Message exchange patterns and the Exchange object
The MEP defines the messaging style used such as one-way (InOnly) or request-reply (InOut),
which means you have IN and optionally OUT messages. This closely maps to other APIs such as WS, WSDL, REST, JBI and the likes.
The Exchange API provides two methods to get a message, either
getIn gets the IN message, and the
getOut gets the OUT message.
Flow of an exchange through a route
- The out message from each step is used as the in message for the next step
- if there is no out message then the in message is used instead
- For the InOut MEP the out from the last step in the route is returned to the producer. In case of InOnly the last out is thrown away
Beware of getOut to check if there is an out message
exchange.getOut creates an out message if there is none. So if you want to check if there is an out message then you should use exchange.hasOut instead
Using getIn or getOut methods on Exchange
Now suppose you want to use a Camel Processor to adjust a message. This can be done as follows:
This seems intuitive and is what you would expect is the right approach to change a message from a Processor.
However there is an big issue - the
getOut method will create a new Message, which means any other information
from the IN message will not be propagated; which means you will lose that data.
To remedy this we'll have to copy the data which is done as follows:
Well that is not all, a Message can also contain attachments so to be sure you need to propagate those as well:
Now we ensure that all additional data is propagated on the new OUT message. But its a shame we need 2 code lines to ensure data is propagated.
What you can do instead is to change the IN message instead, as shown below:
Consider using getIn
As shown above you most often need to alter the existing IN message, than creating a totally new OUT message.
And therefore it's often easier just to adjust the IN message directly.
Changing the IN message directly is possible in Camel as it doesn't mind. Camel will detect that the Exchange has no OUT message
and therefore use the IN message instead.
About Message Exchange Pattern and getOut
So the example code above is possible for any kind of MEP. The MEP is just a flag on the Exchange which the Consumer and Producer adhere to.
You can change the MEP on the Exchange using the
setPattern method. And likewise there is DSL to change it as well.