Home | Back | Contents | Next

# The BeanShell Parser

This BeanShell parser class bsh.Parser is used internally by the BeanShell Interpreter. It is responsible for the lexical parsing of the input text, the application of the grammar structure, and the building of an internal representation of the BeanShell script file called an "abstract syntax tree" (AST).

The Parser just analyzes the language syntax. It knows only how to parse the structure of the language - it does not interpret names, or execute methods or commands. You can use the Parser directly if you have a need to analyze the structure of BeanShell scripts or Java methods and statements in general.

## Validating Scripts With bsh.Parser

You can use the Parser class from the command line to do basic structural validation of BeanShell files without actually executing them. e.g.

    java bsh.Parser [ -p ] file [ file ] [ ... ]

The -p option causes some of the abstract syntax to be printed.

The parser will detect any syntax errors in the script and print an error. Note again that names, imports, and string evaluations are analyzed only for syntax - not content or meaning.

## Parsing and Performance

It is useful to have a high level understanding how BeanShell works with scripts to understand performance issues.

The first time a script is read or sourced into an interpreter, BeanShell uses the parser to parse the script internally to an AST. The AST consists of Java object representations of all of the language structures and objects. The AST consists of Java classes, but is not the same as compiled Java code. When the script is "executed" BeanShell steps through each element of the AST and tells it to perform whatever it does (e.g. a variable assignment, for-loop, etc.). This execution of the ASTs is generally much faster than the original parsing of the text of the method. It is really only limited by the speed of the application calls that it is making, the speed of the Java reflection API, and the efficiency of the implementation of the structures in BeanShell.

When parsing "line by line" through a BeanShell script the ASTs are routinely executed and then thrown away. However the case of a BeanShell method declaration is different. A BeanShell method is parsed only once: when it is declared in the script. It is then stored in the namespace like any variable. Successive invocations of the method execute the ASTs again, but do not re-parse the original text.

This means that successive calls to the same scripted method are as fast as possible - much faster than re-parsing the script each time. You can use this to your advantage when running the same script many times simply by wrapping your code in the form of a BeanShell scripted method and executing the method repeatedly, rather than sourcing the script repeatedly. For example:

    // From Java
    import bsh.Interpreter;
    i=new Interpreter();

    // Declare method or source from file
    i.eval("foo( args ) { ... }");

    i.eval("foo(args)"); // repeatedly invoke the method
    i.eval("foo(args)");
    ...

In the above example we defined a method called foo() which holds our script. Then we executed the method repeatedly. The foo() method was parsed only once: when its declaration was evaluated. Subsequent invocations simply execute the AST.

## Parsing Scripts Procedurally

If you are willing to learn about the BeanShell abstract syntax tree classes you can use the Parser to parse a BeanShell script into its ASTs like this:

    in=new FileReader("somefile.bsh");
    Parser parser = new Parser(in);
    while( !(eof=parser.Line()) ) {
        SimpleNode node = parser.popNode();
        // Use the node, etc. (See the bsh.BSH* classes)
        ...
    }

To learn more about the abstract syntax tree please download the source distribution and consult the source documentation.

Tip: The BshDoc bshdoc.bsh script uses the parser to extract method signatures and comments from a BeanShell file. Check it out for a more realistic example.

Note: Many components of the AST classes are not public at this time. Use setAccessibility(true) to access them.

Home | Back | Contents | Next