SpEL expressions - identify fields declared in an expression?

Question

I have the following code which evaluates a SpEL expression using data values defined in a Map object.

// data map
Map dataMap = new HashMap();
dataMap.put("abc",1);
dataMap.put("def",2);
dataMap.put("xyz",1);
dataMap.put("qwerty",2);

// spel rule expression
String ruleExpression = "#abc+#def";

// Set evaluation context
StandardEvaluationContext stdContext = new StandardEvaluationContext();
stdContext.setVariables(map);

// Evaluate the SpEL expression
ExpressionParser parser = new SpelExpressionParser();
Object returnValue = parser.parseExpression(ruleExpression).getValue(stdContext);

// returnValue = 3 :-)

In the real world our map is populated based in a DB query result set and the 'ruleExpression' is only known at runtime. I have a new requirement to log the values defined in the 'ruleExpression' such that a string like this is generated

abc=1,def=2

A brute force approach might see us parsing the 'ruleExpression' string to identify fieldnames that start with '#' using regex but i could see how this could get messy as the complexity of the ruleExpression increases.

I'm wondering since the SpEl engine must identify the fields declared in the 'ruleExpression' during the parseExpress() phase is there a way for us to reuse this logic?

EDIT - I did come across the VariableScope private inner class on the org.springframework.expression.spel.ExpressionState class which seems to do what i want but alas the it's not accessible.

Answer 1

You can try overriding lookupVariable in StandardExpressionContext and adding your logging in there. Replacing your stdContext with the following will catch each variable as it is used:

    StandardEvaluationContext stdContext = new StandardEvaluationContext() {
        @Override
        public Object lookupVariable(String name) {
            Object value = super.lookupVariable(name);
            //do logging here
            System.out.println(name + "=" + value);
            return value;
        }
    };

This outputs:

abc=1
def=2

This will only catch the values which are used from variables. Anything that comes from a bean or other object etc. will not go through lookupVariable. Neither will variable values which are never used (due to a conditional for instance).