🎯 My Spring Boot Controllers Looked Fine… Until I Understood the Presentation Layer

Source: Dev.to

🧠 What is the Presentation Layer (in simple terms)?

The Presentation Layer is the entry point of your Spring Boot application. It is responsible for:

• Receiving HTTP requests from the client
• Mapping requests to the correct endpoints
• Accepting and converting input data
• Returning responses in JSON/XML

It should not contain business logic.
It should not talk directly to the database.

Its job is simple:
Handle requests → delegate work → return responses

🏗️ The Big Picture: Spring Boot Project Structure

A clean Spring Boot web application usually looks like this:

client
controller
service
dto
repository
entity
database

Key rule:

👉 Controllers talk to Services, not to Databases.

Once I followed this, my code became cleaner overnight.

🎮 Controllers: The Heart of the Presentation Layer

Spring MVC gives us an annotation‑based programming model to build controllers.

Important annotations

• @Controller – used in traditional MVC applications that return views.
• @RestController – a shortcut for @Controller + @ResponseBody. Every method inside a @RestController directly returns JSON/XML to the client. For REST APIs, @RestController is the default choice.

🔗 Request Mapping: How Requests Find Your Code

Spring needs to know which URL maps to which method. That’s where request mappings come in.

@RequestMapping

A generic mapping that supports:

• URL paths
• HTTP methods
• Headers
• Media types

In real projects we mostly use its cleaner alternatives.

🚦 HTTP Method‑Specific Mappings (Use These)

Spring provides shortcut annotations that make APIs readable:

• @GetMapping → fetch data
• @PostMapping → create data
• @PutMapping → update data
• @DeleteMapping → delete data
• @PatchMapping → partial update

Using these properly makes APIs self‑explanatory.

🌍 Dynamic URLs: @PathVariable vs @RequestParam

@PathVariable

/employees/123

Use this when the value is mandatory and uniquely identifies a resource.

@RequestParam

/employees?id=123

Use this when the parameter is optional, e.g., for filtering, sorting, pagination.

Understanding this difference improves API design instantly.

📦 Handling Input Data with @RequestBody

When a client sends data (JSON/XML), Spring converts it into a Java object via @RequestBody. It is commonly used in:

• POST
• PUT
• PATCH

Spring’s message converters (e.g., Jackson) automatically transform JSON into Java objects.

⚠️ The Mistake I Was Making

I used to:

• Put logic inside controllers
• Validate data manually
• Call repositories directly

That broke the separation of concerns. The Presentation Layer is not for business logic. Respecting this boundary makes APIs:

• Cleaner
• Easier to test
• Easier to maintain

🚀 Final Thoughts

The Presentation Layer may look simple, but it decides how clean your backend becomes.

If controllers are clean:

• Services shine
• Bugs reduce
• Scaling becomes easier

Understanding this layer was a turning point in my Spring Boot journey.

This post is part of my learning‑in‑public journey while exploring Spring Boot and backend development.