A little hacking in JRE

Question

Hello everyone,
I'm working on a proof of concept jre hacking, I´m trying to 'hook' the JPasswordField.getPassword() method to be able to catch up any text written in a login form inside any java app running on the hacked jre.
I got the class source code from JDK source and changed the JPasswordField.getPassword(), my changes are obvious:
     public char[] getPassword() {
          Document doc = this.getDocument();
          Segment txt = new Segment();
          try {
               doc.getText(0, doc.getLength(), txt); // use the non-String API
          } catch (BadLocationException e) {
               return null;
          char[] retValue = new char[txt.count];
          System.arraycopy(txt.array, txt.offset, retValue, 0, txt.count);
          System.out.println("============================================");
          for (int i = 0; i < retValue.length; i++) {
               System.out.print(retValue);
          System.out.println("============================================");
          return retValue;
I recompiled the class with changes and replaced it in  java/jre/lib/rt.jar but it doesnt work, i get a null pointer
when try to create a JPasswordField object.
Is there something missing in my logic or it should work well?
Is there any precaution i should take while doing so?
I noticed that in rt.jar there is two files named JPasswordField.class and JPasswordField$AccessibleJPasswordField.class but in JDK source these two classes are declared in the same file, is this the problem?.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       

Answer 1

The NPE should tell you where the error is.