分析：
String sql = "select * from user where username='"+username+"' and password='"+password+"'";String sql = "select * from user where username='tom' and password='123'";select * from user where username='jerry' and password='abc'；//這種情況是不會有問題的//測試如果傳入的是SQL語句的的一部分username:tom' or '1'='1password:????????//這時候的SQL語句為一下這種情況select * from user where username='tom' or '1'='1' and password='?????'；//由於and的執行優先級大於Or 可以看作以下情況select * from user where username='tom' or ('1'='1' and password='?????')；//由於是or 或者的關系這時的SQL語句可以看作是這樣的語句select * from user where username='tom'；
二、解決方案（引入： PreparedStatement對象，使SQL語句進行預編譯 ）
PreparedStatement pstmt = conn.perpareStatement(String sql);
其實statement執行executeQuery內部分為兩步：
第一步：編譯sql
第二步：執行sql
優點：能預編譯sql語句
例子：數據庫客戶端服務端基本模型
登陸端
package com.heima.login.client;import java.util.Scanner;import com.heima.login.bean.User;import com.heima.login.seriver.services;public class login { /** * @param args * @throws Exception */ public static void main(String[] args) throws Exception { // TODO Auto-generated method stub // 創建Scanner接收輸入內容- Scanner sc= new Scanner(System.in);
System.out.println("請輸入用戶名"); String username = sc.nextLine(); System.out.println("請輸入密碼"); String password = sc.nextLine(); // 創建服務端對象使用登陸方法
services s=new services(); User user = s.login(username, password); if(user!=null){ System.out.println(user); }else{ System.out.println("用戶名或密碼錯誤！！！"); } } }
服務端：
package com.heima.login.seriver;import java.sql.Connection;import java.sql.PreparedStatement;import java.sql.ResultSet;import com.heima.Tools.JdbcConnection;import com.heima.login.bean.User;public class services { public User login(String username,String password) throws Exception { //新建user引用，默認值問null User u=null; //使用自定義數據庫連接工具類進行數據庫連接 Connection conn=JdbcConnection.getConnection(); //創建預編譯數據庫連接語句 String sql = "select * from user where username=? and password=?"; //創建PreparedStatement對象對數據庫語句進行預編譯 PreparedStatement stem=conn.prepareStatement(sql); //設置預編譯語句中的查詢值 stem.setString(1, username); stem.setString(2, password); //執行SQL語句 ResultSet rs=stem.executeQuery(); //如果有記錄對User對象進行賦值 if(rs.next()) { u = new User(); u.setAge(rs.getInt("age")); u.setDept(rs.getString("Dept")); u.setEmail(rs.getString("email")); u.setGender(rs.getString("gender")); u.setId(rs.getInt("id")); u.setPassword(rs.getString("password")); u.setRegistTime(rs.getDate("registTime")); u.setSalary(rs.getDouble("salary")); u.setUsername(rs.getString("username")); } //關閉數據庫資源 JdbcConnection.close(conn, stem, rs); return u; }}
User對象：
package com.itheima.login.entity;public class User { private int id; private String username; private String password; private String email; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } public String getEmail() { return email; } public void setEmail(String email) { this.email = email; } @Override public String toString() { return "User [id=" + id + ", username=" + username + ", password=" + password + ", email=" + email + "]"; } }