From 4c087e4e6a138f0200b1185f34b1d09559bb7240 Mon Sep 17 00:00:00 2001 From: mdb Date: Tue, 13 Feb 2001 18:49:41 +0000 Subject: [PATCH] Created an extension to the webmacro servlet that automatically handles exceptions in a user friendly way. git-svn-id: https://samskivert.googlecode.com/svn/trunk@43 6335cc39-0255-0410-8fd6-9bcaacd3b74c --- .../webmacro/FriendlyException.java | 22 +++++ .../samskivert/webmacro/FriendlyServlet.java | 90 +++++++++++++++++++ .../src/java/com/samskivert/webmacro/Makefile | 10 +++ 3 files changed, 122 insertions(+) create mode 100644 projects/samskivert/src/java/com/samskivert/webmacro/FriendlyException.java create mode 100644 projects/samskivert/src/java/com/samskivert/webmacro/FriendlyServlet.java create mode 100644 projects/samskivert/src/java/com/samskivert/webmacro/Makefile diff --git a/projects/samskivert/src/java/com/samskivert/webmacro/FriendlyException.java b/projects/samskivert/src/java/com/samskivert/webmacro/FriendlyException.java new file mode 100644 index 00000000..867446f7 --- /dev/null +++ b/projects/samskivert/src/java/com/samskivert/webmacro/FriendlyException.java @@ -0,0 +1,22 @@ +// +// $Id: FriendlyException.java,v 1.1 2001/02/13 18:49:41 mdb Exp $ + +package com.samskivert.webmacro; + +/** + * The friendly exception provides a mechanism by which a servlet can + * abort its processing and report a human readable error to the servlet + * framework that will be inserted into the context in the appropriate + * place so that the error message will be displayed to the user. Simply + * construct a friendly exception with the desired error message and throw + * it during the call to populateContext. + * + * @see FriendlyServlet + */ +public class FriendlyException extends Exception +{ + public FriendlyException (String message) + { + super(message); + } +} diff --git a/projects/samskivert/src/java/com/samskivert/webmacro/FriendlyServlet.java b/projects/samskivert/src/java/com/samskivert/webmacro/FriendlyServlet.java new file mode 100644 index 00000000..8e64804a --- /dev/null +++ b/projects/samskivert/src/java/com/samskivert/webmacro/FriendlyServlet.java @@ -0,0 +1,90 @@ +// +// $Id: FriendlyServlet.java,v 1.1 2001/02/13 18:49:41 mdb Exp $ + +package com.samskivert.webmacro; + +import org.webmacro.*; +import org.webmacro.servlet.*; + +/** + * The friendly servlet adds a common error handling paradigm to the basic + * webmacro servlet. This paradigm is to catch any exceptions thrown in + * the populateContext method (used instead of the + * handle method) and to convert them into friendly error + * messages that are inserted into the context with the key + * "error" for easy display to the user in the resulting web + * page. + * + *

To make this work, the process of template selection is separated + * from the process of data generation. First the template is selected; if + * that process fails, a standard error response is given to the user as + * we have no template into which to substitute the template selection + * failure error message. Subsequently, the data is generated. If that + * fails, we can substitute a friendly error message into the selected + * template. Finally, the template is executed, which is a standard + * webmacro procedure. + * + *

The process of mapping exceptions to friendly error messages is + * done through a properties file loaded via the classpath. The properties + * file should be named 'exceptionmap.properties' and placed in a + * directory contained in the classpath of the JVM in which the servlet is + * executed. The file should contain mappings from exception classes to + * friendly error messages. For example: + * + * 

+ * com.samskivert.webmacro.FriendlyException = An error occurred while \
+ * processing your request: {m}
+ * java.sql.SQLException = The database is currently unavailable. Please \
+ * try your request again later.
+ * java.lang.Exception = An unexpected error occurred while processing \
+ * your request. Please try again later.
+ *
+ * + * The message associated with the exception will be substituted into the + * error string in place of {m}. + */ +public abstract class FriendlyServlet extends WMServlet +{ + public Template handle (WebContext ctx) throws HandlerException + { + // first we select the template + Template tmpl; + try { + tmpl = selectTemplate(ctx); + } catch (Exception e) { + throw new HandlerException("Unable to load template: " + e); + } + + // then we populate the context with data + try { + populateContext(ctx, tmpl); + } catch (Exception e) { + } + + return tmpl; + } + + /** + * Override this member function with code that selects and loads the + * proper template based on whatever criterion appropriate for this + * servlet. + * + * @param ctx The context of this request. + * + * @return The template to be used in generating the response. + */ + protected abstract Template selectTemplate (WebContext ctx) + throws Exception; + + /** + * Override this member function with code that populates the context + * with whatever data is appropriate for this servlet. Presumably this + * is where the real work of the servlet takes place. + * + * @param ctx The context of this request. + * @param tmpl The webmacro template previously selected by the + * invocation of the selectTemplate method. + */ + protected abstract void populateContext (WebContext ctx, Template tmpl) + throws Exception; +} diff --git a/projects/samskivert/src/java/com/samskivert/webmacro/Makefile b/projects/samskivert/src/java/com/samskivert/webmacro/Makefile new file mode 100644 index 00000000..f00e18b1 --- /dev/null +++ b/projects/samskivert/src/java/com/samskivert/webmacro/Makefile @@ -0,0 +1,10 @@ +# +# $Id: Makefile,v 1.1 2001/02/13 18:49:41 mdb Exp $ + +ROOT = ../../.. + +SRCS = \ + FriendlyException.java \ + FriendlyServlet.java \ + +include $(ROOT)/build/Makefile.java