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Chapter 3.1☕ 14 min read

GET Mapping and PathVariable

Browsers love GET. APIs love @PathVariable.

01The Concept: Path Variables

The Hyderabad Parcel Delivery Analogy:

Imagine you go to the DTDC courier office in Hyderabad to collect a parcel. You don't just walk up to the counter and say: "Give me my parcel." The clerk will ask: "Which parcel? What is the tracking number?" You reply: "Tracking number is 404."

A @PathVariable in a URL is exactly like this tracking number. You send a specific ID in the URL (e.g., /books/404), and Spring extracts the 404 from the URL structure and drops it directly into your Java method's variable (int id).

02Technical Explanation
  1. @GetMapping("/path/{id}"): This tells Spring that this method will handle HTTP GET requests. The {id} part is a placeholder (a URI template variable) inside the URL path.
  2. @PathVariable int id: This annotation tells Spring: "Look at the URL path, find the part that matched {id}, pull it out, and assign it to this int id variable." (The variable name must match exactly, or you must write @PathVariable("id")).
03Full Working Code: Fetching by ID
package com.devinhyderabad;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.ArrayList;
import java.util.List;

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

// 1. REST Controller to handle web requests
@RestController
@RequestMapping("/api/books")
class BookController {

    // Simulating a database with an in-memory list
    private List books = new ArrayList<>(List.of(
        new Book(101, "Hyderabad History", "Sai"),
        new Book(102, "Spring Boot Guide", "Deva")
    ));

    // 2. Maps GET requests like /api/books/101
    @GetMapping("/{id}")
    public Book getBookById(@PathVariable int id) {
        // 3. Spring extracts '101' from URL and puts it in 'id'
        for (Book book : books) {
            if (book.getId() == id) {
                return book; // Spring converts this to JSON
            }
        }
        return null; // If not found
    }
}

// Simple Data Class
class Book {
    private int id;
    private String title;
    private String author;

    public Book(int id, String title, String author) {
        this.id = id;
        this.title = title;
        this.author = author;
    }

    // Getters are REQUIRED for JSON conversion
    public int getId() { return id; }
    public String getTitle() { return title; }
    public String getAuthor() { return author; }
}
04The Book Class & JSON Output

If you run this and go to http://localhost:8080/api/books/101 in your browser or Postman, you will get: {"id":101,"title":"Hyderabad History","author":"Sai"}.

05Why It Matters / Interview Note

Interview Question: "What happens if the URL path variable name and the method parameter name don't match?"

Answer: If the URL is /{bookId} and the Java parameter is int id, Spring will throw an error because it cannot find a matching name. To fix this, you must explicitly map it using @PathVariable("bookId") int id.

Enterprise Note: In real enterprise apps, you rarely return null when an item isn't found. As we will see in the status code chapter, you should return a 404 Not Found HTTP status. Returning null gives the client a 200 OK with an empty body, which is misleading.

Key Takeaways

  • ✅ @GetMapping handles HTTP GET requests
  • ✅ @PathVariable extracts values from URL templates like /{id}
  • ✅ Path variable name must match method parameter name (or use explicit name)
  • ✅ Browsers can directly test GET endpoints — no Postman needed
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