how to find source component that generated a DocumentEvent

You can set a property in the document to tell you which textcomponent the document belongs to:

For example:

final JTextField field = new JTextField("");
field.getDocument().putProperty("owner", field); //set the owner

final JTextField field2 = new JTextField("");
field2.getDocument().putProperty("owner", field2); //set the owner

DocumentListener documentListener = new DocumentListener() {

     public void changedUpdate(DocumentEvent documentEvent) {}

     public void insertUpdate(DocumentEvent documentEvent) {

         //get the owner of this document
         Object owner = documentEvent.getDocument().getProperty("owner");
         if(owner != null){
             //owner is the jtextfield
             System.out.println(owner);
         }
     }

     public void removeUpdate(DocumentEvent documentEvent) {}

     private void updateValue(DocumentEvent documentEvent) {}
};

field.getDocument().addDocumentListener(documentListener);
field2.getDocument().addDocumentListener(documentListener);

Alternatively:

Get the document that sourced the event and compare it to the document of the textfield.

Example:

public void insertUpdate(DocumentEvent documentEvent) {
    if (documentEvent.getDocument()== field.getDocument()){
        System.out.println("event caused by field");
    }
    else if (documentEvent.getDocument()== field2.getDocument()){
        System.out.println("event caused by field2");
    }
}

Rather than add multiple fields to the same listener. Create a custom listener that upon creation takes a reference to the text field. Then create a new instance of the listener each time you add it to a field.