Chapter 3.2☕ 12 min read

Request Parameters

Path variables identify. Query parameters filter.

01The Concept: Query Parameters

The Swiggy Filter Analogy:

When you search for Biryani on Swiggy, you don't just go to /biryani. You apply filters: "Rating must be 4.0+" and "Pure Veg only". Swiggy's URL transforms into something like: /search?food=biryani&rating=4.0&veg=true.

These values after the ? are Query Parameters. @RequestParam in Spring Boot is the tool that reads these filtering values from the URL.

02Technical Explanation

Syntax: The URL format is endpoint?key1=value1&key2=value2.
@RequestParam String author: This extracts the ?author=Deva value from the URL and assigns it to the author variable.
Optional vs Required: By default, @RequestParam is required = true. If the user doesn't provide it, Spring will throw an error. To prevent this, we use @RequestParam(required = false).

03Full Working Code: Filtering Data

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.RequestParam;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

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

@RestController
@RequestMapping("/api/books")
class BookController {

    private List books = new ArrayList<>(List.of(
        new Book(101, "Hyderabad History", "Sai"),
        new Book(102, "Spring Boot Guide", "Deva"),
        new Book(103, "Angular Basics", "Deva")
    ));

    // 1. Maps GET requests to exactly /api/books (no path variables)
    @GetMapping
    public List getBooks(
            // 2. 'required = false' means if user doesn't send ?author=..., it won't crash
            @RequestParam(required = false) String author) {
        
        if (author == null) {
            return books; // Return all books if no author filter is provided
        }

        // 3. Filter the list based on the query parameter
        return books.stream()
                .filter(book -> book.getAuthor().equalsIgnoreCase(author))
                .collect(Collectors.toList());
    }
}

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;
    }

    public int getId() { return id; }
    public String getTitle() { return title; }
    public String getAuthor() { return author; }
}

04Testing the Filter Endpoint

Test this by going to http://localhost:8080/api/books?author=Deva in your browser. You will only see the books written by Deva.

05Why It Matters / Interview Note

Interview Question: "What is the difference between @PathVariable and @RequestParam?"

Answer: @PathVariable extracts values directly from the URI path structure (e.g., /books/101 -> 101). It is used for mandatory resource identifiers. @RequestParam extracts values from the query string after the ? (e.g., /books?author=Deva). It is typically used for filtering, sorting, or pagination, and can be made optional.

Enterprise Note: For pagination, enterprise APIs strictly use @RequestParam. Example: /api/employees?page=0&size=10&sort=lastName,asc. Spring Data JPA (which we will learn in Phase 4) actually provides a Pageable interface that automatically maps these query parameters for you!

Key Takeaways

✅ @RequestParam extracts values from query string after the ?
✅ Use required=false to make query parameters optional
✅ Query parameters are perfect for filtering, sorting, and pagination
✅ Multiple params: /books?author=Deva&rating=4.0&sort=title,asc

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