In the library that comes with Jdk, there are two ways to implement scheduled tasks, one is Timer
, the other is ScheduledThreadPoolExecutor
.
Creating a Timer
creates a thread, which can be used to schedule TimerTask
tasks
Timer
has four construction methods, which can specify the name of the Timer
thread and whether to set it as a daemon thread. The default name is Timer-number
, which is not a daemon thread by default.
There are three main methods:
cancel()
: Terminate task scheduling, cancel all currently scheduled tasks, and running tasks will not be affected
purge()
: Remove all canceled tasks from the task queue
schedule
: Start scheduling tasks, providing several overloaded methods :
schedule(TimerTask task, long delay)
Delayed execution, which means delay
will be executed once after task
task
schedule(TimerTask task, Date time)`Execute at the specified time, and execute `task once at `time` time
#schedule(TimerTask task, long delay, long period)`Delay cycle execution, after ` `task
schedule(TimerTask task, Date firstTime, long period)` is executed periodically after the specified time `firstTime` and is executed once every `period` milliseconds after the specified time `firstTime` is reached.
public class TimerTest { public static void main(String[] args) { Timer timer = new Timer("aa"); Task task = new Task(); timer.schedule(task,new Date(),1000); } } class Task extends TimerTask{ @Override public void run() { System.out.println(new Date()); } }
Output:
Thu Jul 07 14:50:02 CST 2022
Thu Jul 07 14:50:03 CST 2022
Thu Jul 07 14:50:04 CST 2022
Thu Jul 07 14:50:05 CST 2022
…………
Timer is single-threaded and does not throw exceptions. If an exception occurs in a scheduled task, the entire thread will stop, causing the scheduled task to terminate.
Due to the defects of Timer
, it is not recommended to use Timer
. It is recommended to use ScheduledThreadPoolExecutor
.
ScheduledThreadPoolExecutor
is a replacement for Timer
. It was introduced in JDK1.5 and inherits ThreadPoolExecutor
. It is a scheduled task class based on thread pool design. .
Main scheduling method:
schedule
Execute scheduling only once, (task, delay time, delay time unit)
scheduleAtFixedRate
Scheduling at a fixed frequency. If the execution time is too long, the next scheduling will be delayed, (task, delay time of the first execution, period, time unit)
scheduleWithFixedDelay
Delay scheduling, after a task is executed, a delay time is added to execute the next task, (task, delay time of the first execution, interval time, time unit)
public class TimerTest { public static void main(String[] args) throws Exception{ ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(10); scheduledExecutorService.scheduleAtFixedRate( () -> System.out.println(new Date()), 1,3, TimeUnit.SECONDS); } }
Spring scheduled tasks are mainly implemented by @Scheduled
annotations, corn, fixedDelay, fixedDelayString, fixedRate, fixedRateString
The five parameters are required Specifying one of them, passing two or three will throw an exception
@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Repeatable(Schedules.class) public @interface Scheduled { String CRON_DISABLED = ScheduledTaskRegistrar.CRON_DISABLED; // cron表达式 String cron() default ""; // 时区 String zone() default ""; // 从上一次调用结束到下一次调用之间的固定时间 long fixedDelay() default -1; // 和fixedDelay意思相同,只是使用字符传格式,支持占位符。例如:fixedDelayString = "${time.fixedDelay}" String fixedDelayString() default ""; // 两次调用之间固定的毫秒数(不需要等待上次任务完成) long fixedRate() default -1; // 同上,支持占位符 String fixedRateString() default ""; // 第一次执行任务前延迟的毫秒数 long initialDelay() default -1; // 同上,支持占位符 String initialDelayString() default ""; }
@Component @EnableScheduling public class ScheduledTask { @Scheduled(fixedDelay = 1000) public void task(){ System.out.println("aaa"); } }
Project startupScheduledAnnotationBeanPostProcessor
postProcessAfterInitialization ()
Method scans methods with @Scheduled
annotations:
@Override public Object postProcessAfterInitialization(Object bean, String beanName) { if (bean instanceof AopInfrastructureBean || bean instanceof TaskScheduler || bean instanceof ScheduledExecutorService) { // Ignore AOP infrastructure such as scoped proxies. return bean; } Class<?> targetClass = AopProxyUtils.ultimateTargetClass(bean); if (!this.nonAnnotatedClasses.contains(targetClass) && AnnotationUtils.isCandidateClass(targetClass, Arrays.asList(Scheduled.class, Schedules.class))) { Map<Method, Set<Scheduled>> annotatedMethods = MethodIntrospector.selectMethods(targetClass, (MethodIntrospector.MetadataLookup<Set<Scheduled>>) method -> { Set<Scheduled> scheduledMethods = AnnotatedElementUtils.getMergedRepeatableAnnotations( method, Scheduled.class, Schedules.class); return (!scheduledMethods.isEmpty() ? scheduledMethods : null); }); if (annotatedMethods.isEmpty()) { this.nonAnnotatedClasses.add(targetClass); if (logger.isTraceEnabled()) { logger.trace("No @Scheduled annotations found on bean class: " + targetClass); } } else { // Non-empty set of methods annotatedMethods.forEach((method, scheduledMethods) -> // 调用processScheduled方法将定时任务的方法存放到任务队列中 scheduledMethods.forEach(scheduled -> processScheduled(scheduled, method, bean))); if (logger.isTraceEnabled()) { logger.trace(annotatedMethods.size() + " @Scheduled methods processed on bean '" + beanName + "': " + annotatedMethods); } } } return bean; }
protected void processScheduled(Scheduled scheduled, Method method, Object bean) { try { // 创建任务线程 Runnable runnable = createRunnable(bean, method); // 解析到定时任务方式的标记,解析到正确的参数后会设置为TRUE,如果在解析到了其他的参数就会抛出异常 boolean processedSchedule = false; String errorMessage = "Exactly one of the 'cron', 'fixedDelay(String)', or 'fixedRate(String)' attributes is required"; Set<ScheduledTask> tasks = new LinkedHashSet<>(4); // Determine initial delay 解析第一次的延迟(解析initialDelay参数) long initialDelay = scheduled.initialDelay(); String initialDelayString = scheduled.initialDelayString(); if (StringUtils.hasText(initialDelayString)) { // initialDelay不能小于0 Assert.isTrue(initialDelay < 0, "Specify 'initialDelay' or 'initialDelayString', not both"); if (this.embeddedValueResolver != null) { initialDelayString = this.embeddedValueResolver.resolveStringValue(initialDelayString); } if (StringUtils.hasLength(initialDelayString)) { try { initialDelay = parseDelayAsLong(initialDelayString); } catch (RuntimeException ex) { throw new IllegalArgumentException( "Invalid initialDelayString value \"" + initialDelayString + "\" - cannot parse into long"); } } } // Check cron expression 解析cron表达式 String cron = scheduled.cron(); if (StringUtils.hasText(cron)) { // 解析时区 String zone = scheduled.zone(); if (this.embeddedValueResolver != null) { cron = this.embeddedValueResolver.resolveStringValue(cron); zone = this.embeddedValueResolver.resolveStringValue(zone); } if (StringUtils.hasLength(cron)) { Assert.isTrue(initialDelay == -1, "'initialDelay' not supported for cron triggers"); processedSchedule = true; if (!Scheduled.CRON_DISABLED.equals(cron)) { TimeZone timeZone; if (StringUtils.hasText(zone)) { timeZone = StringUtils.parseTimeZoneString(zone); } else { timeZone = TimeZone.getDefault(); } tasks.add(this.registrar.scheduleCronTask(new CronTask(runnable, new CronTrigger(cron, timeZone)))); } } } // 第一次延迟参数小于0,默认为0 // At this point we don't need to differentiate between initial delay set or not anymore if (initialDelay < 0) { initialDelay = 0; } // Check fixed delay 解析fixedDelay参数 long fixedDelay = scheduled.fixedDelay(); if (fixedDelay >= 0) { Assert.isTrue(!processedSchedule, errorMessage); processedSchedule = true; tasks.add(this.registrar.scheduleFixedDelayTask(new FixedDelayTask(runnable, fixedDelay, initialDelay))); } String fixedDelayString = scheduled.fixedDelayString(); if (StringUtils.hasText(fixedDelayString)) { if (this.embeddedValueResolver != null) { fixedDelayString = this.embeddedValueResolver.resolveStringValue(fixedDelayString); } if (StringUtils.hasLength(fixedDelayString)) { Assert.isTrue(!processedSchedule, errorMessage); processedSchedule = true; try { fixedDelay = parseDelayAsLong(fixedDelayString); } catch (RuntimeException ex) { throw new IllegalArgumentException( "Invalid fixedDelayString value \"" + fixedDelayString + "\" - cannot parse into long"); } tasks.add(this.registrar.scheduleFixedDelayTask(new FixedDelayTask(runnable, fixedDelay, initialDelay))); } } // Check fixed rate 解析fixedRate参数 long fixedRate = scheduled.fixedRate(); if (fixedRate >= 0) { Assert.isTrue(!processedSchedule, errorMessage); processedSchedule = true; tasks.add(this.registrar.scheduleFixedRateTask(new FixedRateTask(runnable, fixedRate, initialDelay))); } String fixedRateString = scheduled.fixedRateString(); if (StringUtils.hasText(fixedRateString)) { if (this.embeddedValueResolver != null) { fixedRateString = this.embeddedValueResolver.resolveStringValue(fixedRateString); } if (StringUtils.hasLength(fixedRateString)) { Assert.isTrue(!processedSchedule, errorMessage); processedSchedule = true; try { fixedRate = parseDelayAsLong(fixedRateString); } catch (RuntimeException ex) { throw new IllegalArgumentException( "Invalid fixedRateString value \"" + fixedRateString + "\" - cannot parse into long"); } tasks.add(this.registrar.scheduleFixedRateTask(new FixedRateTask(runnable, fixedRate, initialDelay))); } } // Check whether we had any attribute set // 如果五个参数一个也没解析到,抛出异常 Assert.isTrue(processedSchedule, errorMessage); // Finally register the scheduled tasks // 并发控制将任务队列存入注册任务列表 synchronized (this.scheduledTasks) { Set<ScheduledTask> regTasks = this.scheduledTasks.computeIfAbsent(bean, key -> new LinkedHashSet<>(4)); regTasks.addAll(tasks); } } catch (IllegalArgumentException ex) { throw new IllegalStateException( "Encountered invalid @Scheduled method '" + method.getName() + "': " + ex.getMessage()); } }
After the task is parsed and added to the task queue, it is handed over to the ScheduledTaskRegistrar
class ##scheduleTasksMethod to add (register) scheduled tasks to the environment
protected void scheduleTasks() { if (this.taskScheduler == null) { //获取ScheduledExecutorService对象,实际上都是使用ScheduledThreadPoolExecutor执行定时任务调度 this.localExecutor = Executors.newSingleThreadScheduledExecutor(); this.taskScheduler = new ConcurrentTaskScheduler(this.localExecutor); } if (this.triggerTasks != null) { for (TriggerTask task : this.triggerTasks) { addScheduledTask(scheduleTriggerTask(task)); } } if (this.cronTasks != null) { for (CronTask task : this.cronTasks) { addScheduledTask(scheduleCronTask(task)); } } if (this.fixedRateTasks != null) { for (IntervalTask task : this.fixedRateTasks) { addScheduledTask(scheduleFixedRateTask(task)); } } if (this.fixedDelayTasks != null) { for (IntervalTask task : this.fixedDelayTasks) { addScheduledTask(scheduleFixedDelayTask(task)); } } } private void addScheduledTask(@Nullable ScheduledTask task) { if (task != null) { this.scheduledTasks.add(task); } }
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