadLoop: RuntimeException "
                 +e.getMessage(), e);
     }
     lastAcquireFailed = true;
 }

这段代码是从jobStore里拿到下一个要执行的trigger，一般情况下jobStore使用的是RAMJobStore，即trigger等相关信息存放在内存里，如果需要把任务持久化就得使用可持久化JobStore。继续看代码：

 

 now = System.currentTimeMillis();
 long triggerTime = trigger.getNextFireTime().getTime();
 long timeUntilTrigger = triggerTime - now;
 long spinInterval = 10;
 int numPauses = (int) (timeUntilTrigger / spinInterval);
 while (numPauses >= 0 && !signaled) {
     try {
         Thread.sleep(spinInterval);
     } catch (InterruptedException ignore) {
     }
     now = System.currentTimeMillis();
     timeUntilTrigger = triggerTime - now;
     numPauses = (int) (timeUntilTrigger / spinInterval);
 }
 if (signaled) {
     try {
         qsRsrcs.getJobStore().releaseAcquiredTrigger(
                 ctxt, trigger);
     } catch (JobPersistenceException jpe) {
         qs.notifySchedulerListenersError(
                 "An error occured while releasing trigger '"
                         + trigger.getFullName() + "'",
                 jpe);
         // db connection must have failed... keep
         // retrying until it's up...
         releaseTriggerRetryLoop(trigger);
     } catch (RuntimeException e) {
         getLog().error(
             "releaseTriggerRetryLoop: RuntimeException "
             +e.getMessage(), e);
         // db connection must have failed... keep
         // retrying until it's up...
         releaseTriggerRetryLoop(trigger);
     }
     signaled = false;
     continue;
 }

此段代码是计算下一个trigger的执行时间和现在系统时间的差，然后通过循环线程sleep的方式暂停住此线程，一直等到trigger的执行时间点。继续看代码：

 import org.quartz.core.JobRunShell;
 JobRunShell shell = null;
 try {
     shell = qsRsrcs.getJobRunShellFactory().borrowJobRunShell();
     shell.initialize(qs, bndle);
 } catch (SchedulerException se) {
     try {
         qsRsrcs.getJobStore().triggeredJobComplete(ctxt,
                 trigger, bndle.getJobDetail(), Trigger.INSTRUCTION_SET_ALL_JOB_TRIGGERS_ERROR);
     } catch (SchedulerException se2) {
         qs.notifySchedulerListenersError(
                 "An error occured while placing job's triggers in error state '"
                         + trigger.getFullName() + "'", se2);
         // db connection must have failed... keep retrying
         // until it's up...
         errorTriggerRetryLoop(bndle);
     }
     continue;
 }
 if (qsRsrcs.getThreadPool().runInThread(shell) == false) {
     try {
         getLog().error("ThreadPool.runInThread() return false!");
         qsRsrcs.getJobStore().triggeredJobComplete(ctxt,
                 trigger, bndle.getJobDetail(), Trigger.INSTRUCTION_SET_ALL_JOB_TRIGGERS_ERROR);
     } catch (SchedulerException se2) {
         qs.notifySchedulerListenersError(
                 "An error occured while placing job's triggers in error state '"
                         + trigger.getFullName() + "'", se2);
         // db connection must have failed... keep retrying
         // until it's up...
         releaseTriggerRetryLoop(trigger);
     }
 }

此段代码就是包装trigger，然后通过以JobRunShell为载体，在threadpool里执行trigger所关联的jobDetail。

之后的代码就是清扫战场，就不在累述。