problem with custom annotations

[Ayan mallick]

    import java.lang.annotation.*;  
  
@Retention(RetentionPolicy.RUNTIME)  
@interface Todo {  
    String value();  
} 

import java.lang.annotation.*;  
  
@Todo("Hello world")  
public class Example {  
    public static void main(String[] args) {  
        Todo task = Example.class.getAnnotation(Todo.class);  
        System.out.println("Found annotation: " + task.value());  
    }  
} 

now my problem is i don't want to get a particular annotation or all annotations against a particular class(like here we are providing class object) rather i want want to get all classes in the class path which are annotated with that particular annotation.Then from them based on the parameter value i will find a specific class and i will instantiate that class in runtime.

trying to get an idea how to move from configuration file to annotation where i have to get fully qualified class name or Class object at runtime based on annotation so that i can instantiate that bean and populate.
like JSF @ManagedBean().

please give me some idea on how i can proceed.

[Jayesh A Lalwani]

No there is no direct way to get all classes annottated with an annotation. However there are different strategies that you can use

A) if you already have a factory or a DI framework that instantiates the classes, you can have that framework scan the class that its instantiating.
B) you can scan all the classes loaded by class loader and look for a particular annotation. If you are using spring, spring provides a class for doing this
C) there is nothing stopping you from processing runtime annotations during compile time. At compile time you can generate a property file containing the list of classes to scan. You can use this during runtime

[Jan Hoppmann]

Luckily, there are libraries that can help you. There are Scannotations and the Reflections library. I only have experience with the latter, but it works fine for me.

[Ayan mallick]

Thanks a lot to both.@Jan Hoppmann @Jayesh A Lalwani

I have found one way to do it using libraries
Here is my modified implementation.

@Target({ ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
public @interface IAnnotation {
   public String owner();
}

@IAnnotation(owner="Ayan")
public class CustomAnnotation {
public void printSuccess(){
	System.out.println("success comes at last in custom annotation");
}
	
}

@IAnnotation(owner="Hyan")
public class TwinCustomAnnotation {

	public void printSuccess(){
		System.out.println("success comes at last in Twin custom annotation");
	}
}

public class AnnotationPuzzleCracker {
	 
	public static void main(String [] args) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException{
		Reflections reflections = new Reflections("org.example.annotation");
		
	     Set<Class<?>> annotated = 
             reflections.getTypesAnnotatedWith(IAnnotation.class);
	     
	     System.out.println("printing size========>"+annotated.size());
	    for (Class<?> class1 : annotated) {
	    	System.out.println(class1.getName());
	    	IAnnotation iAnnotation = class1.getAnnotation(IAnnotation.class);
	    	if(iAnnotation.owner().equals("Ayan")){
	    		Object customAnnot = class1.newInstance();
		    	Method [] methodArr = class1.getDeclaredMethods();
		    	for (Method method : methodArr) {
		    		String methodName = method.getName();
		    		System.out.println("methodName===>"+methodName);
		    		if(methodName.equals("printSuccess")){
		    			//Class [] paramArr = method.getParameterTypes();
		    			method.invoke(customAnnot);
		    		}
				}
	    	}
	    	
		}

	     
	}
}

But one confusion regarding this line Reflections reflections = new Reflections("org.example.annotation"); say if i have the frame work and someone else developing classes using that framework then how would i know
runtime the package prefix.Is there anyway to know the package structure runtime.Again looking for your help