Question

I have written a small class, which reads out annotation from methods.
Now I want to extend that class to make it more dynamic.

My class uses at the moment following code for reading out the annotation:

ExtendedCommandAnnotation e = foo.getClass()
                                 .getAnnotation(ExtendedCommandAnnotation.class);
String startTag = e.annoPropStartTag();

That is the simple case with fixed annotation.
In the new version I haven’t any fixed annotation. I will get the annotation ‘ExtendedCommandAnnotation’ in a variable.
So the code above will be edited to:

String className= "ExtendedCommandAnnotation";
??? e = foo.getClass().getAnnotation(Class.forName(className));
String startTag = e.annoPropStartTag();

I don’t know what I shall put instead of the ???. I tried it with Annotation, but then I can’t get the properties with the defined methods.
Is there any way to get this working?

My annotation “class”:

@Retention( RetentionPolicy.RUNTIME )
public @interface ExtendedCommandAnnotation
{
    String   annoPropUseTab() default "0";
    String   annoPropStartTag() default "";
    String   annoPropEndTag() default "";
}

EDIT:

Finally I get something like that:
String[] cmdMethNames = this.getAvailableCommandNames();

    Class<?> annotationClass = Class.forName(this.annotationClassName); 

    for( Method meth : cmdMeth )
    {
        HashMap<String, String> tempAnno = new HashMap<String, String>();
        if (meth.isAnnotationPresent((Class<? extends Annotation>) annotationClass))
        {
            Annotation anno = meth.getAnnotation((Class<? extends Annotation>) annotationClass);
            [...]
        }
        [...]
    }

But the cast to (Class<? extends Annotation>) make following warning: “Type safety: Unchecked cast from Class< capture#4-of ? > to Class< ? extends Annotation >“

Answer 1

/* Foo.java */

@ExtendedCommandAnnotation(annoPropStartTag = "hello")
public class Foo {
}

/* ExtendedCommandAnnotation.java */

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Retention( RetentionPolicy.RUNTIME )
public @interface ExtendedCommandAnnotation {
    String annoPropUseTab() default "0";
    String annoPropStartTag() default "";
    String annoPropEndTag() default "";
}

/* Main.java */

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

public class Main {
    public static void main(String[] args) {
        doOriginalImplementation();     // Prints "hello"
        doReflectionImplementation();   // Prints "hello"
    }

    public static void doOriginalImplementation() {
        Foo foo = new Foo();
        ExtendedCommandAnnotation e = foo.getClass().getAnnotation(ExtendedCommandAnnotation.class);
        String startTag = e.annoPropStartTag();
        System.out.println(startTag);
    }

    public static void doReflectionImplementation() {
        Foo foo = new Foo();

        Annotation[] annotations = foo.getClass().getAnnotations();
        // or the statement below, depends on what you intent to do:
        // Annotation[] annotations = foo.getClass().getDeclaredAnnotations();

        Class classOfExtendedCommandAnnotation = null;
        Annotation annotationOnClassFoo = null;
        for (Annotation a : annotations) {
            Class classA = a.annotationType();
            if ("ExtendedCommandAnnotation".equals(classA.getName())) {
                classOfExtendedCommandAnnotation = classA;
                annotationOnClassFoo = a;
                break;
            }
        }

        Method methodAnnoPropStartTag = null;
        if (classOfExtendedCommandAnnotation != null) {
            try {
                methodAnnoPropStartTag = classOfExtendedCommandAnnotation.getMethod("annoPropStartTag");
            } catch (NoSuchMethodException e) {
                throw new RuntimeException(e);
            }
        }

        if (methodAnnoPropStartTag != null) {
            try {
                String startTag = (String) methodAnnoPropStartTag.invoke(annotationOnClassFoo);
                System.out.println(startTag);
            } catch (ClassCastException e) {
                throw new RuntimeException(e);
            } catch (IllegalAccessException e) {
                throw new RuntimeException(e);
            } catch (IllegalArgumentException e) {
                throw new RuntimeException(e);
            } catch (InvocationTargetException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

In my solution, the class ExtendedCommandAnnotation need not to be present at compile time. However, the class Foo must be present. The solution could be modified a little bit so that the class Foo need not to be present too.