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Topics
  1. Introduction to Spring
  2. Understanding Spring Controllers

Introduction to Spring

What is Spring?

Spring is a popular Java framework used to build web applications. It provides tools and conventions to help developers create applications more efficiently by handling common tasks and allowing them to focus on the core functionality of their apps.

Understanding Spring Controllers

Mapping HTTP Requests

The @RequestMapping annotation is used to connect web requests to specific methods in your controller. It tells Spring which method should handle which URL. 

                                

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class HelloController {
  @RequestMapping("/hello")
  public String sayHello() {
    return "Hello, world";
  }
}
Base Path Mapping

When you use @RequestMapping at the class level, you define a base URL that all methods in that class will use. This helps in organizing URL patterns for different controllers. 

                                

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
@RequestMapping("/recipes")
public class RecipeController {
  @GetMapping()
  public List getAllRecipes() {
    // return a list of recipes
  }
}
Common Request Types

Different types of HTTP requests (GET, POST, PUT, DELETE) are used to perform different actions. For example, GET retrieves data, POST creates new data, PUT updates existing data, and DELETE removes data. 

                                

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.PutMapping;
import org.springframework.web.bind.annotation.DeleteMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class FlowerController {
  @GetMapping("/flowers")
  public List getAllFlowers() {
    // return a list of flowers
  }
  @PostMapping("/flowers")
  public Flower addFlower(@RequestBody Flower flower) {
    // add a new flower
  }
  @PutMapping("/flowers/{id}")
  public Flower updateFlower(@PathVariable Long id, @RequestBody Flower flower) {
    // update an existing flower
  }
  @DeleteMapping("/flowers/{id}")
  public void deleteFlower(@PathVariable Long id) {
    // delete a flower
  }
}
Accessing Request Parameters

You can access parameters from the URL or request body using annotations like @RequestParam and @PathVariable. This helps you work with the data sent by the user. 

                                

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class FruitController {
  @GetMapping("/fruit")
  public Fruit getFruit(@RequestParam String type) {
    // find and return the fruit based on the type
  }
}
REST Controllers

REST controllers handle HTTP requests and send responses in a RESTful way. They use annotations like @RestController and @GetMapping to define how different requests are handled. 

                                

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class LocationController {
  @GetMapping("/locations/{coordinates}")
  public City getLocationByCoordinates(@PathVariable String coordinates) {
    // find and return the city based on coordinates
  }
}
Handling Response Exceptions

You can handle errors and send custom HTTP status codes by throwing exceptions like ResponseStatusException. This helps in managing error responses properly. 

                                

import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.server.ResponseStatusException;
@RestController
public class BookController {
  @GetMapping("/books/{id}")
  public Book getBook(@PathVariable String id) {
    if (id.matches("\d+")) {
      // find and return the book
    } else {
      throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Invalid ID format.");
    }
  }
}
HTTP Status Codes

HTTP status codes are used to indicate the result of an HTTP request. For example, 200 means OK, 404 means not found, and 500 means server error. 

                                

HttpStatus.OK // 200: The request was successful
HttpStatus.NOT_FOUND // 404: The resource was not found
HttpStatus.INTERNAL_SERVER_ERROR // 500: There was a server error
Setting HTTP Status Codes

You can specify the HTTP status code to return with a response by using the @ResponseStatus annotation. This helps indicate the result of an operation clearly. 

                                

import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseStatus;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class BookController {
  @PostMapping("/books")
  @ResponseStatus(HttpStatus.CREATED)
  public void addBook(@RequestParam String title) {
    // add a new book and return status 201 Created
  }
}
Deserializing Request Body

When you receive data in your application, you can deserialize it (convert it from JSON) into Java objects using @RequestBody. This helps in handling complex data structures sent by clients. 

                                

import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class BookController {
  
  @PostMapping("/books")
  public Book getBook(@RequestBody Book book) {
    // find and return the book based on the provided data
    return book; // Just returning the book for this example
  }
}

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