Bean Scripting Framework
Bean Scripting Framework (BSF) is a set of Java classes which provides
scripting language support within Java applications, and
access to Java objects and methods from scripting languages.
BSF Architectural Overview
The two primary components of BSF are the
BSFManager handles all scripting execution engines
running under its control, and maintains the object registry that permits
scripts access to Java objects. By creating an instance of the
BSFManager class, a Java application can gain access to
BSFEngine provides an interface that must be
implemented for a language to be used by BSF. This interface provides
an abstraction of the scripting language's capabilities that permits
generic handling of script execution and object registration within
the execution context of the scripting language engine.
An application can instantiate a single
and execute several different scripting languages identically via the
BSFEngine interface. Furthermore, all of the scripting
languages handled by the
BSFManager are aware of the
objects registered with that
BSFManager, and the execution
state of those scripting languages is maintained for the lifetime of
BSF can be used standalone, as a class library, or as part of an
application server. In order to be used as a class library or as
a standalone system, you simply download a copy of the bsf.jar
file from the
BSF web site
and include it in your classpath, along with any required classes
or jar files for desired languages.
In order to use BSF as part of the
servlet engine, you must currently download patches from the BSF
web site that permit Jasper to call BSF. Instructions for this will be
posted on the website, and will soon be accompanied by prebuilt binaries.
We hope that these changes will be merged into Tomcat in the near
BSF and JSPs
After you set up an application server that is BSF enabled, you can write
JSPs using any of the supported scripting languages. JSPs using scripting
languages differ only slightly from those using Java.
First, you must set the language attribute of the page directive
in the JSP to the desired language. For example,
expressions within the JSP
will be handed off to BSF, which will in turn hand the code over to
Rhino for execution.
The standard set of JSP implicit objects is available within BSF.
These implicit objects must be used for input and output with respect
to the generated page, since the scripting languages do not have any
awareness of having been called within a JSP. For example, in order to
print a line of text into the page generated by the JSP, one must use the
println() method of the
out implicit object.
Multiple languages can be supported within a given JSP; this is
accomplished by using the BSF taglibs, which are available from the
project. BSF taglib provides two tags:
expression. Both of these have a required language attribute,
which is used to specify the language used on a per
Servlets and Other Applications
Using BSF in servlets or applications is also quite simple. In order
to provide an application with scripting support, you need to
import the BSF class hierarchy and instantiate a
object. After instantiating the
register or declare any Java objects to be made available within the
scripting engine. Then call either one of the
exec() BSFManager methods (depending on whether you want to
evaluate a script and have the value of the evaluation returned, or
execute a script). Alternatively, you can call the
loadScriptingEngine() method in order to get an object
BSFEngine interface for the desired
scripting language. You can then call the
eval() methods of
BSFEngine to run the script.
Additionally, BSF declares an object named
bsf within a
scripting engine's execution context, which represents the
BSFManager that is associated with the scripting engine.
This object provides all of the methods and properties
associated with the
BSFManager to the script.
However, the most used method within scripts is usually
lookupBean(), which is used to access objects
in BSF's object registry.
The most important methods within the
BSFManager() - the
eval() - used to evaluate a script and return
exec() - used to execute a script
loadScriptingEngine() - used to return a
BSFEngine for the desired scripting language
registerBean() - adds an object to BSF's object
lookupBean() - retrieves an object from BSF's
declareBean() - creates an implicit object in
the context of any loaded scripting language, which does not have
to be accessed via
Other, less often used methods within the
apply() - used to call anonymous functions
compileExpr() - used to compile an expression into a
compileScript() - similar to compile expression, used to
compile scripts into
compileApply() - similar to both of the above - used to
compile anonymous functions into
For the curious, the
CodeBuffer is a class provided by BSF for
storing generated Java code.
apply() methods (as well as their compile counterparts)
are wrappers over the equivalent methods presented by the
BSFEngine interface. If the programmer explicitly
loads a scripting engine via
can use the
eval() methods of the
BSFEngine as appropriate.
Adding BSF Support for a Scripting Language
In order to incorporate your own scripting language into BSF, you must first
write a class implementing the
BSFEngine interface for the
language; examples are available in the BSF source distribution.
Usually, a scripting language author extends the
BSFEngineImpl class, which implements
and only requires the scripting language author to implement the
eval() method. However, the following methods specified by
BSFEngine interface are the most commonly implemented:
initialize() - used to set up the underlying scripting
call() - used to call functions or methods within the
eval() - used to evaluate a script
exec() - used to execute a script
declareBean() - used to create an implicit object within
the scripting language
undeclareBean() - used to remove an implicit object
from the scripting language
Once you have implemented the wrapper for your language engine, you
BSFManager in your application, and register your
engine with it via the
Afterward, you may use your language within the application through the
usual BSF semantics.
BSF provides a facility for running scripting languages itself. Simply running
java org.apache.bsf.Main will produce a help message, with
instructions on how to run these scripts.