Although there are a lot of evaluators available in Mule CE it is very easy to add your own evaluator. In my case we have a self defined message format that holds some properties in the header of a message (similar to JMS Message, MuleMessage, etc.). To get access to these properties in the Mule config I created a custom evaluator that made this possible. Although there will be other solutions available for this situation, I found this a nice (pragmatic) way to solve it. It also provides a base to start from in case of possible changes in the future.
The XML schema that describes our message looks like:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:tns="http://www.pascalalma.net/message/v01_0"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.pascalalma.net/message/v01_0"
elementFormDefault="qualified">
<xs:element name="message" type="tns:MessageType"/>
<xs:complexType name="MessageHeaderType">
<xs:annotation>
<xs:documentation>The header for a message. Contains all metadata about the message.</xs:documentation>
</xs:annotation>
<xs:sequence>
<xs:element name="messageVersion" type="tns:versionNumberType" minOccurs="1" maxOccurs="1"/>
<xs:element name="PropertySet" type="tns:PropertySetType" minOccurs="0" maxOccurs="1"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="MessageType">
<xs:sequence>
<xs:element name="MessageHeader" type="tns:MessageHeaderType" minOccurs="1" maxOccurs="1"/>
<xs:element name="MessageBody" type="tns:MessageBodyType" minOccurs="0" maxOccurs="1"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="MessageBodyType">
<xs:sequence>
<xs:any minOccurs="0" maxOccurs="1"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="PropertyType">
<xs:attribute name="key" type="xs:string" use="required"/>
<xs:attribute name="value" type="xs:string"/>
</xs:complexType>
<xs:complexType name="PropertySetType">
<xs:annotation>
<xs:documentation>Set of properties</xs:documentation>
</xs:annotation>
<xs:sequence>
<xs:element name="Property" type="tns:PropertyType" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
<xs:simpleType name="versionNumberType">
<xs:annotation>
<xs:documentation>Type used to define a Message versionNumber</xs:documentation>
</xs:annotation>
<xs:restriction base="xs:string">
<xs:pattern value="v[0-9]{2}_[0-9]{1}"/>
<xs:length value="5"/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
And example message looks like this:
<?xml version="1.0" encoding="UTF-8"?>
<tns:message xsi:schemaLocation="http://www.pascalalma.net/message/v01_0 message_v01_0.xsd" xmlns:tns="http://www.pascalalma.net/message/v01_0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<tns:MessageHeader>
<tns:messageVersion>v01_0</tns:messageVersion>
<tns:PropertySet>
<tns:Property key="SOURCE_SYSTEM" value="CRM" />
<tns:Property key="ENTITY" value="Order" />
<tns:Property key="VERSION" value="v01_0" />
</tns:PropertySet>
</tns:MessageHeader>
<tns:MessageBody>
<payload-root>Blablabla</payload-root>
</tns:MessageBody>
</tns:message>
To get access to the ‘Property’ elements in the message-header via an ‘evaluator expression’ I created the following Java class based on the info given here:
package net.pascalalma.evaluator;
import net.pascalalma.cdm.util.JaxbUtil;
import java.util.List;
import javax.xml.bind.JAXBException;
import net.pascalalma.message.v01_0.Message;
import net.pascalalma.message.v01_0.PropertySetType;
import net.pascalalma.message.v01_0.PropertyType;
import org.apache.log4j.Logger;
import org.mule.api.MuleMessage;
import org.mule.api.expression.ExpressionEvaluator;
import org.mule.api.expression.RequiredValueException;
import org.mule.config.i18n.CoreMessages;
/**
*
* @author pascal
*/
public class CdmMessageEvaluator implements ExpressionEvaluator {
public static final String NAME = "cdm-msg-property";
protected static Logger logger = Logger.getLogger(CdmMessageEvaluator.class);
public Object evaluate(String expression, MuleMessage msg) {
Object result = null;
boolean required;
//Is the header optional? the '*' denotes optional
if (expression.endsWith("*")) {
expression = expression.substring(expression.length() - 1);
required = false;
} else {
required = true;
}
Message cdmMsg = null;
//Look up the property on the message
if (msg.getPayload() instanceof Message)
{
cdmMsg = (Message) msg.getPayload();
} else if (msg.getPayload() instanceof String)
{
try {
// Apparantly we are getting the message as an XML String
// So use jaxb to create a Object of it
cdmMsg =
JaxbUtil.unmarshal(Message.class,
msg.getPayload().toString());
} catch (JAXBException ex) {
throw new UnsupportedOperationException("Unexpected String input received (not a Ship Message)");
}
} else {
// It's not an expected payload. Fail this operation.
throw new UnsupportedOperationException("Unexpected inputtype received: " + msg.getPayload().getClass().getName());
}
result = getProperty(cdmMsg.getMessageHeader().getPropertySet(),expression.toUpperCase());
if (result == null && required) {
throw new RequiredValueException(CoreMessages.expressionEvaluatorReturnedNull(NAME, expression));
}
return result;
}
public String getName() {
return NAME;
}
public void setName(String name) {
throw new UnsupportedOperationException("setName");
}
private String getProperty(PropertySetType propertySet, String property)
{
List<propertyType> props = propertySet.getProperties();
String value = null;
for (PropertyType prop: props) {
if (prop.getKey().toUpperCase().equals(property))
{
value = prop.getValue();
break;
}
}
return value;
}
}
I think the code is fairly straightforward. In the method ‘evaluate’ I receive the payload of the MuleMessage and I check to see if the payload is a JAXBObject of the type Message. If it is a String I assume I receive the XML message as payload and transform the String to the expected JAXB object myself (see this post for more details about this subject).
When I have access to the JAXBObject I simply question the properties of the message for the property name that is supplied with the expression. If a value is found this is returned by the expression.
To make use of this evaluator I ‘bootstrapped’ it with the Mule application. This can simply be done by adding the file ‘registry-bootstrap.properties’ to your classpath in the folder ‘META-INF/services/org/mule/config’ (as explained here). In this file I put:
object.1=net.pascalalma.evaluator.CdmMessageEvaluator
And that’s it. To make use of this evaluator I have in my Mule config:
<expression-recipient-list-router evaluator="custom" custom-evaluator="cdm-msg-property" expression="ADDR_LIST" transformer-refs="JaxbObjectToCdmXml" >
This will take the property ‘ADDR_LIST’ out of my Message and use it as outbound Endpoint, but I will explain that in another post.
The Pragmatic Integrator 