Day 6

---

Went to get the third dog bite injection and also do not feel much energetic

So where were we ?

So let’s just start with a Web Server

Step 1 : How to make a Hello World in Java Step 2 : Add Logger in Java Step 3 : Add Request Validator in Java

Once these Done we can think of going to the next steps

• Project Aim
• Procedure
• Result

Link to original

• So let’s make the logger today Add Logger in Java : Done
• Before solving Add a request Validator It’s better to make a post request which takes a username and password and return them.

First Java Post Request

How to Take request in Post Controller

So apparently This is a little different that what we see in Nodejs

• This is called the DTO Pattern Data Transfer Object

// Request DTO: only what client sends
public class LoginRequestDTO {
    private String username;
    private String password;
 
    // Getters and setters
    public String getUsername() { return username; }
    public void setUsername(String username) { this.username = username; }
 
    public String getPassword() { return password; }
    public void setPassword(String password) { this.password = password; }
}
 

public class LoginResponseDTO {
    private String username;
 
    // Constructor
    public LoginResponseDTO(String username) {
        this.username = username;
    }
 
    // Getter
    public String getUsername() { return username; }
}

Now what if there are a specific type

{
	user : {
		name: {
			first_name : "John",
			"last_name" : "doe"
			}
		}
	}
}

public class UserRequestDTO {
    private User user;
 
    // getters and setters
    public User getUser() { return user; }
    public void setUser(User user) { this.user = user; }
 
    public static class User {
        private Name name;
 
        // getters and setters
        public Name getName() { return name; }
        public void setName(Name name) { this.name = name; }
    }
 
    public static class Name {
        private String first_name;
        private String last_name;
 
        // getters and setters
        public String getFirst_name() { return first_name; }
        public void setFirst_name(String first_name) { this.first_name = first_name; }
 
        public String getLast_name() { return last_name; }
        public void setLast_name(String last_name) { this.last_name = last_name; }
    }
}
 

sequenceDiagram
    participant Client
    participant Controller
    participant LoginRequestDTO
    participant LoginResponseDTO

    Client->>Controller: POST / with JSON { username, password }
    Controller->>LoginRequestDTO: Map JSON to LoginRequestDTO
    Controller->>Controller: Process login data
    Controller->>LoginResponseDTO: Create LoginResponseDTO with username only
    Controller->>Client: Return JSON { username }

---

Explanation of the DTOs in this flow:

• LoginRequestDTO:

  • Represents the data sent from the client to the server.

  • Contains username and password fields.

  • Used by Spring to deserialize the incoming JSON request body into a Java object.

  • Helps the controller safely receive only the expected data.

• LoginResponseDTO:

  • Represents the data sent from the server back to the client.

  • In this case, contains only the username.

  • Used by Spring to serialize the Java object back into JSON for the HTTP response.

  • Helps control exactly what data is exposed to the client (e.g., hiding password).

---

Why use separate DTOs?

• Security: You avoid sending sensitive information like passwords back to the client.

• Clear API contract: You define exactly what your API expects and what it returns.

• Decoupling: Allows your internal domain or entity models to change without affecting API contracts.