What is a Processor in Camel and how do you implement a custom Processor?

A Processor is the atomic unit of message manipulation in Camel — any class implementing org.apache.camel.Processor. Every EIP construct compiles to Processors chained in a Pipeline. A custom Processor gives direct access to the full Exchange: body, headers, properties, and exception state.

The interface declares one method:

public interface Processor {
    void process(Exchange exchange) throws Exception;
}

A custom Processor that validates an order and stamps an audit header:

public class OrderValidationProcessor implements Processor {
    @Override
    public void process(Exchange exchange) throws Exception {
        String body = exchange.getIn().getBody(String.class);
        if (body == null || body.isBlank()) {
            exchange.setException(new IllegalArgumentException("Empty payload"));
            return;
        }
        exchange.getIn().setHeader("X-Validated-At",
            java.time.Instant.now().toString());
        exchange.getIn().setBody(body.trim());
    }
}

from("jms:queue:raw-orders")
    .process(new OrderValidationProcessor())
    .to("jms:queue:valid-orders");

// Lambda shorthand:
from("direct:greet")
    .process(e -> e.getIn().setBody("Hello " + e.getIn().getBody(String.class)));

Processor vs Bean: Use Processor when you need direct Exchange-level access (headers, exceptions). Use .bean(MyBean.class) when the logic is pure business code — keeping domain classes free of Camel API imports and independently unit-testable.