Natural Kron

A parser that converts natural (English) language to a cron quartz expressions, written in Kotlin.

Installation

You can add the library to your project using gradle:

implementation("io.github.yamilmedina:natural-kron:2.0.0")

Motif

This library is intended to be used in applications where the user can define cron expressions in a more natural way. Some examples, where you can use this library are, but not limited to:

• Chatbots
• Scheduling applications for non-technical users
• Cron expression generators
• Scheduling tasks in a more human-readable way

Why Quartz like style and not Unix like?

If you worked before with scheduling tasks for the jvm, you probably heard/know about Quartz. Quartz is a powerful library for this job. But there is a small catch, Quartz uses a different cron expression style than Unix.

This means they start counting from seconds, adding an initial field to the left. Also, you can not use the day of the week and day of the month fields at the same time¹.

Usage

To use the library, you can specify the style in the parser, like this:

import io.github.yamilmedina.kron.NaturalKronParser

val input = "every saturday at 10:11"
val cronExpression = NaturalKronParser().parse(input)

assertEquals("0 11 10 ? * SAT", cronExpression) //---> This is a valid Quartz cron expression

Implementation details

This library uses an ANTLR4 grammar to parse the input. The grammar is defined in the CronGrammar.g4 file. The generated files are included in the project, but are not intended to be used directly, that's why they are in the internal package (good old sun days xD), therefore, don't use it, there might be gone, or changed in the future.

Known limitations

This library is a side quest project born into the requirements of a bigger project. This is a starting point for now, so these things are not supported yet (most likely I missed some):

• yearly expressions (e.g. every day during 2005 -> 0 0 0 ? * * 2005)
• step every x minutes/hours/days/weeks/months (e.g. every 5 minutes -> 0 0/5 * ? * *)
• ranges of values (e.g. every day between 10 and 20 -> 0 0 0 10-20 * ?)
• last day of the month (e.g. every last day of the month -> 0 0 0 L * ?)

Note

Up to version 1.0.1, the library was inspired, and a direct port of this library from PHP to Kotlin. The library was refactored to be more Kotlin idiomatic and to add some features, but rewritten completely from scratch, due to lack of test and not scalable code.

In version 2.0.0, the library was refactored to use ANTLR4 to parse the input.

License

The MIT license. See LICENSE.

Footnotes

1. Quartz Notes Section ↩