前言
上一篇文章Quartz数据库表分析介绍了Quartz默认提供的11张表，本文将具体分析Quartz是如何调度的，是如何通过数据库的方式来现在分布式调度。
调度线程
Quartz内部提供的调度类是QuartzScheduler，而QuartzScheduler会委托QuartzSchedulerThread去实时调度；当调度完需要去执行job的时候QuartzSchedulerThread并没有直接去执行job，
而是交给ThreadPool去执行job，具体使用什么ThreadPool，初始化多线线程，可以在配置文件中进行配置：
org.quartz.threadPool.class: org.quartz.simpl.SimpleThreadPool
org.quartz.threadPool.threadCount: 10
org.quartz.threadPool.threadPriority: 5常用的线程池是SimpleThreadPool，这里默认启动了10个线程，在SimpleThreadPool会创建10个WorkerThread，由WorkerThread去执行具体的job；
调度分析
QuartzSchedulerThread是调度的核心类，具体Quartz是如何实现调度的，可以查看QuartzSchedulerThread核心源码：
public void run() {
boolean lastAcquireFailed = false;
while (!halted.get()) {
try {
// check if we're supposed to pause...
synchronized (sigLock) {
while (paused && !halted.get()) {
try {
// wait until togglePause(false) is called...
sigLock.wait(1000L);
} catch (InterruptedException ignore) {
}
}
if (halted.get()) {
break;
}
}
int availThreadCount = qsRsrcs.getThreadPool().blockForAvailableThreads();
if(availThreadCount > 0) { // will always be true, due to semantics of blockForAvailableThreads...
List<OperableTrigger> triggers = null;
long now = System.currentTimeMillis();
clearSignaledSchedulingChange();
try {
triggers = qsRsrcs.getJobStore().acquireNextTriggers(
now + idleWaitTime, Math.min(availThreadCount, qsRsrcs.getMaxBatchSize()), qsRsrcs.getBatchTimeWindow());
lastAcquireFailed = false;
if (log.isDebugEnabled())
log.debug("batch acquisition of " + (triggers == null ? 0 : triggers.size()) + " triggers");
} catch (JobPersistenceException jpe) {
if(!lastAcquireFailed) {
qs.notifySchedulerListenersError(
"An error occurred while scanning for the next triggers to fire.",
jpe);
}
lastAcquireFailed = true;
continue;
} catch (RuntimeException e) {
if(!lastAcquireFailed) {
getLog().error("quartzSchedulerThreadLoop: RuntimeException "
+e.getMessage(), e);
}
lastAcquireFailed = true;
continue;
}
if (triggers != null && !triggers.isEmpty()) {
now = System.currentTimeMillis();
long triggerTime = triggers.get(0).getNextFireTime().getTime();
long timeUntilTrigger = triggerTime - now;
while(timeUntilTrigger > 2) {
synchronized (sigLock) {
if (halted.get()) {
break;
}
if (!isCandidateNewTimeEarlierWithinReason(triggerTime, false)) {
try {
// we could have blocked a long while
// on 'synchronize', so we must recompute
now = System.currentTimeMillis();
timeUntilTrigger = triggerTime - now;
if(timeUntilTrigger >= 1)
sigLock.wait(timeUntilTrigger);
} catch (InterruptedException ignore) {
}
}
}
if(releaseIfScheduleChangedSignificantly(triggers, triggerTime)) {
break;
}
now = System.currentTimeMillis();
timeUntilTrigger = triggerTime - now;
}
// this happens if releaseIfScheduleChangedSignificantly decided to release triggers
if(triggers.isEmpty())
continue;
// set triggers to 'executing'
List<TriggerFiredResult> bndles = new ArrayList<TriggerFiredResult>();
boolean goAhead = true;
synchronized(sigLock) {
goAhead = !halted.get();
}
if(goAhead) {
try {
List<TriggerFiredResult> res = qsRsrcs.getJobStore().triggersFired(triggers);
if(res != null)
bndles = res;
} catch (SchedulerException se) {
qs.notifySchedulerListenersError(
"An error occurred while firing triggers '"
+ triggers + "'", se);
//QTZ-179 : a problem occurred interacting with the triggers from the db
//we release them and loop again
for (int i = 0; i < triggers.size(); i++) {
qsRsrcs.getJobStore().releaseAcquiredTrigger(triggers.get(i));
}
continue;
}
}
for (int i = 0; i < bndles.size(); i++) {
TriggerFiredResult result = bndles.get(i);
TriggerFiredBundle bndle = result.getTriggerFiredBundle();
Exception exception = result.getException();
if (exception instanceof RuntimeException) {
getLog().error("RuntimeException while firing trigger " + triggers.get(i), exception);
qsRsrcs.getJobStore().releaseAcquiredTrigger(triggers.get(i));
continue;
}
// it's possible to get 'null' if the triggers was paused,
// blocked, or other similar occurrences that prevent it being
// fired at this time... or if the scheduler was shutdown (halted)
if (bndle == null) {
qsRsrcs.getJobStore().releaseAcquiredTrigger(triggers.get(i));
continue;
}
JobRunShell shell = null;
try {
shell = qsRsrcs.getJobRunShellFactory().createJobRunShell(bndle);
shell.initialize(qs);
} catch (SchedulerException se) {
qsRsrcs.getJobStore().triggeredJobComplete(triggers.get(i), bndle.getJobDetail(), CompletedExecutionInstruction.SET_ALL_JOB_TRIGGERS_ERROR);
continue;
}
if (qsRsrcs.getThreadPool().runInThread(shell) == false) {
// this case should never happen, as it is indicative of the
// scheduler being shutdown or a bug in the thread pool or
// a thread pool being used concurrently - which the docs
// say not to do...
getLog().error("ThreadPool.runInThread() return false!");
qsRsrcs.getJobStore().triggeredJobComplete(triggers.get(i), bndle.getJobDetail(), CompletedExecutionInstruction.SET_ALL_JOB_TRIGGERS_ERROR);
}
}
continue; // while (!halted)
}
} else { // if(availThreadCount > 0)
// should never happen, if threadPool.blockForAvailableThreads() follows contract
continue; // while (!halted)
}
long now = System.currentTimeMillis();
long waitTime = now + getRandomizedIdleWaitTime();
long timeUntilContinue = waitTime - now;
synchronized(sigLock) {
try {
if(!halted.get()) {
// QTZ-336 A job might have been completed in the mean time and we might have
// missed the scheduled changed signal by not waiting for the notify() yet
// Check that before waiting for too long in case this very job needs to be
// scheduled very soon
if (!isScheduleChanged()) {
sigLock.wait(timeUntilContinue);
}
}
} catch (InterruptedException ignore) {
}
}
} catch(RuntimeException re) {
getLog().error("Runtime error occurred in main trigger firing loop.", re);
}
} // while (!halted)
// drop references to scheduler stuff to aid garbage collection...
qs = null;
qsRsrcs = null;
}1.halted和paused
这是两个boolean值的标志参数，分别表示：停止和暂停；halted默认为false，当QuartzScheduler执行shutdown()时才会更新为true；paused默认是true，当QuartzScheduler执行start()时
更新为false；正常启动之后QuartzSchedulerThread就可以往下执行了；
2.availThreadCount
查询SimpleThreadPool是否有可用的WorkerThread，如果availThreadCount>0可以往下继续执行其他逻辑，否则继续检查；
3.acquireNextTriggers
查询一段时间内将要被调度的triggers，这里有3个比较重要的参数分别是：idleWaitTime，maxBatchSize，batchTimeWindow，这3个参数都可以在配置文件中进行配置：
org.quartz.scheduler.idleWaitTime:30000
org.quartz.scheduler.batchTriggerAcquisitionMaxCount:1
org.quartz.scheduler.batchTriggerAcquisitionFireAheadTimeWindow:0idleWaitTime:在调度程序处于空闲状态时，调度程序将在重新查询可用触发器之前等待的时间量（以毫秒为单位），默认是30秒;
batchTriggerAcquisitionMaxCount:允许调度程序节点一次获取（用于触发）的触发器的最大数量，默认是1;
batchTriggerAcquisitionFireAheadTimeWindow:允许触发器在其预定的火灾时间之前被获取和触发的时间（毫秒）的时间量，默认是0;
往下继续查看acquireNextTriggers方法源码：
public List<OperableTrigger> acquireNextTriggers(final long noLaterThan, final int maxCount, final long timeWindow)
throws JobPersistenceException {
String lockName;
if(isAcquireTriggersWithinLock() || maxCount > 1) {
lockName = LOCK_TRIGGER_ACCESS;
} else {
lockName = null;
}
return executeInNonManagedTXLock(lockName,
new TransactionCallback<List<OperableTrigger>>() {
public List<OperableTrigger> execute(Connection conn) throws JobPersistenceException {
return acquireNextTrigger(conn, noLaterThan, maxCount, timeWindow);
}
},
......
});
}可以发现只有在设置了acquireTriggersWithinLock或者batchTriggerAcquisitionMaxCount>1情况下才使用LOCK_TRIGGER_ACCESS锁，也就是说在默认参数配置的情况下，这里是没有使用锁的，
那么如果多个节点同时去执行acquireNextTriggers，会不会出现同一个trigger在多个节点都被执行？
注：acquireTriggersWithinLock可以在配置文件中进行配置：
org.quartz.jobStore.acquireTriggersWithinLock=trueacquireTriggersWithinLock：获取triggers的时候是否需要使用锁，默认是false，如果batchTriggerAcquisitionMaxCount>1最好同时设置acquireTriggersWithinLock为true；
带着问题继续查看TransactionCallback内部的acquireNextTrigger方法源码：
protected List<OperableTrigger> acquireNextTrigger(Connection conn, long noLaterThan, int maxCount, long timeWindow)
throws JobPersistenceException {
if (timeWindow < 0) {
throw new IllegalArgumentException();
}
List<OperableTrigger> acquiredTriggers = new ArrayList<OperableTrigger>();
Set<JobKey> acquiredJobKeysForNoConcurrentExec = new HashSet<JobKey>();
final int MAX_DO_LOOP_RETRY = 3;
int currentLoopCount = 0;
do {
currentLoopCount ++;
try {
List<TriggerKey> keys = getDelegate().selectTriggerToAcquire(conn, noLaterThan + timeWindow, getMisfireTime(), maxCount);
// No trigger is ready to fire yet.
if (keys == null || keys.size() == 0)
return acquiredTriggers;
long batchEnd = noLaterThan;
for(TriggerKey triggerKey: keys) {
// If our trigger is no longer available, try a new one.
OperableTrigger nextTrigger = retrieveTrigger(conn, triggerKey);
if(nextTrigger == null) {
continue; // next trigger
}
// If trigger's job is set as @DisallowConcurrentExecution, and it has already been added to result, then
// put it back into the timeTriggers set and continue to search for next trigger.
JobKey jobKey = nextTrigger.getJobKey();
JobDetail job;
try {
job = retrieveJob(conn, jobKey);
} catch (JobPersistenceException jpe) {
try {
getLog().error("Error retrieving job, setting trigger state to ERROR.", jpe);
getDelegate().updateTriggerState(conn, triggerKey, STATE_ERROR);
} catch (SQLException sqle) {
getLog().error("Unable to set trigger state to ERROR.", sqle);
}
continue;
}
if (job.isConcurrentExectionDisallowed()) {
if (acquiredJobKeysForNoConcurrentExec.contains(jobKey)) {
continue; // next trigger
} else {
acquiredJobKeysForNoConcurrentExec.add(jobKey);
}
}
if (nextTrigger.getNextFireTime().getTime() > batchEnd) {
break;
}
// We now have a acquired trigger, let's add to return list.
// If our trigger was no longer in the expected state, try a new one.
int rowsUpdated = getDelegate().updateTriggerStateFromOtherState(conn, triggerKey, STATE_ACQUIRED, STATE_WAITING);
if (rowsUpdated <= 0) {
continue; // next trigger
}
nextTrigger.setFireInstanceId(getFiredTriggerRecordId());
getDelegate().insertFiredTrigger(conn, nextTrigger, STATE_ACQUIRED, null);
if(acquiredTriggers.isEmpty()) {
batchEnd = Math.max(nextTrigger.getNextFireTime().getTime(), System.currentTimeMillis()) + timeWindow;
}
acquiredTriggers.add(nextTrigger);
}
// if we didn't end up with any trigger to fire from that first
// batch, try again for another batch. We allow with a max retry count.
if(acquiredTriggers.size() == 0 && currentLoopCount < MAX_DO_LOOP_RETRY) {
continue;
}
// We are done with the while loop.
break;
} catch (Exception e) {
throw new JobPersistenceException(
"Couldn't acquire next trigger: " + e.getMessage(), e);
}
} while (true);
// Return the acquired trigger list
return acquiredTriggers;
}首先看一下在执行selectTriggerToAcquire方法时引入了新的参数：misfireTime=当前时间-MisfireThreshold，MisfireThreshold可以在配置文件中进行配置：
org.quartz.jobStore.misfireThreshold: 60000misfireThreshold：叫触发器超时，比如有10个线程，但是有11个任务，这样就有一个任务被延迟执行了，可以理解为调度引擎可以忍受这个超时的时间；具体的查询SQL如下所示：
SELECT TRIGGER_NAME, TRIGGER_GROUP, NEXT_FIRE_TIME, PRIORITY
FROM qrtz_TRIGGERS
WHERE SCHED_NAME = 'myScheduler'
AND TRIGGER_STATE = 'WAITING'
AND NEXT_FIRE_TIME <= noLaterThan
AND (MISFIRE_INSTR = -1 OR
(MISFIRE_INSTR != -1 AND NEXT_FIRE_TIME >= noEarlierThan))
ORDER BY NEXT_FIRE_TIME ASC, PRIORITY DESC这里的noLaterThan=当前时间+idleWaitTime+batchTriggerAcquisitionFireAheadTimeWindow，
noEarlierThan=当前时间-MisfireThreshold；
在查询完之后，会遍历执行updateTriggerStateFromOtherState()方法更新trigger的状态从STATE_WAITING到STATE_ACQUIRED，并且会判断rowsUpdated是否大于0，这样就算多个节点都查询到相同的trigger，但是肯定只会有一个节点更新成功；更新完状态之后，往qrtz_fired_triggers表中插入一条记录，表示当前trigger已经触发，状态为STATE_ACQUIRED；
4.executeInNonManagedTXLock
Quartz的分布式锁被用在很多地方，下面具体看一下Quartz是如何实现分布式锁的，executeInNonManagedTXLock方法源码如下：
protected <T> T executeInNonManagedTXLock(
String lockName,
TransactionCallback<T> txCallback, final TransactionValidator<T> txValidator) throws JobPersistenceException {
boolean transOwner = false;
Connection conn = null;
try {
if (lockName != null) {
// If we aren't using db locks, then delay getting DB connection
// until after acquiring the lock since it isn't needed.
if (getLockHandler().requiresConnection()) {
conn = getNonManagedTXConnection();
}
transOwner = getLockHandler().obtainLock(conn, lockName);
}
if (conn == null) {
conn = getNonManagedTXConnection();
}
final T result = txCallback.execute(conn);
try {
commitConnection(conn);
} catch (JobPersistenceException e) {
rollbackConnection(conn);
if (txValidator == null || !retryExecuteInNonManagedTXLock(lockName, new TransactionCallback<Boolean>() {
@Override
public Boolean execute(Connection conn) throws JobPersistenceException {
return txValidator.validate(conn, result);
}
})) {
throw e;
}
}
Long sigTime = clearAndGetSignalSchedulingChangeOnTxCompletion();
if(sigTime != null && sigTime >= 0) {
signalSchedulingChangeImmediately(sigTime);
}
return result;
} catch (JobPersistenceException e) {
rollbackConnection(conn);
throw e;
} catch (RuntimeException e) {
rollbackConnection(conn);
throw new JobPersistenceException("Unexpected runtime exception: "
+ e.getMessage(), e);
} finally {
try {
releaseLock(lockName, transOwner);
} finally {
cleanupConnection(conn);
}
}
}大致分成3个步骤：获取锁，执行逻辑，释放锁；getLockHandler().obtainLock表示获取锁txCallback.execute(conn)表示执行逻辑，commitConnection(conn)表示释放锁
Quartz的分布式锁接口类是Semaphore，默认具体的实现是StdRowLockSemaphore，具体接口如下：
public interface Semaphore {
boolean obtainLock(Connection conn, String lockName) throws LockException;
void releaseLock(String lockName) throws LockException;
boolean requiresConnection();
}具体看一下obtainLock()是如何获取锁的，源码如下：
public boolean obtainLock(Connection conn, String lockName)
throws LockException {
if (!isLockOwner(lockName)) {
executeSQL(conn, lockName, expandedSQL, expandedInsertSQL);
getThreadLocks().add(lockName);
} else if(log.isDebugEnabled()) {
}
return true;
}
protected void executeSQL(Connection conn, final String lockName, final String expandedSQL, final String expandedInsertSQL) throws LockException {
PreparedStatement ps = null;
ResultSet rs = null;
SQLException initCause = null;
int count = 0;
do {
count++;
try {
ps = conn.prepareStatement(expandedSQL);
ps.setString(1, lockName);
rs = ps.executeQuery();
if (!rs.next()) {
getLog().debug(
"Inserting new lock row for lock: '" + lockName + "' being obtained by thread: " +
Thread.currentThread().getName());
rs.close();
rs = null;
ps.close();
ps = null;
ps = conn.prepareStatement(expandedInsertSQL);
ps.setString(1, lockName);
int res = ps.executeUpdate();
if(res != 1) {
if(count < 3) {
try {
Thread.sleep(1000L);
} catch (InterruptedException ignore) {
Thread.currentThread().interrupt();
}
continue;
}
}
}
return; // obtained lock, go
} catch (SQLException sqle) {
......
} while(count < 4);
}obtainLock首先判断是否已经获取到锁，如果没有执行方法executeSQL，其中有两条重要的SQL，分别是：expandedSQL和expandedInsertSQL，以SCHED_NAME = ‘myScheduler’为例：
SELECT * FROM QRTZ_LOCKS WHERE SCHED_NAME = 'myScheduler' AND LOCK_NAME = ? FOR UPDATE
INSERT INTO QRTZ_LOCKS(SCHED_NAME, LOCK_NAME) VALUES ('myScheduler', ?)select语句后面添加了FOR UPDATE，如果LOCK_NAME存在，当多个节点去执行此SQL时，只有第一个节点会成功，其他的节点都将进入等待；
如果LOCK_NAME不存在，多个节点同时执行expandedInsertSQL，只会有一个节点插入成功，执行插入失败的节点将进入重试，重新执行expandedSQL；
txCallback执行完之后，执行commitConnection操作，这样当前节点就释放了LOCK_NAME，其他节点可以竞争获取锁，最后执行了releaseLock；
5.triggersFired
表示触发trigger，具体代码如下：
protected TriggerFiredBundle triggerFired(Connection conn,
OperableTrigger trigger)
throws JobPersistenceException {
JobDetail job;
Calendar cal = null;
// Make sure trigger wasn't deleted, paused, or completed...
try { // if trigger was deleted, state will be STATE_DELETED
String state = getDelegate().selectTriggerState(conn,
trigger.getKey());
if (!state.equals(STATE_ACQUIRED)) {
return null;
}
} catch (SQLException e) {
throw new JobPersistenceException("Couldn't select trigger state: "
+ e.getMessage(), e);
}
try {
job = retrieveJob(conn, trigger.getJobKey());
if (job == null) { return null; }
} catch (JobPersistenceException jpe) {
try {
getLog().error("Error retrieving job, setting trigger state to ERROR.", jpe);
getDelegate().updateTriggerState(conn, trigger.getKey(),
STATE_ERROR);
} catch (SQLException sqle) {
getLog().error("Unable to set trigger state to ERROR.", sqle);
}
throw jpe;
}
if (trigger.getCalendarName() != null) {
cal = retrieveCalendar(conn, trigger.getCalendarName());
if (cal == null) { return null; }
}
try {
getDelegate().updateFiredTrigger(conn, trigger, STATE_EXECUTING, job);
} catch (SQLException e) {
throw new JobPersistenceException("Couldn't insert fired trigger: "
+ e.getMessage(), e);
}
Date prevFireTime = trigger.getPreviousFireTime();
// call triggered - to update the trigger's next-fire-time state...
trigger.triggered(cal);
String state = STATE_WAITING;
boolean force = true;
if (job.isConcurrentExectionDisallowed()) {
state = STATE_BLOCKED;
force = false;
try {
getDelegate().updateTriggerStatesForJobFromOtherState(conn, job.getKey(),
STATE_BLOCKED, STATE_WAITING);
getDelegate().updateTriggerStatesForJobFromOtherState(conn, job.getKey(),
STATE_BLOCKED, STATE_ACQUIRED);
getDelegate().updateTriggerStatesForJobFromOtherState(conn, job.getKey(),
STATE_PAUSED_BLOCKED, STATE_PAUSED);
} catch (SQLException e) {
throw new JobPersistenceException(
"Couldn't update states of blocked triggers: "
+ e.getMessage(), e);
}
}
if (trigger.getNextFireTime() == null) {
state = STATE_COMPLETE;
force = true;
}
storeTrigger(conn, trigger, job, true, state, force, false);
job.getJobDataMap().clearDirtyFlag();
return new TriggerFiredBundle(job, trigger, cal, trigger.getKey().getGroup()
.equals(Scheduler.DEFAULT_RECOVERY_GROUP), new Date(), trigger
.getPreviousFireTime(), prevFireTime, trigger.getNextFireTime());
}首先查询trigger的状态是否STATE_ACQUIRED状态，如果不是直接返回null；然后通过通过jobKey获取对应的jobDetail，更新对应的FiredTrigger为EXECUTING状态；最后判定job的DisallowConcurrentExecution是否开启，如果开启了不能并发执行job，那么trigger的状态为STATE_BLOCKED状态，否则为STATE_WAITING；如果状态为STATE_BLOCKED，那么下次调度
对应的trigger不会被拉取，只有等对应的job执行完之后，更新状态为STATE_WAITING之后才可以执行，保证了job的串行；
6.执行job
通过ThreadPool来执行封装job的JobRunShell；
问题解释
在文章Spring整合Quartz分布式调度中，最后做了几次测试分布式调度，现在可以做出相应的解释
1.同一trigger同一时间只会在一个节点执行
上文中可以发现Quartz使用了分布式锁和状态来保证只有一个节点能执行
2.任务没有执行完，可以重新开始
因为调度线程和任务执行线程是分开的，认为执行在Threadpool中执行，互相不影响；
3.通过DisallowConcurrentExecution注解保证任务的串行
在triggerFired中如果使用了DisallowConcurrentExecution，会引入STATE_BLOCKED状态，保证任务的串行；
总结
本文从源码的角度大致介绍了一下Quartz调度的流程，当然太细节的东西没有去深入；通过本文大致可以对多节点调度产生的现象做一个合理的解释。
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