Using Java with Other Languages - Java - page 963

Roll Your Own Scripting Engine

Problem

You like javax.script but there isn't yet a script engine for your favorite language.

Solution

Roll your own: implement ScriptEngine and ScriptEngineFactory and add one trivial
configuration file.

Discussion

Hooking into an existing Scripting language sounds easy. It may or may not be, depending
on the language. Before you do implement one, check the set of languages available at Java's
website .

ScriptEngine is a fairly simple interface, with just over a dozen methods, of which six are
overloads of the popular eval method. There is an AbstractScriptEngine that handles all
these overloads and some other bookkeeping, leaving only four abstract methods. When you
extend AbstractScriptEngine , you only are required to implement four methods:

public public class class CalcScriptEngine CalcScriptEngine extends extends AbstractScriptEngine {

private private ScriptEngineFactory factory ;

CalcScriptEngine ( ScriptEngineFactory factory ) {
super

super ();
this

this . factory = factory ;

}

@Override
public public Object eval ( String script , ScriptContext context )

throws

throws ScriptException {
System . out . println ( "CalcScriptEngine.eval(): Running: " + script );
Stack < Integer > stack = new new Stack <>();

StringTokenizer st = new

new StringTokenizer ( script );

while while ( st . hasMoreElements ()) {

String tok = st . nextToken ();
iif ( tok . equals ( "+" )) {