目录
Sql注入
SQL注入攻击指的是通过构建特殊的输入作为参数传入Web应用程序，而这些输入大都是SQL语法里的一些组合，通过执行SQL语句进而执行攻击者所要的操作，其主要原因是程序没有细致地过滤用户输入的数据，致使非法数据侵入系统。
以模拟登录为例:在前台输入用户名和密码，后台判断信息是否正确，并给出前台反馈信息，前台输出反馈信息。
public class TestInjection {
private static String driver ="com.mysql.cj.jdbc.Driver";
private static String url="jdbc:mysql://127.0.0.1:3306/mydb?useSSL=false&useUnicode=true&characterEncoding=UTF-8&serverTimezone=Asia/Shanghai&allowPublicKeyRetrieval=true";
private static String user="root";
private static String password="root";
public static void main(String[] args) {
Scanner sc =new Scanner(System.in);
System.out.println("请输入用户名");
String username=sc.next();
System.out.println("请输入密码");
String pwd =sc.next();
Account account = getAccount(username, pwd);
System.out.println(null!= account?"登录成功":"登录失败");
sc.close();
}
public static Account getAccount(String username,String pwd){
Connection connection = null;
Statement statement=null;
ResultSet resultSet=null;
Account account =null;
try{
Class.forName(driver);
connection = DriverManager.getConnection(url, user,password);
statement = connection.createStatement();
String sql="select * from account where username ='"+username+"' and password ='"+pwd+"'";
System.out.println(sql);
resultSet = statement.executeQuery(sql);
while(resultSet.next()){
int aid = resultSet.getInt("aid");
String usernamea = resultSet.getString("username");
String pwda = resultSet.getString("password");
double money = resultSet.getDouble("money");
account=new Account(aid,usernamea,pwda,money);
System.out.println(account);
}
}catch (Exception e){
e.printStackTrace();
}finally {
if(null != resultSet){
try {
resultSet.close();
} catch (SQLException e) {e.printStackTrace();
}
}
if(null != statement){
try {
statement.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
if(null != connection){
try {
connection.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
}
return account;
}
}
当输入下面的值时：
username=aaa，pwd=abc'or'aa'='a
即使是错误的，也能登录成功。让登录功能形同虚设，这就是SQL注入风险。这是为什么呢？原因在于SQL语句是字符串拼接的。SQL语句中拼接的内容破坏了SQL语句原有的判断逻辑。
使用PreparedStatement预编译语句对象解决SQL注入攻击
public class TestInjection2 {
private static String driver ="com.mysql.cj.jdbc.Driver";
private static String url="jdbc:mysql://127.0.0.1:3306/mydb?useSSL=false&useUnicode=true&characterEncoding=UTF-8&serverTimezone=Asia/Shanghai&allowPublicKeyRetrieval=true";
private static String user="root";
private static String password="root";
public static void main(String[] args) {
Scanner sc =new Scanner(System.in);
System.out.println("请输入用户名");
String username=sc.next();
System.out.println("请输入密码");
String pwd =sc.next();
Account account = getAccount(username, pwd);
System.out.println(null!= account?"登录成功":"登录失败");
sc.close();
}
public static Account getAccount(String username,String pwd){
Connection connection = null;
PreparedStatement preparedStatement=null;
ResultSet resultSet=null;
Account account =null;
try{
Class.forName(driver);
connection = DriverManager.getConnection(url, user,password);
/*
* 1使用PreparedStatement语句对象防止注入攻击
* 2PreparedStatement 可以使用 ? 作为参数的占位符
* 3使用?作为占位符,即使是字符串和日期类型,也不使用单独再添加 ''
* 4connection.createStatement();获得的是普通语句对象 Statement
* 5connection.prepareStatement(sql);可以获得一个预编译语句对象PreparedStatement
* 6如果SQL语句中有?作为参数占位符号,那么要在执行CURD之前先设置参数
* 7通过set***(问号的编号,数据) 方法设置参数
* */
String sql="select * from account where username = ? and password = ?";
preparedStatement = connection.prepareStatement(sql);//这里已经传入SQL语句
//设置参数
preparedStatement.setString(1,username );
preparedStatement.setString(2,pwd );
//执行CURD
resultSet = preparedStatement.executeQuery();// 这里不需要再传入SQL语句
while(resultSet.next()){
int aid = resultSet.getInt("aid");
String usernamea = resultSet.getString("username");
String pwda = resultSet.getString("password");
double money = resultSet.getDouble("money");
account=new Account(aid,usernamea,pwda,money);
System.out.println(account);
}
}catch (Exception e){
e.printStackTrace();
}finally {
if(null != resultSet){
try {
resultSet.close();
} catch (SQLException e) {e.printStackTrace();
}
}
if(null != preparedStatement){
try {
preparedStatement.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
if(null != connection){
try {
connection.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
}
return account;
}
}
prepareStatment对象在set***方法上,会对单引号进行转译处理,也就是说,?中的数据的单引号 ‘ 会被转义成 \’,这样就单引号就不会破坏sql语句的结构。
SELECT * FROM users WHERE userName = ? AND password = ?
preparedStatement.setString(1,"xiaoming");
preparedStatement.setString(2,'anything' OR 'x'='x');
会被转义为
SELECT * FROM users WHERE userName = 'xiaoming' AND password = 'anything\' OR\'x\'=\'x\''
而不是
SELECT * FROM users WHERE userName = 'xiaoming' AND password = 'anything' OR 'x'='x'
PreparedStatement把值当中的所有单引号给转义了，这就达到了防止sql注入的目的，mysql驱动的PreparedStatement实现类的setString()；方法内部做了单引号的转义。
Statement不能防止sql注入，因为它没有把单引号做转义，而是直接拼接字符串，所以达不到防止sql注入的目的。