cron expression parsing into java date

Question

• my database having 10 18 16 ? * SUN,MON,WED,FRI * cron expression then how to convert into Java date.
• how to comparing with present day time.
• and one more is how to compare to cron expressions i.e. 10 18 16 ? * SUN,MON,WED,FRI * and 0 30 9 30 * ?
• please explain the sample code using quartz or spring scheduling.

Frescobaldi

Answer 1

Please use:

import org.springframework.scheduling.support.CronSequenceGenerator;

final String cronExpression = "0 45 23 * * *";
final CronSequenceGenerator generator = new CronSequenceGenerator(cronExpression);
final Date nextExecutionDate = generator.next(new Date());

...and then I suggest use Joda DateTime for date comparison.

Tisman

Excellent! Thank you very much! – Rosena

Spring implementation of cron actionally lacks one feature - 'day' and 'day-of-week' fields are combined by AND logic, not by OR as in cron. – Sergiosergipe

@Sergiosergipe i don't think tat this is a disadvantage... you can easily simulate "OR"-ed expressions by providing 2 expression. With AND you have the advantage that you can execute e.g. "every 2nd thuesday at..." – Analysis

Deprecated: need to use CronExpression generator = CronExpression.parse(cronExpression) – Allele

Answer 2

I wrote a small class for handling cron expressions, available here: https://github.com/frode-carlsen/cron

Based on Joda-time, but should be fairly easy to port to Java8 time api. This also makes it possible to embed in unit tests, do simulations etc by adjusting the DateTime offset in Joda-time.

It also has pretty good test coverage (was done as TDD Kata).

Update Now supports java 8 time api as well thanks to a contribution from github user https://github.com/zemiak. In both cases, the expression parser is a single, tiny class which can easily be copied into your own project.

Ing

Is it on maven central? – Fore

No, it's just a single class only depending on joda-time, so I haven't bothered. Feel free to fork or copy – Ing

Hey, this very cool. I've used your code to replace cron-utils which depend on guava, an over 2MB monster jar in my project: github.com/actframework/actframework/blob/master/src/main/java/… – Burgoyne

Seems guava dependency is no longer required by cron-utils. Release 5.0.0 relies on JDK8 and no longer needs guava nor jodatime. – Isothermal

This was exactly what I was looking for. A small class that does cron-like parsing. Thank you very much! – Addams

Answer 3

You may want to look into the org.quartz.CronExpression class in the Quartz API.

Please note that you cannot simply compare a cron expression with a date because the cron expression (typically) represents a sequence of various dates. In any case, you may find the following methods useful:

public boolean isSatisfiedBy(Date date)
public Date getNextValidTimeAfter(Date date)

As for comparing two cron expressions, what would you like to compare? The only thing that IMO makes sense to compare are the next 'trigger' dates, i.e. dates obtained from getNextValidTimeAfter([some reference date]) calls.

Marhtamari

Does it based on Joda-Time? or what?! – Kolnos

We want to convert cron to java date. What is the method to achieve? – Kolnos

You typically cannot convert a cron expression to a single Date instance because a cron expression usually represents a series of Dates. To obtain the the Date series, I think you can repeatedly call the getNextValidTimeAfter(Date) method and use the returned values as the input value for the next call. To answer your former question, Quartz API does not use Joda-Time, it uses java.util Date/Calendar APIs. – Marhtamari

Answer 4

Perhaps you can check cron-utils It has some utils to get next/previous execution given certain date, ex.: now. Works with JodaTime, but you could retrieve a JavaDate from there. The library is scheduler agnostic: you just provide a string with a cron expression. Is compatible with JDK6.

Isothermal

cron-utils is nice. The only thing is it has guava dependency which is over 2MB – Burgoyne

Guava dependency was an issue. Since version 5.0.0 they removed it by migrating to JDK8. Now the library has even less dependencies, since they also got rid off jodatime leveraging new Java support for time. – Isothermal

sounds good. But it now locked to Java8. I end up with using this single file parser: github.com/frode-carlsen/cron/blob/master/src/main/java/fc/cron/… – Burgoyne

Sure, is an option :) Eventually you need to bind to some JDK. – Isothermal

Thanks for this suggestion. I needed joda-time support in core-utils. Hence I choose the 4.1.3 version. Works as expected. – Sternberg