JSON对象到Java对象的映射是如何完成的？

Question

I have tested the REST service with Postman API client which passes the JSON object to the service.

The JSON object

{
    "username" : "user",
    "password" : "pass"
}

The mapping to the Java object is done correctly and returned with Response status 200.

How the JSON object to a Java object mapping is done in operation to the REST service method.

UserCl Class

static class UserCl{

        private String uname;
        private String password;
        private String name;

        public UserCl(){
        }

        public String getName() {
            return name;
        }

        public void setName(String pname) {
            this.name = pname;
        }

        public String getUsername() {
            return uname;
        }
        public void setUsername(String pusername) {
            this.uname = pusername;
        }
        public String getPassword() {
            return password;
        }
        public void setPassword(String ppassword) {
            this.password = ppassword;
        }

    }

REST service method

@PermitAll
@POST   
@Consumes(MediaType.APPLICATION_JSON)
@Path("/auth")
public Response login(UserCl usr){

    if(usr.getUsername().compareTo("user") == 0 && usr.getPassword().compareTo("pass") == 0){

    String usernameAndPassword = usr.getUsername() + ":" + usr.getPassword(); 

    try {               

            String base64String = Base64Encoder.encode(usernameAndPassword);
            System.out.println(base64String);                               

            return Response.status(200).build();


        } catch (IOException e) {           
            e.printStackTrace();        
            return Response.status(401).build();                
        }   
    }
    else{       
        return Response.status(401)build();
    }

}   

Answer 1

Resteasy uses Jackson by default under the hood. The basic behavior (as is the case with most serialization frameworks) is to look for JavaBean properties (getters/setters). For deserialization, the setters are introspected. They should follow JavaBean naming convention . If your case

private String username;

public void setUsername(String string) {}

The set is removed from the method name, and first letter lowercases. So setUsername maps to the "username" JSON property.

When serializing, the getters will be used. Just like with deserialization, the get is stripped and the first letter lower cased. So getUsername will add a "username" property to the resulting JSON.

It's also possible to use Jackson annotations to change to property names. For instance

@JsonProperty("name")
private String username;

But this will require you to have Jackson added to your project as a compile-time dependency.

EDIT

Just to add that with Jackson, we don't have to use JavaBean properties. Even with public fields, the name of the fields will be used. Not really recommended, just thought I'd throw it out there.

Answer 2

Generally it is implemented using reflection. Java has ability to access methods and fields of object using their names. This is called reflection. In your case JSON library that you are using does this magic.

First it finds which class must be instantiated. It gets it from the type of argument of method login() . It creates instance using code like

Class.forName("UserCl").newInstance()

(that requires public default constructor. Your class does not define any constructor and therefore has one default constructor implicitly.)

Then it discovers the class UserCl , finds setPassword() and setUsername() and calls them passing values of appropriate JSON attributes.

For more information read about java reflection API.