JDBC学习笔记02【ResultSet类详解、JDBC登录案例练习、PreparedStatement类详解】

矫情吗；* 2022-10-28 10:22 197阅读 0赞

>  *  **黑马程序员-JDBC文档（腾讯微云）JDBC笔记.pdf：[https://share.weiyun.com/Kxy7LmRm][https_share.weiyun.com_Kxy7LmRm]**
> 
> 1.  [JDBC学习笔记01【JDBC快速入门、JDBC各个类详解、JDBC之CRUD练习】][JDBC_01_JDBC_JDBC_JDBC_CRUD]
> 2.  [JDBC学习笔记02【ResultSet类详解、JDBC登录案例练习、PreparedStatement类详解】][JDBC_02_ResultSet_JDBC_PreparedStatement]
> 3.  [JDBC学习笔记03【JDBC事务管理、数据库连接池、JDBCTemplate】][JDBC_03_JDBC_JDBCTemplate]

**目录**

04 ResultSet类详解

JDBC各个类详解\_ResultSet\_基本使用

JDBC各个类详解\_ResultSet\_遍历

05 JDBC登录案例练习

JDBC练习\_select语句

Emp.java // 封装Emp表数据的JavaBean

JDBCDemo8.java

JDBC工具类

JDBC练习\_登录案例

06 PreparedStatement类详解

JDBC各个类详解\_PreparedStatement

登录案例——解决sql注入问题

--------------------

# 04 ResultSet类详解 #

## JDBC各个类详解\_ResultSet\_基本使用 ##

> **4. ResultSet：结果集对象,封装查询结果**  
>         \* boolean next()：游标向下移动一行，判断当前行是否是最后一行末尾(是否有数据)；如果是，则返回false，如果不是则返回true。  
>         \* getXxx(参数):获取数据  
>             \* Xxx：代表数据类型   如：int getInt()、String getString()  
>             \* 参数：  
>                 1. int：代表列的编号，从1开始，如： getString(1)  
>                 2. String：代表列名称，如： getDouble("balance")

> ![20210206103631389.png][]  ![20210206103832567.png][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70][]
> 
> ![20210206104017411.png][]
> 
> ![20210206104136424.png][]

> ![20210206104633170.png][]

package cn.itcast.jdbc;
    
    import java.sql.*;
    
    /**
     * 执行DDL语句
     */
    public class JDBCDemo6 {
        public static void main(String[] args) {
            Connection conn = null;
            Statement stmt = null;
            ResultSet rs = null;
            try {
                //1. 注册驱动
                Class.forName("com.mysql.jdbc.Driver");
                //2.获取连接对象
                conn = DriverManager.getConnection("jdbc:mysql:///db3", "root", "root");
                //3.定义sql
                String sql = "select * from account";
                //4.获取执行sql对象
                stmt = conn.createStatement();
                //5.执行sql
                rs = stmt.executeQuery(sql);
                //6.处理结果
                //6.1 让游标向下移动一行
                rs.next();
                //6.2 获取数据
                int id = rs.getInt(1);
                String name = rs.getString("name");
                double balance = rs.getDouble(3);
                System.out.println(id + "---" + name + "---" + balance);
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                //7.释放资源
                if (rs != null) {
                    try {
                        rs.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if (stmt != null) {
                    try {
                        stmt.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if (conn != null) {
                    try {
                        conn.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }

## JDBC各个类详解\_ResultSet\_遍历 ##

> **4. ResultSet：结果集对象，封装查询结果**  
>         \* boolean next(): 游标向下移动一行，判断当前行是否是最后一行末尾(是否有数据)；如果是，则返回false；如果不是则返回true。  
>         \* getXxx(参数):获取数据  
>             \* Xxx：代表数据类型，如：int getInt()，String getString()  
>             \* 参数：  
>                 1. int：代表列的编号，从1开始，如： getString(1)  
>                 2. String：代表列名称。如： getDouble("balance")  
>           
>         **\* 注意：**  
>             \* 使用步骤：  
>                 1. 游标向下移动一行  
>                 2. 判断是否有数据  
>                 3. 获取数据
> 
>                //6.1 循环判断游标是否是最后一行末尾。  
>                 while(rs.next())\{  
>                     //6.2 获取数据  
>                     int id = rs.getInt(1);  
>                     String name = rs.getString("name");  
>                     double balance = rs.getDouble(3);  
>                     System.out.println(id + "---" + name + "---" + balance);  
>                 \}

> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 1][]
> 
> **报错的代码：**

package cn.itcast.jdbc;
    
    import java.sql.*;
    
    /**
     * 执行DDL语句
     */
    public class JDBCDemo6 {
        public static void main(String[] args) {
            Connection conn = null;
            Statement stmt = null;
            ResultSet rs = null;
            try {
                //1. 注册驱动
                Class.forName("com.mysql.jdbc.Driver");
                //2.获取连接对象
                conn = DriverManager.getConnection("jdbc:mysql:///db3", "root", "root");
                //3.定义sql
                String sql = "select * from account";
                //4.获取执行sql对象
                stmt = conn.createStatement();
                //5.执行sql
                rs = stmt.executeQuery(sql);
                //6.处理结果
                //6.1 让游标向下移动一行
                rs.next();
                //6.2 获取数据
                int id = rs.getInt(1);
                String name = rs.getString("name");
                double balance = rs.getDouble(3);
                System.out.println(id + "---" + name + "---" + balance);
    
                //6.1 让游标向下移动一行
                rs.next();
                //6.2 获取数据
                int id2 = rs.getInt(1);
                String name2 = rs.getString("name");
                double balance2 = rs.getDouble(3);
                System.out.println(id2 + "---" + name2 + "---" + balance2);
    
                //6.1 让游标向下移动一行
                rs.next();
                //6.2 获取数据
                int id3 = rs.getInt(1);
                String name3 = rs.getString("name");
                double balance3 = rs.getDouble(3);
                System.out.println(id3 + "---" + name3 + "---" + balance3);
    
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                //7.释放资源
                if (rs != null) {
                    try {
                        rs.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if (stmt != null) {
                    try {
                        stmt.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if (conn != null) {
                    try {
                        conn.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }

> ![20210206105611357.png][]

package cn.itcast.jdbc;
    
    import java.sql.*;
    
    /**
     * 执行DDL语句
     */
    public class JDBCDemo7 {
        public static void main(String[] args) {
            Connection conn = null;
            Statement stmt = null;
            ResultSet rs = null;
            try {
                //1. 注册驱动
                Class.forName("com.mysql.jdbc.Driver");
                //2.获取连接对象
                conn = DriverManager.getConnection("jdbc:mysql:///db3", "root", "root");
                //3.定义sql
                String sql = "select * from account";
                //4.获取执行sql对象
                stmt = conn.createStatement();
                //5.执行sql
                rs = stmt.executeQuery(sql);
                //6.处理结果
                //6.1 循环判断游标是否是最后一行末尾。
                while (rs.next()) {
                    //6.2 获取数据
                    int id = rs.getInt(1);
                    String name = rs.getString("name");
                    double balance = rs.getDouble(3);
                    System.out.println(id + "---" + name + "---" + balance);
                }
    
               /* //6.1 让游标向下移动一行
                if(rs.next()){
                    //判断是否有数据
                    //6.2 获取数据
                    int id = rs.getInt(1);
                    String name = rs.getString("name");
                    double balance = rs.getDouble(3);
                    System.out.println(id + "---" + name + "---" + balance);
                }
    
                //6.1 让游标向下移动一行
                if(rs.next()){
                    //判断是否有数据
                    //6.2 获取数据
                    int id = rs.getInt(1);
                    String name = rs.getString("name");
                    double balance = rs.getDouble(3);
                    System.out.println(id + "---" + name + "---" + balance);
                }
    
                //6.1 让游标向下移动一行
                if(rs.next()){
                    //判断是否有数据
                    //6.2 获取数据
                    int id = rs.getInt(1);
                    String name = rs.getString("name");
                    double balance = rs.getDouble(3);
                    System.out.println(id + "---" + name + "---" + balance);
                }
    
                //6.1 让游标向下移动一行
                if(rs.next()){
                    //判断是否有数据
                    //6.2 获取数据
                    int id = rs.getInt(1);
                    String name = rs.getString("name");
                    double balance = rs.getDouble(3);
    
                    System.out.println(id + "---" + name + "---" + balance);
                }*/
    
              /*  //6.1 让游标向下移动一行
                rs.next();
                //6.2 获取数据
                int id2 = rs.getInt(1);
                String name2 = rs.getString("name");
                double balance2 = rs.getDouble(3);
                System.out.println(id2 + "---" + name2 + "---" + balance2);
    
                //6.1 让游标向下移动一行
                rs.next();
                //6.2 获取数据
                int id3 = rs.getInt(1);
                String name3 = rs.getString("name");
                double balance3 = rs.getDouble(3);
                System.out.println(id3 + "---" + name3 + "---" + balance3);*/
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                //7.释放资源
                if (rs != null) {
                    try {
                        rs.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if (stmt != null) {
                    try {
                        stmt.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if (conn != null) {
                    try {
                        conn.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }

# 05 JDBC登录案例练习 #

## JDBC练习\_select语句 ##

> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 2][]  ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 3][]

> ![20210206110812627.png][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 4][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 5][]  ![20210206111618855.png][]

### Emp.java // 封装Emp表数据的JavaBean ###

package cn.itcast.domain;
    
    import java.util.Date;
    
    /**
     * 封装Emp表数据的JavaBean
     */
    public class Emp {
        private int id;
        private String ename;
        private int job_id;
        private int mgr;
        private Date joindate;
        private double salary;
        private double bonus;
        private int dept_id;
    
        public int getId() {
            return id;
        }
    
        public void setId(int id) {
            this.id = id;
        }
    
        public String getEname() {
            return ename;
        }
    
        public void setEname(String ename) {
            this.ename = ename;
        }
    
        public int getJob_id() {
            return job_id;
        }
    
        public void setJob_id(int job_id) {
            this.job_id = job_id;
        }
    
        public int getMgr() {
            return mgr;
        }
    
        public void setMgr(int mgr) {
            this.mgr = mgr;
        }
    
        public Date getJoindate() {
            return joindate;
        }
    
        public void setJoindate(Date joindate) {
            this.joindate = joindate;
        }
    
        public double getSalary() {
            return salary;
        }
    
        public void setSalary(double salary) {
            this.salary = salary;
        }
    
        public int getDept_id() {
            return dept_id;
        }
    
        public void setDept_id(int dept_id) {
            this.dept_id = dept_id;
        }
    
        public double getBonus() {
            return bonus;
        }
    
        public void setBonus(double bonus) {
            this.bonus = bonus;
        }
    
        @Override
        public String toString() {
            return "Emp{" +
                    "id=" + id +
                    ", ename='" + ename + '\'' +
                    ", job_id=" + job_id +
                    ", mgr=" + mgr +
                    ", joindate=" + joindate +
                    ", salary=" + salary +
                    ", bonus=" + bonus +
                    ", dept_id=" + dept_id +
                    '}';
        }
    }

### JDBCDemo8.java ###

package cn.itcast.jdbc;
    
    import cn.itcast.domain.Emp;
    import cn.itcast.util.JDBCUtils;
    
    import java.sql.*;
    import java.util.ArrayList;
    import java.util.List;
    
    /**
     * * 定义一个方法，查询emp表的数据将其封装为对象，然后装载集合，返回。
     */
    public class JDBCDemo8 {
    
        public static void main(String[] args) {
            List<Emp> list = new JDBCDemo8().findAll();
            System.out.println(list);
            System.out.println(list.size());
            for (Emp x : list) {
                System.out.println(x);
            }
        }
    
        /**
         * 查询所有emp对象
         *
         * @return
         */
        public List<Emp> findAll() {
            Connection conn = null;
            Statement stmt = null;
            ResultSet rs = null;
            List<Emp> list = null;
            try {
                //1.注册驱动
                Class.forName("com.mysql.jdbc.Driver");
                //2.获取连接
                conn = DriverManager.getConnection("jdbc:mysql:///db3", "root", "root");
                //3.定义sql
                String sql = "select * from emp";
                //4.获取执行sql的对象
                stmt = conn.createStatement();
                //5.执行sql
                rs = stmt.executeQuery(sql);
                //6.遍历结果集，封装对象，装载集合
                Emp emp = null;
                list = new ArrayList<Emp>();
                while (rs.next()) {
                    //获取数据
                    int id = rs.getInt("id");
                    String ename = rs.getString("ename");
                    int job_id = rs.getInt("job_id");
                    int mgr = rs.getInt("mgr");
                    Date joindate = rs.getDate("joindate");
                    double salary = rs.getDouble("salary");
                    double bonus = rs.getDouble("bonus");
                    int dept_id = rs.getInt("dept_id");
                    // 创建emp对象,并赋值
                    emp = new Emp();
                    emp.setId(id);
                    emp.setEname(ename);
                    emp.setJob_id(job_id);
                    emp.setMgr(mgr);
                    emp.setJoindate(joindate);
                    emp.setSalary(salary);
                    emp.setBonus(bonus);
                    emp.setDept_id(dept_id);
                    //装载集合
                    list.add(emp);
                }
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                if (rs != null) {
                    try {
                        rs.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if (stmt != null) {
                    try {
                        stmt.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if (conn != null) {
                    try {
                        conn.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }
            return list;
        }
    
    }

## JDBC工具类 ##

> **抽取JDBC工具类 ： JDBCUtils**
> 
> **\* 目的：简化书写**  
> \* **分析：**  
>     1. 注册驱动也抽取  
>     2. 抽取一个方法获取连接对象  
>         \* 需求：不想传递参数（麻烦），还得保证工具类的通用性。  
>         \* 解决：配置文件  
>             jdbc.properties  
>                 url=...  
>                 user=...  
>                 password=...
> 
>     3. 抽取一个方法释放资源  
>         \* 代码实现：...

> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 6][]  ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 7][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 8][]
> 
> **[Java19-day10【标准输入输出流、字节字符打印流、对象序列化-反序列化流、serialVersionUID&transient、Properties】][Java19-day10_-_serialVersionUID_transient_Properties]**
> 
> **Properties介绍**
> 
>  *  是一个Map体系的集合类。
>  *  Properties可以保存到流中或从流中加载。
>  *  属性列表中的每个键及其对应的值都是一个字符串。
> 
> ![2021020611400627.png][]
> 
>  
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 9][]  ![20210206114112377.png][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 10][]

> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 11][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 12][]
> 
> ![20210206115227269.png][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 13][]

package cn.itcast.util;
    
    import java.io.FileReader;
    import java.io.IOException;
    import java.net.URL;
    import java.sql.*;
    import java.util.Properties;
    
    /**
     * JDBC工具类
     */
    public class JDBCUtils {
    
        private static String url;
        private static String user;
        private static String password;
        private static String driver;
    
        /**
         * 文件的读取，只需要读取一次即可拿到这些值。使用静态代码块
         * 静态代码块，随着类的加载而执行（只会执行一次）
         */
        static {
            //读取资源文件，获取值。
            try {
                //1. 创建Properties集合类。
                Properties pro = new Properties();
                //获取src路径下的文件的方式--->ClassLoader 类加载器：加载字节码文件进内存、获取src下资源文件的路径
                //获取ClassLoader要先获取其对应的字节码文件对象
                ClassLoader classLoader = JDBCUtils.class.getClassLoader();
                //以src为相对的根路径 统一资源定位符URL
                URL res = classLoader.getResource("jdbc.properties"); // 传文件名，获取resource资源
                String path = res.getPath();
                // System.out.println(path);///D:/IdeaProjects/itcast/out/production/day04_jdbc/jdbc.properties
                //2. 加载文件
                // pro.load(new FileReader("D:\\IdeaProjects\\itcast\\day04_jdbc\\src\\jdbc.properties"));
                pro.load(new FileReader(path));
                //3. 获取数据，赋值
                url = pro.getProperty("url");
                user = pro.getProperty("user");
                password = pro.getProperty("password");
                driver = pro.getProperty("driver");
                //4. 注册驱动
                Class.forName(driver);
            } catch (IOException e) {
                e.printStackTrace();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        }
    
        /**
         * 获取连接
         *
         * @return 连接对象
         */
        public static Connection getConnection() throws SQLException {
            return DriverManager.getConnection(url, user, password);
        }
    
        /**
         * 释放资源
         *
         * @param stmt
         * @param conn
         */
        public static void close(Statement stmt, Connection conn) {
            if (stmt != null) {
                try {
                    stmt.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            if (conn != null) {
                try {
                    conn.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }
    
        /**
         * 释放资源
         *
         * @param stmt
         * @param conn
         */
        public static void close(ResultSet rs, Statement stmt, Connection conn) {
            if (rs != null) {
                try {
                    rs.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            if (stmt != null) {
                try {
                    stmt.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            if (conn != null) {
                try {
                    conn.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }
    
    }

## JDBC练习\_登录案例 ##

> \* 练习：  
>     **\* 需求：**  
>         1. 通过键盘录入用户名和密码  
>         2. 判断用户是否登录成功  
>             \* select \* from user where username = "" and password = "";  
>             \* 如果这个sql有查询结果，则成功，反之，则失败
> 
>     **\* 步骤：**  
>         1. 创建数据库表 user  
>             CREATE TABLE USER(  
>                 id INT PRIMARY KEY AUTO\_INCREMENT,  
>                 username VARCHAR(32),  
>                 PASSWORD VARCHAR(32)  
>             );
> 
>             INSERT INTO USER VALUES(NULL,'zhangsan','123');  
>             INSERT INTO USER VALUES(NULL,'lisi','234');
> 
>         2. 代码实现：...

> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 14][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 15][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 16][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 17][]

# 06 PreparedStatement类详解 #

## JDBC各个类详解\_PreparedStatement ##

> 5. PreparedStatement：执行sql的对象  
>         **1. SQL注入问题：**在拼接sql时，有一些sql的特殊关键字参与字符串的拼接，会造成安全性问题。  
>             1. 输入用户随便，输入密码：a' or 'a' = 'a  
>             2. sql：select \* from user where username = 'fhdsjkf' and password = 'a' or 'a' = 'a' 
> 
>         **2. 解决sql注入问题：**使用PreparedStatement对象来解决  
>         **3. 预编译的SQL：**参数使用?作为占位符  
>         **4. 步骤：**  
>             1. 导入驱动jar包 mysql-connector-java-5.1.37-bin.jar  
>             2. 注册驱动  
>             3. 获取数据库连接对象 Connection  
>             4. 定义sql  
>                 \* 注意：sql的参数使用？作为占位符， 如：select \* from user where username = ? and password = ?;  
>             5. 获取执行sql语句的对象 PreparedStatement  Connection.prepareStatement(String sql)   
>             6. 给？赋值：  
>                 \* 方法： setXxx(参数1,参数2)  
>                     \* 参数1：？的位置编号 从1 开始  
>                     \* 参数2：？的值  
>             7. 执行sql，接受返回结果，不需要传递sql语句  
>             8. 处理结果  
>             9. 释放资源
> 
>         **5. 注意：**后期都会使用PreparedStatement来完成增删改查的所有操作  
>             1. 可以防止SQL注入  
>             2. 效率更高

> ![20210206133314210.png][]  ![20210206133434719.png][]
> 
> ![20210206133743873.png][]

### 登录案例——解决sql注入问题 ###

> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 18][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDk0OTEzNQ_size_16_color_FFFFFF_t_70 19][]

package cn.itcast.jdbc;
    
    import cn.itcast.util.JDBCUtils;
    
    import java.sql.*;
    import java.util.Scanner;
    
    /**
     * 练习：
     * * 需求：
     * 1. 通过键盘录入用户名和密码
     * 2. 判断用户是否登录成功
     */
    public class JDBCDemo9 {
    
        public static void main(String[] args) {
            //1.键盘录入，接受用户名和密码
            Scanner sc = new Scanner(System.in);
            System.out.println("请输入用户名：");
            String username = sc.nextLine();
            System.out.println("请输入密码：");
            String password = sc.nextLine();
            //2.调用方法 非静态方法，需要创建对象来调用
            boolean flag = new JDBCDemo9().login(username, password);
            //3.判断结果，输出不同语句
            if (flag) {
                //登录成功
                System.out.println("登录成功！");
            } else {
                System.out.println("用户名或密码错误！");
            }
        }
    
        /**
         * 登录方法
         */
        public boolean login(String username, String password) {
            if (username == null || password == null) {
                return false;
            }
            //连接数据库判断是否登录成功
            Connection conn = null;
            Statement stmt = null;
            ResultSet rs = null;
            //1.获取连接
            try {
                conn = JDBCUtils.getConnection();
                //2.定义sql
                String sql = "select * from user where username = '" + username + "' and password = '" + password + "' ";
                System.out.println(sql);
                //3.获取执行sql的对象
                stmt = conn.createStatement();
                //4.执行查询
                rs = stmt.executeQuery(sql);
                //5.判断
               /* if(rs.next()){//如果有下一行，则返回true
                    return true;
                }else{
                    return false;
                }*/
                return rs.next();//如果有下一行，则返回true
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                JDBCUtils.close(rs, stmt, conn);
            }
            return false;
        }
    
        /**
         * 登录方法,使用PreparedStatement实现
         */
        public boolean login2(String username, String password) {
            if (username == null || password == null) {
                return false;
            }
            //连接数据库判断是否登录成功
            Connection conn = null;
            PreparedStatement pstmt = null;
            ResultSet rs = null;
            //1.获取连接
            try {
                conn = JDBCUtils.getConnection();
                //2.定义sql
                String sql = "select * from user where username = ? and password = ?";
                //3.获取执行sql的对象
                pstmt = conn.prepareStatement(sql);
                //给?赋值
                pstmt.setString(1, username);
                pstmt.setString(2, password);
                //4.执行查询,不需要传递sql
                rs = pstmt.executeQuery();
                //5.判断
               /* if(rs.next()){//如果有下一行，则返回true
                    return true;
                }else{
                    return false;
                }*/
                return rs.next();//如果有下一行，则返回true
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                JDBCUtils.close(rs, pstmt, conn);
            }
            return false;
        }
    
    }

> 加油加油加油，宁死不破~~~

[https_share.weiyun.com_Kxy7LmRm]: https://share.weiyun.com/Kxy7LmRm
[JDBC_01_JDBC_JDBC_JDBC_CRUD]: https://blog.csdn.net/weixin_44949135/article/details/113701706
[JDBC_02_ResultSet_JDBC_PreparedStatement]: https://blog.csdn.net/weixin_44949135/article/details/113708612
[JDBC_03_JDBC_JDBCTemplate]: https://blog.csdn.net/weixin_44949135/article/details/113724109
[20210206103631389.png]: https://img-blog.csdnimg.cn/20210206103631389.png
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[Java19-day10_-_serialVersionUID_transient_Properties]: https://blog.csdn.net/weixin_44949135/article/details/107968971
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