README.md

Custom Field Builder

This project demonstrates the use of custom field builders. You can easily set up a logger that can render complex domain objects and have them rendered in structured logging:

public class CustomFieldMain {

  // Use a logger with a custom field builder that can log a Person object.
  private static final Logger<PersonFieldBuilder> logger = LoggerFactory.getLogger(CustomFieldMain.class, PersonFieldBuilder.instance())

  public static void main(String[] args) {
    Person abe = new Person("Abe", 1, "yodelling");
    abe.setFather(new Person("Bert", 35, "keyboards"));
    abe.setMother(new Person("Candace", 30, "iceskating"));

    Condition ageCondition =
        (level, context) ->
            context
                .findNumber("$.person.mother.age")
                .filter(age -> age.intValue() > 30)
                .isPresent();

    // this should not print out because Candace is 30
    logger.info(
        ageCondition,
        "Prints if person's mother age is more than 30",
        fb -> fb.person("person", abe));

    // [[yodelling], [keyboards], [iceskating]]
    // Note we get the entire array back here for every match
    Condition interestsCondition =
        (level, context) ->
            context.findList("$..interests").stream()
                .anyMatch(
                    i -> (i instanceof List) ? ((List<?>) i).get(0).equals("iceskating") : false);

    // log with a field builder i.e. Logger<PersonFieldBuilder>
    logger.info(
        interestsCondition, "Prints if someone likes iceskating", fb -> fb.person("person", abe));
  }
}

This is fixed to a PersonFieldBuilder:

public class PersonFieldBuilder implements Field.Builder {

  static PersonFieldBuilder instance() {
    return new PersonFieldBuilder() {};
  }

  // Renders a `Person` as an object field.
  public Field person(String fieldName, Person p) {
    return keyValue(fieldName, personValue(p));
  }

  public Value.ObjectValue personValue(Person p) {
    // Note that properties must be broken down to the basic JSON types,
    // i.e. a primitive string/number/boolean/null or object/array.
    Field name = string("name", p.name());
    Field age = number("age", p.age());
    Field father = keyValue("father", Value.optional(p.getFather().map(this::personValue)));
    Field mother = keyValue("mother", Value.optional(p.getMother().map(this::personValue)));
    Field interests = array("interests", p.interests());
    return Value.object(name, age, father, mother, interests);
  }
}