SQL 주입 빈틈 프로 세 스 실례 및 해결 방안

public class JDBCDemo3 {
  public static void demo3_1(){
    boolean flag=login("aaa' OR ' ","1651561");  //      ，                   
    if (flag){
      System.out.println("    ");
    }else{
      System.out.println("    ");
    }

  }
  public static boolean login(String username,String password){
    Connection conn=null;
    Statement stat=null;
    ResultSet rs=null;
    boolean flag=false;
    try {
      conn=JDBCUtils.getConnection();
      String sql="SELECT * FROM user WHERE username='"+username+"'AND password='"+password+"'"; //   SQL       ，         ，        ：SELECT * FROM user WHERE username='aaa' OR '' AND password='1651561',             ，      
      stat=conn.createStatement();
      rs=stat.executeQuery(sql);
      if(rs.next()){
        flag=true;
      }else{
        flag=false;
      }
    } catch (SQLException e) {
      e.printStackTrace();
    }
    return flag;
  }

public static void demo3_1(){
    boolean flag=login1("aaa' OR ' ","1651561");
    if (flag){
      System.out.println("    ");
    }else{
      System.out.println("    ");
    }

  }
  public static boolean login1(String username,String password){
    Connection conn=null;
    PreparedStatement pstat=null;
    ResultSet rs=null;
    boolean flag=false;

    try {
      conn=JDBCUtils.getConnection();
      String sql="SELECT * FROM user WHERE username=? AND password=?"; //  ?    ，     sql  ，     sql       sql  
      pstat=conn.prepareStatement(sql);
      pstat.setString(1,username); //      
      pstat.setString(2,password);
      rs=pstat.executeQuery();
      if(rs.next()){
        flag=true;
      }else{
        flag=false;
      }
    } catch (SQLException e) {
      e.printStackTrace();
    }
    return flag;
  }
}