How can I define a Java interface in JRuby rather than implement one?

Question

I would like to invoke a Java API from JRuby that requires a Java interface, not a concrete class. The Java API uses java.lang.reflect.proxy to implement functionality based on the interface. I have found numerous examples of implementing a java interface with JRuby. However, I need to simply define a java interface in JRuby, not implement one. I can of course define an interface in Java and use this as the interface argument to the API via intf.java_class. However, for convenience, I'd like to be able to directly define the interface from JRuby code.

Update: Here's the Java API I want to invoke from JRuby:

public interface Query {
    <T> Collection<T> execute(Class<T> intf, String sql, Object... args);
}

The Java API requires that the intf argument be an interface, not a class. This is due to the fact that the implementation uses java.lang.reflect.proxy to provide a collection of data objects that can be accessed via the provided interface. Java's java.lang.reflect.proxy will only work with interfaces. As an example, suppose I were reading rows from a database that represented log statements. If I were invoking this from java I might define my data via

public interface LogRecord {
    int id();
    Timestamp when();
    String msg();
    String level();
    String logger();
    String thread();
}

and pass LogRecord.class as the first argument to the Query.execute method.

could you maybe show us some simplified code what you are trying to achieve ... from the description it seems quite redundant to introduce an interface solely in/for JRuby — kares, Mar 07 '14 at 07:30
Agree on seeing some sample code. The reflection API doesn't make a distinction between interfaces and classes, so you can just create a JRuby class with the right methods and pass it in. — Shepmaster, Mar 12 '14 at 00:58

score 0 · Answer 1 · answered Apr 16 '14 at 05:31

I don't think it can be done, because the Java class would have to inherit from the ruby interface, and apparently Java classes can't inherit from a JRuby class:

https://github.com/jruby/jruby/wiki/CallingJavaFromJRuby#java-classes-cant-inherit-from-a-jruby-class

How can I define a Java interface in JRuby rather than implement one?

1 Answers1