java - 从 java 设置 MySql 连接的问题

Question

在 Netbeans 11.1 中，我试图将我的 Java Maven 代码连接到 MySql 数据库，但每次我尝试根据 Internet 教程设置连接时，我都会遇到错误。我在 Dependencies 文件夹中有 mysql-connector-java-8.0.17.jar。我是 Java 的新手，我的高级项目团队成员在确定连接时也遇到了问题。

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class NewJFrame extends javax.swing.JFrame {

//connect to DB
    private final String ConnectionString = "jdbc:mysql://54.235.42.66:3306/CIS470db";
    private final String UserName = "team6";
    private final String Password = "CIS470team6";
   //next line has error
 private final DBConnect dbConnect;
    //next line has error
    dbConnect = new DBConnect(ConnectionString, UserName, Password);

第一个错误: 找不到标志 符号:类 DBConnect

位置:NewJFrame 类

第二个错误: 预计

找不到符号 符号:类 DBConnect

位置:NewJFrame 类

Answer 1

您还没有在您的类中导入 DBConnect，这就是编译器不知道在哪里可以找到 DBConnect 甚至不知道它是什么的原因。

您还需要这样做，否则会导致 SQLException:

try {
    dbConnect = new DBConnect(ConnectionString, UserName, Password);
} catch (SQLException e) {
    e.printStackTrace();
}

---

另外，尝试实现 Singleton Pattern与数据库连接时。跟随起来真的很方便。

这是我在我的一个旧项目中的做法:

DatabaseConnection.java

package com.stackoverflow.utility;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

import com.stackoverflow.constant.Credentials;

public class DatabaseConnection implements Credentials {
    private static DatabaseConnection database;
    Connection connection;

    public static DatabaseConnection getDatabase() {
        if (database == null) {
            database = new DatabaseConnection();
        }

        return database;
    }

    public Connection getConnection() throws ClassNotFoundException, SQLException {
        if (connection == null || connection.isClosed()) {
            Class.forName(JDBC_DRIVER);
            connection = DriverManager.getConnection(DB_URL, USER, PWD);
        }

        return connection;
    }
}

CloseConnection.java

package com.stackoverflow.utility;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class CloseConnection {
    public static void close(ResultSet resultSet) {
        if (resultSet != null) {
            try {
                resultSet.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public static void close(PreparedStatement preparedStatement) {
        if (preparedStatement != null) {
            try {
                preparedStatement.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public static void close(Connection connection) {
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public static void close(PreparedStatement preparedStatement, ResultSet resultSet) {
        close(resultSet);
        close(preparedStatement);
    }
}

Credentials.java

package com.stackoverflow.constant;

public interface Credentials {
    static final String JDBC_DRIVER = "com.mysql.cj.jdbc.Driver"; // Database driver
    static final String DB_URL = "jdbc:mysql://116.72.10.78:3352/MohitDB"; // Database url

    // Database credentials
    static final String USER = "user";
    static final String PWD = "password";
}

查询.java

package com.stackoverflow.constant;

public interface Queries {
    public static final String AUTHENTICATION_QUERY = "SELECT * FROM TableUser WHERE username = ? AND password = ?";
}

认证方式

public boolean getAuthentication(Login login) {
    try {
        PreparedStatement preparedStatement = DatabaseConnection.getDatabase().getConnection()
                .prepareStatement(AUTHENTICATION_QUERY);
        preparedStatement.setString(1, login.getUsername());
        preparedStatement.setString(2, login.getPassword());

        ResultSet resultSet = preparedStatement.executeQuery();

        if (resultSet.next()) {
            close(preparedStatement, resultSet);

            return true;
        }
    } catch (Exception e) {
        e.printStackTrace();
    }

    return false;
}