Default JMS function selector

Default function selector

There has to be some mechanism that can be used to determine which of the possible JMS or MQ JMS export components the message should be delivered to, and which SCA operation the message corresponds with. A JMS or MQ JMS message contains only data, and the message may contain an indication about the target operation, but there is not a standard way to retrieve this information. Therefore, we need to map a certain message to a certain operation of the targeted service interface. The JMS or MQ JMS export listens to a particular destination. When a message arrives to that destination, the function selector determines the target operation.

In the export binding, the method binding has a native method , which contains an identifier representing a JMS or MQ JMS message. Use of the native method allows for a level of indirection, which enables rename of an operation, without affecting the mapping.

By default, the value of native method is set to the interface operation name when the export is created. The export utilizes a function selector to inspect an incoming message in the message header or the message body. The export provides a default function selector that returns the value of JMS String Property TargetFunctionName from the header. So to identify the JMS or MQ JMS messages, the default behavior is for the import to set the JMS header property TargetFunctionName to the name of the interface operation name, and the default function selector used by the export will extract the TargetFunctionName property from the JMS header to correctly identify the incoming message. If the manner in which the native method name is to be found does not use the TargetFunctionName property, then a FunctionSelector implementation must be provided to correctly map the incoming JMS message to its intended SCA operation.

The commonj.connector.runtime.FunctionSelector interface presented below is used to extract the native function name or identifier (event ID) from the native data flowing inbound from the resource adapter using an EIS export.

public interface FunctionSelector {

	public String generateEISFunctionName(Object[] argObjects)
			throws SelectorException;

}

The implementation of this interface has to be provided by a resource adapter or third party who understands how to locate in the native data the appropriate identifier.

The implementation for the default JMS function selector basically looks like the following:

public class JMSFunctionSelectorImpl implements FunctionSelector {

	public String generateEISFunctionName(Object[] argObjects)
			throws SelectorException {

		try {
			Message message = (Message) argObjects[0];
			String functionName = message
					.getStringProperty("TargetFunctionName");
			return functionName;
		} catch (Throwable e) {
			throw new SelectorException(e);
		}
	}
}

Some other possible implementations for a FunctionSelector could use the JMSType header property or hard code the name of the method on the interface. Why would you hard code the name of the method on the interface? You would only hard code the name of the method if there were only one target SCA operation and that operation had a single input or output. Using the header property would typically be done for routing the message, performing some parsing and applying some logic to the parsed message. But suppose your application does not require any routing? A hard coded value in this case would make sense.

public class JMSFunctionSelectorImpl implements FunctionSelector {

    public String generateEISFunctionName(Object[] argObjects) throws SelectorException {
		
	return "handleMessage";
}

Another possible implementation is to have the SCA application handle the routing. The mapped SCA operation would decide how to dispatch the message and then another part of the application would parse the message contents represented by its Service Data Object (SDO).

The following method demonstrates how a JMSType function selector could be used to create a name for the function:

public String generateEISFunctionName(Object[] arg)
			throws SelectorException {

		try {
			Message message = (Message) arg[0];
			String type = message.getJMSType();
			return type;
		} catch (JMSException e) {
			// Todo - Auto-generated catch block
			throw new SelectorException(e);
		}
	}