Error Handling

Here we describe how to handle errors. An error is caused by an exception that is not caught by the application. An exception might be thrown in two situations: when loading a ZUML document, and when serving an AU request (aka, an Ajax request).

Error Handling When Loading ZUML Documents

If an un-caught exception is thrown when loading a ZUML document, it is handled directly by the Web server. In other words, the handling is no different from other servlets.

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

You can customize the error handling by specifying the error page in WEB-INF/web.xml as follows.

<!-- WEB-INF/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.

Request Attribute	Type
javax.servlet.error.status_code	java.lang.Integer
javax.servlet.error.exception_type	java.lang.Class
javax.servlet.error.message	java.lang.String
javax.servlet.error.exception	java.lang.Throwable
javax.servlet.error.request_uri	java.lang.String
javax.servlet.error.servlet_name	java.lang.String

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>

Tips:

• The error page can be any kind of servlets. In addition to ZUML, you can use JSP or whatever servlet you preferred.

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