DevTut

# JSON in Java

JSON (JavaScript Object Notation) is a lightweight, text-based, language-independent data exchange format that is easy for humans and machines to read and write. JSON can represent two structured types: objects and arrays. JSON is often used in Ajax applications, configurations, databases, and RESTful web services. The Java API for JSON Processing (opens new window) provides portable APIs to parse, generate, transform, and query JSON.

# Using Jackson Object Mapper

Pojo Model

public class Model {
    private String firstName;
    private String lastName;
    private int age;
    /* Getters and setters not shown for brevity */        
}

Example: String to Object

Model outputObject = objectMapper.readValue(
     "{\"firstName\":\"John\",\"lastName\":\"Doe\",\"age\":23}",
     Model.class);
System.out.println(outputObject.getFirstName());
//result: John

Example: Object to String

String jsonString = objectMapper.writeValueAsString(inputObject));
//result: {"firstName":"John","lastName":"Doe","age":23}

# Details

Import statement needed:

import com.fasterxml.jackson.databind.ObjectMapper;

Maven dependency: jackson-databind (opens new window)

# ObjectMapper instance

//creating one
ObjectMapper objectMapper = new ObjectMapper();
  • ObjectMapper is threadsafe
  • recommended: have a shared, static instance

# Deserialization:

<T> T readValue(String content, Class<T> valueType)
  • valueType needs to be specified -- the return will be of this type
  • Throws
    • IOException - in case of a low-level I/O problem
    • JsonParseException - if underlying input contains invalid content
    • JsonMappingException - if the input JSON structure does not match object structure

    Usage example (jsonString is the input string):

    Model fromJson = objectMapper.readValue(jsonString, Model.class);

    # Method for serialization:

    String writeValueAsString(Object value)

  • Throws
      - `JsonProcessingException` in case of an error - Note: prior to version 2.1, throws clause included IOException; 2.1 removed it.

      # JSON To Object (Gson Library)

      Lets assume you have a class called Person with just name

      private class Person {
    public String name;

    public Person(String name) {
        this.name = name;
    }
}

      Code:

      Gson gson = new Gson();
String json = "{\"name\": \"John\"}";

Person person = gson.fromJson(json, Person.class);
System.out.println(person.name); //John

      You must have gson library (opens new window) in your classpath.

      # JSONObject.NULL

      If you need to add a property with a null value, you should use the predefined static final JSONObject.NULL and not the standard Java null reference.

      JSONObject.NULL is a sentinel value used to explicitly define a property with an empty value.

      JSONObject obj = new JSONObject();
obj.put("some", JSONObject.NULL);   //Creates: {"some":null}
System.out.println(obj.get("some"));//prints: null

      Note

      JSONObject.NULL.equals(null); //returns true

      Which is a clear violation of Java.equals() (opens new window) contract:

      For any non-null reference value x, x.equals(null) should return false

      # JSON Builder - chaining methods

      You can use method chaining (opens new window) while working with JSONObject and JSONArray.

      JSONObject example

      JSONObject obj = new JSONObject();//Initialize an empty JSON object 
//Before: {}
obj.put("name","Nikita").put("age","30").put("isMarried","true");
//After: {"name":"Nikita","age":30,"isMarried":true}

      JSONArray

      JSONArray arr = new JSONArray();//Initialize an empty array
//Before: []
arr.put("Stack").put("Over").put("Flow");
//After: ["Stack","Over","Flow"]

      # Object To JSON (Gson Library)

      Lets assume you have a class called Person with just name

      private class Person {
    public String name;

    public Person(String name) {
        this.name = name;
    }
}

      Code:

      Gson g = new Gson();

Person person = new Person("John");
System.out.println(g.toJson(person)); // {"name":"John"}

      Of course the Gson (opens new window) jar must be on the classpath.

      # JSON Iteration

      Iterate over JSONObject properties

      JSONObject obj = new JSONObject("{\"isMarried\":\"true\", \"name\":\"Nikita\", \"age\":\"30\"}");
Iterator<String> keys = obj.keys();//all keys: isMarried, name & age
while (keys.hasNext()) {                      //as long as there is another key
      String key = keys.next();               //get next key 
      Object value = obj.get(key);            //get next value by key
      System.out.println(key + " : " + value);//print key : value
}

      Iterate over JSONArray values

      JSONArray arr = new JSONArray();        //Initialize an empty array
//push (append) some values in:
arr.put("Stack");
arr.put("Over");
arr.put("Flow");
for (int i = 0; i < arr.length(); i++) {//iterate over all values
    Object value = arr.get(i);          //get value
    System.out.println(value);          //print each value
}

      # optXXX vs getXXX methods

      JSONObject and JSONArray have a few methods that are very useful while dealing with a possibility that a value your are trying to get does not exist or is of another type.

      JSONObject obj = new JSONObject();
obj.putString("foo", "bar");

// For existing properties of the correct type, there is no difference
obj.getString("foo");        // returns "bar"
obj.optString("foo");        // returns "bar"
obj.optString("foo", "tux"); // returns "bar"

// However, if a value cannot be coerced to the required type, the behavior differs
obj.getInt("foo");      // throws JSONException
obj.optInt("foo");      // returns 0
obj.optInt("foo", 123); // returns 123

// Same if a property does not exist
obj.getString("undefined");        // throws JSONException
obj.optString("undefined");        // returns ""
obj.optString("undefined", "tux"); // returns "tux"

      The same rules apply to the getXXX / optXXX methods of JSONArray.

      # Encoding data as JSON

      If you need to create a JSONObject and put data in it, consider the following example:

      // Create a new javax.json.JSONObject instance.
JSONObject first = new JSONObject();

first.put("foo", "bar");
first.put("temperature", 21.5);
first.put("year", 2016);

// Add a second object.
JSONObject second = new JSONObject();
second.put("Hello", "world");
first.put("message", second);

// Create a new JSONArray with some values
JSONArray someMonths = new JSONArray(new String[] { "January", "February" });
someMonths.put("March");
// Add another month as the fifth element, leaving the 4th element unset.
someMonths.put(4, "May");

// Add the array to our object
object.put("months", someMonths);

// Encode
String json = object.toString();

// An exercise for the reader: Add pretty-printing!
/* {
       "foo":"bar",
       "temperature":21.5,
       "year":2016,
       "message":{"Hello":"world"},
       "months":["January","February","March",null,"May"]
   }
*/

      # Decoding JSON data

      If you need to get data from a JSONObject, consider the following example:

      String json = "{\"foo\":\"bar\",\"temperature\":21.5,\"year\":2016,\"message\":{\"Hello\":\"world\"},\"months\":[\"January\",\"February\",\"March\",null,\"May\"]}";

// Decode the JSON-encoded string
JSONObject object = new JSONObject(json);

// Retrieve some values
String foo = object.getString("foo");
double temperature = object.getDouble("temperature");
int year = object.getInt("year");

// Retrieve another object
JSONObject secondary = object.getJSONObject("message");
String world = secondary.getString("Hello");

// Retrieve an array
JSONArray someMonths = object.getJSONArray("months");
// Get some values from the array
int nMonths = someMonths.length();
String february = someMonths.getString(1);

      # Extract single element from JSON

      String json = "{\"name\": \"John\", \"age\":21}";

JsonObject jsonObject = new JsonParser().parse(json).getAsJsonObject();

System.out.println(jsonObject.get("name").getAsString()); //John
System.out.println(jsonObject.get("age").getAsInt()); //21

      # JsonArray to Java List (Gson Library)

      Here is a simple JsonArray which you would like to convert to a Java ArrayList:

      {
    "list": [
                "Test_String_1",
                "Test_String_2"
            ] 
}

      Now pass the JsonArray 'list' to the following method which returns a corresponding Java ArrayList:

      public ArrayList<String> getListString(String jsonList){
    Type listType = new TypeToken<List<String>>() {}.getType();
    //make sure the name 'list' matches the name of 'JsonArray' in your 'Json'.
    ArrayList<String> list = new Gson().fromJson(jsonList, listType);    
    return list;
}

      You should add the following maven dependency to your POM.xml file:

      <!-- https://mvnrepository.com/artifact/com.google.code.gson/gson -->
<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.7</version>
</dependency>

      Or you should have the jar com.google.code.gson:gson:jar:<version> in your classpath.

      # Deserialize JSON collection to collection of Objects using Jackson

      Suppose you have a pojo class Person

      public class Person {
    public String name;

    public Person(String name) {
        this.name = name;
    }
}

      And you want to parse it into a JSON array or a map of Person objects. Due to type erasure you cannot construct classes of List<Person> and Map<String, Person> at runtime directly (and thus use them to deserialize JSON). To overcome this limitation jackson provides two approaches - TypeFactory and TypeReference.

      TypeFactory

      The approach taken here is to use a factory (and its static utility function) to build your type for you. The parameters it takes are the collection you want to use (list, set, etc.) and the class you want to store in that collection.

      TypeReference

      The type reference approach seems simpler because it saves you a bit of typing and looks cleaner. TypeReference accepts a type parameter, where you pass the desired type List<Person>. You simply instantiate this TypeReference object and use it as your type container.

      Now let's look at how to actually deserialize your JSON into a Java object. If your JSON is formatted as an array, you can deserialize it as a List. If there is a more complex nested structure, you will want to deserialize to a Map. We will look at examples of both.

      # Deserializing JSON array

      String jsonString = "[{\"name\": \"Alice\"}, {\"name\": \"Bob\"}]"

      # TypeFactory approach

      CollectionType listType = 
    factory.constructCollectionType(List.class, Person.class);
List<Preson> list = mapper.readValue(jsonString, listType);

      # TypeReference approach

      TypeReference<Person> listType = new TypeReference<List<Person>>() {};
List<Person> list = mapper.readValue(jsonString, listType);

      # Deserializing JSON map

      String jsonString = "{\"0\": {\"name\": \"Alice\"}, \"1\": {\"name\": \"Bob\"}}"

      # TypeFactory approach

      CollectionType mapType = 
    factory.constructMapLikeType(Map.class, String.class, Person.class);
List<Person> list = mapper.readValue(jsonString, mapType);

      # TypeReference approach

      TypeReference<Person> mapType = new TypeReference<Map<String, Person>>() {};
Map<String, Person> list = mapper.readValue(jsonString, mapType);

      # Details

      Import statement used:

      import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.type.CollectionType;

      Instances used:

      ObjectMapper mapper = new ObjectMapper();
TypeFactory factory = mapper.getTypeFactory();

      # Note

      While TypeReference approach may look better it has several drawbacks:

      1. TypeReference should be instantiated using anonymous class
      2. You should provide generic explicity

      Failing to do so may lead to loss of generic type argument which will lead to deserialization failure.

      # Remarks

      This example focuses on parsing and creating JSON in Java using various libraries such as the Google Gson (opens new window) library, Jackson Object Mapper, and others..

      Examples using other libraries could be found here: How to parse JSON in Java (opens new window)

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