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.
BeanShell is a natural scripting language for Java.
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 data elements. You might be thinking that there is a more obvious
difference here - that of "interpreted" code vs. compiled code. But
the compiler 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 of a language 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 labeling things can be tedious. It can be especially
tedious during certain kinds of development or special applications where
it is flexibility and not program structure that is paramount.
There are times where simplicity and ease of use is a more important
criterion.
This is not just rationalization to cover some underlying 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 ratio in some developer's mind
reached a certain threshold.
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".
Give 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.
Customizability through scripting also opens the door to applications that
are more powerful than the sum of their parts. When users can extend, enhance,
and add to their applications they use them in new and unexpected ways.
Scripting is powerful.
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 sufficient for many kinds of scripting.
Many scripting languages operate in a loose, unstructured land - a place
dominated by text and course-grained tools.
As such these scripting languages have evolved
sophisticated mechanisms for working with these simple types (regular
expressions, pipes, etc.).
As a result there has developed a casm between the scripting languages
and the application languages created by the collapse of the type system
in-between.
The scripting languages have remained a separate species, isolated and
speaking a different dialect from their brothers the application languages.
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.
What seems like an eternity ago, back in the summer of 1993, I was working at
Southwestern Bell Technology Resources and I was infatuated with the Tcl/Tk
scripting language.
On the advice of someone at Sun I also began playing around a bit with the
Oak language written by James Gosling.
Little did I know that within just a few years Oak, which would become Java,
would not only spark a revolution, but that I would be writing one of the
first books on the new Java language (Exploring Java,
O'Reilly & Associates) and creating
Java's first scripting language, BeanShell, drawing inspiration 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
as a language became 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.
