SnakeYAML by example - groovy

Question

I am trying to read and parse a YAML file with SnakeYAML and turn it into a config POJO for my Java app:
// Groovy pseudo-code
class MyAppConfig {
List<Widget> widgets
String uuid
boolean isActive
// Ex: MyAppConfig cfg = new MyAppConfig('/opt/myapp/config.yaml')
MyAppConfig(String configFileUri) {
this(loadMap(configFileUri))
}
private static HashMap<String,HashMap<String,String>> loadConfig(String configFileUri) {
Yaml yaml = new Yaml();
HashMap<String,HashMap<String,String>> values
try {
File configFile = Paths.get(ClassLoader.getSystemResource(configUri).toURI()).toFile();
values = (HashMap<String,HashMap<String,String>>)yaml.load(new FileInputStream(configFile));
} catch(FileNotFoundException | URISyntaxException ex) {
throw new MyAppException(ex.getMessage(), ex);
}
values
}
MyAppConfig(HashMap<String,HashMap<String,String>> yamlMap) {
super()
// Here I want to extract keys from 'yamlMap' and use their values
// to populate MyAppConfig's properties (widgets, uuid, isActive, etc.).
}
}
Example YAML:
widgets:
- widget1
name: blah
age: 3000
isSilly: true
- widget2
name: blah meh
age: 13939
isSilly: false
uuid: 1938484
isActive: false
Since it appears that SnakeYAML only gives me a HashMap<String,<HashMap<String,String>> to represent my config data, it seems as though we can only have 2 nested mapped properties that SnakeYAML supports (the outer map and in the inner map of type <String,String>)...
But what if widgets contains a list/sequence (say, fizzes) which contained a list of, say, buzzes, which contained yet another list, etc? Is this simply a limitation of SnakeYAML or am I using the API incorrectly?
To extract values out of this map, I need to iterate its keys/values and (seemingly) need to apply my own custom validation. Does SnakeYAML provide any APIs for doing this extraction + validation? For instance, instead of hand-rolling my own code to check to see if uuid is a property defined inside the map, it would be great if I could do something like yaml.extract('uuid'), etc. And then ditto for the subsequent validation of uuid (and any other property).
YAML itself contains a lot of powerful concepts, such as anchors and references. Does SnakeYAML handle these concepts? What if an end user uses them in the config file - how am I supposed to detect/validate/enforce them?!? Does SnakeYAML provide an API for doing this?

Answer 1

Do you mean like this:
#Grab('org.yaml:snakeyaml:1.17')
import org.yaml.snakeyaml.*
import org.yaml.snakeyaml.constructor.*
import groovy.transform.*
String exampleYaml = '''widgets:
| - name: blah
| age: 3000
| silly: true
| - name: blah meh
| age: 13939
| silly: false
|uuid: 1938484
|isActive: false'''.stripMargin()
#ToString(includeNames=true)
class Widget {
String name
Integer age
boolean silly
}
#ToString(includeNames=true)
class MyConfig {
List<Widget> widgets
String uuid
boolean isActive
static MyConfig fromYaml(yaml) {
Constructor c = new Constructor(MyConfig)
TypeDescription t = new TypeDescription(MyConfig)
t.putListPropertyType('widgts', Widget)
c.addTypeDescription(t);
new Yaml(c).load(yaml)
}
}
println MyConfig.fromYaml(exampleYaml)
Obviously, that's a script to run in the Groovy console, you wouldn't need the #Grab line, as you probably already have the library in your classpath ;-)