Java新手需要帮助数据库连接 [英] Java newbie needs help in database connection

问题描述

我是Java的新手，甚至更新到java数据库连接。我设法创建一个数据库连接和查询表，当我把它放在Main类中。现在我把它移动到一个新类名为Connection我收到错误：

I'm new to Java and even newer to java database connections. I've managed to create a database connection and query a table when I put it in the Main class. Now that I've moved it into a new class called Connection I am getting errors:

package lokate;

import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.ResultSet;

public class Connection {

private static Statement stmt = null;
private static ResultSet rs = null;
private static Connection con = null;

public Connection() throws SQLException {
    try {
        Class.forName("com.mysql.jdbc.Driver");
        String connectionUrl = "jdbc:mysql://localhost:3306/Lokate?" +
                               "user=root&password=";
        con = DriverManager.getConnection(connectionUrl);
        stmt = con.createStatement();
        retriveData("SELECT * FROM Users");
        int rowsEffected = 0;
    } catch (SQLException sqlEx) {
        System.out.println("SQL Exception: "+ sqlEx.toString());
    } catch (ClassNotFoundException classEx) {
        System.out.println("Class Not Found Exception: "+ classEx.toString());
    } catch (Exception Ex) {
        System.out.println("Exception: "+ Ex.toString());
    }
}

public static void retriveData(String SQL) throws Exception {
    rs = stmt.executeQuery(SQL);
    while (rs.next()) 
    {
        System.out.println(rs.getString("fname") + " : " + rs.getString("lname"));
    }
}

}

m得到一个错误说找不到符号。符号：方法createStatement（）和无法比较的类型为con = DriveManager .....

I'm getting an error saying cannot find symbol. Symbol:method createStatement() and incomparable types for con = DriveManager.....

任何人都可以帮助？

另外，最好的做法是将连接放在类中，这样每次我想用db做一些事情时调用一个新对象。

Also, is it best practice to put the connection in the class like this then call a new object every time I want to do something with the db?

比利

推荐答案

的许多最坏的做法。让我来计算一下方法：

I'd say your code is an example of many worst practices. Let me count the ways:

1. 你的Connection类是一个糟糕的抽象，不提供java.sql.Connection。 / li>
   
2. 如果你使用你的类，你永远不会利用连接池。


3. 你连接你的驱动类，等等。你不能在不编辑和重新编译的情况下改变它。


4. 在catch块中打印错误消息远远少于提供整个堆栈跟踪的信息。


5. 你的代码伤害了我的眼睛。它不遵循Sun Java编码标准。


6. 您的 retrieveData 方法是毫无价值的。你将如何处理所有这些印刷的声明？


7. 这是 rowsAffected - 影响是动词，效果是名词。

1. Your Connection class is a poor abstraction that offers nothing over and above that of java.sql.Connection.
2. If you use your class, you'll never get to take advantage of connection pooling.
3. You hard wire your driver class, your connection URL, etc. You can't change it without editing and recompiling. A better solution would be to externalize such things.
4. Printing an error message in the catch blocks is far less information than supplying the entire stack trace.
5. Your code hurts my eyes. It doesn't follow the Sun Java coding standards.
6. Your retrieveData method is utterly worthless. What will you do with all those printed statements? Wouldn't it be better to load them into a data structure or object so the rest of your code might use that information?
7. It's rowsAffected - "affect" is the verb, "effect" is the noun. Another variable that's not doing any good.

错误的轨道。请重新思考。

You're on the wrong track. Rethink it.

我认为您会发现此代码更有帮助。

I think you'll find this code more helpful.

package persistence;

import java.sql.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class DatabaseUtils
{
    public static Connection createConnection(String driver, String url, String username, String password) throws ClassNotFoundException, SQLException
    {
        Class.forName(driver);

        if ((username == null) || (password == null) || (username.trim().length() == 0) || (password.trim().length() == 0))
        {
            return DriverManager.getConnection(url);
        }
        else
        {
            return DriverManager.getConnection(url, username, password);
        }
    }

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


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

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

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

    public static List<Map<String, Object>> map(ResultSet rs) throws SQLException
    {
        List<Map<String, Object>> results = new ArrayList<Map<String, Object>>();

        try
        {
            if (rs != null)
            {
                ResultSetMetaData meta = rs.getMetaData();
                int numColumns = meta.getColumnCount();
                while (rs.next())
                {
                    Map<String, Object> row = new HashMap<String, Object>();
                    for (int i = 1; i <= numColumns; ++i)
                    {
                        String name = meta.getColumnName(i);
                        Object value = rs.getObject(i);
                        row.put(name, value);
                    }
                    results.add(row);
                }
            }
        }
        finally
        {
            close(rs);
        }

        return results;
    }
}

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