List free variables in an EL expression

Question

I have an application which contains some EL evaluation used for programmatic configuration. Given an EL expression I want to get what free variables it contains without actually evaluating it. The intention is to provide a UI where end users can bind values to free variables before pressing an "evaluate" button.

Unfortunately javax.el.ValueExpression does not provide this functionality, so I may need to use vendor-specific API. It is quite early in the development, so I have not yet fixed my choice of implementation. I have thought of MVEL, JUEL and SpEL, but of course whatever I chose should have the functionality I described above.

Answer 1

MVEL's ParserContext can tell you all about the variables organized by locals and inputs.

ParserContext ctx = ParserContext.create();
MVEL.analysisCompile("a = 0; b = 0;", ctx);

HashMap<String, Class> vars = ctx.getVariables();

assert vars.containsKey("a") && Number.class.isAssignableFrom(vars.get("a"));
assert vars.containsKey("b") && Number.class.isAssignableFrom(vars.get("b"));

Answer 2

How about this...

    SpelExpression parseExpression = (SpelExpression) new SpelExpressionParser().parseExpression(expressionString);
    SpelNode node = parseExpression.getAST();
    List<String> vars = getVars(node);

...


private List<String> getVars(SpelNode node) {
    List<String> vars = new ArrayList<String>();
    for (int i = 0; i < node.getChildCount(); i++) {
        SpelNode child = node.getChild(i);
        if (child.getChildCount() > 0) {
            vars.addAll(getVars(child));
        }
        else {
            if (child instanceof VariableReference) {
                vars.add(child.toStringAST());
            }
        }
    }
    return vars;
}

Answer 3

Gary's answer is good but it didn't work for me when the expression contained a single variable, e.g. "#var" (one node with no children). A small change:

private Set<String> getVars(SpelNode node) {
    Set<String> vars = new HashSet<String>();
    if (node == null) {
        return vars;
    }

    if (node instanceof VariableReference) {
        // Remove the "#" to get the actual variable name
        vars.add(StringUtils.remove(node.toStringAST(), "#"));
    }

    for (int i = 0; i < node.getChildCount(); i++) {
        SpelNode child = node.getChild(i);
        vars.addAll(getVars(child));
    }

    return vars;
}