Return json encoded payload from exception strategy

Question

I am experimenting with using Mule 3.4.0CE to provide a RESTful API and evaluating both the Jersey and Rest-router modules to handle this. That is mostly going well but I am not finding very much in terms of concrete/complete examples of implementing RESTful APIs in Mule.

At present I have simple GET and PUT endpoints for an entity working using the rest-router. The PUT flow is successfully passing stuff through to JDBC but I am fizzy about how to handle the case where the entity already exists.

I am ok with relying on SqlException to catch the pk constraint violation and have an exception strategy handling that:

<catch-exception-strategy when="#[exception.causedBy(java.sql.SQLException) and exception.getCauseException().getMessage().contains('Duplicate entry')]" doc:name="Duplicate_entry1">
          <set-payload value="The request cannot be processed, the error is #[exception.getSummaryMessage()]" doc:name="Set Payload"/> <!-- [1] -->
          <set-property propertyName="http.status" value="400" doc:name="Property"/> <!-- [2] -->
</catch-exception-strategy>

but am confused about 2 things:

1) Catching a more specific Exception? I am able to get the exception strategy to match on java.sql.SQLException but would rather match on the root cause of com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException. Using that class and the various forms of casusedBy(), causedExactlyBy() and casueMatches() dont seem to find anything other than the outer SQLException.

and

2) How to return a simple json encoded payload in response to this error? What I would like to do inside the catch-exception-strategy is to create a map of KV pairs something like status="error" and error_message="entity XX already exists" and have that json encoded as the mule payload/ response.

I am embarrassed that I cant seem to get my head around a way to do that simply with MEL or the various components in MuleStudio. Looking for pointers or docs on how to do this. I am resisting building a custom component to return the map I want and have that json encoded on the way out of Mule.

Answer 1

For 1) In your when clause you can make use of containsType() method of org.mule.util.ExceptionUtils (build on top of Apache ExceptionUtils class). It checks whole stacktrace for presence of specific exception.
If you don't want to provide fully qualified class name in MEL (for ExceptionUtils), you can use global imports feature, described in last part of MEL Cheat Sheet.

I have made two blog posts (here & here) about RESTful services on Mule. Maybe, you find them useful.

Answer 2

For 2) You can look into <json:object-to-json-transformer doc:name=“Object to JSON”/>

http://mule3.wordpress.com/2012/10/14/mule-object-to-json/

http://svn.muleforge.org/json-support/trunk/src/main/java/org/mule/module/json/transformers/JsonToObject.java

http://www.mulesoft.org/docs/site/current/apidocs/org/mule/module/json/transformers/package-summary.html

http://www.mulesoft.org/documentation/display/current/Native+Support+for+JSON#NativeSupportforJSON-Examples