JDBC连接数据库查询信息的步骤（提取成配置文件方式）

硬编码格式的弊端：数据库发生改变时，要重新修改代码，重新编译和部署

解决方法：将数据库信息写在配置文件当中，让程序通过读取配置文件来获得这些信息

jdbc.driver.class=com.mysql.jdbc.Driver
jdbc.connection.url=jdbc:mysql://127.0.0.1:3306/tree
jdbc.connection.username=root
jdbc.connection.password=123321

编写配置文件database.properties

创建包com.pb.tree.util，编写类ConfigManager 读取属性文件

package cn.tree.util;

import java.io.IOException;

import java.io.InputStream;

import java.util.Properties;

//读取配置文件的工具类-单例模式

public class ConfigManager {

// 读取配置文件properties.load(inputstream)

private static ConfigManager configManager;

private static Properties properties;

private ConfigManager() {

String configFile = "database.properties";

properties = new Properties();

InputStream is = ConfigManager.class.getClassLoader()

.getResourceAsStream(configFile);

try {

properties.load(is);

is.close();

} catch (IOException e) {

e.printStackTrace();

}

}

public static ConfigManager getInstance() {

if (configManager == null) {

configManager = new ConfigManager();

}

return configManager;

}

public String getString(String key) {

return properties.getProperty(key);

}

}

修改MenuDao类的查询信息的方法

package cn.tree.util;

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

//读取配置文件的工具类-单例模式
public class ConfigManager {
  // 读取配置文件properties.load(inputstream)

  private static ConfigManager configManager;
  private static Properties properties;

  private ConfigManager() {
    String configFile = "database.properties";
    properties = new Properties();
    InputStream is = ConfigManager.class.getClassLoader()
        .getResourceAsStream(configFile);

    try {
      properties.load(is);
      is.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  public static ConfigManager getInstance() {
    if (configManager == null) {
      configManager = new ConfigManager();
    }
    return configManager;
  }

  public String getString(String key) {
    return properties.getProperty(key);
  }

}

　　

package cn.tree.dao;

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

import cn.tree.util.ConfigManager;

public class MenuDao {

  // 查询主目录
  public void getMainList() {
    Connection connection = null;
    Statement st = null;
    ResultSet rs = null;
    String driver = ConfigManager.getInstance().getString(
        "jdbc.driver.class");
    String url = ConfigManager.getInstance().getString(
        "jdbc.connection.url");
    String username = ConfigManager.getInstance().getString(
        "jdbc.connection.username");
    String password = ConfigManager.getInstance().getString(
        "jdbc.connection.password");

    try {
      // 1、加载驱动
      Class.forName(driver);
      // 2、获取数据库连接
      connection = DriverManager.getConnection(url, username, password);
      // 3、获取Statement对象，执行sql语句
      String sql = "select * from main";
      st = connection.createStatement();
      rs = st.executeQuery(sql);
      // 4、处理sql执行结果
      while (rs.next()) {
        int id = rs.getInt("id");
        String name = rs.getString("name");
        System.out.println("id: " + id + "\t" + "name: " + name);
      }
    } catch (ClassNotFoundException e) {
      e.printStackTrace();
    } catch (SQLException e) {
      e.printStackTrace();
    } finally {
      try {
        // 5、释放资源
        rs.close();
        st.close();
        connection.close();
      } catch (SQLException e) {
        e.printStackTrace();
      }
    }
  }

  public static void main(String[] args) {
    MenuDao menuDao = new MenuDao();
    menuDao.getMainList();
  }
}