JDBC

JDBC(Java Database Connect)

Java是一种面向对象的语言
||语言不同，需要交流，找个翻译（java只出标准，数据库实现）
Mysql是结构化查询语言

正式连接数据库

java连接数据的步骤

1. 把Mysql的jar包拿过来加入到工程下
2. 把该jar包解压到本工程下，build path
   上面两步在同一个工程下执行一次就行了
3. 加载驱动。Class.forName("com.mysql.jdbc.Driver");
4. 获取连接对象。Connection conn = DriverManager.getConnection(url,user,password); 需要导包:import.java.sql.Connection; import java.sql.DriverManager;
String url = "jdbc:mysql://IP:3306/数据库名";
String user = "root";
String password = "数据库密码";
5. 获取执行sql语句的对象
   Statement st = conn.createStatement(); 需要导包 import java.sql.Statement;
6. 执行sql语句。增删改
   int row = st.executeUpdate(sql语句); row返回值表示影响的行数
   jdbc：后面加mysql：
7. 执行sql语句，查询
   ResultSet rs = st.executeQuery(sql);
   遍历rs对象
   while(rs.next()){
   xx 变量名 = rs.getxx("字段名"); //获取该记录下指定字段的内容
   };
   

常见的错误

练习：CRUD，使用方法并且关闭资源

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class CRUD {
    public static void main(String[] args) {
        //add("上海");
        //delete(4);
        //update("南京",4);
        selectall();
    }
    public static void add(String cityname) {
        Connection conn = null;
        Statement st = null;
        try {
           Class.forName("com.mysql.jdbc.Driver");
           conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/emp","root","123456");
           st = conn.createStatement();
           int row = st.executeUpdate("insert into city(cityname) values ('"+cityname+"')");
           System.out.println("受影响的行数为："+row);
        } catch (ClassNotFoundException e) {
           e.printStackTrace();
        } catch (SQLException e) {
           e.printStackTrace();
        }finally {
           if(conn != null) {
               try {
                   conn.close();
               } catch (SQLException e) {
                   e.printStackTrace();
               }
           }
           if(st != null) {
               try {
                   st.close();
               } catch (SQLException e) {
                   e.printStackTrace();
               }
           }
        }
    }
    public static void delete(int cityid) {
        Connection conn = null;
        Statement st = null;
        try {
           Class.forName("com.mysql.jdbc.Driver");
           conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/emp","root","123456");
           st = conn.createStatement();
           int row = st.executeUpdate("delete from city where cityID = "+cityid);
           System.out.println("受影响的行数为："+row);
        } catch (ClassNotFoundException e) {
           e.printStackTrace();
        } catch (SQLException e) {
           e.printStackTrace();
        }finally {
           if(conn != null) {
               try {
                   conn.close();
               } catch (SQLException e) {
                   e.printStackTrace();
               }
           }
           if(st != null) {
               try {
                   st.close();
               } catch (SQLException e) {
                   e.printStackTrace();
               }
           }
        }
    }
    public static void update(String cityname,int cityid) {
        Connection conn = null;
        Statement st = null;
        try {
           Class.forName("com.mysql.jdbc.Driver");
           conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/emp","root","123456");
           st = conn.createStatement();
           int row = st.executeUpdate("update city set cityname = '"+cityname+"' where cityid = "+cityid);
           System.out.println("受影响的行数为："+row);
        } catch (ClassNotFoundException e) {
           e.printStackTrace();
        } catch (SQLException e) {
           e.printStackTrace();
        }finally {
           if(conn != null) {
               try {
                   conn.close();
               } catch (SQLException e) {
                   e.printStackTrace();
               }
           }
           if(st != null) {
               try {
                   st.close();
               } catch (SQLException e) {
                   e.printStackTrace();
               }
           }
        }
    }
    public static void selectall() {
        Connection conn = null;
        Statement st = null;
        ResultSet rs = null;
        try {
           Class.forName("com.mysql.jdbc.Driver");
           conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/emp","root","123456");
           st = conn.createStatement();
           rs = st.executeQuery("select * from city");
           while(rs.next()) {
           int cityid = rs.getInt("cityid");
           String cityname = rs.getString("cityname");
           System.out.println(cityid+"======>"+cityname);
           }
        } catch (ClassNotFoundException e) {
           e.printStackTrace();
        } catch (SQLException e) {
           e.printStackTrace();
        }finally {
           if(conn != null) {
               try {
                   conn.close();
               } catch (SQLException e) {
                   e.printStackTrace();
               }
           }
           if(st != null) {
               try {
                   st.close();
               } catch (SQLException e) {
                   e.printStackTrace();
               }
           }
           if(rs != null) {
               try {
                  rs.close();
               } catch (SQLException e) {
                  e.printStackTrace();
               }
           }
        }   
    }
}

企业级开发

一张表对应一个操作类（crud），操作类中包含的是CRUD
例如：对emp表操作，那么java中对应的类就是EmpDao（Data Access Object） 这个类尽量放到dao包下
包名：企业域名反向写 www.baidu.com====>com.baidu + 开发人员名称缩写 ====> com.baidu.wlq.dao

练习：抽取一个BaseDao父类（7个属性，一个方法）
写一个员工表的操作类EmpDao 继承Base Dao。包含

1. 添加员工（ename,sal,job）
2. 删除员工（empNo）
3. 修改（ename，sal，job 根据empNo修改）
4. 查询 （根据empno查）
5. 查询所有员工

并写一个测试类来测试上面5个方法

package com.alibaba.wlq.dao;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class BaseDao {
    protected Connection conn = null;
    protected Statement st = null;
    protected ResultSet rs = null;
    protected String driver = "com.mysql.jdbc.Driver";
    protected String url = "jdbc:mysql://localhost:3306/emp";
    protected String user = "root";
    protected String password = "123456";
    public void closeAll() {
        if(conn != null) {
           try {
               conn.close();
           } catch (SQLException e) {
               e.printStackTrace();
           }
        }
        if(st != null) {
           try {
               st.close();
           } catch (SQLException e) {
               e.printStackTrace();
           }
        }
        if(rs != null) {
           try {
               rs.close();
           } catch (SQLException e) {
               e.printStackTrace();
           }
        }
    }
}

package com.alibaba.wlq.dao;
import java.sql.DriverManager;
import java.sql.SQLException;
public class EmpDao extends BaseDao{
    /**
     * 增加员工信息
     * @param ename 员工姓名
     * @param sal  员工薪水
     * @param job  员工职位
     */
    public void add(String ename,Double sal,String job) {
           try {
               Class.forName(driver);
               conn = DriverManager.getConnection(url,user,password);
               st = <u>conn</u>.createStatement();
               String sql = "insert into emp(ename,sal,job,hiredate) values ('"+ename+"',"+sal+",'"+job+"',now())";
               int row = st.executeUpdate(sql);
               System.out.println("影响了"+row+"行");
           } catch (ClassNotFoundException e) {
               e.printStackTrace();
           } catch (SQLException e) {
               e.printStackTrace();
           }
    }
    /**
     * 根据员工编号删除员工的信息
     * @param empno    员工编号
     */     
    public void delete(int empno) {
        try {
           Class.forName(driver);
           conn = DriverManager.getConnection(url,user,password);
           st = conn.createStatement();
           String sql = "delete from emp where empno ="+empno;
           int row = st.executeUpdate(sql);
           System.out.println("影响了"+row+"行");
        } catch (ClassNotFoundException e) {
           e.printStackTrace();
        } catch (SQLException e) {
           e.printStackTrace();
        }
    }
    /**
     * 根据员工编号查询员工信息
     * @param empno        员工编号
     */
    public void select(int empno) {
        try {
           Class.forName(driver);
           conn = DriverManager.getConnection(url,user,password);
           st = conn.createStatement();
           String sql = "select * from emp where empno = " + empno;
           rs = st.executeQuery(sql);
           while(rs.next()) {
               System.out.println("员工编号："+rs.getInt("empno"));
               System.out.println("员工姓名："+rs.getString("ename"));
               System.out.println("员工薪资："+rs.getDouble("sal"));
               System.out.println("员工入职时间："+rs.getDate("hiredate"));
           }
        } catch (ClassNotFoundException e) {
           e.printStackTrace();
        } catch (SQLException e) {
           e.printStackTrace();
        }finally {
           closeAll();
        }
    }
    /**
     * 根据员工编号修改员工姓名、薪资、职位
     * @param ename    员工姓名
     * @param sal  薪资
     * @param job  职位
     * @param empno    员工编号
     */
    public void update(String ename,Double sal,String job,int empno) {
        try {
           Class.forName(driver);
           conn = DriverManager.getConnection(url,user,password);
           st = conn.createStatement();
           String sql = "update emp set ename = '"+ename+"',sal = '"+sal+"',job='"+job+"' where empno = "+empno;
           int row = st.executeUpdate(sql);
           System.out.println("影响了"+row+"行");
        } catch (ClassNotFoundException e) {
           e.printStackTrace();
        } catch (SQLException e) {
           e.printStackTrace();
        }finally {
           closeAll();
        }
    }
    /**
     * 查询所有员工的信息
     * @param empno员工编号
     */
    public void selectALL() {
        try {
           Class.forName(driver);
           conn = DriverManager.getConnection(url,user,password);
           st = conn.createStatement();
           String sql = "select * from emp";
           rs = st.executeQuery(sql);
           while(rs.next()) {
               System.out.print("员工编号："+rs.getInt("empno")+"	");
               System.out.print("员工姓名："+rs.getString("ename")+"	");
               System.out.print("员工领导编号："+rs.getInt("mgr")+"	");
               System.out.print("员工薪资："+rs.getDouble("sal")+"	");
               System.out.println("员工入职时间："+rs.getDate("hiredate"));
           }
        } catch (ClassNotFoundException e) {
           e.printStackTrace();
        } catch (SQLException e) {
           e.printStackTrace();
        }finally {
           closeAll();
        }
    }
}

package com.alibaba.wlq.dao;
public class Test1 {
    public static void main(String[] args) {
        EmpDao emp = new EmpDao();
        //emp.add("刘德华",20000.0,"演员");
        //emp.delete(1016);
        //emp.select(1001);
        //emp.update("刘德华", 20000.0, "演员", 1015);
        emp.selectALL();
    }
}