In JRuby (master branch)
Responds to method_missing true
Responds to my_test_method true

In JRuby (1_7 branch) & In JRuby (1.7.18)
esponds to method_missing false
Responds to my_test_method true

In MRI (Ruby 2.0.0) & In MRI (Ruby 2.1.0)
Responds to method_missing true
Responds to my_test_method true

I suppose the conditional expression in core/src/main/java/org/jruby/runtime/Helpers.java causes this issue. Regardless of the declaration of method_missing, the visibility of method_missing is always Visibility.PRIVATE.
This is a my patch to fix this issue on jruby-1_7 branch.

diff --git a/core/src/main/java/org/jruby/runtime/Helpers.java b/core/src/main/java/org/jruby/runtime/Helpers.java
index 5404190..daf3bb6 100644
--- a/core/src/main/java/org/jruby/runtime/Helpers.java
+++ b/core/src/main/java/org/jruby/runtime/Helpers.java
@@ -2039,7 +2039,7 @@ public class Helpers {
         DynamicMethod method;
         final Ruby runtime = containingClass.getRuntime();

-        if ("initialize".equals(name) || "initialize_copy".equals(name) || "method_missing".equals(name) || visibility == Visibility.MODULE_FUNCTION) {
+        if ("initialize".equals(name) || "initialize_copy".equals(name) || visibility == Visibility.MODULE_FUNCTION) {
             visibility = Visibility.PRIVATE;
         } else if (runtime.is2_0() && ("respond_to_missing?".equals(name) || "initialize_clone".equals(name) || "initialize_dup".equals(name))) {
             visibility = Visibility.PRIVATE;

@enebo @headius please review my patch. Thank you.

@k77ch7 Looking...

Weird...I can find no place in MRI where they special case either method_missing or respond_to_missing? to force them to always be private. In fact, I can't find any code in MRI 2.2 that does this for initialize forms either.

Testing across versions:

~/projects/ruby $ rvm ruby-2.2 do ruby -e "class Foo; def method_missing(*); end; def respond_to_missing?(*); end; def initialize; end; end; Foo.new.method_missing; Foo.new.respond_to_missing?; Foo.new.initialize"

~/projects/ruby $ rvm ruby-2.2 do ruby -e "class Foo; def method_missing(*); end; def respond_to_missing?(*); end; def initialize; end; end; Foo.new.method_missing; Foo.new.respond_to_missing?; Foo.new.initialize"

~/projects/ruby $ rvm ruby-2.1 do ruby -e "class Foo; def method_missing(*); end; def respond_to_missing?(*); end; def initialize; end; end; Foo.new.method_missing; Foo.new.respond_to_missing?; Foo.new.initialize"

~/projects/ruby $ rvm ruby-2.0 do ruby -e "class Foo; def method_missing(*); end; def respond_to_missing?(*); end; def initialize; end; end; Foo.new.method_missing; Foo.new.respond_to_missing?; Foo.new.initialize"

~/projects/ruby $ rvm ruby-1.9 do ruby -e "class Foo; def method_missing(*); end; def respond_to_missing?(*); end; def initialize; end; end; Foo.new.method_missing; Foo.new.respond_to_missing?; Foo.new.initialize"

~/projects/ruby $ rvm ruby-1.8 do ruby -e "class Foo; def method_missing(*); end; def respond_to_missing?(*); end; def initialize; end; end; Foo.new.method_missing; Foo.new.respond_to_missing?; Foo.new.initialize"

~/projects/ruby $

I have maintained this private logic for years, and now I can find no evidence that MRI ever did it. I must be going mad.

Here's a diff that removes the private-forcing logic. I do not have cycles this evening to make sure that MRI does not still have private-forcing logic somewhere.

diff --git a/core/src/main/java/org/jruby/runtime/Helpers.java b/core/src/main/java/org/jruby/runtime/Helpers.java
index 9d01515..4ed539c 100644
--- a/core/src/main/java/org/jruby/runtime/Helpers.java
+++ b/core/src/main/java/org/jruby/runtime/Helpers.java
@@ -2039,12 +2039,6 @@ public class Helpers {

         DynamicMethod method;
         final Ruby runtime = containingClass.getRuntime();
-
-        if ("initialize".equals(name) || "initialize_copy".equals(name) || "method_missing".equals(name) || visibility == Visibility.MODULE_FUNCTION) {
-            visibility = Visibility.PRIVATE;
-        } else if (runtime.is2_0() && ("respond_to_missing?".equals(name) || "initialize_clone".equals(name) || "initialize_dup".equals(name))) {
-            visibility = Visibility.PRIVATE;
-        }

         if (RubyInstanceConfig.LAZYHANDLES_COMPILE) {
             method = factory.getCompiledMethodLazily(

Weird... it looks like this logic was added or modified by @tduehr in 9c758b4.

@tduehr Did you see this logic in MRI?

I did that based on the pickaxe it looks like that chunk was in error. It starts as a private method but when re-defined, is public. it seems this change was in error.

@headius @tduehr Thank you. I hope that @headius 's patch is applied to jruby-1_7 branch.

correct (respond_to_missing related) logic has been backported at some point. should be good with 1.7.26

actually it fails on 1.7.26 (its doing fine on 9K e.g. 9.1.7.0 returning true, true) :

Responds to method_missing false
Responds to my_test_method true

I'm not sure we're going to bother fixing this after so long, especially since it works properly on 9k. Please upgrade. If you are really stuck by this, open a new issue.