BeanShell
Simple Java Scripting
version 1.3
1
Table of Contents
Table of Contents•
Introduction•
Scripting vs. Application Languages♦
Tearing Down the Barriers♦
History♦
Conclusion♦
Quick Start•
Download and Run BeanShell♦
The BeanShell GUI♦
Java Statements and Expressions♦
Useful BeanShell Commands♦
Scripted Methods♦
Implementing Interfaces♦
Scripted Objects♦
Calling BeanShell From Your Application♦
Conclusion♦
Basic Syntax•
Standard Java Syntax♦
Loosely Typed Java Syntax♦
Exception Handling♦
Basic Scoping of Variables♦
Variable Modifiers♦
Convenience Syntax♦
Auto Boxing and Unboxing♦
Importing Classes and Packages♦
Document Friendly Entities♦
Scripted Methods•
Scoping of Variables and Methods♦
Scope Modifier: 'super'♦
Scripted Objects•
The 'this' reference♦
Scope Modifiers•
'this', 'super', and 'global'♦
Synchronized Methods Revisited♦
Scripting Interfaces•
Anonymous Inner−Class Style♦
'this' references as Interface Types♦
Interface Types and Casting♦
"Dummy" Adapters and Incomplete Interfaces♦
Threads − Scripting Runnable♦
Limitations♦
Special Variables and Values•
Special Members of 'this' type References♦
Undefined Variables♦
Setting the Command Prompt♦
BeanShell Commands•
Commands Overview♦
Adding BeanShell Commands•
Hello World♦
Compiled Commands♦
Table of Contents 2
User Defined Commands with invoke()♦
Commands Scope♦
Getting the Caller Context♦
setNameSpace()♦
Getting the Invocation Text♦
Working with Dirctories and Paths♦
Working With Class Identifiers♦
Working with Iterable Types♦
Strict Java Mode•
Class Loading and Class Path Management•
Changing the Class Path♦
Auto−Importing from the Classpath♦
Reloading Classes♦
Setting the Default ClassLoader♦
Class Loading in Java♦
Class Loading in BeanShell♦
Modes of Operation•
Standalone♦
Remote♦
Interactive Use♦
The .bshrc Init File♦
Embedding BeanShell in Your Application•
The BeanShell Core Distribution♦
Calling BeanShell From Java♦
eval()♦
EvalError♦
source()♦
Multiple Interpreters vs. Multi−threading♦
Serializing Interpreters and Scripted Objects♦
Remote Server Mode•
Web Browser Access♦
Example♦
Telnet Access♦
BshServlet and Servlet Mode Scripting•
Deploying BshServlet♦
Running Scripts♦
The Script Environment♦
BshServlet Parameters♦
The BeanShell Demo Applet•
BeanShell Desktop•
Shell Windows♦
Editor Windows♦
The Class Browser♦
BshDoc − Javadoc Style Documentation•
BshDoc Comments♦
BshDoc XML Output♦
The bshcommands.xsl stylesheet♦
The BeanShell Parser•
Validating Scripts With bsh.Parser♦
Parsing and Performance♦
Parsing Scripts Procedurally♦
Table of Contents 3
Using JConsole•
ConsoleInterface♦
Reflective Style Access to Scripted Methods•
eval()♦
invokeMethod()♦
Method Lookup♦
BshMethod♦
Uses♦
Executable scripts under Unix•
BSF Bean Scripting Framework•
Ant♦
Learning More•
Helping With the Project♦
Credit and Acknowledgments•
License and Terms of Use♦
BeanShell Commands Documentation•
Table of Contents 4 − − one of the foundations of writing good code − − − a place dominated by text and
course−grained tools. As such these scripting languages have evolved sophisticated mechanisms for working
Introduction 5
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. −
History 6
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.
History 7
Quick Start
Welcome to BeanShell. This is a crash course to get you going. We'll leave out many important options and
details. Please see the rest of the user's guide for more information.
Download and Run BeanShell
Download the latest JAR file from and start up BeanShell either in the graphical
desktop mode or on the command line.
If you just want to start playing around you may be able to launch the BeanShell desktop by simply double
clicking on the BeanShell JAR file. More generally however you'll want to add the jar to your classpath so
that you can work with your own classes and applications easily.
To do this you can either drop the BeanShell JAR file into your Java extensions folder or add it to your
classpath. (Important: If you put BeanShell in the extensions folder and wish to use it with BSF applications
like Jakarta Ant you must install the bsf.jar in the same location).
To install as an extension place the bsh.jar file in your
$JAVA_HOME/jre/lib/ext folder. (OSX users: place the bsh.jar in
/Library/Java/Extensions or ~/Library/Java/Extensions for individual users.)
Or add BeanShell to your classpath like this:
unix: export CLASSPATH=$CLASSPATH:bsh−xx.jar
windows: set classpath %classpath%;bsh−xx.jar
Tip:
You can modify the classpath from within BeanShell using the addClassPath() and setClassPath()
commands.
You can then run BeanShell in either a GUI or command line mode:
java bsh.Console // run the graphical desktop
or
java bsh.Interpreter // run as text−only on the command line
or
java bsh.Interpreter filename [ args ] // run script file
It's also possible to call BeanShell from within your own Java applications, to reach it in a remote server mode
for debugging, to use it as a servlet, or even in an applet. See "BeanShell Modes of Operation" for more
details.
The BeanShell GUI
The BeanShell GUI desktop is meant to allow some experimentation with the features of BeanShell. It is not
intended to be a replacement for a full featured IDE. Please check out the jEdit editor for an example of a full
featured development environment based in part on BeanShell scripting capabilities.
Upon starting the BeanShell in GUI mode a console window will open. By right clicking on the desktop
background you can open additional console windows and other tools such as a simple class browser.
Quick Start 8
Each console window runs a separate instance of the BeanShell interpreter. The graphical console supports
basic command history, line editing, cut and paste, and even class and variable name completion. From the
console you can open a simple editor window. In it you can write scripts and use the 'eval' option to evaluate
the text in the attached console's workspace or a new workspace.
Java Statements and Expressions
BeanShell understands standard Java statements, expressions, and method declarations. Statements and
expressions are all of the normal things that you'd say inside a Java method such as variable declarations and
assignments, method calls, loops, and conditionals.
You can use these exactly as they would appear in Java, however in BeanShell you also have the option of
working with "loosely typed" variables. That is, you can simply omit the types of variables that you use (both
primitives and objects). BeanShell will only signal an error if you attempt to misuse the actual type of the
variable.
Here are some examples:
foo = "Foo";
four = (2 + 2)*2/2;
print( foo + " = " + four ); // print() is a BeanShell command
// Do a loop
for (i=0; i<5; i++)
print(i);
// Pop up a frame with a button in it
button = new JButton( "My Button" );
frame = new JFrame( "My Frame" );
frame.getContentPane().add( button, "Center" );
frame.pack();
frame.setVisible(true);
Useful BeanShell Commands
In the previous example we used a convenient "built−in" BeanShell command called print(), to display values.
print() does pretty much the same thing as System.out.println() except that it insures that the output always
goes to the command line. print() also displays some types of objects (such as arrays) more verbosely than
Java would. Another related command is show(), which toggles on and off automatic display of the result of
every line you type.
Here are a few other examples of BeanShell commands:
source(), run() − Read a bsh script into this interpreter, or run it in a new interpreter•
frame() − Display a GUI component in a Frame or JFrame.•
load(), save() − Load or save serializable objects to a file.•
cd(), cat(), dir(), pwd(), etc. − Unix−like shell commands•
exec() − Run a native application•
javap() − Print the methods and fields of an object, similar to the output of the Java javap command.•
setAccessibility() − Turn on unrestricted access to private and protected components.•
See the complete list of BeanShell Commands for more information.
Java Statements and Expressions 9
Tip:
BeanShell commands are not really "built−in" but are simply BeanShell scripts that are automatically loaded
from the classpath. You can add your own scripts to the classpath to extend the basic command set.
Scripted Methods
You can declare and use methods in BeanShell just as you would in a Java class.
int addTwoNumbers( int a, int b ) {
return a + b;
}
sum = addTwoNumbers( 5, 7 ); // 12
Bsh methods may also allow dynamic (loose) argument and return types.
add( a, b ) {
return a + b;
}
foo = add(1, 2); // 3
foo = add("Oh", " baby"); // "Oh baby"
Implementing Interfaces
Note: implementing arbitrary interfaces requires BeanShell be run under a Java 1.3 or higher environment.
You can use the standard Java anonymous inner class syntax to implement an interface type with a script. For
example:
ActionListener scriptedListener = new ActionListener() {
actionPerformed( event ) { ... }
}
You don't have to script all of the methods of an interface. You can opt to script only those that you intend to
call if you want to. The calling code will simply throw an exception if it tries to invoke a method that isn't
defined. If you wish to override the behavior of a large number of methods − say to produce a "dummy"
adapter for logging − you can implement a special method signature: invoke(name, args) in your scripted
object. The invoke() method is called to handle any undefined method invocations:
ml = new MouseListener() {
mousePressed( event ) { ... }
// handle the rest
invoke( name, args ) { print("Method: "+name+" invoked!");
}
Scripted Objects
In BeanShell, as in JavaScript and Perl, method "closures" allow you to create scripted objects. You can turn
the results of a method call into an object reference by having the method return the special value this. You
can then use the reference to refer to any variables set during the method call. Useful objects need methods of
Scripted Methods 10
course, so in BeanShell scripted methods may also contain methods at any level. For example:
foo() {
print("foo");
x=5;
bar() {
print("bar");
}
return this;
}
myfoo = foo(); // prints "foo"
print( myfoo.x ); // prints "5"
myfoo.bar(); // prints "bar"
If this "closure" thing seems strange to don't worry. It's just an evolutionary step that languages acquired along
the path to Objects. Please see the user's manual for a more thorough explanation.
Within your scripts, BeanShell scripted objects (i.e. any 'this' type reference like myFoo in the previous
example) can automatically implement any Java interface type. When Java code calls methods on the interface
the corresponding scripted methods will be invoked to handle them. BeanShell will automatically "cast" your
scripted object when you attempt to pass it as an argument to a method that takes an interface type. For
passing script references outside of BeanShell, you can perform an explicit cast where necessary. Please see
the user manual for full details.
Calling BeanShell From Your Application
You can evaluate text and run scripts from within your application by creating an instance of the BeanShell
interpreter and using the eval() or source() commands. You may pass in variable references to objects you
wish to use in scripts via the set() method and retrieve results with the get() method.
import bsh.Interpreter;
Interpreter i = new Interpreter(); // Construct an interpreter
i.set("foo", 5); // Set variables
i.set("date", new Date() );
Date date = (Date)i.get("date"); // retrieve a variable
// Eval a statement and get the result
i.eval("bar = foo*10");
System.out.println( i.get("bar") );
// Source an external script file
i.source("somefile.bsh");
Tip:
In the above example the Interpreter's eval() method also returned the value of bar as the result of the
evaluation.
Calling BeanShell From Your Application 11
Conclusion
We hope this brief introduction gets you started. Please see the full user manual for more details. Please
consult the mailing list archives for more useful information.
Conclusion 12
Basic Syntax
BeanShell is, foremost, a Java interpreter. So you probably already know most of what you need to start
scripting with BeanShell. This section describes specifically what portion of the Java language BeanShell
interprets and how BeanShell extends it or "loosens" it to be more scripting−language−like.
Standard Java Syntax
In a BeanShell script (and on the command line) you can type normal Java statements and expressions and
display the results. Statements and expressions are the kinds of things you normally find inside of a Java
method: variable assignments, method calls, math expressions, for−loops, etc.
Here are some examples:
/*
Standard Java syntax
*/
// Use a hashtable
Hashtable hashtable = new Hashtable();
Date date = new Date();
hashtable.put( "today", date );
// Print the current clock value
print( System.currentTimeMillis() );
// Loop
for (int i=0; i<5; i++)
print(i);
// Pop up a frame with a button in it
JButton button = new JButton( "My Button" );
JFrame frame = new JFrame( "My Frame" );
frame.getContentPane().add( button, "Center" );
frame.pack();
frame.setVisible(true);
You can also define your own methods and use them just as you would inside a Java class. We'll get to that in
a moment.
Loosely Typed Java Syntax
In the examples above, all of our variables have declared types. e.g. "JButton button". Beanshell will enforce
these types, as you will see if you later try to assign something other than a JButton to the variable "button"
(you will get an error message). However BeanShell also supports "loose" or dynamically typed variables.
That is, you can refer to variables without declaring them first and without specifying any type. In this case
BeanShell will do type checking where appropriate at runtime. So, for example, we could have left off the
types in the above example and written all of the above as:
/*
Loosely Typed Java syntax
*/
// Use a hashtable
Basic Syntax 13
hashtable = new Hashtable();
date = new Date();
hashtable.put( "today", date );
// Print the current clock value
print( System.currentTimeMillis() );
// Loop
for (i=0; i<5; i++)
print(i);
// Pop up a frame with a button in it
button = new JButton( "My Button" );
frame = new JFrame( "My Frame" );
frame.getContentPane().add( button, "Center" );
frame.pack();
frame.setVisible(true);
This may not seem like it has saved us a great deal of work. But you will see the difference when you come to
rely on scripting as part of your development and testing process; especially for in interactive use.
When a "loose" variable is used you are free to reassign it to another type of Java object later. Untyped
BeanShell variables can also freely hold Java primitive values like int and boolean. Don't worry, BeanShell
always knows the real types and only lets you use the values where appropriate. For primitive types this
includes doing the correct numeric promotion that the real Java language would do when you use them in an
expression.
Exception Handling
Exception handling using try/catch blocks works just as it does in Java. For example:
try {
int i = 1/0;
} catch ( ArithmeticException e ) {
print( e );
}
But you can loosely type your catch blocks if you wish:
try {
...
} catch ( e ) {
print( "caught exception: "+e );
}
Basic Scoping of Variables
Note:
As of BeanShell version 1.3 the default scoping of loosely typed variables was changed to be
more consistent with Java. BeanShell still supports an alternate scoping used in earlier versions.
This mode can be enabled for legacy code by setting the system property "localscoping" to true.
See appendix "Local Scoping".
Exception Handling 14
Variable scoping in BeanShell behaves, wherever possible, just like that in Java. Ordinary Java, however,
does not offer "loose" variables (variables that can be used without being declared first). So we must define
their behavior within BeanShell. We'll see in the next section that untyped variables − variables that are not
declared and not assigned a value elsewhere − default to the local scope. This means that, in general, if you
assign a value to a variable without first declaring it, you are creating a new local variable in the current
scope.
Blocks
Blocks are statements between curly braces {}. In BeanShell, as in Java, blocks define a level of scope for
typed variables: typed variables declared within a block are local to the block. Other assignments within the
block occur, as always, wherever the variable was defined.
Untyped variables in BeanShell, however, are not constrained by blocks. Instead they act as if they were
declared at the outer (enclosing) scope's level. With this in mind, BeanShell code looks just like Java code. In
BeanShell if you declare a typed variable within a block it is local to the block. But if you use an untyped
variable (which looks just like an ordinary assignment in Java) it behaves as an assignment to the enclosing
scope.
This will make sense with a few examples:
// Arbitrary code block
{
y = 2; // Untyped variable assigned
int x = 1; // Typed variable assigned
}
print( y ); // 2
print( x ); // Error! x is undefined.
// Same with any block statement: if, while, try/catch, etc.
if ( true ) {
y = 2; // Untyped variable assigned
int x = 1; // Typed variable assigned
}
print( y ); // 2
print( x ); // Error! x is undefined.
Variables declared in the for−init area of a for−loop follow the same rules as part of the block:
for( int i=0; i<10; i++ ) { // typed for−init variable
j=42;
}
print( i ); // Error! 'i' is undefined.
print( j ); // 42
for( z=0; z<10; z++ ) { } // untyped for−init variable
print( z ); // 10
Variable Modifiers
The standard Java variable modifiers may be used on typed variables: private / protected / public, final,
transient, volatile, static. Only 'final' is currently implemented. The others are currently ignored.
Modifiers may not be applied to untyped variables.
Blocks 15
Convenience Syntax
In BeanShell you may access JavaBean properties as if they were fields:
button = new java.awt.Button();
button.label = "my button"; // Equivalent to: b.setLabel("my button");
print( button.label ); // Equivalent to print( b.getLabel() );
JavaBean properties are simply pairs of "setter" and "getter" methods that adhere to a naming convention. In
the above example BeanShell located a "setter" method with the name "setLabel()" and used it to assign the
string value. It then found the method named getLabel() to retrieve the value.
Boolean properties may optionally use the syntax "is" for their "getter". e.g.
Float f = new Float(42f);
print( f.infinite ); // Equivalent to print( f.isInfinite() ); // false
If there is any ambiguity with an actual Java field name of the object (e.g. label in the above example) then the
actual field name takes precedence. If you wish to avoid any ambiguity BeanShell provides an additional,
uniform syntax for accessing both Java Bean properties and Hashtable or Map entries. You may use the "{}"
curly brace construct with a String identifier as a qualifier on any variable of the appropriate type:
b = new java.awt.Button();
b{"label"} = "my button"; // Equivalent to: b.setLabel("my button");
h = new Hashtable();
h{"foo"} = "bar"; // Equivalent to: h.put("foo", "bar");
Where the java.util.Collections API is available, Maps are also supported.
Enhanced 'for' Loop
BeanShell supports the Java 1.5 style enhanced for−loop for iterating over collections and array types. (Note
that you do not have to be running Java 1.5 to use this feature)..
JDK 1.1+ − (no collections): Enumeration, arrays, Vector, String, StringBuffer•
Convenience Syntax 16
JDK 1.2+ − (w/collections): Collections, Iterator•
See also the BshIterator API which supports the ehanced for−loop and allows iteration over these types using
the dynamically loaded BeanShell Collection manager.
Switch Statements
In BeanShell, the switch statement may be used not only with numeric types but with objects. For example,
you may switch on Dates and Strings which are compared for equality with their equals() methods:
dateobj = new Date();
switch( dateobj )
{
case newYears:
break;
case christmas:
break;
default:
}
Auto Boxing and Unboxing
"Boxing" and "Unboxing" are the terms used to describe automatically wrapping a primitive type in a wrapper
class and unwrapping it as necessary. Boxing is a feature of Java (SDK1.5) and has been supported in
BeanShell for many years.
BeanShell supports boxing and unboxing of primitive types. For example:
int i=5;
Integer iw = new Integer(5);
print( i * iw ); // 25
Vector v = new Vector();
v.put(1);
int x = v.getFirstElement();
Importing Classes and Packages
In BeanShell as in Java, you can either refer to classes by their fully qualified names, or you can import one or
more classes from a Java package.
// Standard Java
import javax.xml.parsers.*;
import mypackage.MyClass;
In BeanShell import statements may appear anywhere, even inside a method, not just at the top of a file. In the
event of a conflict, later imports take precedence over earlier ones.
A somewhat experimental feature is the "super import". With it you may automatically import the entire
classpath, like so:
Switch Statements 17
import *;
The first time you do this BeanShell will map out your entire classpath; so this is primarily intended for
interactive use. Note that importing every class in your classpath can be time consuming. It can also result in a
lot of ambiguities. Currently BeanShell will report an error when resolving an an ambiguous import from
mapping the entire classpath. You may disambiguate it by importing the class you intend.
Tip:
The BeanShell which() command will use the classpath mapping capability to tell you where exactly in your
classpath a specified class is located:
bsh % which( java.lang.String );
Jar: file:/usr/java/j2sdk1.4.0/jre/lib/rt.jar
See "Class Path Management" for information about modifying the BeanShell classpath at run−time with the
addClassPath() or setClassPath() commands.
Also see "BeanShell Commands" for information about importing new BeanShell commands from the
classpath.
Default Imports
By default, common Java core and extension packages are imported for you. They are, in the order in which
they are imported:
javax.swing.event•
javax.swing•
java.awt.event•
java.awt•
java.net•
java.util•
java.io•
java.lang•
Two BeanShell package classes are also imported by default:
bsh.EvalError•
bsh.Interpreter•
Finally, we should mention that BeanShell commands may be imported from the classpath. The default
commands are imported in the following way:
importCommands("/bsh/commands");
We will discuss how to import your own commands in a later section.
Tip:
The classes java.awt.List and java.util.List are both imported by default. Because java.util.List is imported
later, as part of the java.util package, it takes precedence. To access java.awt.List simply import it in, or the
java.awt package again your script. Later imports take precedence.
Default Imports 18
Document Friendly Entities
BeanShell supports special overloaded text forms of all common operators to make it easier to embed
BeanShell scripts inside other kinds of documents (e.g XML).
@gt >
@lt <
@lteq <=
@gteq >=
@or ||
@and &&
@bitwise_and &
@bitwise_or |
@left_shift <<
@right_shift >>
@right_unsigned_shift >>>
@and_assign &=
@or_assign |=
@left_shift_assign <<=
@right_shift_assign >>=
@right_unsigned_shift_assign >>>=
Document Friendly Entities 19
Scripted Methods
You can define define methods in BeanShell, just as they would appear in Java:
int addTwoNumbers( int a, int b ) {
return a + b;
}
And you can use them in your scripts just as you would any Java method or "built−in" BeanShell command:
sum = addTwoNumbers( 5, 7 );
Just as BeanShell variables may be dynamically typed, methods may have dynamic argument and return
types. We could, for example, have declared our add() method above like so:
add( a, b ) {
return a + b;
}
In this case, BeanShell would dynamically determine the types when the method is called and attempt to "do
the right thing":
foo = add(1, 2);
print( foo ); // 3
foo = add("Oh", " baby");
print( foo ); // Oh baby
In the first case Java performed arithmetic addition on the integers 1 and 2. (By the way, if we had passed in
numbers of other types BeanShell would have performed the appropriate numeric promotion and returned the
correct Java primitive type.) In the second case BeanShell performed the usual string concatenation for String
types and returned a String object. This example is a bit extreme, as there are no other overloaded operators
like string concatenation in Java. But it serves to emphasize that BeanShell methods can work with loose
types.
Methods with unspecified return types may return any type of object (as in the previous example).
Alternatively they may also simply issue a "return;" without a value, in which case the effective type of the
method is "void" (no type). In either case, the return statement is optional. If the method does not perform an
explicit "return" statement and the return type is not explicitly set to void, the value of the last statement or
expression in the method body becomes the return value (and must adhere to any declared return typing).
Method Modifiers and 'throws' Clauses
The standard Java modifiers may be applied to methods: private / protected / public, synchronized, final,
native, abstract, and static.
The synchronized modifier is the only modifier currently implemented. The others are ignored. The 'throws'
clause of methods is checked for valid class type names, but is not otherwise enforced.
Synchronized methods are synchronized on the object representing the method's common parent scope, so
they behave like Java methods contained in a class. We will return to this topic after discussing scripted
Scripted Methods 20
objects and "closures".
// foo() and bar() are synchronized as if they were in a common class
synchronized foo() { }
synchronized bar() { }
Scoping of Variables and Methods
As in Java, a method can refer to the values of variables and method names from the enclosing scope (in Java
the "enclosing scope" would be a class). For example:
a = 1;
anotherMethod() { ... }
foo() {
print( a );
a = a+1;
anotherMethod();
}
// invoke foo()
foo(); // prints 1
print( a ); // prints 2
Variables and methods are "inherited" from the parent scope in the usual way. In the example above there are
just two levels of scope: the top or "global" scope and the scope of the method foo(). Later we'll talk about
scripting objects in BeanShell and see that there can be arbitrary levels of scoping involved. But the rules will
be the same.
As in Java, a typed variable is not visible outside the scope in which it is declared. So declaring a variable
with a type is a way to limit its scope or make a local variable. In BeanShell using an untyped or "loosely"
typed variable is also equivalent to declaring a local variable. That is, if you use a variable that has not been
defined elsewhere, it defaults to the local scope:
a = 1;
foo() {
a = a + 1; // a is defined in parent scope
b = 3; // undefined, defaults local scope
int c = 4; // declared local scope
}
// invoke foo()
print( a ); // prints 2
print( b ); // ERROR! b undefined
print( c ); // ERROR! c undefined
In the above example the variable 'a' is declared in the global scope. When its value is read and assigned
inside of foo() the global value of 'a' will be affected.
The variable 'b' is a usage of an untyped variable. Since 'b' has not been declared or assigned a value in any
enclosing scope, it becomes a local variable 'b' in the scope of foo. The variable 'c' is explicitly declared (with
a type) in the scope of foo() and is therefore, of course, local to foo().
Scoping of Variables and Methods 21
Later we'll see that BeanShell allows arbitrary nesting of methods. If we were to declare another method
inside of foo() it could see all of these variables (a, b, and c) as it is also in the scope of foo().
Scoping of Loosely Typed Variables
As in Java, declaring a variable with a type will always make it local. Even if the variable exists in the outer
scope, it will be hidden by the local variable declaration. But what of loosely typed variables? As we've seen,
untyped variable usage looks just like an ordinary Java assignment. What do we do if we want to make a local
variable with the same name as a global one? One answer would be to resort to declaring the variable with a
type. But if we wish to continue working with loosely typed variables in this case we have two options: We
can explicitly declare a loosely typed variable with the BeanShell 'var' type. Or we can simply qualify our
assignment with the 'this.' qualifier.
If you wish to, you can explicitly declare an untyped variable (making it local) using the special type 'var'. e.g.
foo() {
var a = 1;
}
foo();
print( a ); // ERROR! a is undefined!
'var' is a magic type in BeanShell that represents a loose (untyped) variable. The default value of a variable
declared with 'var' is null.
Alternately, you can use the scope modifier 'this' to explicitly qualify the variable assignment and make it
local.
foo() {
this.a = 1;
}
foo();
print( a ); // ERROR! a is undefined!
In this example we used the modifier 'this' to qualify an untyped variable's scope and make it local. We will
explain 'this' and what it means in BeanShell scripted methods in the next section on Scripted Objects.
Scope Modifier: 'super'
Within a method, it is possible to explicitly qualify a variable or method reference with the identifier 'super' in
order to refer to a variable or method defined in an enclosing scope (the scope in which the method is defined
or "higher"). e.g.
int a = 42;
foo() {
int a = 97;
print( a );
print( super.a );
}
foo(); // prints 97, 42
Scoping of Loosely Typed Variables 22
As in Java, the 'super' modifiers tells the scoping to begin its search for the variable or method in the parent
scope. In the case above, the variable 'a' by default refers to the variable in the local scope. By qualifying 'a'
with 'super' we can refer to the variable 'a' in the global scope (the "topmost" scope).
So, we've seen that 'super' can be used to refer to the method's parent context. We'll see in the next section
how 'this' and 'super' are used in scripting Objects in BeanShell.
Scoping of Loosely Typed Variables 23
Scripted Objects
Many people who use BeanShell use it to write scripts that work with existing Java classes and APIs, or
perform other kinds of dynamic activities for their own applications at run−time without the aid of a compiler.
Often this means writing relatively unstructured code − for example, a sequence of method invocations or
loops, all contained in a single script file or eval() statement. In the previous section we saw that BeanShell is
also capable of scripting methods, just like Java. Creating methods and new BeanShell commands (which are
just methods in their own files) is the natural progression of organizing your scripts into re−usable and
maintainable components.
Beyond methods and structured programming lie, of course, objects and the full breadth of object oriented
programming. In Java objects are the products of classes. While BeanShell is compatible with standard Java
syntax for statements, expressions, and methods, you can't yet script new Java classes within BeanShell.
Instead, BeanShell allows you to script objects as "method closures", similar to the way it is done in Perl 5.x,
JavaScript, and other object−capable scripting languages. This style of scripting objects (which we'll describe
momentarily) is simple and flows very naturally from the style of scripting methods. The syntax, as you'll see,
is a straightforward extension of the standard Java concept of referring to an object with a 'this' reference.
Note:
In standard Java, a method inside of an object (an instance method) may refer to the enclosing
object using the special variable 'this'. For example:
// MyClass.java
MyClass {
Object getObject() {
return this; // return a reference to our object
}
}
In the example above, the getObject() method of MyClass returns a reference to its own object
instance (an instance of the MyClass object) using 'this'.
The 'this' reference
As in most languages, an executing method in BeanShell has its own "local" scope that holds argument
(parameter) variables and locally declared variables. For example, in the following code segment any
variables that we might use within the foo() method will normally only be visible within the scope of foo()
and for the lifetime of one particular foo() method invocation:
// Define the foo() method:
foo() {
int bar = 42;
print( bar );
}
// Invoke the foo() method:
foo(); // prints 42
print( bar ); // Error, bar is undefined here
In the above, the bar variable is local to foo() and therefore not available outside of the method invocation − it
is thrown away when the method exits, just like a standard Java local variable.
Scripted Objects 24
Now comes the twist − In BeanShell you have the option to "hang on" to the scope of a method invocation
after exiting the method by referring to the special 'this' reference. As in Java, 'this' refers to the current object
context. The only difference is that in this case the context is associated with the method and not a class
instance.
By saving the 'this' reference after the method returns, you can continue to refer to variables defined within the
method, using the standard Java "." notation:
foo() {
int bar = 42;
return this;
}
fooObject = foo();
print( fooObject.bar ); // prints 42!
In the above, the value returned by the foo() method (the 'this' reference) can be thought of as an instance of a
"foo" object. Each foo() method invocation effectively creates a new object; foo() is now not just a method,
but a kind of object constructor.
In the above case our foo object is not so much an object, but really more of a structure. It contains variables
(bar) but no "behavior". The next twist that we'll introduce is that BeanShell methods are also allowed to
contain other methods:
foo() {
bar() {
...
}
}
Scripted methods may define any number of nested methods in this way, to an arbitrary depth. The methods
are "local" to the method invocation.
Statements and expressions within the enclosing BeanShell method can call their "local" methods just like any
other method. (Locally declared methods override outer−more methods like local variables hide instance
variables in Java.) The enclosed methods are not directly visible outside of their enclosing method. However,
as you might expect, we can invoke them as we would on a Java object, through an appropriate object
reference:
foo() {
int a = 42;
bar() {
print("The bar is open!");
}
bar();
return this;
}
// Construct the foo object
fooObject = foo(); // prints "the bar is open!"
// Print a variable of the foo object
print ( fooObject.a ) // 42
// Invoke a method on the foo object
fooObject.bar(); // prints "the bar is open!"
Scripted Objects 25
Methods declared inside block structures within methods behave just as if they were declared directly in the
method. i.e. there are no block−local methods. For example:
foo() {
bar() { }
if ( true ) {
bar2() { }
}
return this;
}
In the above example the methods bar() and bar2() are both defined within foo().
In the next section we'll return to the topic of variable scoping and go into more depth about how to work with
scripted methods and objects.
Scripted Objects 26
Scope Modifiers
Now that we've seen how methods can be nested and treated as objects, we can revisit the topic of variable
scope and scope modifiers.
'this', 'super', and 'global'
In the "Scripted Methods" section we described the use of 'super' to refer to a method's parent scope (the scope
in which the method is defined). And in the previous section we talked about super's brother 'this', which
refers to the current method's scope, allowing us to think of a method scope as an object. Now we can see how
these concepts are related. Any method scope, and indeed the 'global' scope, can be thought as an object
context. A scripted object can be thought of as encapsulated in a parent scope that determines its
"environment" − its inherited variables and methods.
The references 'this', 'super', and 'global' are really the same kind of reference − references to BeanShell
method contexts, which can be used as scripted objects. From here on We'll refer to 'this', 'super', 'global', and
any other reference to a scripted object context in general as a 'this' type reference.
Note:
If you print a 'this' type reference you'll see what it refers to:
BeanShell 1.3 − by Pat Niemeyer (pat@pat.net)
bsh % print( this );
'this' reference (XThis) to Bsh object: global
bsh % foo() { print(this); print(super); }
bsh % foo();
'this' reference (XThis) to Bsh object: foo
'this' reference (XThis) to Bsh object: global
The above note shows that the foo() method's 'this' reference is local (named 'foo') and that it's parent is the
global scope; the same scope in which foo is defined.
'global'
The scope modifier 'global' allows you to always refer to the top−most scope. In the previous note you can see
that the top level script context is called "global" and that it appears again as the 'super' of our foo() method.
The global context is always the top scope of the script. It is the global namespace of the current interpreter.
Referring to 'super' from the top scope simply returns the same 'global' again.
global.foo = 42;
Global variables are not special in any way. Their visibility derives simply from the fact that they are in the
topmost scope. However, for those who do not like the idea of qualifying anything with "global". You can
always use a more object oriented approach like the following.
// Create a top level object to hold some state
dataholder = object();
foo() {
...
bar() {
dataholder.value = 42;
Scope Modifiers 27
}
bar();
print( dataholder.value );
}
In the above example we used a global object to hold some state, rather than putting the 'value' variable
directly in the global scope.
Tip:
In the above example we used the BeanShell object() command to create an "empty" BeanShell scripted
object context in which to hold some data. The object() command is just a standard empty method named
object() that returns 'this'. The variable 'dataholder' above is a 'this' type reference and has all of the
properties of any other BeanShell object scope.
Synchronized Methods Revisited
Now that we have covered the meaning of 'this' and 'super' with respect to BeanShell methods we can define
the meaning of the 'synchronized' modifier for BeanShell methods. Synchronized BeanShell methods behave
as if they were in a common class by synchronizing on their common 'super' reference object. For example, in
the four cases in the following example, synchronization occurs on the same Java object. That object is the
'this' type reference of the global scope (a Beanshell object of type bsh.This):
print( this ); // 'this' reference (XThis) to Bsh object: global
// The following cases all synchronize on the same lock
synchronized ( this ) { } // synchronized block
synchronized int foo () { } // synchronized method foo()
synchronized int bar () { } // synchronized method bar()
int gee() {
synchronized( super ) { } // synchronized blockinside gee()
}
Synchronized Methods Revisited 28
Scripting Interfaces
One of the most powerful features of BeanShell is the ability to script Java interfaces. This feature allows you
to write scripts that serve as event handlers, listeners, and components of other Java APIs. It also makes
calling scripted components from within your applications easier because they can be made to look just like
any other Java object.
Anonymous Inner−Class Style
One way to get a scripted component to implement a Java interface is by using the standard Java anonymous
inner class syntax to construct a scripted object implementing the interface type. For example:
buttonHandler = new ActionListener() {
actionPerformed( event ) {
print(event);
}
};
button = new JButton();
button.addActionListener( buttonHandler );
frame(button);
In the above example we have created an object that implements the ActionListener interface −.
'this' references as Interface Types
The anonymous inner class style syntax which we just discussed allows you to explicitly create an object of a
specified interface type, just as you would in Java. But BeanShell is more flexible than that. In fact, within
your BeanShell scripts, any 'this' type script reference can automatically implement any interface type, as
needed. This means that you can simply use a 'this' reference to your script or a scripted object anywhere that
you would use the interface type. BeanShell will automatically "cast" it to the correct type and perform the
method delegation for you.
For example, we could script an event handler for our button even more simply using just a global method,
like this:
Scripting Interfaces 29
actionPerformed( event ) {
print( event );
}
button = new JButton("Foo!");
button.addActionListener( this );
frame( button );
Here, instead of making a scripted object to hold our actionPerformed() method we have simply placed the
method in the current context (the global scope) and told BeanShell to look there for the method.
Just as before, when ActionEvents are fired by the button, your actionPerformed() method will be
invoked. The BeanShell 'this' reference to our script implements the interface and directs method invocations
to the appropriately named method, if it exists.
Note:
If you want to have some fun, try entering the previous example interactively in a shell or on the
command line. You'll see that you can then redefine actionPerformed() as often as you like by
simply entering the method again. Each button press will find the current version in your shell. In
a sense, you are working inside a dynamic Java object that you are creating and modifying as you
type. Neat, huh? Be the Bean!
Of course, you don't have to define all of your interface methods globally. You can create references in any
scope, as we discussed in "Scripting Objects". For example, the following code creates a scripted message
button object which displays a message when its pushed. The scripted object holds its own actionPerformed()
method, along with a variable to hold the Frame used for the GUI:
messageButton( message ) {
JButton button = new JButton("Press Me");
button.addActionListener( this );
JFrame frame = frame( button );
actionPerformed( e ) {
print( message );
frame.setVisible(false);
}
}
messageButton("Hey you!");
messageButton("Another message...");
The above example creates two buttons, with separate messages. Each button prints its message when pushed
and then dismisses itself. The buttons are created by separate calls to the messageButton() method, so each
will have its own method context, separate local variables, and a separate instance of the ActionListener
interface handler. Each registers itself (its own method context) as the ActionListener for its button, using its
own 'this' reference.
In this example all of the "action" is contained in messageButton() method context. It serves as a scripted
object that implements the interface and also holds some state, the frame variable, which is used to dismiss the
GUI. More generally however, as we saw in the "Scripting Objects" section, we could have returned the 'this'
reference to the caller, allowing it to work with our messageButton object in other ways.
Scripting Interfaces 30
Interface Types and Casting
It is legal, but not usually necessary to perform an explicit cast of a BeanShell scripted object to an interface
type. For example:
actionPerformed( event ) {
print( event );
}
button.addActionListener(
(ActionListener)this ); // added cast
In the above, the cast to ActionListener would have been done automatically by BeanShell when it tried to
match the 'this' type argument to the signature of the addActionListener() method.
Doing the cast explicitly has the same effect, but takes a different route internally. With the cast, BeanShell
creates the necessary adapter that implements the ActionListener interface first, at the time of the cast, and
then later finds that the method is a perfect match.
What's the difference? Well, there are times where performing an explicit cast to control when the type is
created may be important. Specifically, when you are passing references out of your script, to Java classes that
don't immediately use them as their intended type. In our earlier discussion we said that automatic casting
happens "within your BeanShell scripts". And in our examples so far BeanShell has always had the
opportunity to arrange for the scripted object to become the correct type, before passing it on. But it is
possible for you to pass a 'this' reference to a method that, for example, takes the type 'Object', in which case
BeanShell would have no way of knowning what it was destined for later. You might do this, for example, if
you were placing your scripted objects into a collection (Map or List) of some kind. In that case, you can
control the process by performing an explicit cast to the desired type before the reference leaves your script.
Another case where you may have to perform a cast is where you are using BeanShell in an embedded
application and returning a scripted object as the result of an eval() or a get() variable from the Interpreter
class. There again is a case where BeanShell has no way of knowing the intended type within the script. By
performing an explicit cast you can create the type before the reference leaves your script.
We'll discuss embedded applications of BeanShell in the "Embedding BeanShell" section a bit later, along
with the Interpreter getInterface() method, which is another way of accomplishing this type of cast from
outside a script.
"Dummy" Adapters and Incomplete Interfaces
It is common in Java to see "dummy" adapters created for interfaces that have more than one method. The job
of a dummy adapter is to implement all of the methods of the interface with stubs (empty bodies), allowing
the developer to extend the adapter and override just the methods of interest.
We hinted in our earlier discussion that BeanShell could handle scripted interfaces that implement only the
subset of methods that are actually used and that is indeed the case. You are free in BeanShell to script only
the interface methods that you expect to be called. The penalty for leaving out a method that is actually
invoked is a special run−time exception: java.lang.reflect.UndeclaredThrowableException, which the caller
will receive.
The UndeclaredThrowableException is an artifact of Java Proxy API that makes dynamic interfaces possible.
It says that an interface threw a checked exception type that was not prescribed by the method signature. This
Interface Types and Casting 31
is a situation that cannot normally happen in compiled Java. So the Java reflection API handles it by wrapping
the checked exception in this special unchecked (RuntimeException) type in order to throw it. You can get the
underlying error using the exception's getCause() method, which will, in this case, reveal the BeanShell
EvalError exception, reporting that the scripted method of the correct signature was not found.
The invoke() Meta−Method
BeanShell provides a very simple short−hand mechanism for scripting interfaces with large numbers of
methods. You can implement the special method invoke( name, args ) in any scripted context. The invoke()
method will be called to handle the invocation of any method of the interface that is not defined. For example:
mouseHandler = new MouseListener() {
mousePressed( event ) {
print("mouse button pressed");
}
invoke( method, args ) {
print("Undefined method of MouseListener interface invoked:"
+ name +", with args: "+args
);
}
};
In the above example we have neglected to implement four of the five methods of the MouseListener
interface. They will be handled by the invoke() method, which will simply print the name of the method and
its arguments. However since mousePressed() is defined it will be called for the interface.
Here is a slightly more realistic example of where this comes in handy. Let's use the invoke() method to print
the names of methods called via the ContentHandler interface of the Java SAX API, while parsing an XML
document.
import javax.xml.parsers.*;
import org.xml.sax.InputSource;
factory = SAXParserFactory.newInstance();
saxParser = factory.newSAXParser();
parser = saxParser.getXMLReader();
parser.setContentHandler( this );
invoke( name, args ) {
print( name );
}
parser.parse( new InputSource(bsh.args[0]) );
By running this script with the XML file as an argument, we can see which of the dozen or so methods of the
SAX API are being exercised by the structure of the document, without having to write a stub for each of
them.
Tip:
You can use the invoke( name, args ) meta−method directly in your own scope or in the global scope as
well, in which case you can handle arbitrary "unknown" method invocations yourself, perhaps to implement
your own "virtual" commands. Try typing this on the command line:
invoke(name,args) { print("Command: "+name+" invoked!"); }
The invoke() Meta−Method 32
noSuchMethod(); // prints "Command: noSuchMethod() invoked!"
Threads − Scripting Runnable
BeanShell 'this' type references can implement the standard java.lang.Runnable interface. So you can declare a
"run()" method in your bsh objects and make it the target of a Thread:
foo() {
run() {
// do work...
}
return this;
}
foo = foo();
// Start two threads on foo.run()
new Thread( foo ).start();
new Thread( foo ).start();
BeanShell is thread−safe internally, so as long as your scripts do not explicitly do anything ordinarily
non−thread safe (e.g. access shared variables or objects) you can write multi−threaded scripts.
Note:
You can use the bg() "background" command to run an external script in a separate thread. See
bg().
Limitations
When running under JDK 1.3 or greater BeanShell can script any kind of Java interface. However when
running under JDK 1.2 (or JDK1.1 + Swing) only the core AWT and Swing interfaces are available. To
support those legacy cases a special extension of the 'this' reference implementation (the bsh.This class) is
loaded which implements these interfaces along with Runnable, statically.
Threads − Scripting Runnable 33
Special Variables and Values
In addition to the scope modifiers: 'this', 'super', 'global', BeanShell supports a number of pre−defined system
variables, "magic" values, and methods.
Special Values
$_ − The value of the last expression evaluated. The strange construct for this is drawn from Perl, but
the idea exists in many scripting languages. It is useful for getting back the last result when you are
working interactively.
•
$_e − The last uncaught exception object thrown. This is useful in interactive use for retrieving the
last exception to inspect it for details.
•
bsh − The BeanShell root system object, containing system information and variables.•
bsh.args − An array of Strings passed as command line arguments to the BeanShell interpreter.•
bsh.shared − A special static space which is shared across all interpreter instances. Normally each
bsh.Interpreter instance is entirely independent; having its own unique global namespace and settings.
bsh.shared is implemented as a static namespace in the bsh.Interpreter class. It was added primarily to
support communication among instances for the GUI desktop.
•
bsh.console − If BeanShell is running in its GUI desktop mode, this variable holds a reference to the
current interpreter's console, if it has one.
•
bsh.appletcontext − If BeanShell is running inside an Applet, the current applet context, if one exists.•
bsh.cwd − A String representing the current working directory of the BeanShell interpreter. This is
used or manipulated by the cd(), dir(), pwd(), and pathToFile() commands.
•
bsh.show − A boolean value used by the show() command. It indicates whether results are always
printed, for interactive use.
•
bsh.interactive − A boolean indicating whether this interpreter running in an interactive mode•
bsh.evalOnly − A boolean indicating whether this interpreter has an input stream or whether is it only
serving as an engine for eval() operations (e.g. for embedded use).
•
Note:
The choice of "bsh" for the root system object name was somewhat unfortunate because it
conflicts with the current package name for BeanShell (also bsh). This means that if you wish to
work with BeanShell classes explicitly from BeanShell scripts (e.g. bsh.Interpreter) you must
first import them, e.g.:
import bsh.Interpreter;
i=new Interpreter();
Special Members of 'this' type References
'this' type references have several "magic" members:
this.variables − An array of Strings listing the variables defined in the current method context
(namespace).
•
this.methods − An array of Strings listing the methods defined the current method context
(namespace).
•
this.interpreter − A bsh.Interpreter reference to the currently executing BeanShell Interpreter object.•
this.namespace − A bsh.NameSpace reference to the BeanShell NameSpace object of the current
method context. See "Advanced Topics".
•
this.caller − A bsh.This reference to the calling BeanShell method context. See "Variables and Scope
Modifiers".
•
Special Variables and Values 34
this.callstack − An array of bsh.NameSpace references representing the "call stack" up to the current
method context. See "Advanced Topics".
•
These magic references are primarily used by BeanShell commands.
Undefined Variables
You can test to see if a variable is defined using the special value void. For example:
if ( foobar == void )
// undefined
You can return a variable to the undefined state using the unset() command:
a == void; // true
a=5;
unset("a"); // note the quotes
a == void; // true
Setting the Command Prompt
Users may set the command line prompt string for use in interactive mode by setting the value of or command does not exist, throws an exception, or does not
return a String, a default prompt of "bsh % " will be used.
Undefined Variables 35
BeanShell Commands
BeanShell commands appear to the user as pre−defined methods such as print(), load(), or save(). BeanShell
Commands can be implemented as scripted methods or compiled Java classes which are dynamically loaded
on demand from the classpath. We'll talk about adding your own commands in the next section "Adding
BeanShell Commands".
Tip:
You can easily override any BeanShell command simply by defining the method yourself in your script. For
example:
print( arg ) {
System.out.println( "You printed: " + arg );
}
If you define the method in the global scope it will apply everywhere. If you define it local to a scripted
object it will only apply in that object context.
Commands Overview
This is a high level overview of the BeanShell command set. You can find full documentation for all
BeanShell commands in the "BeanShell Commands Documentation" section of this manual. See also the
"BshDoc" section which covers javadoc style documentation of BeanShell script files.
Interpreter Modes
The following commands affect general modes of operation of the interpreter.
exit() Exit the interpreter. (Also Control−D).
show() Turn on "show" mode which prints the result of every evaluation that is not of void type.
setAccessibility() Turn on access to private and protected members of Java classes.
server()
Launch the remote access mode, allowing remote access to the interpreter from a web
browser or telnet client.
debug()
Turns on debug mode. Note: this is very verbose, unstructured output and is primarily of
interest to developers.
setStrictJava()
Turn on "strict Java" mode which enforces Java compatibility by dissallowing loose
types and undeclared variables.
Output
The following commands are used for output:
print(),
error()
Print output to standard out or standard error. print() always goes to the console, whereas
System.out may or may not be captured by a GUI console or servlet.
frame() Display the AWT or Swing component in a Frame
BeanShell Commands 36
Source and Evaluation
The following commands are used for evaluation or to run external scripts or applications:
eval() Evaluate a string as if it were typed in the current scope.
source(),
sourceRelative()
Read an external script file into the interpreter and evaluate it in the current scope
run(), bg()
Run an external file in a subordinate interpreter or in a background thread in a
subordinate interpreter.
exec() Run a native executable in the host OS
Utilities
The following commands are useful utilities:
javap() Print the methods and fields of an object, similar to the output of javap
which()
Like the Unix 'which' command for executables. Map the classpath and determine the location
of the specified class.
load(),
save()
load a serializable object from a file or save one to a file. Special handling is provided for
certain objects.
object() Create an "empty" object context to hold variables; analogous to a Map.
Variables and Scope
The following commands affect the current scope:
clear() Clear all variables, methods and imports from the current scope.
unset() Remove a variable from the current scope. (Return it to the "undefined" state).
setNameSpace()
Set the current namespace to a specified scope. Effectively bind the current scope to a
new parent scope.
Classpath
The following commands manipulate or access the classpath:
addClassPath(), setClassPath(),
getClassPath()
Modify the BeanShell classpath.
reloadClasses() Reload a class or group of classes.
getClass()
Load a class explicitly taking into account the BeanShell
classpath.
getResource() Get a resource from the classpath.
Source and Evaluation 37
Files and Directories
The following commands work with files, directories, and the working directory:
cd(), pwd(), dir(), rm(), mv(), cat() Unix Style file commands.
pathToFile()
Translate a relative path to an absolute path taking into account the
BeanShell current working directory.
Desktop and Class Browser
The following commands work with GUI tools:
classBrowser(), browseClass() Open a class browser window or browse a specific class or object.
desktop() Launch the BeanShell GUI desktop.
setNameCompletion() Turn on or off name completion in the GUI console.
Note:
The dir() command is written in Java; primarily as a demonstration of how to do this when
desired.
Files and Directories 38
Adding BeanShell Commands
BeanShell Commands are scripted methods or compiled Java classes which are dynamically loaded from the
classpath to implement a method. All of the standard commands we discuss in this manual live in the
BeanShell JAR file under the path /bsh/commands.
Adding to the set of "prefab" commands supplied with BeanShell is as easy as writing any other BeanShell
methods. You simply have to place your script into a file named with the same name as the command and
place the file in the classpath. You may then "import" the commands with the importCommands() method.
Command files can be placed anywhere in the BeanShell classpath. You can use even use the addClassPath()
or setClassPath() commands to add new command directories or JARs containing commands to your script at
any time.
Hello World
For example, let's make a helloWorld() command:
// File: helloWorld.bsh
helloWorld() {
print("Hello World!");
}
Place the command file helloWorld.bsh in a directory or JAR in your classpath and import it with the
importCommands() command. You can either set the classpath externally for Java or inside of BeanShell with
the addClassPath() command. For example, suppose we have placed the file in the path:
/home/pat/mycommands/helloWorld.bsh. We could then do:
addClassPath("/home/pat"); // If it's not already in our classpath
importCommands("/mycommands");
We can now use helloWorld() just like any other BeanShell command.
helloWorld(); // prints "Hello World!"
importCommands() will accept either a "resource path" style path name or a Java package name. Either one is
simply converted to a resource path or Java package name as required to load scripts or compiled BeanShell
command classes. A relative path (e.g. "mycommands") is turned into an absolute path by prepending "/".
You may import "loose" commands (like unpackaged classes) at the top of the classpath by importing "/".
If for example you have placed BeanShell commands along with your other classes in a Java package called
com.xyz.utils in your classpath, you can import those commands with:
// equivalent
importCommands("com.xyz.utils");
importCommands("/com/xyz/utils");
Imported commands are scoped just like imported classes. So if you import commands in a method or object
they are local to that scope.
Adding BeanShell Commands 39
Overloaded Commands
BeanShell command scripts can contain any number of overloaded forms of the command method, e.g.:
// File: helloWorld.bsh
helloWorld() {
print("Hello World!");
}
helloWorld( String msg ) {
print("Hello World: "+msg);
}
BeanShell will select the appropriate method based on the usual rules for methods selection.
Compiled Commands
You can also implement BeanShell commands as compiled classes instead of scripts if you wish. Your class
name must simply be the name of the command (matching case as well) and it must implement one or more
static invoke() methods who's signatures match a pattern. The first two arguments of the invoke() method
must be the bsh.Interpreter and bsh.CallStack objects that provide context to all BeanShell scripts. Then any
number (possibly zero) of arguments, which are the arguments of the command may follow. BeanShell will
select the appropriate method based on the usual rules for methods selection.
The dir() command is an example of a BeanShell command that is implemented in Java. Let's look at a
snippet from it to see how it implements a pair of invoke() methods for the dir() and dir(path) commands.
/**
Implement dir() command.
*/
public static void invoke( Interpreter env, CallStack callstack )
{
String dir = ".";
invoke( env, callstack, dir );
}
/**
Implement dir( String directory ) command.
*/
public static void invoke(
Interpreter env, CallStack callstack, String dir )
{
...
}
User Defined Commands with invoke()
It is useful to note that the invoke() meta−method which we described in the section "Scripting Interfaces" can
be used directly in scope as well as through an object reference and one could use this to load arbitrary
commands or implement arbitrary behavior for commands (undefined method calls). For example:
invoke( String methodName, Object [] arguments ) {
print("You invoked the method: "+ methodName );
}
Overloaded Commands 40
// invoke() will be called to handle noSuchMethod()
noSuchMethod("foo");
invoke() is called to handle any method invocations for undefined methods within its scope. In this case we
have declared it at the global scope.
Commands Scope
Scripted BeanShell commands are loaded when no existing method matches the command name. When a
command script is loaded it is sourced (evaluated) in the 'global' scope of the interpreter. This means that once
the command is loaded the methods declared in the command script are then defined in the interpreter's global
scope and subsequent calls to the command are simply handled by the those methods as any other scripted
method.
Note:
Note that this means that currently scripted commands may only be loaded once and then they
are effectively cached.
Getting the Caller Context
A useful feature of BeanShell for command writers is the 'this.caller' reference, which allows you to create
side effects (set or modify variables) in the method caller's scope. For example:
fooSetter() {
this.caller.foo=42;
}
The above command has the effect that after running it the variable 'foo' will be set in the caller's scope. e.g.:
fooSetter();
print( foo ); // 42
It may appear that we could simply have used the 'super' modifier to accomplish this and in this case it would
have worked. However it would not have been correct in general because the 'super' of fooSetter() always
points to the same location − the scope in which it was defined. We would like fooSetter() to set the variable
in whatever scope it was called from.
To reiterate: The 'super' of a method is always the context in which the method was defined. But the caller
may be any context in which the method is used. In the following example, the parent context of foo() and the
caller context of foo() are the same:
foo() { ... }
foo();
But this is not always the case, as for bar() in the following example:
foo() {
bar() { ... }
...
}
Commands Scope 41
// somewhere
fooObject.bar();
The special "magic" field reference: 'this.caller' makes it possible to reach the context of whomever called
bar(). The 'this.caller' reference always refers to the calling context of the current method context.
The diagram above shows the foo() and bar() scopes, along with the caller's scope access via 'this.caller'.
This is very useful in writing BeanShell commands. BeanShell command methods are always loaded into the
global scope. If you refer to 'super' from your command you will simply get 'global'. Often it is desirable to
write commands that explicitly have side effects in the caller's scope. The ability to do so makes it possible to
write new kinds of commands that have the appearance of being "built−in" to the language.
A good example of this is the eval() BeanShell command. eval() evaluates a string as if it were typed in the
current context. To do this, it sends the string to an instance of the BeanShell interpreter. But when it does so
it tells the interpreter to evaluate the string in a specific namespace: the namespace of the caller; using
this.caller.
eval("a=5");
print( a ); // 5
The eval() command is implemented simply as:
eval( String text ) {
this.interpreter.eval( text, this.caller.namespace );
}
As a novelty, you can follow the call chain further back if you want to by chaining the '.caller' reference, like
so:
this.caller.caller...;
Or, more generally, another magic reference 'this.callstack' returns an array of bsh.NameSpace objects
representing the full call "stack". This is an advanced topic for developers that we'll discuss in another
location.
Commands Scope 42
setNameSpace()
In the previous discussion we used the this.caller reference to allow us to write commands that have side
effects in the caller's context. This is a powerful tool. But what happens when one command calls another
command that intends to do this? That would leave the side effects in the first command's context, not it's
original caller. Fortunately this doesn't come up all that often. But there is a general way to solve this problem.
That is to use the powerful setNameSpace() method to "step into" the caller's context. After that we may set
variables and call methods exactly as if we were in the caller's context (because we are). If all commands did
this there would be no need to use the this.caller reference explicitly (indeed, we may make it idiomatic for all
commands to do this in the future).
myCommand() {
// "Step into" the caller's namespace.
setNameSpace( this.caller.namespace );
// work as if we were in the caller's namespace.
}
You can try out the setNameSpace() command with arbitrary object scope's as well. For example:
object = object();
// save our namespace
savedNameSpace = this.namespace;
// step into object's namespace
setNameSpace( object.namespace );
// Work in the object's scope
a=1;
b=2;
// step back
setNameSpace( savedNameSpace );
print( object.a ); // 1
print( object.b ); // 2
print( a ); // ERROR! undefined
Getting the Invocation Text
You can get specific information about the invocation of a method using namespace.getInvocationLine() and
namespace.getInvocationText(). The most important use for this is in support of the ability to write an assert()
method for unit tests that automatically prints the assertion text.
assert( boolean condition )
{
if ( condition )
print( "Test Passed..." );
else {
print(
"Test FAILED: "
+"Line: "+ this.namespace.getInvocationLine()
+" : "+this.namespace.getInvocationText()
+" : while evaluating file: "+getSourceFileInfo()
setNameSpace() 43
);
super.test_failed = true;
}
}
Working with Dirctories and Paths
BeanShell supports the notion of a current working directory for commands that work with files. The cd()
command can be used to change the working directory and pwd() can be used to display the current value.
The BeanShell current working directory is stored in the variable bsh.cwd.
All commands that work with files respect the working directory, including the following:
dir()•
source()•
run(),•
cat()•
load()•
save()•
mv()•
rm()•
addClassPath()•
pathToFile()
As a convenience for writing your own scripts and commands you can use the pathToFile() command to
translate a relative file path to an absolute one relative to the current working directory. Absolute paths are
unmodified.
absfilename = pathToFile( filename );
Path Names and Slashes
When working with path names you can generally just use forward slashes in BeanShell. Java localizes
forward slashes to the appropriate value under Windows environments. If you must use backslashes remember
to escape them by doubling them:
dir("c:/Windows"); // ok
dir("c:\\Windows"); // ok
Working With Class Identifiers
You may have noticed that certain BeanShell commands such as javap(), which(), and browseClass() which
take a class as an argument can accept any type of argument, including a plain Java class identifier. For
example, all of the following are legal:
javap( Date.class ); // use a class type directly
javap( new Date() ); // uses class of object
javap( "java.util.Date" ); // Uses string name of class
javap( java.util.Date ); // Use plain class identifier
Working with Dirctories and Paths 44
In the last case above we used the plain Java class identifier java.util.Date. In Beanshell this resolves to a
bsh.ClassIdentifier reference. You can get the class represented by a ClassIdentifier using the
Name.identifierToClass() method. Here is an example of how to work with all of the above, converting the
argument to a class type:
import bsh.ClassIdentifier;
if ( o instanceof ClassIdentifier )
clas = this.namespace.identifierToClass(o);
if ( o instanceof String)
clas = this.namespace.getClass((String)o);
else if ( o instanceof Class )
clas = o;
else
clas = o.getClass();
Working with Iterable Types
In conjunction with the enhanced for−loop added in BeanShell version 1.3 a unified API was added to provide
support for iteration over composite types. The bsh.BshIterator interface provides the standard hasNext() and
next() methods of the java.util.Iterator interface, but is available in all versions of Java and can be created for
all composite types including arrays.
The BeanShell CollectionManager is used to get a BshIterator for an interable object or array. It is a
dynamically loaded extension, so it provides support for the java.util.Collections API when available, but
does not break compatability for Java 1.1 applications. You can use this in the implementation of BeanShell
commands to iterate over Enumeration, arrays, Vector, String, StringBuffer and (when the java.util.collections
API is present) Collections and Iterator.
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