Processing YAML

Groovy has an optional groovy-yaml module which provides support for converting between Groovy objects and YAML. The classes dedicated to YAML serialisation and parsing are found in the groovy.yaml package.

1. YamlSlurper

YamlSlurper is a class that parses YAML text or reader content into Groovy data structures (objects) such as maps, lists and primitive types like Integer, Double, Boolean and String.

The class comes with a bunch of overloaded parse methods plus some special methods such as parseText and others. For the next example we will use the parseText method. It parses a YAML String and recursively converts it to a list or map of objects. The other parse* methods are similar in that they return a YAML String but for different parameter types.

        def ys = new YamlSlurper()
        def yaml = ys.parseText '''
language: groovy
sudo: required
dist: trusty

    - jdk: openjdk10
    - jdk: oraclejdk9
    - jdk: oraclejdk8

  - |
    unset _JAVA_OPTIONS


        assert 'groovy' == yaml.language
        assert 'required' == yaml.sudo
        assert 'trusty' == yaml.dist
        assert ['openjdk10', 'oraclejdk9', 'oraclejdk8'] ==  yaml.matrix.include.jdk
        assert ['unset _JAVA_OPTIONS'] == yaml.before_script*.trim()

Notice the result is a plain map and can be handled like a normal Groovy object instance. YamlSlurper parses the given YAML as defined by the YAML Ain’t Markup Language (YAML™).

As YamlSlurper is returning pure Groovy object instances without any special YAML classes in the back, its usage is transparent. In fact, YamlSlurper results conform to GPath expressions. GPath is a powerful expression language that is supported by multiple slurpers for different data formats (XmlSlurper for XML being one example).

For more details please have a look at the section on GPath expressions.

The following table gives an overview of the YAML types and the corresponding Groovy data types:

YAML Groovy




java.lang.BigDecimal or java.lang.Integer












java.util.Date based on the yyyy-MM-dd’T’HH:mm:ssZ date format

Whenever a value in YAML is null, YamlSlurper supplements it with the Groovy null value. This is in contrast to other YAML parsers that represent a null value with a library-provided singleton object.

1.1. Builders

Another way to create YAML from Groovy is to use YamlBuilder. The builder provide a DSL which allows to formulate an object graph which is then converted to YAML.

        def builder = new YamlBuilder()
        builder.records {
            car {
                name 'HSV Maloo'
                make 'Holden'
                year 2006
                country 'Australia'
                homepage new URL('')
                record {
                    type 'speed'
                    description 'production pickup truck with speed of 271kph'

        assert builder.toString() == '''---
    name: "HSV Maloo"
    make: "Holden"
    year: 2006
    country: "Australia"
    homepage: ""
      type: "speed"
      description: "production pickup truck with speed of 271kph"