Integration / Apache Camel Interview Questions

What is the Message in Camel (body, headers, attachments) and the difference between In and Out messages?

A Camel Message (org.apache.camel.Message) is the envelope carried inside an Exchange. It has three parts: body (any Java object — String, byte[], InputStream, POJO, DOM — type-converted transparently via getBody(TargetClass.class)), headers (a Map<String, Object> of metadata auto-populated by transport components), and attachments (named DataHandler objects for SOAP/MIME multipart messages).

In vs Out messages: In an InOnly exchange, only the In message exists; JMS publish, Kafka produce, and file write are InOnly by default. In an InOut exchange, the caller sends an In message and waits for a reply via the Out message; HTTP REST calls and synchronous direct: calls use InOut.

In Camel 3.x, the idiomatic reply approach is to mutate exchange.getIn() with the response body rather than writing to exchange.getOut(). Calling getOut() creates a fresh Message and silently discards all headers set by prior processors — a common source of bugs during Camel 3.x migrations.

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Which ExchangePattern is used by default for a Kafka produce call (to(kafka:...))? InOnly — fire-and-forget, no reply expected

✓ Well done! Kafka produce is one-way; no consumer response is expected by default.

InOut — request-reply, waits for a response

✗ Try again — InOut expects a reply; Kafka produce is one-way.

OutOnly — a non-standard push-only pattern

✗ Try again — OutOnly is not a standard Camel ExchangePattern.

What does Message.getBody(String.class) return when the body is byte[] and a converter is registered? ClassCastException is thrown

✗ Try again — the type converter handles the conversion transparently.

null, because the types do not match

✗ Try again — if a converter is registered, the conversion succeeds.

The byte[] converted to String by the registered type converter

✓ Well done! Camel type converter framework transparently converts byte[] to String.

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Integration / Apache Camel Interview Questions

What is the Message in Camel (body, headers, attachments) and the difference between In and Out messages?

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