Reflective Style Access to Scripted Methods

The following examples show how to work with BeanShell methods dynamically from within scripts, using the equivalent of reflective style access in Java. This is an advanced topic primarily of interest to developers who wish to do tight integration of BeanShell scripts with their application environment.

eval()

The simplest form of reflective style access to scripts is through the eval() command. With eval() you can evaluate any text just as if it had appeared in the current scope. For example:

eval("a=5;");
print( a ); // 5

So, if you know the signature (argument types) of a method you wish to work with you can simply construct a method call as a string and evaluate it with eval() as in the following:

// Declare methods foo() and bar( int, String )
foo() { ... }
bar( int arg1, String arg2 ) { ... }

// Invoke a no-args method foo() by its name using eval()
name="foo";
// invoke foo() using eval()
eval( name+"()");

// Invoke two arg method bar(arg1,arg2) by name using eval()
name="bar";
arg1=5;
arg2="stringy";
eval( name+"(arg1,arg2)");

You can get the names of all of the methods defined in the current scope using the 'this.methods' magic reference, which returns an array of Strings:

// Print the methods defined in this namespace
print( this.methods );

We'll talk about more powerful forms of method lookup in a moment.

invokeMethod()

You can explicitly invoke a method by name with arguments through a 'this' type reference using the invokeMethod() method:

this.invokeMethod( "bar", new Object [] { new Integer(5), "stringy" } );

Arguments are passed as an array of objects. Primitive types must be wrapped in their appropriate wrappers. BeanShell will select among overloaded methods using the standard Java method resolution rules. (JLS 15.11.2).

Method Lookup

The previous section showed how to invoke a method by name when we know the argument types. Of course, in general we'd like to be able to find out what methods are defined in the current script or to look up a method by its signature.

You can get "handles" to all of the methods defined in a context using the namespace getMethods() method. getMethods() returns an array of bsh.BshMethod objects, which are wrappers for the internally parsed representation of BeanShell scripted methods:

foo() { ... }
foo( int a ) { ... }
bar( int arg1, String arg2 ) { ... }

print ( this.namespace.getMethods() );

// Array: [Lbsh.BshMethod;@291aff {
//   Bsh Method: bar
//   Bsh Method: foo
//   Bsh Method: foo
// }

We'll talk about what you can do with a BshMethod in a moment.

Alternately, you can use the namespace getMethod() method to search for a specific method signature. The method signature is a set of argument types represented by an array of Classes:

name="bar";
signature = new Class [] { Integer.TYPE, String.class };

// Look up a method named bar with arg types int and String
bshMethod = this.namespace.getMethod( name, signature );

print("Found method: "+bshMethod);

Tip:
The Java reflection API uses special class values to represent primitive types such as int, char, an boolean. These types are static fields in the respective primitive wrapper classes. e.g. Integer.TYPE, Character.TYPE, Boolean.TYPE.

In the above snippet we located the bar() method by its signature. If there had been overloaded forms of bar() getMethod() would have located the most specific one according to the standard Java method resolution rules (JLS 15.11.2). The result of the lookup is a bsh.BshMethod object, as before.

BshMethod

You can inspect a BshMethod object to determine its method name and argument types:

name = bshMethod.getName();
Class [] types = bshMethod.getArgumentTypes();
Class returnType = bshMethod.getReturnType();

To invoke the BshMethod, call its invoke() method, passing an array of arguments, an interpreter reference, and a "callstack" reference.

// invoke the method with arg
bshMethod.invoke( new Object [] { new Integer(1), "blah!" }, 
    this.interpreter, this.callstack );

For the interpreter and callstack references you can simply pass along the current context's values via 'this.interpreter' and 'this.callstack', as we did above. The arguments array may be null or empty for no arguments.

Uses

Why would anyone want to do this? Well, perhaps you are sourcing a script created by a user and want to automatically begin using methods that they have defined. Perhaps the user is allowed to define methods to take control of various aspects of your application. With the tools we've described in this section you can list the methods they have defined and invoke them dynamically.

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