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Message exchange patterns and the Exchange object

The Camel API is influenced by APIs such as JBI specification, CXF which defines a concept called Message Exchange Patterns (MEP for short).

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Flow of an exchange through a route

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• 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

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Now suppose you want to use a Camel Processor to adjust a message. This can be done as follows:

public void process(Exchange exchange) throws Exception {
   String body = exchange.getIn().getBody(String.class);
   // change the message to say Hello
   exchange.getOut().setBody("Hello " + body);
}

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:

public void process(Exchange exchange) throws Exception {
   String body = exchange.getIn().getBody(String.class);
   // change the message to say Hello
   exchange.getOut().setBody("Hello " + body);
   // copy headers from IN to OUT to propagate them
   exchange.getOut().setHeaders(exchange.getIn().getHeaders());
}

Well that is not all, a Message can also contain attachments so to be sure you need to propagate those as well:

public void process(Exchange exchange) throws Exception {
   String body = exchange.getIn().getBody(String.class);
   // change the message to say Hello
   exchange.getOut().setBody("Hello " + body);
   // copy headers from IN to OUT to propagate them
   exchange.getOut().setHeaders(exchange.getIn().getHeaders();
   // copy attachements from IN to OUT to propagate them
   exchange.getOut().setAttachments(exchange.getIn().getAttachments());
}

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:

public void process(Exchange exchange) throws Exception {
   String body = exchange.getIn().getBody(String.class);
   // change the existing message to say Hello
   exchange.getIn().setBody("Hello " + body);
}

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