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YamlBeans makes it easy to serialize and deserialize Java object graphs to and from YAML, a human-friendly data format. Replace XML and properties files with YAML for more expressive power (lists, maps, anchors, etc) and easier hand-editing.

Maven Central: https://repo1.maven.org/maven2/com/esotericsoftware/yamlbeans/yamlbeans/

The YamlReader class is used to deserialize YAML to Java objects. The following YAML defines a Map with four entries. The "phone numbers" entry is a List of two items, each of which is a Map.

    name: Nathan Sweet
    age: 28
    address: 4011 16th Ave S
    phone numbers:
     - name: Home
       number: 206-555-5138
     - name: Work
       number: 425-555-2306

The "read" method reads the next YAML document and deserializes it into HashMaps, ArrayLists, and Strings. Since we know the root object defined in the YAML of our example is a Map, we can cast the object and make use of it.

    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"));

A stream of YAML can contain more than one YAML document. Each call to YamlReader#read() deserializes the next document into an object. YAML documents are delimited by "---" (this is optional for the first document).

    name: Nathan Sweet
    age: 28
    ---
    name: Some One
    age: 25

This prints the String "28" then "25":

    YamlReader reader = new YamlReader(new FileReader("contact.yml"));
    while (true) {
    	Map contact = reader.read();
    	if (contact == null) break;
    	System.out.println(contact.get("age"));
    }

There are two ways to deserialize something other than HashMaps, ArrayLists, and Strings. Imagine this YAML document and Java class:

    name: Nathan Sweet
    age: 28

    public class Contact {
    	public String name;
    	public int age;
    }

The "read" method can be passed a class, so the YamlReader knows what it is deserializing:

    YamlReader reader = new YamlReader(new FileReader("contact.yml"));
    Contact contact = reader.read(Contact.class);
    System.out.println(contact.age);

The YamlReader creates an instance of the Contact class and sets the "name" and "age" fields. The YamlReader determines the "age" value in the YAML needs to be converted into a int. Deserialization would have failed if the age was not a valid int. The YamlReader can set public fields and bean setter methods.

Instead of telling the YamlReader what type to deserialize, the type can alternatively be specified in the YAML using a tag:

    !com.example.Contact
    name: Nathan Sweet
    age: 28

The YamlWriter class is used to serialize Java objects to YAML. The "write" method automatically handles this by recognizing public fields and bean getter methods.

    Contact contact = new Contact();
    contact.name = "Nathan Sweet";
    contact.age = 28;
    YamlWriter writer = new YamlWriter(new FileWriter("output.yml"));
    writer.write(contact);
    writer.close();

This outputs:

    !com.example.Contact
    name: Nathan Sweet
    age: 28

The tags are automatically output as needed so that the YamlReader class will be able to reconstruct the object graph. For example, serializing this ArrayList does not output any tag for the list because YamlReader uses an ArrayList by default.

    List list = new ArrayList();
    list.add("moo");
    list.add("cow");

    - moo
    - cow

If 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
    - cow

Note that it is not advisable to subclass Collection or Map. YamlBeans will only serialize the collection or map and its elements, not any additional fields.

YamlBeans can serialize any object graph.

    public class Contact {
    	public String name;
    	public int age;
    	public List phoneNumbers;
    }
    
    public class Phone {
    	public String name;
    	public String number;
    }

    friends:
      - !com.example.Contact
        name: Bob
        age: 29
        phoneNumbers:
            - !com.example.Phone
              name: Home
              number: 206-555-1234
            - !com.example.Phone
              name: Work
              number: 206-555-5678
      - !com.example.Contact
        name: Mike
        age: 31
        phoneNumbers:
            - !com.example.Phone
              number: 206-555-4321
    enemies:
      - !com.example.Contact
        name: Bill
        phoneNumbers:
            - !com.example.Phone
              name: Cell
              number: 206-555-1234

This is a map of lists of contacts, each with a list of phone numbers. Again, the public fields could also have been bean properties.

Tags can be lengthy sometimes and can clutter up the YAML. Alternate tags can be defined for a class and will be used instead of the full class name.

    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 classname for the Contact class.

    !contact
    name: Nathan Sweet
    age: 28

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.

When an object graph contains multiple references to the same object, an anchor may be used so that the object only needs to be defined once in the YAML.

    oldest friend:
        &1 !contact
        name: Bob
        age: 29
    best friend: *1

In this map, the "oldest friend" and "best friend" keys reference the same object. The YamlReader automatically handles the anchors in the YAML when rebuilding the object graph. By default, the YamlWriter automatically outputs anchors when writing objects.

    Contact contact = new Contact();
    contact.name = "Bob";
    contact.age = 29;
    Map map = new HashMap();
    map.put("oldest friend", contact);
    map.put("best friend", contact);

By default, the behaviour of this YAML parser is to ignore duplicate keys if you have. e.g if you have the following

    name: Nathan Sweet
    age: 28
    address:
      line1: 485 Madison Ave S
      line1: 711 3rd Ave S
      line2: NYC

The above YAML will give you an address object with attribute line1 set to 711 3rd Ave S. This is because the key line1 in the above YAML is duplicated and thus the last value of line1 will be retained. YAML parser will not complain about it. However, if your business logic requires you to validate YAML for such duplicates, then you can still do using allowDuplicates option of the YamlConfig object. Following is how its done:

    try {
        YamlConfig yamlConfig = new YamlConfig();
        yamlConfig.setAllowDuplicates(false); // default value is true
        YamlReader reader = new YamlReader(new FileReader("contact.yml"), yamlConfig);
        Object object = reader.read();
        System.out.println(object);
        Map map = (Map)object;
        System.out.println(map.get("address"));
    } catch (YamlException ex) {
        ex.printStackTrace();
        // or handle duplicate key case here according to your business logic
    }

The above code will not print anything, but throw YamlReaderException at line 5 saying, Duplicate key found 'line1'.

The YAML tokenizer, parser, and emitter are based on those from the JvYAML project. They have been heavily refactored, bugs fixed, etc. The rest of the JvYAML project was not used because of its complexity. YamlBeans strives for the simplest possible thing that works, with the goal being to make it easy to use the YAML data format with Java.

YamlBeans supports YAML version 1.0 and 1.1.

See the javadocs for various other features available on the YamlConfig class.