Java 后端开发技术学习总结：实用代码示例与项目实操经验大公开

int num = 10;
double pi = 3.14;
String name = "John";

int score = 85;
if (score >= 90) {
    System.out.println("优秀");
} else if (score >= 80 && score < 90) {
    System.out.println("良好");
} else {
    System.out.println("再接再厉");
}

int[] arr = {1, 2, 3, 4, 5};
for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}

int[] numbers = new int[5];
numbers[0] = 1;
// 遍历数组
for (int num : numbers) {
    System.out.println(num);
}

ArrayList<String> list = new ArrayList<>();
list.add("apple");
list.add("banana");
// 访问元素
System.out.println(list.get(0));

@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;
    // 业务逻辑
}

@RestController
@RequestMapping("/api")
public class UserController {
    @Autowired
    private UserService userService;

    @GetMapping("/users")
    public List<User> getUsers() {
        return userService.getAllUsers();
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//XML" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.dao.UserDao">
    <select id="getUserById" resultType="com.example.entity.User">
        SELECT * FROM users WHERE id = #{id}
    </select>
</mapper>

@Mapper
public interface UserDao {
    User getUserById(int id);
}

Class.forName("com.mysql.cj.jdbc.Driver");

Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb", "root", "password");

Statement statement = connection.createStatement();
ResultSet resultSet = statement.executeQuery("SELECT * FROM users");
while (resultSet.next()) {
    System.out.println(resultSet.getString("name"));
}

HikariConfig hikariConfig = new HikariConfig();
hikariConfig.setJdbcUrl("jdbc:mysql://localhost:3306/mydb");
hikariConfig.setUsername("root");
hikariConfig.setPassword("password");
HikariDataSource dataSource = new HikariDataSource(hikariConfig);

@Transactional
public void transferMoney(int fromUserId, int toUserId, double amount) {
    // 从一个用户账户扣除金额
    userRepository.decreaseBalance(fromUserId, amount);
    // 向另一个用户账户增加金额
    userRepository.increaseBalance(toUserId, amount);
}

private static final Logger logger = LoggerFactory.getLogger(MyClass.class);
public void doSomething() {
    logger.info("开始执行 doSomething 方法");
    // 业务逻辑
    logger.info("doSomething 方法执行完毕");
}

@ControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(Exception.class)
    public ResponseEntity<String> handleException(Exception e) {
        logger.error("系统出现异常：", e);
        return new ResponseEntity<>("系统繁忙，请稍后再试", HttpStatus.INTERNAL_SERVER_ERROR);
    }
}