JRuby Complex Classes in Java Method Signatures

As documented in the JRuby wiki, java_signature changes a method’s signature to match the signature string passed to it:

	require 'jruby/core_ext'

	class Foo

	java_signature 'void bar(int, int)'
	def bar(a, b)
	puts a + b
	end

	end

	Foo.become_java!.declared_methods.each { \|m\|
	puts m
	}

view raw example(1).rb hosted with ❤ by GitHub

Observe that the classes in the method signature are primitive. What if we use a complex class as a parameter type?

	require 'jruby/core_ext'

	class MyClass
	attr_accessor :b

	def initialize(b)
	@b = b
	end
	end

	class Foo

	java_signature 'void bar(int, MyClass)'
	def bar(a, my_class)
	puts a + my_class.b
	end

	end

	Foo.become_java!.declared_methods.each { \|m\|
	puts m
	}

view raw example(2).rb hosted with ❤ by GitHub

Running the above code will give you the following NoMethodError message:

	NoMethodError: undefined method `start_with?' for #<Java::OrgJrubyAstJava_signature::ReferenceTypeNode:0x3b9fa8f7>
	as_java_type at /opt/jruby-1.7.4/lib/ruby/shared/jruby/compiler/java_signature.rb:20
	parameters at /opt/jruby-1.7.4/lib/ruby/shared/jruby/compiler/java_signature.rb:48
	each at file:/opt/jruby-1.7.4/lib/jruby.jar!/jruby/java/java_ext/java.util.rb:7
	map at org/jruby/RubyEnumerable.java:713
	parameters at /opt/jruby-1.7.4/lib/ruby/shared/jruby/compiler/java_signature.rb:48
	types at /opt/jruby-1.7.4/lib/ruby/shared/jruby/compiler/java_signature.rb:54
	java_signature at /opt/jruby-1.7.4/lib/ruby/shared/jruby/core_ext/class.rb:44
	Foo at lib/example.rb:12
	(root) at lib/example.rb:11

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The way I went about using complex classes in signatures is to utilise add_method_signature instead of java_signature:

	require 'jruby/core_ext'

	class MyClass
	attr_accessor :b

	def initialize(b)
	@b = b
	end
	end

	class Foo

	add_method_signature('bar', [java.lang.Void, Java::java.lang.Integer::TYPE, MyClass.become_java!(false)])
	def bar(a, my_class)
	puts a + my_class.b
	end

	end

	Foo.become_java!.declared_methods.each { \|m\|
	puts m
	}

view raw example(3).rb hosted with ❤ by GitHub

add_method_signature expects the first argument to be the name of the method that will have its signature changed. For the second argument, it expects it to be a list of classes. The list’s first item is the return class (e.g., void) while the subsequent items are the signature’s parameter classes (e.g., int and MyClass). Note that I invoke become_java! on the complex class. This tells MyClass to materialize itself into a Java class. The false flag is needed so that JRuby’s main class loader is used to load the class. Without it, you’ll be greeted by a java.lang.ClassNotFoundException.