Subject: Re: [Bitcoin-development] Introducing BitcoinKit.framework

From: Mike Hearn <[email-masked]>
Date: Mon, 15 Jul 2013 17:48:41 +0200

Oracle provide an OSX JVM and will do so for the forseeable future, it's
also open source, so the community could carry on if they stopped. The
primary problem with the Oracle JVM is lack of retina support for Swing,
but if you'd write a Cocoa UI yourself then it doesn't matter of course as
Java won't handle any GUI stuff. Retina support for JavaFX2 (the
current-gen gui toolkit) is available in Java 8 so it's definitely being
actively developed, it's not abandoned or anything.

So the question then becomes, which is better:

a) Take bitcoinj completely out of the Java world via native compilation or
transpilation to C++
b) Embed the JVM and link the two worlds together?

(b) is much less ambitious, especially if you're OK with writing a bit of
Java code to keep the interface thin. Basically the Java side calls into
your app when interesting user-visible things happen, like new transactions
appearing, then your GUI can call into the java side to send money. There
are auto-translators that make the glue work easy, like
https://code.google.com/p/javacpp/. You probably wouldn't want to expose
the entire bitcoinj API that way because it's very large, but the code
needed to bring up a wallet app is very small. I knocked one up this
weekend in about one evenings worth of coding, completed with nice
animations. The interfaces you'd need are basically some Objective-C++
methods that receive information from the Bitcoin side, like the balance
having changed, a list of transactions, etc, and then a callback into the
Java side to send money. If you look at the javacpp site you can see
example code for making calls both ways.

If I were in your shoes, I'd go for (b) because it is the most well trodden
path and will let you achieve the best user visible results quickly. The
JVM can be bundled with your app and stripped down if you're worried about
download size.

If it's unclear how the code would look, let me know and I'll try and knock
up a really simple prototype.

There's also (a). I'm investigating transpilation for a few reasons, one of
which is to do with a private project. I'm working with the author of j2c:
https://code.google.com/a/eclipselabs.org/p/j2c/. It's a rather
sophisticated transpiler that converts Java to clean, readable C++11 that
looks much like code a human would write. It's complete enough to transpile
the entire standard Java class library, including all the GUI toolkits and
other things - so, pretty amazing piece of code. However it's incomplete
because where the Java code calls native methods (that would be provided by
the JVM) it just spits out stubs you're expected to fill out yourself, for
starting threads and so on. As there's no JVM it's just like using a C++
library that is missing a "portability layer".

I'm working on this myself and don't really need much help at the moment,
I'm just making steady progress towards getting something up and running. I
can let you know once I reach some interesting milestones. The point of
this is that whilst you don't need access to most of the API to write a
wallet app, I'd like to make every kind of app easy from C++, not just GUI
wallets. Then the compile-to-C++ approach is much more appealing, even
though it's also more work.

On Mon, Jul 15, 2013 at 4:39 PM, Wendell <[email-masked]> wrote:

> Hi Mike,
>
> You are absolutely right about the synchronize time, it's one of our main
> frustration points right now and we clearly won't deliver the kind of user
> experience we want, without fixing this. Actually we were thinking of
> extending Jeff Garzik's picocoin as time permits, but the plan is far from
> concrete at the moment.
>
> What you say about trans-piling bitcoinj is _really_ appealing. We
> discounted Java simply because an OS X JVM is no longer guaranteed, but
> otherwise bitcoinj is ideal for our purposes. How can we assist or
> otherwise accelerate such an effort?
>
> -w
>
> On Jul 15, 2013, at 3:19 PM, Mike Hearn wrote:
>
> > That's great! I'm all for more wallets, especially user friendly UIs.
> >
> > However being based on bitcoind means it will take a very long time to
> synchronize for new users. We know a lot of users drop out. The best fix
> for this is SPV mode. Do you have any plans in this direction?
> >
> > So far, the only SPV mode implementation I know about is bitcoinj. I am
> experimenting with trans-piling bitcoinj to C++ to make it usable from
> Objective-C++ exactly with your use case in mind.
>
>