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%

My code then parses these fields and renames file accoringly. But I want the user to be able to use an 'expression languge' so they can say things like:

$if(%disctotal% >= 01,%discno%) 

Use ifelse, compare length and case and so on.

I dont want to write this from scratch, is there something that offers me this that I can just plugin to my code?

EDIT:I think Ive had some good replies, but my knowledge is letting me down. Lets simplify the issue I want the user to be able to write

$if(%disctotal% >= 01,%discno%)

into a field in a gui.

Then later on in my Java code I want to be able to apply this expression to a set of files, so for each file I have the value of disctotal, and discno but I want to be able to convert the expression into something like

if(discTotal >=1) { return discNo } else { return "" }

What I dont want to do is have to write the code that recognises the string (" $if(%disctotal% >= 01,%discno%)" is an if statement because this is akward to do.

Then expanding on this I would like the expression to allow things such as capitalizing oof fields, checking lengths of fields and so on.

Alternatively: Perhaps it should work this way, the user enters the expression, then at a later date for each file the Java code replaces each variable with the real value

i.e $if(%disctotal% >= 01,%discno%) -> $if(2 >= 01,1)

then this is passed to the exopression language to parse and give a result,

Is this the Javascript idea ?

Faqir

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.

Aldous

This might be the right solution, but I dont know Im struggling to get my head round this. – Faqir

OK I will give more details in answer – Aldous

Answer 2

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.

Geniagenial

Maybe but I dont want to output something from Java, I want to go the other way, user enters an expression that is converted into java ( I think) – Faqir

Answer 3

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.

Arak

Could be, depending on your familiarity with grammars. Whether it's merited depends partly on the complexity of the grammar you want to support. – Arak

Answer 4

I guess you are looking for a template engine

Strophanthus