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# 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.

### 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 );
    }

    // 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() { ... }
        ...
    }

    // 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.

## 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()
            );
            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

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 compatibility 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.

    cm = CollectionManager.getCollectionManager();
    if ( cm.isBshIterable( myObject ) )
    {
    	BshIterator iterator = cm.getBshIterator( myObject );
    	while ( iterator.hasNext() )
    		i = iterator.next();
    }

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