Parse Nested User-Defined Functions using Spring Expression Language (SpEL)

Spring Expression Language (SpEL) is a powerful expression language that supports querying and manipulation of objects during runtime. Among its many capabilities, SpEL supports invoking methods and evaluating nested expressions, which can be particularly useful when working with user-defined functions.

Here's a guide on how to parse nested user-defined functions using SpEL:

1. Setting up a Spring Boot Application:

If you haven't already, set up a basic Spring Boot project using the Spring Initializr with the Spring Web dependency. This step is crucial to leverage Spring's capabilities.

2. Define Your User-Defined Functions:

For this example, let's define two functions:

concatenate: Concatenates two strings.
toUpperCase: Converts a string to uppercase.

@Component
public class CustomFunctions {

    public String concatenate(String str1, String str2) {
        return str1 + str2;
    }

    public String toUpperCase(String str) {
        return str.toUpperCase();
    }
}

3. Use SpEL to Parse Nested Functions:

To evaluate nested expressions, you can use SpEL's Expression and ExpressionParser classes. Let's define a service that uses SpEL to parse nested functions:

@Service
public class ExpressionEvaluator {

    @Autowired
    private CustomFunctions customFunctions;

    public String evaluateNestedFunctions(String expression) {
        ExpressionParser parser = new SpelExpressionParser();
        StandardEvaluationContext context = new StandardEvaluationContext();
        context.setRootObject(customFunctions);

        return parser.parseExpression(expression, new TemplateParserContext()).getValue(context, String.class);
    }
}

In the evaluateNestedFunctions method, the context is set with the root object as customFunctions, so we can access the user-defined methods in our SpEL expressions. The TemplateParserContext is used to parse expressions within ${...} delimiters.

4. Testing the Nested Functions:

Now, let's test our nested functions by invoking concatenate and toUpperCase together:

@RestController
public class TestController {

    @Autowired
    private ExpressionEvaluator evaluator;

    @GetMapping("/test")
    public String test() {
        // This will concatenate "Hello " and "world", and then convert the result to uppercase
        return evaluator.evaluateNestedFunctions("${toUpperCase(concatenate('Hello ', 'world'))}");
    }
}

When you navigate to /test, you should see the result HELLO WORLD.

Conclusion:

SpEL offers a flexible way to parse and evaluate nested expressions, allowing users to chain or nest methods easily. By defining a context and using an appropriate parser context, SpEL provides a powerful mechanism for evaluating complex expressions at runtime.