Putting a simple expression language into java

Question

Im my Java application, users can specify how to name their files from a series of metadata fields. i.e

%artist% - %album% - %disctotal%

Answer 1

How about using the JavaScript engine that is bundled with Java 1.6?

You can see how you would pass in all the parameters:

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
engine.put("artist", artist);
...

You would read back the value using

ScriptEngine.get(...)

The final bit of glue would be to surround the user's expression with a function declaration, and write an expression that calls the function and assigns the result to a well known variable.

So to start experimenting, lets have a function to test expressions out:

private static void printResult(final ScriptEngine jsEngine, String name, String expr) throws ScriptException {
    Object result = jsEngine.eval(expr);
    System.out.println(name + " result: " + result + "; expr: " + expr);
}

Now let's call it:

public static void main(String[] args) throws Exception {
    ScriptEngineManager sem = new ScriptEngineManager();
    ScriptEngine jsEngine = sem.getEngineByName("JavaScript");
    printResult(jsEngine, "Hello World", "'Hello World'");
    printResult(jsEngine, "Simple Math", "123 + 456");
}

This produces:

Hello World result: Hello World; expr: 'Hello World'
Simple Math result: 579.0; expr: 123 + 456

Now lets try with your usecase:

public static void main(String[] args) throws Exception {
    ScriptEngineManager sem = new ScriptEngineManager();
    ScriptEngine jsEngine = sem.getEngineByName("JavaScript");

    String expr = "artist + '-' + album + (disktotal > 1 ? ('-D' + diskno) : '')";

    jsEngine.put("artist", "U2");
    jsEngine.put("album", "The Joshua Tree");
    jsEngine.put("disktotal", 1);
    jsEngine.put("diskno", 1);
    printResult(jsEngine, "Single Disk", expr);

    jsEngine.put("artist", "Tori Amos");
    jsEngine.put("album", "To Venus and Back");
    jsEngine.put("disktotal", 2);
    jsEngine.put("diskno", 2);
    printResult(jsEngine, "Muti-Disk", expr);
}

Produces result:

Single Disk result: U2-The Joshua Tree; expr: ...
Muti-Disk result: Tori Amos-To Venus and Back-D2; expr: ...

Notice how Tori's has 'D2' at the end.

Answer 2

One option would be to use a parser generator.

ANTLR is one such tool for Java. If you're a novice at creating grammars, you'll find the grammar IDE ANTLRWorks useful.

Answer 3

I guess you are looking for a template engine

Answer 4

I'd look into FreeMarker. It is a templating engine. You can pass your string with the el constructs to Freemarker and give it the objects you want to bind to your string and it will give you a string back.