Creating an Interactive JRuby Console for the Eclipse Environment
Musings on Language and Design
Creating an Interactive JRuby Console for the Eclipse Environment
by Jeremy Meyer
April 4, 2008
Summary
Excited by the possibilities afforded by JRuby and the ability to plug it into various different Java environments, I decided to try implementing a JRuby interactive console in Eclipse.
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Plugging in JRuby
Like so many others my
imagination has been captured by Ruby. Perhaps it is because of the steep rise
in its awareness the software development community has undergone since Rails,
perhaps it is a combination of its quirky constructs, enthusiastic (and often
surreal) proponents, and the fact that DSLs are
gaining respect and momentum. At the
same time, the JVM is becoming an obvious platform choice and Java itself is
starting to look a little tired and lost as a language.
Perhaps Java is reaching its
sell-by date, after all, there do seem to be natural universal laws about these things; and
they apply to a great many domains. Fashion, empires and programming languages
are three that I can think of which adhere to these rules. They rise, dominate
and fall. Sometimes they come back, (like flared trousers and interpreters) but
even if they die, they always leave their legacy, like the Roman
Empire, or Cobol.
Anyway, it is obvious that however long it stays in vogue, one such mark that will be left by Java is the JVM as a platform, and fascinated as I am by Ruby, I think that JRuby is
(together with Rails) what will really keep it on the map. I find myself more
and more regularly explaining to colleagues and clients (partly to prevent them
rolling their eyes as one does when faced with a religious fanatic, or a pushy
salesperson) that I only really love Ruby and crowbar it into every discussion
I can, because compiled languages like
Java exist. JRuby makes using Ruby sensible (and
cool).
Why is this cool? Because
like so many consultants, it is my job it is to help people come up with general,
repeatable solutions to their problems, so I am always on the lookout for some
sort of lazy reuse idea. Since all clients want those three very simple and clearly
mutually exclusive features that constantly haunt us, i.e. good, fast and
cheap, I find myself drawn to the model of leveraging powerful application
libraries with glue code or something minimal, scriptable and clever.
JRuby fits that bill quite nicely.
No Silver Bullet
I wouldn’t propose anything
as a silver bullet, but I have seen so many projects fail for the same reasons, that
I am tempted by anything that can cut out some of the badness.
This means doing all sorts of things, ranging from trying to improve communication of requirements down to proposing
better techniques of project management (so agile methodologies are always a
popular choice). But anything that helps to cut out as many middle-men as
possible, and allows the people with more domain knowledge to get closer to the engineering of the solutions has to be a good thing.
Or does it? Isn’t this falling into the old trap of trying getting non-programmers to write programs? Haven’t previous attempts at this
failed? Languages like BPEL and Visual
Basic and even (looking back a bit further) Cobol have
all, in their own way, in their own domains tried to make programming easy.
These have all ended up producing horrible languages. In the case of the latter
two they have also produced code bases huge and prevalent by virtue of their
easy proliferation, not their suitability for solving the problem. Not popular
with OO purists and lovers of aesthetically pleasing languages!
Well now we have more languages
like JRuby(and Jython and
Groovy etc.) which allow us pleasing constructs, great power and efficient
syntax together with access to Java libraries and access to any platform which
has a JVM implementation. More importantly, perhaps, is that this translates into access to any Java middleware. Excellent! This means that
the powerful application stuff can be written by the software engineers and the
business logic can be written (at least initially) by the domain experts, using
a domain specific language (or domain specific flavour, if language isn't subtle enough).
This should mean that we can give an expert in the domain of feet enough power to shoot
themselves in the foot with a homing missile, and they wouldn't have to be a rocket scientist.
What is doubly pleasing of course, is the fact that Ruby by its nature is dynamic (as are some of the other languages which have JVM
implementations), and building a domain specific
language to solve a problem can be implemented quite elegantly. So helping
prevent our domain experts from getting in too much trouble is a bit easier. We can expose the important bits to them and shield the complexity from them. Very importantly though, they will always have the full power of
the language at their finger tips should they need it.
Triply pleasing perhaps, is
the fact that you can do all of this in an interactive shell (jirb in JRuby) if you want to, which is a great and agile way to
wire up existing domain libraries, or produce "glue code".
You can embark on a learning adventure with a
framework, or library and produce a solution that can form the basis of
something permanent. For example, you
can experiment with the creation of swing applications using
JRuby from the interactive shell. This is so easy that even
a sock puppet could do it (and did, see
here!).
Scripting in an Eclipse Environment
I got to thinking that one
of the environments it would be great to play with was the Eclipse
environment. It is
a mature, fairly solid platform, with a sound plugin model
and some very powerful development tools available in it. What would be great,
I thought, would be if you could script in it, or create macros, even. Creating
an interactive shell would give you access to any plugins
you liked. Certainly it seemed it would be a worthwhile effort plugging in an
interactive shell to see what would happen. As a self-confessed
non-expert-but-fascinated wannabe Ruby-ist, I thought
I could kill two birds with one (precious) stone, i.e. learn more about the language
and learn more about the Eclipse platform at the same time.
How did I do this? Some of
the highlights follow below. If you
really can’t bear the thought of looking through code, or want to see
everything in its entirety, then download this zip which has the Eclipse
plugin and the full source included:
The obvious place to start looking
at how to do this was jirb,
the interactive Ruby shell written for JRuby. Of
course, I thought, this would be a Java console that read in a line of text and
passed it to the Ruby interpreter for evaluation, so it couldn’t be too hard to
re-implement this in an Eclipse app.
Turns out I was slightly wrong. The jirb command, it seems, is just a batch file that runs
JRuby and points it to irb. So irb, the interactive Ruby shell,
is just a Ruby program and jirb ,
the interactive JRuby
shell is just irb
running in JRuby.
The Ruby Code
After some head scratching,
I realised that this was very cunning and would actually help make my life
easier. I did some investigation into the irb code, and with help from a good
(albeit old) article
by Leonard Richardson about unit testing the Ruby Cookbook source code, I
discovered that redirecting irb input and output is fairly straightforward. Turns out that
all you need to do is extend the Ruby Irb class and
get it to use our own input and output streams. The Irb
class implements the strategy pattern to read from its input stream using an
InputMethod. Creating an Input Method is as simple as
providing a Ruby class which has a gets operation and a prompt= setter method. The
prompt= setter method is necessary, because irb will throw exceptions
without it, (although I admit I am stumped as to why it is there, it doesn’t
seem to do anything other than pass in an empty string.)
I needed to customise my prompt,
and get the line of text typed, so:
class EclipseConsoleInputMethod
# echo the prompt and get a line of input.
def gets
$stdout.print 'eIrb:> '
$stdin.gets
end
def prompt=(x)
end
end
And now, I could minimally extend
the Ruby Irb class to give me a custom
Irb class, which has the right context and some useful
configurations:
class EclipseConsoleIrb < IRB::Irb
def initialize(ec_inputmethod)
IRB.setup(__FILE__)
IRB.conf[:VERBOSE] = false
super(nil, ec_inputmethod)
end
def run
IRB.conf[:MAIN_CONTEXT] = self.context
eval_input
end
end
Not too hard at all!
Now what I needed to do was get an Eclipse
console to provide said input and output streams and pass the typed Ruby code into
the former and echo the results into the latter.
The Java Code
Eclipse’s IO Console does that
job very well, it displays in the normal Eclipse Console area, provides and
input stream for you, and can have multiple output streams directed to it, perfect.
(You can even set colours for the different streams!)
I created a simple Eclipse
Plugin application using the basic Eclipse "new project" wizard.
All I had to do was create a subclass of an IOConsole
that had an instance of the RubyInterpreter class in
it ..
Highlights below:
import org.jruby.Ruby;
public class RubyConsole extends IOConsole implements Runnable {
public void run() {
..
RubyInstanceConfig conf = new RubyInstanceConfig() {
public InputStream getInput() {return in;}
public PrintStream getOutput() {return out;}
public PrintStream getError() {return err;}
..
}
..
try {
Ruby rubyRuntime = Ruby.newInstance(conf);
String jRubyHome = System.getProperty("jruby.home");
String jRubyVersion = System.getProperty("jruby.version");
rubyRuntime.evalScriptlet("$:.insert(0,"+jRubyHome+"\\lib\\ruby\\"+jRubyVersion+"')");
rubyRuntime.evalScriptlet("$:.insert(0,'"+jRubyHome+"\\lib')");
rubyRuntime.evalScriptlet("require 'jruby';");
rubyRuntime.evalScriptlet("require 'eclipse_console_irb';");
} catch (Exception e) {
e.printStackTrace();
}
}
}
I got the input output and
error streams from the superclass IOConsole,
and used them to create an inner config class of type
RubyInstanceConfig. I then used that to make the Ruby
Interpreter. You will notice that once I
create the new interpreter I call the evalScriptlet
method with some Ruby script code. First I add the paths to the Ruby load path
by inserting into the special Ruby array $: . I then
issue two require statements. One to start
JRuby and one to load up my Eclipse console Ruby script,
which contains the Ruby code already shown above.
The Console can be added to
the GUI by a very simple piece of code. I chose to add it by creating an action
that creates a new Console, so I have a new menu
entitled Ruby Console
Highlights below:
..
static RubyConsole ruby = new RubyConsole();
..
ConsolePlugin.getDefault().
getConsoleManager().addConsoles(new IConsole[]{ ruby });
ConsolePlugin.getDefault().getConsoleManager().showConsoleView(ruby);
..
And that is it.
There really wasn’t much more to it than
that, so when I say highlights I actually mean almost everything. Most of
what I have left out is the Eclipse plugin library
code. All the Ruby code I needed is shown above.
After I build the
plugin from my project, and deploy it, I get the rather
pleasing result of a Ruby console in my Eclipse workbench with
a eIrb>: prompt at the
console, and the interpreter’s results shown as I type in commands. It even has
nice colours. Shown here:
The Result
What can you do with this
though? Well anything you like (within
reason) but provided your plugin has added upstream
plugins to its dependency list, you can load any Java
classes you like from that plugin and work with them.
For example, I have added the “org.eclipse.core.resources”
to my RubyConsole plugin’s
dependency list, and so I can access the Eclipse Workspace by typing:
eIrb:> include_class 'org.eclipse.core.resources.ResourcesPlugin'
irb responds with:
=> ["org.eclipse.core.resources.ResourcesPlugin"]
Indicating that I have loaded up the class..
eIrb:> workspace = ResourcesPlugin.get_workspace
Yields:
=> #<Java::OrgEclipseCoreInternalResources::Workspace:0x25abb1 @java_object=org.eclipse.core.internal.resources.Workspace@12605d>
So I now have the workspace object stored in workspace. The great thing about this environment of course, is that should I feel I can’t be bothered to look in the Javadocs for the methods available on the Eclipse Workspace class, I can make use of Ruby’s reflection by just typing:
eIrb:> workspace.methods
Which returns a huge array of all the methods (too big to list here). I can then experiment with the sensible looking ones, and soon discover that:
eIrb:> projects = workspace.get_root.get_projects
eIrb:> projects.each { |p|
eIrb:> puts p.get_name
eIrb:> }
..will list all of the names of the projects in my Eclipse workspace, while:
eIrb:> projects[0].build(0,nil)
..will force the first project in the workspace to build.
eIrb:> projects[0].close(0,nil)
..will close the project, while:
eIrb:> workspace.get_root.get_project('project1').open nil
.. would open a project in my workspace called ‘project1’. Of course this is not good coding style at all, and if you didn’t have a project with this name, you would get a Java exception reported at the Ruby prompt, but you get the idea.
All in all, this is a really agile way to develop and a great way to test and experiment with libraries and middleware. Of course, once you have a piece of Ruby code that does something useful, you can save it in a .rb file and use the Ruby’s require statement at the prompt to load it up. You would then be able to continue typing interactively.
eIrb:> require 'useful.rb'
I hope you have fun playing around with this. Enjoy!