POJO To JSON
By default, when you use the @ResponseBody and @Controller annotations, or @RestController, Spring will look for the Jackson-Databind Library.
If Spring finds the library it will convert the POJO returned by the controller method to its JSON representation.
Practice Exercise¶
JSON is not the only format you can return. Some REST API's deal with XML or a number of other formats. You can find a full list of returnable MediaTypes here.
If we define a User class in Java like so:
public class User {
private String name;
private String email;
private int age;
// ...
}
The JSON representation of an instance of this class would look like this:
User kris = new User("Kris", "kkane@gmail.com", 28);
{
"name":"Kris",
"email":"kkane@gmail.com",
"age":28
}
How Does Jackson Do This?¶
Jackson accomplishes this conversion by using the public getters on the Java class to return values.
- It then removes the 'get' and uses the method name as the key for the returned value.
JSON To POJO¶
Assume that we had a form on a web page that would take in the necessary information to construct a new User object, in this case a name, email, and age.
We can use a class called ObjectMapper to map the JSON fields to the Java object property.
We would use syntax like this to use the ObjectMapper class defined in the Jackson library:
ObjectMapper mapper = new ObjectMapper();
User user = mapper.readValue(UserJSONString, User.class);
Similar to the JSON to POJO conversion, the Jackson Library uses the JSON key values to determine what public setter method to call.
It then sets the corresponding value into the Java object.
Where would we get the JSON String from?¶
In order to have access to the JSON, we need to access the request body sent by the client's POST, PUT, or PATCH.
We can use Spring's @RequestBody annotation to access this.
This annotation injects the JSON content sent in the request body into a defined variable.
The following is an example of a POST request where we access the body.
@RequestMapping(path="users", method=RequestMethod.POST)
public void create(@RequestBody String userJson){
ObjectMapper mapper = new ObjectMapper();
try{
User newUser = mapper.readValue(userJson, User.class);
quizDAO.addUser(newUser);
}
catch(Exception e){
e.printStackTrace();
}
}
However, we don't need to do this ourselves.
Spring will automatically convert the JSON request body to the POJO type of the parameter.
-
This is similar to how Spring creates a command object from form input fields.
-
If the JSON can't be converted to the parameter object type an exception will occur.
@RequestMapping(path="users", method=RequestMethod.POST)
public void create(@RequestBody User user){
quizDAO.addUser(user);
}