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

The getOut method is only needed in special use cases and by certain Camel component such as the JBI or NMR components.

If you use the getOut method, then data from the in message would be lost. For example several Camel end users who got started with Camel, couldn't understand why their message headers was lost during route in Camel. The problem was they used getOut method in a Camel Processor. They should as said use the getIn method.

Obviously the getIn gets the IN message, and the getOut gets the OUT message.

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);
}

About Message Exchange Pattern and getOut

If the Exchange is using InOnly as the MEP, then you may think that the Exchange has no OUT message. But you can still invoke the getOut method on Exchange; Camel will not barf. 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 wellIf you decide to use the getOut method and want to keep headers, attachments and the likes, then you must manually copy those information from the in to the out message.