How to check if a parameter of the current method has an annotation and retrieve that parameter value in Java?

Question

Consider this code:

public example(String s, int i, @Foo Bar bar) {
  /* ... */
}

I want to check if the method has an annotation @Foo and get the argument or throw an exception if no @Foo annotation is found.

My current approach is to first get the current method and then iterate through the parameter annotations:

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;

class Util {

    private Method getCurrentMethod() {
        try {
            final StackTraceElement[] stes = Thread.currentThread().getStackTrace();
            final StackTraceElement ste = stes[stes.length - 1];
            final String methodName = ste.getMethodName();
            final String className = ste.getClassName();   
            final Class<?> currentClass = Class.forName(className);
            return currentClass.getDeclaredMethod(methodName);
        } catch (Exception cause) {
            throw new UnsupportedOperationException(cause);
        }  
    }

    private Object getArgumentFromMethodWithAnnotation(Method method, Class<?> annotation) {
        final Annotation[][] paramAnnotations = method.getParameterAnnotations();    
            for (Annotation[] annotations : paramAnnotations) {
                for (Annotation an : annotations) {
                    /* ... */
                }
            }
    }

}

Is this the right approach or is there a better one? How would the code inside the forach loop look like? I'm not sure if I have understood the what getParameterAnnotations actually returns...

Answer 1

The outer for loop

for (Annotation[] annotations : paramAnnotations) {
   ...
}

should use an explicit counter, otherwise you don't know what parameter you are processing right now

final Annotation[][] paramAnnotations = method.getParameterAnnotations();
final Class[] paramTypes = method.getParameterTypes();
for (int i = 0; i < paramAnnotations.length; i++) {
    for (Annotation a: paramAnnotations[i]) {
        if (a instanceof Foo) {
            System.out.println(String.format("parameter %d with type %s is annotated with @Foo", i, paramTypes[i]);
        }
    }
}

Also make sure your annotation type is annotated with @Retention(RetentionPolicy.RUNTIME)

From your question it is not entirely clear what you are trying to do. We agree on the difference of formal parameters vs. actual arguments:

void foo(int x) { }

{ foo(3); }

where x is a parameter and 3 is an argument?

It is not possible to get the arguments of methods via reflection. If it is possible at all, you would have to use the sun.unsafe package. I can't tell you much about that though.