YAML(Ain't Markup Language) is a human-readable data serialization language.
It is commonly used for configuration files and in applications where data is being stored for transmitted.
It uses both Python-style indentations to indicate nesting and a more compact format that uses [...] for lists and {...} for maps making YAML 1.2 a superset of JSON.
YAML natively encodes scalars(such as strings, integers, and floats), lists, and associative arrays(also known as maps, dictionaries, or hashes).
YAML is intended to be read and written in streams, a feature inspired bySAX.
Syntax
- Whitespace indentation. Not tab characters.
- Comments begin with #. Can start anywhere, without inside of a string.
- List members are denoted by -, and a list can be specified by square brackets[...] (like JSON).
- An associative array entry is represented using colon space in the form key: value.
- A question mark can be used in front of a key, like "?key: value" to allow the key to containing leading dashes, square brackets, etc., without quotes.
- An associative array can also be specified in curly braces{...}, for JSON compatibility.
- Block scalars are delimited with indentation with optional modifiers to preserve (|) or fold (>) newlines. eg:
- Multiple documents within a single stream are separated by three hyphens(---)
- Three periods(...) optionally end a document within a stream.
- Repeated nodes are initially denoted by an ampersand(&) and thereafter referenced with an asterisk(*).
- Nodes may be labeled with a type or tag using the exclamation point(!!) followed by a string, which can be expanded into a URL.
- YAML documents in a stream may be preceded by 'directives' composed of a percent sign(%) followed by a name and space-delimited parameters. Two directives are defined in YAML 1.1
- Two additional sign characters are reserved in YAML for possible future specification: the at sign @ and backtick `.
Advanced components
#define anchor and references.
#YAML autodetects the datatype of the entity,
Comparison with JSON
JSON syntax is a basis of YAML version 1.2,
most JSON documents can be parsed by some YAML parsers such as Syck.
YAML has many additional features lacking in JSON, including comments, extensible data types, relational anchors, strings without quotation marks, and mapping types preserving key order.
Basic deserialization
contact.yml:
name: Nathan Sweet age: 28 address: 4011 16th Ave S phone numbers: - name: Home number: 206-555-5138 - name: Work number: 425-555-2306
In Java:
YamlReader reader = new YamlReader(new FileReader("contact.yml")); Object object = reader.read(); System.out.println(object); Map map = (Map)object; System.out.println(map.get("address"));
Multiple objects
contact2.yml
name: Nathan Sweet age: 28 --- name: Some One age: 25
In Java:
YamlReader reader = new YamlReader(new FileReader("contact.yml")); while (true) { Map contact = reader.read(); if (contact == null) break; System.out.println(contact.get("age")); } #print: 28 25
Deserializing other classes
contact2.yml
name: Nathan Sweet age: 28
JavaBean:
public class Contact { public String name; public int age; }
Deserializing to object instance:
YamlReader reader = new YamlReader(new FileReader("contact.yml")); Contact contact = reader.read(Contact.class); System.out.println(contact.age);
print result:
!com.example.Contact name: Nathan Sweet age: 28
Serializing objects
Contact contact = new Contact(); contact.name = "Nathan Sweet"; contact.age = 28; YamlWriter writer = new YamlWriter(new FileWriter("output.yml")); writer.write(contact); writer.close();
Print output.yml:
!com.example.Contact name: Nathan Sweet age: 28
yamlReader uses an ArrayList by default.
List list = new ArrayList();
list.add("moo");
list.add("cow"); - moo
- cowIf the list was a LinkedList, then YamlWriter knows that a tag is needed and outputs:
List list = new LinkedList();
list.add("moo");
list.add("cow"); !java.util.LinkedList
- moo
- cowTag shortcuts
YamlWriter writer = new YamlWriter(new FileWriter("output.yml")); writer.getConfig().setClassTag("contact", Contact.class); writer.write(contact); writer.close();
The output no longer contains the full class name for the Contact class.
!contact
name: Nathan Sweet
age: 28Lists and maps
When reading or writing a List or Map, YamlBeans cannot know what type of objects are supposed to be in the List or Map, so it will write out a tag.
!com.example.Contact name: Bill phoneNumbers: - !com.example.Phone number: 206-555-1234 - !com.example.Phone number: 206-555-5678 - !com.example.Phone number: 206-555-7654
This can make the YAML less readable. To improve this, you may define what element type should be expected for a List or Map field on your object.
YamlWriter writer = new YamlWriter(new FileWriter("output.yml")); writer.getConfig().setPropertyElementType(Contact.class, "phoneNumbers", Phone.class); writer.write(contact); writer.close();
Now YamlBeans knows what to expect for elements of the "phoneNumbers" field, so extra tags will not be output.
!com.example.Contact name: Bill phoneNumbers: - number: 206-555-1234 - number: 206-555-5678 - number: 206-555-7654
Setting the element type for a Map field tells YamlBeans what to expect for values in the Map. Keys in a Map are always Strings.
Refer yamlbeas: https://github.com/EsotericSoftware/yamlbeans
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