DynamicForm submit ( DSCallback ) is not work

[wonderman882]

SmartGWT 2.5 used in this case

Hi, I try the following code and ensure the form can submit and output message in the servlet side.

DynamicForm uploadForm = new DynamicForm();
uploadForm.setEncoding(Encoding.MULTIPART);
uploadForm.setAction("/test/TestServlet");
uploadForm.submit ( new DSCallback(){
    System.out.println("Callback Successfully");
} );

After finishing the servlet side I expect program going to callback which will log the above message. However, no any message is logged.

Do I miss anything or have any misunderstand on submit?