Introduction
This document is about BeanShell.
BeanShell is a small, free, embeddable, Java source interpreter with
object scripting language features, written in Java. BeanShell executes
standard Java statements and expressions but also extends Java into the
scripting domain with common scripting language conventions and syntax.
Traditionally, the primary difference between a scripting language and
a compiled language has been in its type system: the way in which you define
and use variables. You might be thinking that there is a more obvious
difference here - that of "interpreted" code vs. compiled code. But
compilation in and of itself does not fundamentally change the way you work
with a language. Nor does interpreting a language necessarily make
it more useful for what we think of as "scripting".
It is the type system that makes it possible for a compiler
to analyze the structure of an application for correctness.
Without types, compilation is reduced to just a grammar check and an
optimization for speed.
From the developer's perspective, it is also the type system that
characterizes the way in which we interact with the code.
Types are good. Without strongly type languages it would be very hard to
write large scale systems and make any assertions about their correctness
before they are run. But working with types imposes a burden on the developer.
Types are labels and labelling things can be tedious. It can be especially
tedious during certain kinds of development or special applications where
program structure is not paramount. There are times where maximizing
flexibility and productivity is a more important criterion.
This is not just hand waving to cover our laziness. Productivity affects
what people do and more importantly do *not* do in the real world, much more
than you might think. There is a lot of important software that exists in
the world today only because the cost/benefit ration in some developer's mind
reached a certain threshhold.
Unit testing - one of the foundations of
writing good code - is a prime example. Unit tests for well written code are,
in general, vitally important as a collective but almost insignificant
individually. It's a "tragedy of the commons" that leads individual
developers to repeatedly weigh the importance of writing another unit test
with working on "real code".
This is an example where scripting can really pay off. If developers have
a tool that makes it easy to perform a test with a line or two of code they
will probably use it. If, moreover, it is also a tool that they enjoy using
during their development process - that saves the time, they will be even
more inclined to use it. Scripting is good (too).
Traditionally (again, I'll say traditionally) scripting languages have traded
in the power of types for simplicity. Most scripting languages distill the
type system to just one or a handful of types such as strings, numbers, or
simple lists. This is often sufficient for some kinds of scripting.
Many scripts live in a desolate, unstructured land; a place full of text
and course grained tools. As such these scripting languages have evolved
sophisticated mechanisms for working with these simple types (regular
expressions, pipes, etc.). But the fact remains that they are still a
separate species, isolated from their brethren, the application languages.
There is a casm between the scripting languages
and the application languages created by the collapse of the type system
in-between.
BeanShell is a new kind of scripting language. BeanShell begins with
the standard Java language and bridges it into the scripting domain in a
natural way, but allowing the developer to relaxing types where appropriate.
It is possible to write BeanShell scripts that look exactly like Java
method code. But it's also possible to write scripts that look more like
a traditional scripting language, while still maintaining the framework
of the Java syntax.
BeanShell emulates typed variables and parameters when they are used. This
allows you to "seed" your code with strong types where appropriate. You can
"shore up" repeatedly used methods as you work on them, migrating them
closer to Java. Eventually you may find that you want to compile
these methods and maintain them in standard Java. With BeanShell this is
easy. BeanShell does not impose a syntactic boundary between your
scripts and Java.
But the bridge to Java extends much deeper than simple code similarity.
BeanShell is one of a new breed of scripting languages made possible by
Java's advanced reflection capabilities. Since BeanShell can run in the same
Java virtual machine as your application, you can freely work with real,
live, Java objects - passing them into and out of your scripts. Combined
with BeanShell's ability to implement Java interfaces, you can achieve
seamless and simple integration of scripting into your Java applications.
BeanShell does not impose a type boundary between your scripts and Java.
A long time ago, back in the summer of 1993 while working at Southwestern
Bell Technology Resources, I was infatuated with the Tcl/Tk scripting
language. At the time I was also playing around a bit with a
language called Oak written by James Gosling at Sun.
Little did I know that within just a few years Oak would not only spark the
Java revolution, but that I would be writing one of the first books on
the Java language (Exploring Java, O'Reilly & Associates) and creating
Java's first scripting language, BeanShell, drawing motivation from Tcl.
BeanShell's first public release was not until 1997, but I had been
poking at it in one form or another for some time before that. BeanShell
became truly practical when Sun added reflection to the Java language in
version 1.1. After that, and after having seen its value in helping me create
examples and snippets for the second edition of my book, I decided to try to
polish it up and release it.
BeanShell has slowly, but steadily gained popularity since then. It has
grown in fits and spurts as its contributor's time has allowed. But recently
BeanShell has achieved a sort of critical mass. BeanShell is distributed with
Emacs as part of the JDE and with Sun Microsystem's NetBeans / Forte for Java
IDEs. BeanShell is also bundled by BEA with their Weblogic application
server. We've had reports of BeanShell being used everywhere from
the high energy physics laboratory CERN, to classrooms teaching programming
to nine year olds. BeanShell is being used in everything from large
financial applications all the way down to embedded systems floating in Buoys
in the pacific ocean. I attribute this success to the power of the open
source development model and owe many thanks to everyone who has contributed.
I believe that BeanShell is the simplest and most natural scripting language
for Java because it is, foremost, Java. BeanShell draws on a rich history
of scripting languages for its scripting syntax and uses it to very
conservatively extend the Java language into this new domain.
I hope that you have half as much fun using BeanShell as I have had working
on it and I welcome all comments and suggestions.