如何在基于 servlet 的 Web 应用程序中运行后台任务？

I would suggest using a library like quartz in order to run the task at regular intervals. What does the servlet really do ? It sends you a report ?

Your problem is that you misunderstand the purpose of the servlet. It's intented to act on HTTP requests, nothing more. You want just a background task which runs once on daily basis.

EJB available? Use @Schedule

If your environment happen to support EJB (i.e. a real Java EE server such as WildFly, JBoss, TomEE, Payara, GlassFish, etc), then use @Schedule instead. Here are some examples:

@Singleton
public class BackgroundJobManager {


@Schedule(hour="0", minute="0", second="0", persistent=false)
public void someDailyJob() {
// Do your job here which should run every start of day.
}


@Schedule(hour="*/1", minute="0", second="0", persistent=false)
public void someHourlyJob() {
// Do your job here which should run every hour of day.
}


@Schedule(hour="*", minute="*/15", second="0", persistent=false)
public void someQuarterlyJob() {
// Do your job here which should run every 15 minute of hour.
}


@Schedule(hour="*", minute="*", second="*/5", persistent=false)
public void someFiveSecondelyJob() {
// Do your job here which should run every 5 seconds.
}


}

Yes, that's really all. The container will automatically pickup and manage it.

EJB unavailable? Use ScheduledExecutorService

If your environment doesn't support EJB (i.e. you're not using not a real Java EE server, but a barebones servletcontainer such as Tomcat, Jetty, etc), then use ScheduledExecutorService. This can be initiated by a ServletContextListener. Here's a kickoff example:

@WebListener
public class BackgroundJobManager implements ServletContextListener {


private ScheduledExecutorService scheduler;


@Override
public void contextInitialized(ServletContextEvent event) {
scheduler = Executors.newSingleThreadScheduledExecutor();
scheduler.scheduleAtFixedRate(new SomeDailyJob(), 0, 1, TimeUnit.DAYS);
scheduler.scheduleAtFixedRate(new SomeHourlyJob(), 0, 1, TimeUnit.HOURS);
scheduler.scheduleAtFixedRate(new SomeQuarterlyJob(), 0, 15, TimeUnit.MINUTES);
scheduler.scheduleAtFixedRate(new SomeFiveSecondelyJob(), 0, 5, TimeUnit.SECONDS);
}


@Override
public void contextDestroyed(ServletContextEvent event) {
scheduler.shutdownNow();
}


}

Where the job classes look like this:

public class SomeDailyJob implements Runnable {


@Override
public void run() {
// Do your daily job here.
}


}

public class SomeHourlyJob implements Runnable {


@Override
public void run() {
// Do your hourly job here.
}


}

public class SomeQuarterlyJob implements Runnable {


@Override
public void run() {
// Do your quarterly job here.
}


}

public class SomeFiveSecondelyJob implements Runnable {


@Override
public void run() {
// Do your quarterly job here.
}


}

Do not ever think about using java.util.Timer/java.lang.Thread in a Java EE / Servlet based environment

Last but not least, never directly use java.util.Timer and/or java.lang.Thread in Java EE. This is recipe for trouble. An elaborate explanation can be found in this JSF-related answer on the same question: Spawning threads in a JSF managed bean for scheduled tasks using a timer.

You can use cron4j. http://www.sauronsoftware.it/projects/cron4j/manual.php

In a production system that may have multiple non-jee containers running. Use anot enterprise scheduler like Quartz scheduler which can be configured to use a database for task maamgememt.

Implement two classes and call startTask() in main.

public void startTask()
{
// Create a Runnable
Runnable task = new Runnable() {
public void run() {
while (true) {
runTask();
}
}
};


// Run the task in a background thread
Thread backgroundThread = new Thread(task);
// Terminate the running thread if the application exits
backgroundThread.setDaemon(true);
// Start the thread
backgroundThread.start();
}


public void runTask()
{
try {
// do something...
Thread.sleep(1000);


} catch (InterruptedException e) {
e.printStackTrace();
}
}