xxljob可以对定时任务进行调度，现在看下定时任务调度的过程。XxlJobAdminConfig实现了InitializingBean接口，spring会调用afterPropertiesSet()进行初始化。大致有以下几个过程：
admin服务端初始化
JobTriggerPoolHelper.java#toStart()方法中会初始化两个调用任务的线程池，快线程池最大线程数为200，慢线程池最大线程数为100。然后启动线程定时轮询需要调度的定时任务。首先计算每秒能处理的定时任务数量，公式为(快线程池的最大线程数+满线程池的最大线程数)*20(1000ms/每个任务处理的时长50ms)，最多为6000。从数据库中加锁查出任务触发时间<当前时间+预读时间(5s)的任务，然后分情况处理。
- 当前时间大于任务触发时间+预读时间，即任务触发时间已经过期超过5s，此时不做任何处理，只刷新任务下次触发时间
- 当前时间大于任务触发时间但不超过5s，即任务虽然过期但是过期时间不到5s，此时触发任务，将任务数据保存到ringData
private volatile static Map<Integer, List<Integer>> ringData = new ConcurrentHashMap<>();，ringData的key是秒数，value是jobid，然后刷新任务的下次触发时间 - 当前时间小于任务触发时间，即还没到任务的触发时间，此时也会将任务写道ringData中，等到期就会进行处理，因为在内存中查询任务比到数据库查询要快很多。
java
int preReadCount = (XxlJobAdminConfig.getAdminConfig().getTriggerPoolFastMax() + XxlJobAdminConfig.getAdminConfig().getTriggerPoolSlowMax()) * 20;
while (!scheduleThreadToStop) {
boolean preReadSuc = true;
try {
preparedStatement = conn.prepareStatement( "select * from xxl_job_lock where lock_name = 'schedule_lock' for update" );
preparedStatement.execute();
// 1、pre read
long nowTime = System.currentTimeMillis();
List<XxlJobInfo> scheduleList = XxlJobAdminConfig.getAdminConfig().getXxlJobInfoDao().scheduleJobQuery(nowTime + PRE_READ_MS, preReadCount);
if (scheduleList!=null && scheduleList.size()>0) {
for (XxlJobInfo jobInfo: scheduleList) {
if (nowTime > jobInfo.getTriggerNextTime() + PRE_READ_MS) {
refreshNextValidTime(jobInfo, new Date());
} else if (nowTime > jobInfo.getTriggerNextTime()) {
JobTriggerPoolHelper.trigger(jobInfo.getId(), TriggerTypeEnum.CRON, -1, null, null);
refreshNextValidTime(jobInfo, new Date());
if (jobInfo.getTriggerStatus()==1 && nowTime + PRE_READ_MS > jobInfo.getTriggerNextTime()) {
int ringSecond = (int)((jobInfo.getTriggerNextTime()/1000)%60);
pushTimeRing(ringSecond, jobInfo.getId());
refreshNextValidTime(jobInfo, new Date(jobInfo.getTriggerNextTime()));
}
} else {
int ringSecond = (int)((jobInfo.getTriggerNextTime()/1000)%60);
pushTimeRing(ringSecond, jobInfo.getId());
refreshNextValidTime(jobInfo, new Date(jobInfo.getTriggerNextTime()));
}
}最后判断任务调度状态,TimeUnit.MILLISECONDS.sleep((preReadSuc?1000:PRE_READ_MS) - System.currentTimeMillis()%1000);，有任务需要调度则下一秒继续扫描，如果没有发现任务则睡眠5s(PRE_READ_MS)。
刚才说到待执行的任务会加入ringData，现在往下看怎么处理ringData的。这里会回退一秒，因为可能出现任务超时的情况，导致任务处理时遗漏。处理的逻辑很简单，到了某秒时，根据秒数取出对应的jobid集合，然后依次处理触发每个任务即可。触发任务的逻辑我们稍微再说。
java
List<Integer> ringItemData = new ArrayList<>();
int nowSecond = Calendar.getInstance().get(Calendar.SECOND); // 避免处理耗时太长，跨过刻度，向前校验一个刻度；
for (int i = 0; i < 2; i++) {
List<Integer> tmpData = ringData.remove( (nowSecond+60-i)%60 );
if (tmpData != null) {
ringItemData.addAll(tmpData);
}
}
if (ringItemData.size() > 0) {
for (int jobId: ringItemData) {
JobTriggerPoolHelper.trigger(jobId, TriggerTypeEnum.CRON, -1, null, null);
}
ringItemData.clear();
}客户端初始化
客户端创建定时任务只要在bean中添加@XxlJob注解即可，调度任务是通过XxlJobSpringExecutor实现的。过程是到spring容器中获取所有bean，找出对方法使用了@XxlJob的bean，然后使用MethodJobHandler进行封装，注册到jobHandlerRepositoryprivate static ConcurrentMap<String, IJobHandler> jobHandlerRepository = new ConcurrentHashMap<String, IJobHandler>();。
java
String[] beanDefinitionNames = applicationContext.getBeanDefinitionNames();
for (String beanDefinitionName : beanDefinitionNames) {
Object bean = applicationContext.getBean(beanDefinitionName);
Method[] methods = bean.getClass().getDeclaredMethods();
for (Method method: methods) {
XxlJob xxlJob = AnnotationUtils.findAnnotation(method, XxlJob.class);
if (xxlJob != null) {
String name = xxlJob.value();
method.setAccessible(true);
if(xxlJob.init().trim().length() > 0) {
initMethod = bean.getClass().getDeclaredMethod(xxlJob.init());
initMethod.setAccessible(true);
}
if(xxlJob.destroy().trim().length() > 0) {
destroyMethod = bean.getClass().getDeclaredMethod(xxlJob.destroy());
destroyMethod.setAccessible(true);
}
registJobHandler(name, new MethodJobHandler(bean, method, initMethod, destroyMethod));
}
}
}客户端会启动一个netty服务器，xxl-job底层的核心就是netty，监听${xxl.job.executor.port}配置的端口，等待来自服务端的调度。
java
ServerBootstrap bootstrap = new ServerBootstrap();
((ServerBootstrap)bootstrap.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class)).childHandler(new ChannelInitializer<SocketChannel>() {
public void initChannel(SocketChannel channel) throws Exception {
channel.pipeline().addLast(new ChannelHandler[]{new IdleStateHandler(0L, 0L, 90L, TimeUnit.SECONDS)}).addLast(new ChannelHandler[]{new HttpServerCodec()}).addLast(new ChannelHandler[]{new HttpObjectAggregator(5242880)}).addLast(new ChannelHandler[]{new NettyHttpServerHandler(xxlRpcProviderFactory, serverHandlerPool)});
}
}).childOption(ChannelOption.SO_KEEPALIVE, true);
ChannelFuture future = bootstrap.bind(xxlRpcProviderFactory.getPort()).sync();
NettyHttpServer.logger.info(">>>>>>>>>>> xxl-rpc remoting server start success, nettype = {}, port = {}", NettyHttpServer.class.getName(), xxlRpcProviderFactory.getPort());
NettyHttpServer.this.onStarted();
future.channel().closeFuture().sync();服务端触发任务
触发任务是从JobTriggerPoolHelper.java#addTrigger()中开始的。默认是快线程池触发，如果1min内执行时间超过500ms的次数大于10，则改为满线程池。
java
public void addTrigger(final int jobId, final TriggerTypeEnum triggerType, final int failRetryCount, final String executorShardingParam, final String executorParam) {
ThreadPoolExecutor triggerPool_ = fastTriggerPool;
AtomicInteger jobTimeoutCount = jobTimeoutCountMap.get(jobId);
if (jobTimeoutCount!=null && jobTimeoutCount.get() > 10) { // job-timeout 10 times in 1 min
triggerPool_ = slowTriggerPool;
}
triggerPool_.execute(new Runnable() {
@Override
public void run() {
long start = System.currentTimeMillis();
try {
// do trigger
XxlJobTrigger.trigger(jobId, triggerType, failRetryCount, executorShardingParam, executorParam);
} catch (Exception e) {
logger.error(e.getMessage(), e);
} finally {
long minTim_now = System.currentTimeMillis()/60000;
if (minTim != minTim_now) {
minTim = minTim_now;
jobTimeoutCountMap.clear();
}
long cost = System.currentTimeMillis()-start;
if (cost > 500) { // ob-timeout threshold 500ms
AtomicInteger timeoutCount = jobTimeoutCountMap.putIfAbsent(jobId, new AtomicInteger(1));
if (timeoutCount != null) {
timeoutCount.incrementAndGet();
}真正执行是在processTrigger()方法中，先根据调度策略获取处理任务的客户端地址，默认是轮询策略。先获取任务id，然后找到任务对应的客户端索引，通过nextInt()方法找到下个索引，再到客户端地址列表中根据索引获取地址。
java
private static void processTrigger(XxlJobGroup group, XxlJobInfo jobInfo, int finalFailRetryCount, TriggerTypeEnum triggerType, int index, int total){
XxlJobLog jobLog = new XxlJobLog();
XxlJobAdminConfig.getAdminConfig().getXxlJobLogDao().save(jobLog);
TriggerParam triggerParam = new TriggerParam();
String address = null;
routeAddressResult = executorRouteStrategyEnum.getRouter().route(triggerParam, group.getRegistryList());
if (routeAddressResult.getCode() == ReturnT.SUCCESS_CODE) {
address = routeAddressResult.getContent();
}
triggerResult = runExecutor(triggerParam, address);
//轮询策略调度任务
private static int count(int jobId) {
// cache clear
if (System.currentTimeMillis() > CACHE_VALID_TIME) {
routeCountEachJob.clear();
CACHE_VALID_TIME = System.currentTimeMillis() + 1000*60*60*24;
}
// count++
Integer count = routeCountEachJob.get(jobId);
count = (count==null || count>1000000)?(new Random().nextInt(100)):++count; // 初始化时主动Random一次，缓解首次压力
routeCountEachJob.put(jobId, count);
return count;
}
@Override
public ReturnT<String> route(TriggerParam triggerParam, List<String> addressList) {
String address = addressList.get(count(triggerParam.getJobId())%addressList.size());
return new ReturnT<String>(address);
} 处理任务时通过proxy进行动态代理，在XxlRpcReferenceBean.class#getObject为调度的定时任务生成了动态代理对象，在InvocationHandler的invoke()方法中实现了逻辑增强，最终到NettyHttpClient#asyncSend()将消息发送到客户端netty服务器。
客户端执行定时任务
客户端是在NettyHttpServerHandler#channelRead0()中处理定时任务的，先对服务器的字节流进行反序列化，在XxlRpcProviderFactory.class#invokeService()以反射方式远程调用ExecutorBizImpl.java#run()方法。
java
Class<?> serviceClass = serviceBean.getClass();
String methodName = xxlRpcRequest.getMethodName();
Class<?>[] parameterTypes = xxlRpcRequest.getParameterTypes();
Object[] parameters = xxlRpcRequest.getParameters();
Method method = serviceClass.getMethod(methodName, parameterTypes);
method.setAccessible(true);
Object result = method.invoke(serviceBean, parameters);
xxlRpcResponse.setResult(result);在run方法中启动处理任务的JobThread进行处理，JobThread中就是根据定时任务名获取对应的MethodJobHandler，取出要执行的Method，再反射执行即可。
java
IJobHandler newJobHandler = XxlJobExecutor.loadJobHandler(triggerParam.getExecutorHandler());
if (jobThread == null) {
jobThread = XxlJobExecutor.registJobThread(triggerParam.getJobId(), jobHandler, removeOldReason);
}
ReturnT<String> pushResult = jobThread.pushTriggerQueue(triggerParam);总结下，xxl-job首先在服务端启动线程轮询要执行的定时任务，计算定时任务的触发时间，然后后获取代理对象，将要执行的任务信息通过netty发送到客户端，客户端以反射方式执行定时任务。有不对的地方请大神指出，欢迎大家一起讨论交流，共同进步。