Thanks for your reply, but in the action method for this button I need
a server side validation code , for example:

 public String button1_action()
 {
     boolean test = false;

     test = this.getSessionBean1.validateForm(this.layoutPanel1);

     if (test)
     /* Here need to call the following code
     <SCRIPT language="JavaScript">
     <!-- hide from old browsers
       alert('Ok');
      //-->
      </SCRIPT>
      */
     else
     /* Here need to call the following code
     <SCRIPT language="JavaScript">
     <!-- hide from old browsers
       alert('Error');
      //-->
      </SCRIPT>
      */

     return null;
 }

Ricardo Ríos writes:

JSF tags support the onclick attribute, too, e.g.,
<http://java.sun.com/javaee/javaserverfaces/1.2/docs/tlddocs/h/commandButton.html>

This is a legitimate Java question in my book, since it's about JSP / JSF tag
libraries.  The question is a bit misleading from the solution, though, making
it superficially resemble a JS question.  In actuality, the solution Carl
recommends, and I endorse, is not to have "the action method for this button
... call" any Java, but conversely to have Java generate the JS to be called
directly on the client side.  It should produce the user experience you seek.