create or replace and resolve java source named XTest as
public class XTest {
  public static String xString(java.sql.SQLXML x) throws Exception {
    
    return "["+x.getString()+"]";
  }; 
}
/

create or replace function xstring(x XMLType) return varchar2
  as language java name 'XTest.xString(java.sql.SQLXML) return java.lang.String';
/

select length(xs) len, lengthb(xs) lenb, length2(xs) len2, xs, xstring(xmltype(xs)) xx, ascii(xmltype(xs).extract('/X/text()')) ascii
from (select '<X>'||chr(4036991128)||'</X>' xs from dual);

       LEN       LENB       LEN2 XS                   XX                        ASCII
---------- ---------- ---------- -------------------- -------------------- ----------
         8         11          9 <X>&#128536;</X>            [<X>&#128536;</X]           4036991128
--                                      тэг не закрыт [<X>&#128536;</X>]