Java Web项目常见错误排查与解决方案：500错误、空指针异常及资源管理

public class LoginDAO {
    public User login(String username, String password) {
        Connection con = null;
        PreparedStatement pst = null;
        ResultSet rs = null;
        User user = null; // 初始化为null

        try {
            // 假设这里是获取数据库连接的方法，需要确保其健壮性
            con = DatabaseUtil.getConnection(); 
            if (con == null) {
                // 记录日志或抛出自定义异常，表示数据库连接失败
                System.err.println("Error: Database connection failed.");
                return null; 
            }

            String sql = "SELECT id, username, email FROM users WHERE username = ? AND password = ?";
            pst = con.prepareStatement(sql);
            pst.setString(1, username);
            pst.setString(2, password);

            rs = pst.executeQuery();

            if (rs.next()) {
                user = new User();
                user.setId(rs.getInt("id"));
                user.setUsername(rs.getString("username"));
                user.setEmail(rs.getString("email"));
            }
        } catch (SQLException e) {
            System.err.println("Database error during login: " + e.getMessage());
            e.printStackTrace();
        } finally {
            // 确保资源被关闭，将在下一节详细介绍
            closeResources(con, pst, rs);
        }
        return user;
    }

    // 辅助方法，用于关闭资源
    private void closeResources(Connection con, PreparedStatement pst, ResultSet rs) {
        try {
            if (rs != null) rs.close();
            if (pst != null) pst.close();
            if (con != null) con.close();
        } catch (SQLException e) {
            System.err.println("Error closing database resources: " + e.getMessage());
            e.printStackTrace();
        }
    }
}

public class LoginDAO {
    public User login(String username, String password) {
        User user = null;
        String sql = "SELECT id, username, email FROM users WHERE username = ? AND password = ?";

        // 使用 try-with-resources 自动关闭 Connection, PreparedStatement, ResultSet
        try (Connection con = DatabaseUtil.getConnection(); // 确保 getConnection() 返回有效的连接
             PreparedStatement pst = con.prepareStatement(sql)) {

            pst.setString(1, username);
            pst.setString(2, password);

            try (ResultSet rs = pst.executeQuery()) {
                if (rs.next()) {
                    user = new User();
                    user.setId(rs.getInt("id"));
                    user.setUsername(rs.getString("username"));
                    user.setEmail(rs.getString("email"));
                }
            }
        } catch (SQLException e) {
            System.err.println("Database error during login: " + e.getMessage());
            e.printStackTrace();
        }
        return user;
    }
}

<dependency>
    <groupId>com.zaxxer</groupId>
    <artifactId>HikariCP</artifactId>
    <version>5.0.1</version> <!-- 使用最新稳定版本 -->
</dependency>

import com.zaxxer.hikari.HikariConfig;
import com.zaxxer.hikari.HikariDataSource;
import java.sql.Connection;
import java.sql.SQLException;

public class DatabaseUtil {
    private static HikariDataSource dataSource;

    static {
        HikariConfig config = new HikariConfig();
        config.setJdbcUrl("jdbc:mysql://localhost:3306/your_database?useSSL=false&serverTimezone=UTC");
        config.setUsername("root");
        config.setPassword("password");
        config.addDataSourceProperty("cachePrepStmts", "true");
        config.addDataSourceProperty("prepStmtCacheSize", "250");
        config.addDataSourceProperty("prepStmtCacheSqlLimit", "2048");

        dataSource = new HikariDataSource(config);
    }

    public static Connection getConnection() throws SQLException {
        return dataSource.getConnection();
    }

    // 在应用关闭时，需要关闭数据源
    public static void closeDataSource() {
        if (dataSource != null && !dataSource.isClosed()) {
            dataSource.close();
        }
    }
}