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In this section, we will cover the essential steps and concepts required to develop a web application using Java. We will use the Spring Boot framework, which simplifies the development of web applications by providing a comprehensive infrastructure.

Objectives

By the end of this section, you will:

Understand the basics of web application architecture.
Set up a Spring Boot project.
Create RESTful web services.
Implement a simple web interface using Thymeleaf.
Connect to a database using Spring Data JPA.

Web Application Architecture

A typical web application consists of three main layers:

Presentation Layer: The user interface, usually built with HTML, CSS, and JavaScript.
Business Logic Layer: The core functionality, implemented in Java.
Data Access Layer: Interacts with the database to store and retrieve data.

Setting Up a Spring Boot Project

Step 1: Create a New Spring Boot Project

Go to Spring Initializr.
Select the following options:
- Project: Maven Project
- Language: Java
- Spring Boot: 2.5.4 (or the latest stable version)
- Project Metadata: Fill in the Group, Artifact, and other fields as needed.
Add the following dependencies:
- Spring Web
- Spring Data JPA
- H2 Database (for simplicity, we will use an in-memory database)
- Thymeleaf
Click "Generate" to download the project, then unzip it and open it in your favorite IDE.

Step 2: Project Structure

Your project should have the following structure:

src
├── main
│   ├── java
│   │   └── com.example.demo
│   │       ├── DemoApplication.java
│   │       ├── controller
│   │       ├── model
│   │       └── repository
│   └── resources
│       ├── application.properties
│       ├── static
│       └── templates

Creating RESTful Web Services

Step 1: Define the Model

Create a User class in the model package:

package com.example.demo.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String name;
    private String email;

    // Getters and Setters
}

Step 2: Create the Repository

Create a UserRepository interface in the repository package:

package com.example.demo.repository;

import com.example.demo.model.User;
import org.springframework.data.jpa.repository.JpaRepository;

public interface UserRepository extends JpaRepository<User, Long> {
}

Step 3: Create the Controller

Create a UserController class in the controller package:

package com.example.demo.controller;

import com.example.demo.model.User;
import com.example.demo.repository.UserRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/api/users")
public class UserController {

    @Autowired
    private UserRepository userRepository;

    @GetMapping
    public List<User> getAllUsers() {
        return userRepository.findAll();
    }

    @PostMapping
    public User createUser(@RequestBody User user) {
        return userRepository.save(user);
    }

    @GetMapping("/{id}")
    public User getUserById(@PathVariable Long id) {
        return userRepository.findById(id).orElseThrow(() -> new RuntimeException("User not found"));
    }

    @PutMapping("/{id}")
    public User updateUser(@PathVariable Long id, @RequestBody User userDetails) {
        User user = userRepository.findById(id).orElseThrow(() -> new RuntimeException("User not found"));
        user.setName(userDetails.getName());
        user.setEmail(userDetails.getEmail());
        return userRepository.save(user);
    }

    @DeleteMapping("/{id}")
    public void deleteUser(@PathVariable Long id) {
        User user = userRepository.findById(id).orElseThrow(() -> new RuntimeException("User not found"));
        userRepository.delete(user);
    }
}

Implementing a Simple Web Interface Using Thymeleaf

Step 1: Create Thymeleaf Templates

Create an index.html file in the templates folder:

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head>
    <title>User Management</title>
</head>
<body>
    <h1>User Management</h1>
    <form th:action="@{/api/users}" method="post">
        <input type="text" name="name" placeholder="Name" required />
        <input type="email" name="email" placeholder="Email" required />
        <button type="submit">Add User</button>
    </form>
    <ul>
        <li th:each="user : ${users}">
            <span th:text="${user.name}">Name</span> - <span th:text="${user.email}">Email</span>
        </li>
    </ul>
</body>
</html>

Step 2: Create a Controller for the Web Interface

Create a WebController class in the controller package:

package com.example.demo.controller;

import com.example.demo.model.User;
import com.example.demo.repository.UserRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;

@Controller
public class WebController {

    @Autowired
    private UserRepository userRepository;

    @GetMapping("/")
    public String index(Model model) {
        model.addAttribute("users", userRepository.findAll());
        return "index";
    }
}

Connecting to a Database Using Spring Data JPA

Step 1: Configure the Database

Edit the application.properties file in the resources folder:

spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=password
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
spring.h2.console.enabled=true

Step 2: Run the Application

Run the DemoApplication class, which contains the main method:

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {

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

Conclusion

In this section, we covered the basics of developing a web application using Java and Spring Boot. We set up a Spring Boot project, created RESTful web services, implemented a simple web interface using Thymeleaf, and connected to a database using Spring Data JPA. This foundation will help you build more complex web applications and explore additional features of the Spring framework.

Developing a Web Application

Objectives

Web Application Architecture

Setting Up a Spring Boot Project

Step 1: Create a New Spring Boot Project

Step 2: Project Structure

Creating RESTful Web Services

Step 1: Define the Model

Step 2: Create the Repository

Step 3: Create the Controller

Implementing a Simple Web Interface Using Thymeleaf

Step 1: Create Thymeleaf Templates

Step 2: Create a Controller for the Web Interface

Connecting to a Database Using Spring Data JPA

Step 1: Configure the Database

Step 2: Run the Application

Conclusion

Java Programming Course

Module 1: Introduction to Java

Module 2: Control Flow

Module 3: Object-Oriented Programming

Module 4: Advanced Object-Oriented Programming

Module 5: Data Structures and Collections

Module 6: Exception Handling

Module 7: File I/O

Module 8: Multithreading and Concurrency

Module 9: Networking

Module 10: Advanced Topics

Module 11: Java Frameworks and Libraries

Module 12: Building Real-World Applications