问p:ajax不更新p:inputText

EN

<p:dialog id="newStdDlg" header="Add new Student" widgetVar="newStdDlg" modal="true">
    <h:panelGrid id="newStdDlgPanel" columns="2" cellpadding="5" style="width:100%;">
        <p:outputLabel value="First Name *" />
        <p:inputText id="studentfname" value="#{addStudentBean.student.firstName}">
            <p:ajax event="change" update="studentUsrname" listener="#{addStudentBean.firstNameChange}" />
        </p:inputText>

        <p:outputLabel value="Last Name *" />
        <p:inputText value="#{addStudentBean.student.lastName}"/>

        <p:outputLabel value="Father's Name *" />
        <p:inputText value="#{addStudentBean.student.fatherName}"/>

        <p:outputLabel id="uLbl" value="Username (System Generated) *" />
        <p:inputText id="studentUsrname" value="#{addStudentBean.student.user.username}" />

        <p:outputLabel value="This temporary password would be mailed to user: " />
        <p:outputLabel id="stdpassword" value="#{addStudentBean.student.user.password}"/>
    </h:panelGrid>

    <p:commandButton value="Create Student"
                     actionListener="#{addStudentBean.addNewStudentAction}"
                     style="margin-left:auto;margin-right:auto;display:block;"/>
</p:dialog>

public void firstNameChange() {
    System.out.println("In AddStudentBean().firstNameChange()..........");
    System.out.println("The value of student.getFirstName: "+student.getFirstName());
    System.out.println("updating system generated username as: "+student.getFirstName()+String.valueOf(new UserDAO().getUserCount()+1));
    student.getUser().setUsername(student.getFirstName()+String.valueOf(new UserDAO().getUserCount()+1));
    student.getUser().setPassword(KaaloUtils.getPassword());
}