Declare an interface with gen-interface that throws an Exception ?

Question

I'm trying to replicate a Java interface using gen-interface, which has some Java classes implementing it where they define their overrides as throws Exception. But I do not know how in Clojure to specify that in the signature?

Example:

public class Someclass implements MyInterface {
  public void perform(CommandLine line) throws Exception {
     ...
  }
}

-

(gen-interface
 :name some.place.MyInterface
 :methods [[perform [org.apache.commons.cli.CommandLine] void]])

This fails with in javac with: overridden method does not throw Exception

Is there any way to create an interface which declares a throws clause on the methods in Clojure using gen-interface?

Answer 1

This is not supported and is unlikely to be added.

The best option to create custom interfaces like this is to define them in Java.

Answer 2

I don't think either gen-class or gen-interface support checked exceptions. I could be wrong about that though. Why do you want a general "throws Exception" declaration?

Personally my approach (because I think checked exceptions were a poor idea) is whenever I have code that could throw one I just wrap it in a try catch that wraps the checked exception in a RuntimeException and throws that.

Comments

It's mostly for backward compatibility. There's a lot of existing Java code that use that interface and implement it where their declaration throws exceptions and their implementations throw checked exceptions. Would be a lot of work to go and change all of it.

And the reason I am re-writing the interface in Java, is because I want to implement this interface in Clojure, but the Java code already uses Clojure, so there is a circular dependency, where Java needs Clojure to be built first, and Clojure needs Java to be built first. I could extract it out into a 3rd library, but that's also annoying, so I was hoping I could just create the interface in Clojure.

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Could you post some more example code of what you're trying to achieve? Clojure forbids circular dependencies for namespaces, but they're actually fine usually in java interface/class land

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It's at the build level. I have a mixed Clojure/Java project. Java calls the Clojure code through some gen-class, thus I have to build Clojure first so that when Java compiles it finds the gen-classes to link against. But, there's a place where I want to have a Clojure gen-class that implements a Java interface. At this point, when Clojure compiles the gen-class, it fails, because it can't find the interface class file, because Java hasn't been built yet.

So I tried moving the interface to Clojure land, but it defines methods that throw exceptions, so it seems that's not possible.

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You can control your build order more granularly than that using leiningen or boot. Compile the specific clojure code that generates your gen-class, then the java, then the rest of the clojure.