SQL injection vulnerability process example and solution

public class JDBCDemo3 {
  public static void demo3_1(){
    boolean flag = login("aaa' OR ' ","1651561"); //If the username is known, this method can be used to log in successfully without knowing the password if (flag) {
      System.out.println("Login successful");
    }else{
      System.out.println("Login failed");
    }

  }
  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+"'"; //This is the key to the SQL injection vulnerability. Because it is a string concatenation, the query statement will become: SELECT * FROM user WHERE username='aaa' OR '' AND password='1651561'. This query statement can get a result set, so this vulnerability occursstat=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("Login successful");
    }else{
      System.out.println("Login failed");
    }

  }
  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=?"; //Use ? instead of parameter, pre-set sql format, even if you enter sql keywords, sql will not recognize it pstat=conn.prepareStatement(sql);
      pstat.setString(1,username); //Set the value of the question mark pstat.setString(2,password);
      rs=pstat.executeQuery();
      if(rs.next()){
        flag=true;
      }else{
        flag=false;
      }
    } catch (SQLException e) {
      e.printStackTrace();
    }
    return flag;
  }
}