Usage of expression in Mule

1. Ravinder

2.  This component evaluates an expression.
 It accepts either a user-defined expression or a reference to a file.
 The processor view is as below.

3.  This component evaluates a range of expressions.
 The Evaluator and Expression properties are required.

4.  The Expression Transformer executes one or more expressions on the
current message.
 The result of these expressions becomes the payload of the current
message.
Minimum Configuration:
 An expression evaluator and the expression itself.
 In the below figure, Check the Return source if Null box if you want the
message payload source to be returned without modification when all
expressions evaluate to null.
 Use the Advanced tab to optionally configure the return arguments.

5.  For each return argument, you enter or select from the pull-down list its
expression evaluator. Then enter the expression to use. If you set Evaluator
to custom, you also need to specify the custom evaluator. If you are using a
custom expression evaluator, you must first have registered the custom
evaluator with the Expression Evaluator Manager. Expression syntax varies
depending on the evaluator.

6.  When you have multiple expressions for return arguments, by default
expression evaluation returns an error and stops when an expression
evaluates to null. Check the Optional box if you want expression evaluation
to continue to the next expression when an expression evaluates to null.
 The evaluators should be of this enumeration '[attachment, attachments,
attachments-list, bean, endpoint, exception-type, function, groovy, header,
headers, headers-list, json, json-node, jxpath, map-payload, message,
mule, ognl, payload, process, regex, string, variable, xpath, xpath2, xpath-
node, custom]'.

7. Example:

8. Description:
 1. Use http connector to trigger the flow.
 2. Expression component to set the Employee Details as outbound
properties.
 3. Message Properties component to set a Session Variable.
 4. Pass all the properties to another flow using a http outbound end point
and add session properties to the header as session expires after every flow.
 5. The data received will be of Byte Array Stream so use an Object to String
Transformer.

9.  6. Check the attached Session variable using the "#[message]" MEL in Logger
component.
 7. Get all the details from the inbound properties and use a map object to
set-payload. In a similar fashion List can also be used.
 8. Transform the payload type from Object to String.
 9. Evaluate if the payload type is of String or not using Expression Filter. If
the payload is of type String the flow execution forwards.
 10. Use choice router to check for a specific text in the payload and print
his Server IP using Mule Expression Transformer.

10. Thank You All