First of all: I created a very simple project to demonstrate the bugs I stumpled into using Play 2.3.5. (After running activator new routesTest play-java I only added some basic routes in conf/routes and added the accompanying actions in app/controllers/Application.java with this commit).

Now Step-by-Step:

1) There are some JavaScript literal binders missing
Have a look at the routes file. When you want to compile this routes file you get following errors:

No JavaScript literal binder found for type play.libs.F.Option[Integer]. Try to implement an implicit JavascriptLiteral for this type.

and

No JavaScript literal binder found for type Long. Try to implement an implicit JavascriptLiteral for this type.

and

No JavaScript literal binder found for type Boolean. Try to implement an implicit JavascriptLiteral for this type.

(You would also get an error for the missing UUID JavaScript literal binder - I already submitted the PR #3508 for this missing UUID JavaScript literal binder yesterday)

So with this PR I added the missing JavaScript literal binders for play.libs.F.Option, java.lang.Long and java.lang.Boolean.

2) NullPointerExceptions occur when assigning null as a default to Java Types in a route
After implementing the missing binders (including #3508), the project will now fail with a NullPointerException in the already existing java.lang.Integer JavaScript literal binder when assigning null as a default in a route (see line 13). If you have a look at my fix, you will see that's because .toString is called without checking if the default value is null before. (FYI: To actually get this NullPointerException you actually have to call the JavaScript route like in line 53 of the Application.java. If you don't call it you won't experience the NullPointerException).

3) When assigning null as a default to a String in a route, it gets converted to a JavaScript "null" String instead of a Javascript null value
See line 9 in the routes file. Without this PR following JavaScript is generated for JavaScript reverse routing (in case you run my test project you could just have a look at /assets/routes.js):

if (id == "null") {
return _wA({method:"GET", url:"/" + "stringNull"})
}

With my PR it correctly becomes:

if (id == null) {
return _wA({method:"GET", url:"/" + "stringNull"})
}

Please also backport this PR and #3508 to 2.3.x (be aware to rename JavascriptLiteral to JavascriptLitteral for the 2.3.x branch).
Thanks!