When writing some routes with Camel’s Java DSL I came across this exception:

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Caused by: java.lang.IllegalStateException: No method invocation could be created, no matching method could be found on: null

After a lot of tracing I figured out that it was related to me calling the .bean(...) method with a class that was actually just an interface. What was happening was that Camel wants to instantiate this class, usually using Spring, but cannot do that if it isn’t a concrete implementation.

This proved to be a real problem because I had an interface that had two implementations. One of these implementations is used for debugging and the other is used for production. I didn’t want to have to manually select which one I was using in my code because that’s Spring’s job so I came up with a way to do it.

For the complete background here’s what my interface looks like:

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import org.apache.camel.Processor;

public interface ProtobufToWire extends Processor {
}

This converts a Protobuf to our “wire” format. That format could be the native protobuf binary format or JSON. I implement this empty interface in two classes called ProtobufToBinary and ProtobufToJson and I want to use the JSON one only for debugging.

To be clear doing this always fails with the exception I listed above:

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from(SOME_URI).bean(ProtobufToWire.class);

To fix this I added this to my Java-based Spring config:

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@Bean
public ProtobufToWire protobufToWire() {
    return new ProtobufToBinary();
}

Now because, I believe, that Camel’s bean(...) method doesn’t look up the beans with Spring this still fails. What I needed to where I am defining my routes is this:

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    @Autowired
    private ProtobufToWire protobufToWire;

    @Override
    public void configure() {
      from(SOME_URI).bean(protobufToWire.getClass());
    }

What I’m doing here is getting Spring to autowire an instance of that interface into a private variable and then asking it for its real concrete type. Part of me says that I shouldn’t have to do this but this is what works for me.

Did this help you out? Do you have a better way to do it? Post in the comments!