Consider the following code implementation. The goal is to automatically identify and process beans with in a specified package, where the total number of beans and their annotation status are unknown befroehand.

package annotation.processor;

import java.io.File;
import java.net.URL;
import java.util.HashMap;
import java.util.Map;

public class BeanAnnotationScanner {
    public static void main(String[] args) throws Exception {
        Map<String, Object> beanRegistry = new HashMap<>();
        String targetPackage = "com.example.beans";
        String packageAsPath = targetPackage.replaceAll("\.", "/");
        URL packageResource = ClassLoader.getSystemClassLoader().getResource(packageAsPath);
        String absolutePath = packageResource.getPath();
        File directory = new File(absolutePath);
        File[] classFiles = directory.listFiles();
        
        for (File file : classFiles) {
            String fileName = file.getName();
            String className = targetPackage + "." + fileName.split("\\.")[0];
            Class<?> beanClass = Class.forName(className);
            if (beanClass.isAnnotationPresent(Component.class)) {
                Component componentAnnotation = beanClass.getAnnotation(Component.class);
                String beanId = componentAnnotation.identifier();
                Object beanInstance = beanClass.getDeclaredConstructor().newInstance();
                beanRegistry.put(beanId, beanInstance);
            }
        }
        System.out.println(beanRegistry);
    }
}

Executnig this code may result in an error:

Exception in thread "main" java.lang.InstantiationException: com.example.beans.User
    at java.lang.Class.newInstance(Class.java:427)
    at annotation.processor.BeanAnnotationScanner.main(BeanAnnotationScanner.java:28)
Caused by: java.lang.NoSuchMethodException: com.example.beans.User.<init>()
    at java.lang.Class.getConstructor0(Class.java:3082)
    at java.lang.Class.newInstance(Class.java:412)
    ... 1 more

The error com.example.beans.User.<init>() indicates a failure during object instantiation of the User class via newInstance(). This method relies on a no-argument constructor. The issue arises if the User class has a custom constructor that overrides the default no-argument constructor.

Solution: Either remove the custom constructor from the User class or explicitly define a no-argument constructor.