Simple example on annotations

ClassPreamble.java

package com.learnquest;

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;

@Target(ElementType.TYPE)
public @interface ClassPreamble {
	String[] authors();
	double version() default 1.0;
}

Person.java

package com.learnquest;

@ClassPreamble(authors = "Alice")
public class Person {
	@Value("Sarah")
	private String firstName;

	public Person(String firstName) {
		this.firstName = firstName;
	}

	public String getFirstName() {
		return firstName;
	}

	public void setFirstName(String firstName) {
		this.firstName = firstName;
	}

	@Override
	public String toString() {
		return super.toString();
	}
}

Test.java

package com.learnquest;

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

public class Test {
	public static void main(String[] args) throws Exception {
		Person p = new Person(null);
		System.out.println(p.getFirstName());
		performDependencyInjection(p);
		System.out.println(p.getFirstName());
	}
	private static void performDependencyInjection(Object obj) throws Exception{
		Field[] fields = obj.getClass().getDeclaredFields();
		for (Field field : fields) {
			if(field.getType().equals(String.class)){
				Annotation[] annotations = field.getDeclaredAnnotations();
				for (Annotation annotation : annotations) {
					if(annotation.annotationType().equals(Value.class)){
						// set a private field to be accessible
						boolean accessible = field.isAccessible();
						if(!accessible){
							field.setAccessible(true);
						}
						// retrieve the value from annotation
						Value valueAnnotation = (Value) annotation;
						String value = valueAnnotation.value();
						// set value on the field
						field.set(obj, value);
						// set accessibility back to private
						field.setAccessible(accessible);
					}
				}
			}
		}
	}
}

Value.java

package com.learnquest;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Value {
	String value();
}