Reflection based JSON reading and writing

Json

• Overview
• Writing object graphs
• Reading object graphs
• Customizing serialization
• Event based parsing

Overview

Json is a lightweight library that makes it easy to serialize and deserialize Java object graphs to and from JSON. The JAR is 45k and has minimal dependencies. It is not designed to be fast or optimized, but rather to be simple and easy to understand. Four small classes make up the important parts of the library:

• JsonWriter: A builder style API for emitting JSON.
• JsonReader: Parses JSON and builds a DOM of JsonValue objects.
• JsonValue: Describes a JSON object, array, string, float, long, boolean, or null.
• Json: Reads and writes arbitrary object graphs using JsonReader and JsonWriter.

Writing object graphs

The Json class uses reflection to automatically serialize objects to JSON. For example, here are two classes (getters/setters and constructors omitted):

    public class Person {
       private String name;
       private int age;
       private ArrayList numbers;
    }
    
    public class PhoneNumber {
       private String name;
       private String number;
    }

Example object graph using these classes:

    Person person = new Person();
    person.setName("Nate");
    person.setAge(31);
    ArrayList numbers = new ArrayList();
    numbers.add(new PhoneNumber("Home", "206-555-1234"));
    numbers.add(new PhoneNumber("Work", "425-555-4321"));
    person.setNumbers(numbers);

The JsonBeans code to serialize this object graph:

    Json json = new Json();
    System.out.println(json.toJson(person));

{"numbers":[{"class":"com.example.PhoneNumber","name":"Home","number":"206-555-1234"},{"class":"com.example.PhoneNumber","name":"Work","number":"425-555-4321"}],"age":31,"name":"Nate"}

That is compact, but hardly legible. The prettyPrint method can be used:

    Json json = new Json();
    System.out.println(json.prettyPrint(person));

    {
    "name": "Nate",
    "age": 31,
    "numbers": [
       {
          "name": "Home",
          "class": "com.example.PhoneNumber",
          "number": "206-555-1234"
       },
       {
          "name": "Work",
          "class": "com.example.PhoneNumber",
          "number": "425-555-4321"
       }
    ]
    }

Note that the class for the PhoneNumber objects in the ArrayList numbers field appears in the JSON. This is required to recreate the object graph from the JSON because ArrayList can hold any type of object. Class names are only output when they are required for deserialization. If the field was ArrayList<PhoneNumber> numbers then class names would only appear when an item in the list extends PhoneNumber. If you know the concrete type or aren't using generics, you can avoid class names being written by telling the Json class the types:

    Json json = new Json();
    json.setElementType(Person.class, "numbers", PhoneNumber.class);
    System.out.println(json.prettyPrint(person));

    {
    "name": "Nate",
    "age": 31,
    "numbers": [
       {
          "name": "Home",
          "number": "206-555-1234"
       },
       {
          "name": "Work",
          "number": "425-555-4321"
       }
    ]
    }

When writing the class cannot be avoided, an alias can be given:

    Json json = new Json();
    json.addClassTag("phoneNumber", PhoneNumber.class);
    System.out.println(json.prettyPrint(person));

    {
    "name": "Nate",
    "age": 31,
    "numbers": [
       {
          "name": "Home",
          "class": "phoneNumber",
          "number": "206-555-1234"
       },
       {
          "name": "Work",
          "class": "phoneNumber",
          "number": "425-555-4321"
       }
    ]
    }

Json can write and read both JSON and a couple JSON-like formats. It supports "javascript", where the object property names are only quoted when needed. It also supports a "minimal" format, where both object property names and values are only quoted when needed.

    Json json = new Json();
    json.setOutputType(OutputType.minimal);
    json.setElementType(Person.class, "numbers", PhoneNumber.class);
    System.out.println(json.prettyPrint(person));

    {
    name: Nate,
    age: 31,
    numbers: [
       {
          name: Home,
          number: "206-555-1234"
       },
       {
          name: Work,
          number: "425-555-4321"
       }
    ]
    }

Reading object graphs

The Json class uses reflection to automatically deserialize objects from JSON. Here is how to deserialize the JSON from the previous examples:

    Json json = new Json();
    String text = json.toJson(person);
    Person person2 = json.fromJson(Person.class, text);

The type passed to fromJson is the type of the root of the object graph. From this, Json can determine the types of all the fields and all other objects encountered, recursively. The "knownType" and "elementType" of the root can be passed to toJson. This is useful if the type of the root object is not known:

    Json json = new Json();
    json.setOutputType(OutputType.minimal);
    String text = json.toJson(person, Object.class);
    System.out.println(json.prettyPrint(text));
    Object person2 = json.fromJson(Object.class, text);

    {
    class: com.example.Person,
    name: Nate,
    age: 31,
    numbers: [
       {
          name: Home,
          class: com.example.PhoneNumber,
          number: "206-555-1234"
       },
       {
          name: Work,
          class: com.example.PhoneNumber,
          number: "425-555-4321"
       }
    ]
    }

To read the JSON as a DOM of maps, arrays, and values, the JsonReader class can be used:

    Json json = new Json();
    String text = json.toJson(person, Object.class);
    JsonValue root = new JsonReader().parse(text);

The JsonValue describes a JSON object, array, string, float, long, boolean, or null.

Customizing serialization

Serialization can be customized by either having the class to be serialized implement the Json.Serializable interface, or by registering a Json.Serializer with the Json instance. This example writes the phone numbers as an object with a single field:

    static public class PhoneNumber implements Json.Serializable {
       private String name;
       private String number;
    
       public void write (Json json) {
          json.writeValue(name, number);
       }
    
       public void read (Json json, JsonValue jsonMap) {
          name = jsonMap.child().name();
          number = jsonMap.child().asString();
       }
    }
    
    Json json = new Json();
    json.setElementType(Person.class, "numbers", PhoneNumber.class);
    String text = json.prettyPrint(person);
    System.out.println(text);
    Person person2 = json.fromJson(Person.class, text);

    {
    "name": "Nate",
    "age": 31
    "numbers": [
       { 
          "Home": "206-555-1234"
       },
       {
          "Work": "425-555-4321"
       }
    ]
    }

In the JsonSerializable interface methods, the Json instance is given. It has many methods to read and write data to the JSON. When using JsonSerializable, the surrounding JSON object is handled automatically in the write method. This is why the read method always receives a JsonMap.

JsonSerializer provides more control over what is output, requiring writeObjectStart and writeObjectEnd to be called to achieve the same effect. A JSON array or a simple value could be output instead of an object. JsonSerializer also allows the object creation to be customized.

    Json json = new Json();
    json.setSerializer(PhoneNumber.class, new JsonSerializer<PhoneNumber>() {
       public void write (Json json, PhoneNumber number, Class knownType) {
          json.writeObjectStart();
          json.writeValue(number.name, number.number);
          json.writeObjectEnd();
       }
    
       public PhoneNumber read (Json json, JsonValue jsonData, Class type) {
          PhoneNumber number = new PhoneNumber();
          number.setName(jsonData.child().name());
          number.setNumber(jsonData.child().asString());
          return number;
       }
    });
    json.setElementType(Person.class, "numbers", PhoneNumber.class);
    String text = json.prettyPrint(person);
    System.out.println(text);
    Person person2 = json.fromJson(Person.class, text);

Event based parsing

The JsonReader class reads JSON and has protected methods that are called as JSON objects, arrays, strings, numbers, and booleans are encountered. By default, these methods build a DOM out of JsonValue objects. These methods can be overridden to do your own event based JSON handling.

Maven Info

<dependencies>
    ...
    <dependency>
      <groupId>com.dorkbox</groupId>
      <artifactId>Json</artifactId>
      <version>1.9</version>
    </dependency>
</dependencies>

Gradle Info

dependencies {
    ...
    implementation("com.dorkbox:Json:1.9")
}

License

This project is © 2023 dorkbox llc, and is distributed under the terms of the Apache v2.0 License. See file "LICENSE" for further references.