If an un-caught exception is thrown when loading a ZUML page, it is handled directly by the Web server. In other words, its handling is no different from other pages, such as JSP.

By default, the Web server displays an error page showing the error message and stack trace.

You can customize the error handling by specifying the error page in WEB-INF/web.xml as follows. Refer to Java Servlet Specification for more details.

<!-- web.xml -->
<error-page>
    <exception-type>java.lang.Throwable</exception-type>    
    <location>/WEB-INF/sys/error.zul</location>    
</error-page>

Then, when an error occurs in loading a page, the Web server forwards the error page you specified, /error/error.zul. Upon forwarding, the Web server passes a set of request attributes to the error page to describe what happens. These attributes are as follows.

Then, in the error page, you can display your custom information by use of these attributes. For example,

<window title="Error ${requestScope['javax.servlet.error.status_code']}">
    Cause: ${requestScope['javax.servlet.error.message']}    
</window>

Tip: The error page can be any kind of servlets. In addition to ZUL, you can use JSP or whatever you preferred.

Tip: After forwarded, the error page is displayed as the main page, so you don't need to specify the modal or overlapped mode for the main window, if any.

//error.zul
<zk>
    <zscript>    
    if (Executions.getCurrent().isMilDevice())    
        Executions.forward("error.mil");        
    </zscript>    
    <window>    
    ....error message in ZUL    
    </window>    
</zk>