JRuby Serial Interview 3

Posted by Nick Sieger Thu, 18 Jan 2007 03:59:31 GMT

This is part 3 in our ongoing conversation tracking the development of JRuby.

Official Rails support in February? That’s not far away! What do you mean by “official”?

Thomas Enebo: Largely, we just want to spend some extra TLC on fixing up various Rails issues between now and February. Basically, get rid of the remaining known issues with running Rails from JRuby. I think beyond marshalling we are very close to saying that today. We also want to provide a better deployment picture for Rails by then. So we will need to spend some time on that as well (the community has been doing a great job spearheading this).

Software is never perfect, so we know that there will continue to be Rails issues after we say it is supported. By setting this goal, we should give ourselves some pressure to polish what we have and also get a larger number of people some incentive to kick the tires.

Ola Bini: That’s a question of interpretation. In my view, “official” is some kind of high number. 95% of all test cases in 1.1.6, maybe? But the more important part of it is that all the common use cases should work. You should be able to work through AWDwR and everything should work.

It’s ambitious, but we can do it. What’s needed soon is to decide what needs to be done with ActiveRecord-JDBC, and do that, since AR-JDBC is one of the larger points in our Rails support, and sometimes I feel that the support there is our weak link.

Charles Nutter: We could probably say we support Rails today, with a whole list of caveats. Rails runs right now, people are using JRuby on Rails today (in some cases for production apps!), and things largely will “just work”. There’s also a lot of community effort behind alternative deployment scenarios like within a WAR file or behind a fast HTTP server like Grizzly. Rails does run on JRuby today.

Our challenge before making a big official announcement about “Rails support” is to shrink that list of caveats down as small as possible. We want marshalling to work so sessions function correctly. We want AR-JDBC to be cleaned up a bit more, with more testing and even wider database support. We want any remaining core library issues resolved. We want those peripheral deployment projects to work perfectly. There’s a lot of work to get there, but it’s now simply an incremental process; Rails runs today, and will run better tomorrow.

What’s this about YARV instruction interoperability?

Readers: here’s an IRC recap for you:

[1:31pm] olabini: HAHA
[1:31pm] olabini: YEAH BABY.
[1:31pm] olabini: hehe.
[1:31pm] olabini: echo 'puts "Hello world"' > test1.rb
[1:33pm] olabini: ruby-yarv ~/src/yarv/tool/compile.rb -o test1.rbc test1.rb
[1:33pm] olabini: jruby -y test1.rbc #=> "Hello world"
[1:33pm] headius: hah, awesome 
[1:33pm] nicksieger: no way!

So this is awesome that you guys are able to track so closely with YARV’s progress, but why do it? Hedge your bets?

Ola Bini: First of all, it shows the maturity of the JRuby runtime that we can implement basic parts of the YARV VM this easily. Second, it can be very interesting for us to try out parts of how 1.9/2.0 will work out within the current JRuby system. Third, we don’t have a YARV compiler yet, but being able to run files compiled with YARV ensures that we stay on track for Ruby compatibility. Fourth: Yeah, hedging your bets is always a good idea. Diversity breeds evolution. I believe where on the right track with the current AOT and JIT compiler works, but there is always a good idea to implement these things in more than one way. And of course, it ‘s just fun!

The next step for this will be to get the loading to handle more things. At the moment, it only runs extremely simple scripts. But I’m planning on handling the more complex things soon too, adding support for labels and defining YARV methods and other such things. The very next step to handle is a compiled recursive fibonacci script. The YARVMachine already has most things needed for it, but the YARV emitted by the compiler contains some tricks that needs to be fixed.

Charles Nutter: I’ve been the primary person responsible for our various interpreter rewrites over the past year or two. Originally I modified it to be mostly “stackless”, using an Instruction object for each element in the AST and pushing down an external stack of previous instructions to maintain context. That seemed like a neat idea, and it certainly did move toward a stackless design (I actually demoed a recursive fib(100_000) at RubyConf 2005), but it was rather slow and complicated enough that only I could maintain it. So in October of 2006 I did another rewrite, basically simplifying the interpreter down to a “big switch statement” that could quickly traverse the AST without a lot of objects and stack manipulation. This new C-like interpreter engine was quite a bit faster, but unfortunately the tradeoff was that we were again burning Java stack frames when we had to dig deeper into the AST, and our maximum stack depth suffered.

I’ve still always wanted the stackless design back in JRuby, and started to think about alternate routes to get there. The most obvious was having our own bytecode engine. The most readily-available set of bytecodes for Ruby...was YARV’s.

Implementing a stack machine is pretty trivial. You need an operand stack, instructions for manipulating it, and instructions that consume values from it. Over the past year, YARV’s core set of bytecodes have started to solidify to the point that I figured implementing a YARV machine in JRuby would be a good idea. So I did a very simple initial implementation that just handled local variables, method calls, while loops, and so on, to see if it would be feasible. I got it as far as running an iterative fib, and the results were very promising: it was quite a bit faster than the current interpreter.

I ended up putting that down for a while to look at Java bytecode compilation, which has been coming along very nicely. Recently, Ola decided to pick up the YARV machine work, and made it double cool by loading real compiled YARV bytecodes into it. And if that wasn’t good enough, the original partial machine I’d implemented was able to run them without modification!

We’re looking toward the future with this work. We know that YARV is now “The” Ruby VM, and that eventually people will start to run compiled Ruby bytecodes. We also know that JRuby will never be able to fully escape interpreted-mode execution. I believe both goals are answered by having a fast Ruby bytecode machine that works in concert with our Ruby-to-Java compilers. And that’s the current roadmap for JRuby’s execution model.

Thomas Enebo: Personally, I would like to see us move from walking our current tree to walking a set of instructions. I think dynamic optimizations (as well as static) will be much easier using instructions and we can also create a simpler AST/parser (the traditional AST in Ruby does quite a bit of static optimization which I think makes the grammar a little tougher to wrap your head around).

Anything YARV-related sort of hits my sweet spot since it may nudge us in this direction. JRuby will probably always mix execution between compiled and interpreted code. I think interpretation will be easier to support at an instruction level. It will also give us a good opportunity to flatten the Java stack more. YARV is a good place to start. It may be right solution for us too. Who knows? I think experimentation is the name of the game for this stuff. So I love to see it happening :)

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Self-cloning JRuby and RubyGems in a Jar

Posted by Nick Sieger Fri, 12 Jan 2007 04:27:07 GMT

As part of some work I’m doing to make JRuby more portable and easier to run standalone without all the $JRUBY_HOME launcher scripts, it’s now easier than ever to get up and running with JRuby, or to launch in your build scripts or IDE in a platform-neutral way. (Note: as of this writing, this feature is in 0.9.3 jruby-complete snapshots older than 2007/01/11 only.)

Here, give it a try:

IRB in a jar

$ curl -o jruby-complete.jar http://snapshots.repository.codehaus.org/org/jruby/jruby-complete/0.9.3-SNAPSHOT/jruby-complete-0.9.3-20070112.032908-4.jar
$ java -jar jruby-complete.jar --command irb

RubyGems in a jar

RubyGems needs a place to unpack and run gems, so JRuby currently will hide all that away from you in ~/.jruby. (Also, unfortunately we exceed Java’s default memory size when downloading the RubyGems index file, so you’ll have to add the -Xmx256m argument for now to avoid an out of memory condition.)

$ java -Xmx256m -jar jruby-complete.jar --command gem install tattle -y --no-rdoc --no-ri
creating /Users/nicksieger/.jruby/bin/gem
... more files extracted ...
copying /Users/nicksieger/jruby-complete.jar to /Users/nicksieger/.jruby/lib
Bulk updating Gem source index for: http://gems.rubyforge.org
Successfully installed tattle-1.0.1
Successfully installed hoe-1.1.7
Successfully installed rubyforge-0.4.0
Successfully installed rake-0.7.1

You can still have JRuby unpack to a shared directory if you like, and use the regular shell scripts for launching JRuby. In this case, JRuby is actually replicating itself into the directory you choose. Simply add the bin subdirectory to your $PATH, and continue to use JRuby just as you would a regular Ruby installation.

$ sudo java -jar jruby-complete.jar --command extract /opt/local/jruby
creating /opt/local/jruby/bin/gem
... more files extracted ...
copying /Users/nicksieger/jruby-complete.jar to /opt/local/jruby/lib
$ PATH=/opt/local/jruby/bin:$PATH
$ which gem
$ jirb


The --command argument is not limited to just gem and irb. Once you’ve installed any gem that has an accompanying executable script, you can simply pass that argument as the --command:

$ java -jar jruby-complete.jar --command tattle report
ruby_install_name, jruby
LIBRUBY, jruby
target, java
arch, java
host_vendor, Apple Computer, Inc.
key, b1bd5eaf4254d9874ca297995b906be6f4975d395dd0136432f859c62a33cc8c
host_os, Mac OS X
ruby_version, 1.8.5
build, java
target_cpu, i386
prefix, /Users/nicksieger/.jruby
report_time, Thu Jan 11 22:21:57 CST 2007
rubygems_version, 0.9.0
host_cpu, i386
SHELL, /bin/sh
$ java -jar jruby-complete.jar --command tattle
Posting information to Tattle server.  Thanks!

And lo and behold, there’s a ruby_install_name of java at the new Gem Tattle homepage!

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JRuby Serial Interview 2

Posted by Nick Sieger Thu, 11 Jan 2007 04:03:52 GMT

This is part 2 in our ongoing conversation tracking the development of JRuby.

It’s been exciting to see all the discussion between the JRuby developers and Rubinius developers, especially in #rubinius. What benefits do you see coming from this kind of cooperation?

Charles Nutter: Evan and I have been talking since this summer, actually, when he was working on early versions of Rubinius. We both have gone through many of the same growing pains dealing with Ruby’s quirkier features and evaluating interpreter/VM design options. I don’t know how Evan feels about it, but I was very glad to find someone else who was interested in such things.

Now that Rubinius is in the public eye and has some real momentum, there’s more sharing going on. We’ve been talking about those same design options, weighing them together and coming up with new choices we can both implement. Others on the Rubinius team have forwarded the idea of reusing portions of the Rubinius source in JRuby.

It’s also become apparent that we’re trying to solve very similar problems from different directions. In JRuby’s case, we already have a fully-functional Ruby interpreter, functional enough to run apps as complicated as Rails. Our challenge is to keep JRuby running well and evolve a working interpreter toward a more future proof, performant, and maintainable design. Rubinius is really just starting out, only to the point of running a small portion of the Ruby corpus, but the design is easier to follow and simpler to evolve. Their challenge is to keep the design simple while expanding compatibility and improving speed. JRuby is mostly Java with some Ruby code, though we’d like to make it more Ruby code in the future. Rubinius is obviously mostly Ruby code with some C, but they’re interested in being able to migrate it to other underlying languages. We’re both interested in a Java-based Rubinius.

I think it’s been very positive for everyone involved to have this level of cooperation, and it’s helped us all understand Ruby better.

Ola Bini: Well, the exchange with Rubinius is obviously very valuable. The more people working on implementations and sharing information, the better for all the implementations, of course. I also see Rubinius as something very intriguing, and it would be very interesting to see how much we could port to JRuby.

Thomas Enebo: I have not personally been in contact with Rubinius developers (Charles has though), but I can say more generally that many implementations will necessitate some level of cooperation. As we discover differences between implementations, we need dialogue to help understand why those differences exist. It is possible this dialogue will end up identifying poorly-identified cross-platform issues or general mis-features.

At an implementation level it will yield suggestions across the fence for how to do things differently. A thread in ruby-core a month or so ago had some exchanges on how MRI could be optimized. Charles and I chimed in about some of those ideas because we had considered and implemented some of them in JRuby. I think as time goes on, this exchange of ideas will increase.

The biggest news in Ruby land recently is probably the announcement of a fully merged YARV. What affect does this have on JRuby?

Thomas Enebo: This seems like great news for Ruby, but I am not sure it has much affect currently on JRuby. We are still focused on 1.8.x support. It is getting easier for us to change language semantics now and when 1.9/2 starts getting closer to release I feel comfortable that we can spin a Ruby 2 branch pretty quickly.

From a personal standpoint, it is great to see Ruby hit this next milestone. I think the perception of progress is pretty important and merging YARV will give Ruby 2 development a nice perceptual boost. Also this will mean many more people pounding on YARV, which will help run it through its paces better.

Ola Bini: YARV is important news. Very much so. But at the moment the effects will not be that noticeable. Right now we’re still working hard to get 1.8-compatibility complete. But, Charles have begun work on a YARVMachine that runs some basic scripts (including the famous iterative fib bench). I’ve started looking on this the last few days, and have some ideas. My first priority will probably be to implement a reader for YARV bytecode. This will make it easier to test our machine, since we can compile with YARV and then run the compiled files with JRuby. Alongside with that I am going to start tinkering on a new backend to Charles current compiler, so it will emit YARV bytecode instead. I’m not sure exactly when this is going to happen, though, but we try to stay on top of YARV.

Charles Nutter: The merging of YARV (no longer “yet another” Ruby VM but instead “the” Ruby VM) is a very big event for the Ruby world. Koichi has worked long and hard on it, and I’m very glad to see it’s now officially part of Ruby core. I had some time to talk with Koichi and Matz about implementation challenges at RubyConf 2006, and we came to agreement on a number of items, most prominently that critical= needs to go away. Again, more cross-project sharing.

We’re watching the newly-reset 2.0 design process closely, since we know it will eventually affect JRuby’s future. In the interim, however, we’re trying to solve at the 1.8 level many issues YARV is designed to solve in 1.9 and 2.0. So many of the design choices made by Koichi and Matz for 1.9 play directly into how we tackle those same decisions in JRuby.

I think in general the merging of YARV shows that Ruby is moving forward and evolving on all fronts.

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JRuby Serial Interview 1

Posted by Nick Sieger Sat, 06 Jan 2007 18:21:56 GMT

In cooperation with Pat Eyler, we present this conversation as a parallel thread to his recent string of Rubinius “serial” (ongoing) interviews. We aim to bring short, frequent, looks at the two alternate Ruby implementations’ developments, and have the conversations intersect from time to time.

A lot has happened since the last JRuby interview with Pat -- Java was open-sourced, you’ve been to Javapolis, pushed out another release, and a slew of new contributions have poured in. How have your plans and goals for JRuby changed (if at all) since then?

Charles Nutter: For me the biggest items are the following, in order:

  • We need to announce full Rails support as soon as possible
  • We need to resolve the remaining runtime performance bottlenecks
  • We need to keep working on the compiler

All these of these have been the hot topics on the JRuby mailing lists lately. I’ve launched into a newly-refactored compiler that’s showing great performance gains. Many of us have discussed how to speed method dispatch and finally push interpreted-mode performance up to or beyond Ruby’s speed. And we’ve started to test and track Rails 1.2, while continuing to resolve remaining issues running Rails 1.1.6.

Now there’s a lot of work to do, but we’ve seen steady, continuous growth among JRuby contributors. Just in the past week we’ve had a number of new names on the mailing lists, we’ve added a new committer (Nick Sieger), and we’ve seen patches pouring in for more and more bugs. Things are going great.

Ola Bini: I was very happy about the last release. It fulfilled the goals I had set for it, which was to get OpenSSL and complete Java-backed YAML into it. Besides that, we also got seriously many bugs fixed in just a few months. Since I have a tendency to plan mostly for the next release, what I want to see in 0.9.3 is support for Sandbox, all those strange block scope bugs that surface in Rails gone, our load times improved (by refactoring the LoadService code), and finalizers finally working.

The Sandbox stuff is mostly done, and I also hacked a Generator that is so much faster than MRI that we come out faster, all in all, in a test case with generators.

Thomas Enebo: Largely, I think three goals are important: 1. Support Rails well enough where people do not need to ask us if it is ready for prime time; 2. Round out java integration support to do what most people expect it to do; 3. Make the runtime ‘fast’ enough. These three goals existed before JavaPolis, so I do not feel much has changed goal-wise.

How much closer have you come to achieving them? What is your perception of delta in growth in the JRuby community and acceptance of JRuby as a viable alternative to MRI?

Ola Bini: What I like about the last few months - since RubyConf - is that it really feels like JRuby and the other implementations will actually be viable alternatives and that there is something really useful going on. With Java open source, one of the major roadblocks for adoption in certain circumstances has all but disappeared. The contributions we have gotten is mostly visible in how many bug reports we get. That is really great, because that makes it that much easier to fix things. So I would say that the future looks brighter than ever.

Thomas Enebo: If you look at the amount of time between releases then you get a better idea of how much development has sped up. The time between 0.9.1 and 0.9.2 was a little under two months. This is the shortest development cycle to date and it seemed like we got so much done. Sun hiring us obviously had something to do with this, but also our community involvement is at an all time high. We get so many emails, bug reports, patches, tests, ideas, and enthusiasm coming in from the JRuby community. The community impact on JRuby is huge.

I think acceptance of JRuby as a Ruby interpreter is certain. Compatibility keeps improving, we keep getting faster, and we also offer integration with Java. If you look at the trend of how often JRuby is mentioned in blogs or the volume of email on our mailing lists, then I think you can get a picture as to whether people are willing to accept JRuby as a Ruby runtime.

Charles Nutter: Rails is probably the most visible measure of success for JRuby right now, and I’d say we’re able to run something like 75% of Rails 1.1.6 code and test cases. We’ve been using Rails’ own test suite as a yardstick for compatibility, with the idea that if we can run all the Rails test cases, we can say we support it. And that 75% is better than it might sound, since it’s the most heavily-used functions.

Now this might change, but we’re really hoping to claim full Rails support some time in February. We’re not sure if that will mean 90% of 1.1.6 test cases or 100% of 1.2 test cases, but we’re weighing options now. Finally having Rails support behind us will let us change focus toward outward to other applications and inward to improving JRuby internals and performance.

We’re also seeing daily gains in the performance area, with more to come. Since this past summer, we’ve managed to eliminate all the major performance bottlenecks seen when profiling. The only remaining area is the interpreter itself. My work on the compiler will eventually resolve that, and our work to improve the performance of method lookup and dispatch will help both interpreted and compiled execution. Everything’s moving very fast now. It’s going to be a great Spring for JRuby.

Probably the most intriguing change of the past month is the support from Aslak Hellesøy, creator of RSpec. Aslak has helped us get JRuby running RSpec extremely well, and we’re looking forward to the RSpec team using JRuby as part of their regression testing. I hope more Ruby app developers will take this same path, since their users are going be running JRuby more and more. Compatibility and regression testing for those apps should include JRuby just like it includes different Ruby versions and host operating systems.

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RSpec, JRuby, Mocking, and Multiple Interfaces

Posted by Nick Sieger Fri, 01 Dec 2006 18:56:00 GMT

The prospect of doing behavior-driven development in Java has just taken a step closer with the news of RSpec running on JRuby. This is already a big step that will have an impact on Ruby and Java programmers alike in a number of ways.

However, it could be even better. RSpec has a nice, intuitive mocking API, which will unfortunately, at the present time, be useless when working with java objects. It would be awesome to try to get it to work, though. Some possibilities:

  1. Map to JMock and use JMock under the hood. Not a very attractive option for a number of reasons, but mainly because add-on bridging layers are complex and should be avoided.
  2. Improve ability for JRuby to implement any number of Java interfaces dynamically.

This second option is something Charlie, Tom and I talked about on Tuesday night, that could have a much broader impact on Java integration in JRuby.

Consider this spec. It’s trivial, but bear with me.

context "A TaskRunner" do
  setup do
    @task = mock("Runnable")
    @task_runner = TaskRunner.new(@task)

  specify "runs a task when executed" do

This spec might be satisfied by the following Java code:

public class TaskRunner {
  private Runnable task;
  public TaskRunner(Runnable r) {
    this.task = r;

  public void execute() {

Notice how I defined the @task in the spec above. This is the normal way of mocking in RSpec, and the example illustrates how I think JRuby should handle interfaces in Java: by duck-typing them.

Basically, the RSpec mock should act like a Java Runnable because I’ve defined a run method on it (in this case implicitly with @task.should_receive(:run)). JRuby could wrap a dynamic invocation proxy around any Ruby object just before passing it into a Java method invocation. Without doing any type- or method-checking up front. Just define the proxy as implementing the interface required by the Java method signature, and let the JRuby runtime do its thing, and attempt to resolve methods as they’re invoked. Possibly falling back to method_missing, even!

Note that this would also make moot the multiple interface syntax discussion, because you’d never have to declare an object in JRuby as implementing any particular interface. Just define the appropriately named methods with the proper arity, and you’re done. Maybe you don’t even need to declare all of them, if they never get called for your usage! This is the Ruby Way, and would be a completely natural extension to the way Java objects are manipulated in JRuby today, not to mention extremely concise and powerful.

This would also allow RSpec mocking to just work, at least for Java interface types, which would be way cool.

Charlie has a Swing demo that he frequently gives when talking about JRuby. Under the new proposal, it would look more like this:

require 'java'
frame = javax.swing.JFrame.new("Hello")

button = javax.swing.JButton.new("OK!")

def actionPerformed(event)
  event.source.text = "Pressed!"
button.addActionListener self

With luck, this approach will be coming to JRuby very soon.

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Embedding a Complete JRuby Interpreter

Posted by Nick Sieger Tue, 28 Nov 2006 04:24:00 GMT

The other day I was setting up a demo installation of FishEye at work. I don’t have root on the box where it’s running, so none of the built-in authentication schemes were acceptable. Hey, there is already user and password information stored in the CVSROOT directory of the repository (we use the pserver access method). But, ughh, I’d have to read and parse files in Java, and grab a public domain crypt implementation since it’s not included in the Java standard library. Certainly do-able, but not my idea of fun.

So why not use Ruby for this? Painless IO and text-munging? Check. Crypt? Check. In Java? Since Charlie’s Ruby-in-a-Jar, this one is a slam-dunk!

For the implementation, have a look at CVSPasswdFisheyeAuthenticator.java and authenticator.rb. The basic idea is to use the Java class as the embedding wrapper and leave all the main logic in Ruby. The Java wrapper is still a little heavy, since the actual interface is implemented by the Java class, and Ruby is invoked by evaluating a statement inside each interface method. Some notable elements:

  • CVSPasswdFisheyeAuthenticator#init: The interpreter is initialized here. I pass authenticator properties to the interpreter by way of globals here -- not the cleanest, but easy. The Ruby code is loaded as a classpath resource, a nice feature of JRuby’s require that allows Ruby scripts to be jarred up with the rest of your code.
  • CVSPasswdFisheyeAuthenticator#rubyEval: This is your boilerplate-invoking-the-interpreter code here.
  • CVS::Authenticator#initialize: Reading and parsing of the CVS passwd and users files is done here in five succinct lines, much better than you could do with Java.
  • CVS::AuthToken: Here I’m implementing a Java interface in Ruby, and passing it back to Java! (See the CVS::Authenticator#recreate_auth method.) This is probably the nicest single aspect of JRuby’s Java integration, and it will only get better.

What would be nice for a future implementation is to make the Java class a factory and a delegate, and let JRuby implement the entire interface itself directly. This would avoid some ugly boundary-crossing issues (see the hand-crafted Ruby strings passed into the intepreter and rubyToJava conversions on the way out). As soon as JRuby gets better support for multiple interfaces and extending abstract classes, this sort of pattern could probably be templated rather easily.

If you want to build the whole thing, check out the source with Subversion and follow the README file for instructions.

For another example of this kind of embedding of JRuby in the wild, see the JRuby support in NanoContainer.

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JRuby and Sun

Posted by Nick Sieger Fri, 08 Sep 2006 15:11:00 GMT

The noise is deafening by now, but I’m feeling the desire to chime in publicly with my congratulations and support for Charlie and Tom. JRuby has come a long way in the past six months, and this is strong validation of that fact.

As I hinted back in May, this is getting big, and it’s been a pleasure to have been on the JRuby train! The future is bright for Ruby and Java the platform, and JRuby is leading the way.

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Haikus for thought

Posted by Nick Sieger Thu, 18 May 2006 02:48:00 GMT

Open source Java.
Who could possibly fork it
What? Not IBM!

Open source Java
Rather be hacking Ruby
Too late to matter

Open source Java
Need dynamic languages
Use JRuby now

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JRuby on Rails and ActiveRecord on JDBC

Posted by Nick Sieger Mon, 15 May 2006 03:12:00 GMT

Tom and Charlie have just experienced what can only be described as a watershed moment in the grand scheme of dynamic languages on the JVM. The Rails experience may soon be visiting a Java application server near you! Even though JRuby will be in perpetual catch-up mode with C Ruby, Tom and Charlie and the rest of the JRuby contributors have shown incredible perseverance in tracking the Ruby language despite the lack of any formal specification. Maybe Rails will never be mainstream, but the possibilities just got a whole lot more interesting. I agree with Obie that this could be a game-changer.

Now, a few comments about the ActiveRecord JDBC adapter. This code can still be considered alpha quality at best. It’s awesome that Tom and Charlie will be able to demo a top-to-bottom, working Rails app on JRuby, but don’t jump to conclusions yet that this will be anything like a write-once, deploy-to-any-database kind of experience. But you didn’t think it would, did you? We all learned that about Java a long time ago, right?

You can check out the code here. At the moment, I’ve only tried it with MySQL. Most of the problems with it come from the lossy mapping from ActiveRecord’s abstraction of the database to JDBC’s. (Although I suspect as more JDBC drivers are tried that there will eventually be compatibility issues with different implementations of the JDBC spec.)

Probably the thorniest issue is the one of type conversion. ActiveRecord has a fairly simple notion of types: :string, :text, :integer, :float, :datetime etc. Compare this to JDBC’s. What a mess! Right now there are arrays of procs for each AR type that try to guess the best JDBC type to use. This will certainly need improvement to become a more robust solution.

Probably the most promising approach may be to create a patch that refactors much of the AR adapters’ type conversion methods into separate modules that could be included into instances of the JDBC adapter depending on the underlying database. Then the JDBC types wouldn’t be needed at all -- the existing AR database metadata could be reused. Presumably this would require some petitioning of the Rails core team to accept the changes even though the changes don’t buy AR itself any additional flexibility.

If you have a chance to try out the code or can think of any additional tricks that would help the ActiveRecord JDBC implementation along, let me know!

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