Cool usage of TimeCategory in Groovy

Groovy, the programming language based on JVM implements a feature called Categories. It is originally borrowed from Objective-C . Simple explanation for this feature can be the ability to implement new methods in existing classes without modifying their original code which in some way is injecting new methods through a Category class. For more information official documentation can be found here .

Rather interesting for me was playing with the TimeCategory class for writing a short and easy script for fixing some datetime columns in database. This class offers a convenient way of Date and Time manipulation.

General syntax for categories is the following:

use ( Class ) {
// Some code

Concrete usage of TimeCategory:

use ( TimeCategory ) {
// application on numbers:
println 10.hours.ago
// application on dates
def someDate = new Date()
println someDate - 3.months

Seems weird? From when Integer has months, minutes, hours etc. methods ? Well it still doesn’t have any of that, however those methods are dynamically added with the TimeCategory use.

If you are interested how is this possible I suggest you to go through TimeCategory API and source code if possible. Also this forum post can be useful for deeper understanding of the groovy magic.

And last but not least, an example groovy script for your pleasure.

@Grab(group='mysql', module='mysql-connector-java', version='5.1.27')

import groovy.time.TimeCategory
import java.sql.Timestamp

sql = groovy.sql.Sql.newInstance(

def rows= [:]

// Select Data
sql.eachRow("select * from Table_Name"){
def impDates = new ImportedDates() // This is some custom Class found in the same package/directory if script
impDates.dateColumn = it.dateColumn

impDates.dateColumn = impDates.dateColumn - // Shift dateColumn for one day backwards in time

rows.put(it.UID,impDates) // Put private key and ImportedDate object in Map


// Update Data
rows.each {row->
ImportedDates id = row.value
// Check if value is different from null, if it is convert it to Timestamp(we use datetime column in db) and execute update query
dateColumn  = null
if(id.dateColumn) dateColumn = new Timestamp(id.dateColumn.getTime())

// Actual update query
sql.executeUpdate('update Table_Name set dateColumn = ? ' +
'where UID like ?',
[dateColumn, row.key.toString()])



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