Java seek a method with specific annotation and its annotation element

Question

Suppose I have this annotation class

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MethodXY {
    public int x();
    public int y();
}

public class AnnotationTest {
    @MethodXY(x=5, y=5)
    public void myMethodA(){ ... }

    @MethodXY(x=3, y=2)
    public void myMethodB(){ ... }
}

So is there a way to look into an object, "seek" out the method with the @MethodXY annotation, where its element x = 3, y = 2, and invoke it?

Thanks

Answer 1

Here is a method, which returns methods with specific annotations:

public static List<Method> getMethodsAnnotatedWith(final Class<?> type, final Class<? extends Annotation> annotation) {
    final List<Method> methods = new ArrayList<Method>();
    Class<?> klass = type;
    while (klass != Object.class) { // need to traverse a type hierarchy in order to process methods from super types
        // iterate though the list of methods declared in the class represented by klass variable, and add those annotated with the specified annotation
        for (final Method method : klass.getDeclaredMethods()) {
            if (method.isAnnotationPresent(annotation)) {
                Annotation annotInstance = method.getAnnotation(annotation);
                // TODO process annotInstance
                methods.add(method);
            }
        }
        // move to the upper class in the hierarchy in search for more methods
        klass = klass.getSuperclass();
    }
    return methods;
}

It can be easily modified to your specific needs. Pls note that the provided method traverses class hierarchy in order to find methods with required annotations.

Here is a method for your specific needs:

public static List<Method> getMethodsAnnotatedWithMethodXY(final Class<?> type) {
    final List<Method> methods = new ArrayList<Method>();
    Class<?> klass = type;
    while (klass != Object.class) { // need to iterated thought hierarchy in order to retrieve methods from above the current instance
        // iterate though the list of methods declared in the class represented by klass variable, and add those annotated with the specified annotation
        for (final Method method : klass.getDeclaredMethods()) {
            if (method.isAnnotationPresent(MethodXY.class)) {
                MethodXY annotInstance = method.getAnnotation(MethodXY.class);
                if (annotInstance.x() == 3 && annotInstance.y() == 2) {         
                    methods.add(method);
                }
            }
        }
        // move to the upper class in the hierarchy in search for more methods
        klass = klass.getSuperclass();
    }
    return methods;
}

For invocation of the found method(s) pls refer a tutorial. One of the potential difficulties here is the number of method arguments, which could vary between found methods and thus requiring some additional processing.

Answer 2

try this code sample:

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.annotation.ElementType;
import java.lang.reflect.InvocationTargetException;

class AnotTest {
    public static void main(String... args) {
        AnnotationTest at = new AnnotationTest();
        for (Method m : at.getClass().getMethods()) {
           MethodXY mXY = (MethodXY)m.getAnnotation(MethodXY.class);
           if (mXY != null) {
               if (mXY.x() == 3 && mXY.y() == 2){
                   try {
                       m.invoke(at);
                   } catch (IllegalAccessException e) {
                       //do nothing;
                   } catch (InvocationTargetException o) {
                       //do nothing;
                   }
               }
           }
        }
    }
    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.METHOD)
    static public @interface MethodXY {
        public int x();
        public int y();
    }

    static class AnnotationTest {
        @MethodXY(x=5, y=5)
        public void myMethodA() {
            System.out.println("boo");
        }

        @MethodXY(x=3, y=2)
        public void myMethodB() {
            System.out.println("foo");
        }
    }
}

Answer 3

private static final Method getMethodByAnnotation(Class<?> cla, Class<? extends Annotation> annotation,
        int methodIndex) {
    Stream<Method> stream = Arrays.stream(cla.getDeclaredMethods())
            .filter(m -> m.isAnnotationPresent(annotation));
    return methodIndex == 0 ? stream.findFirst().get() : stream.toArray(Method[]::new)[methodIndex];
}