Java language support for InvokeDynamic? (or using InvokeDynamic in place of codegen)

[Rémi Forax]

On 12/08/2010 11:18 PM, Toby Reyelts wrote:

Hi Toby,
First let me say that I'm a big fan of Retroweaver.

> I'm interested in understanding where Java language support is going 
> for InvokeDynamic. The context is that I'd like to replace a system 
> that forward-generates API with a system based on InvokeDynamic. Silly 
> hypothetical example follows:
>
> // colors.prop
> color1: Red
> color2: Green
>
> preprocessor < colors.prop > Colors.java
>
> // generated API
> class Colors {
>   Color color1() { return Color.red; }
>   Color color2() { return Color.green; }
> }
>
> You get the benefits of static analysis (type-errors, code-completion, 
> etc), but you have messy pre-processing / multi-staged compilation to 
> deal with.
>
> One possible example of what I'd like to be able to do:
>
> Dynamic colors = ColorFactory.create("colors.prop");
> Color red = colors.color1();
> Color green = colors.color2();
>
> With the right tooling support, I think I could provide the same kinds 
> of static analysis benefits without the messiness of the preprocessor. 
> I don't know a ton about C#, but I believe that this looks fairly 
> achievable with the "dynamic" type.

I don't get it. You want to do a static analysis and for that you want 
to use the 'dynamic' type.
Hugh ?

Your code above is mostly valid in C# but the dynamic invocation is done
by the compiler. So if you take a to the corresponding bytecode,
it's something like this:

Object colors;
SiteContainer0.site1.bind(null, new Object[]{"colors.prop"}, ..., 
&colors);  // call create
Object tmp1;
SiteContainer0.site2.bind(colors, new Object[]{}, ..., &tmp1); // get 
attribute color1
Object red;
SiteContainer0.site2.bind(tmp1, new Object[]{}, ..., &red); // convert 
to color
...

As you see, the generated code is not usable to do a static analysis.

>
> I've already read through the coin proposal 
> <http://wikis.sun.com/display/mlvm/InterfaceDynamic> and 
> InterfaceDynamic 
> <http://wikis.sun.com/display/mlvm/InterfaceDynamic>. My understanding 
> is that there is no language support at all for JDK 7 (correct?), but 
> I've been playing around with the jdk7 compiler which does support the 
> InvokeDynamic static call syntax. Unlike C#'s dynamic, it seems like 
> there are a lot of barriers to making the above syntax work with (the 
> proposed) Java language support.

That's correct there is no equivalent of dynamic in Java.
Not because it's useless but because it's hard to do it right.

> Examples I ran into with a couple of hours of playing:
>
> 1) You have to apply @BootstrapMethod to every class.
> 2) InvokeDynamic invocations are static.
> 3) InvokeDynamic calls all throw Throwable.

latest spec doesn't use @BootstrapMethod anymore.
2) and 3) are still valid.

>
> These are all deal breakers when it comes to users using InvokeDynamic 
> directly. Essentially, the current picture I'm seeing is that the 
> planned language support isn't useful at all in API, as compared to 
> implementation for alternate language runtimes. I realize that's the 
> primary usecase, but it feels like we're missing out on a lot that 
> invokedynamic could do for us. Are there any plans to provide anything 
> in that direction?

Not now.
As I said, dynamic is useless for what you want to do.

Rémi
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