How to create custom annotation in java?

Question

I want to create custom annotation in java for DirtyChecking. Like I want to compare two string values using this annotation and after comparing it will return a

Answer 1

First you need to mark if annotation is for class, field or method. Let's say it is for method: so you write this in your annotation definition:

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface DirtyCheck {
    String newValue();
    String oldValue();
}

Next you have to write let's say DirtyChecker class which will use reflection to check if method has annotation and do some job for example say if oldValue and newValue are equal:

final class DirtyChecker {

    public boolean process(Object instance) {
        Class clazz = instance.getClass();
        for (Method m : clazz.getDeclaredMethods()) {
            if (m.isAnnotationPresent(DirtyCheck.class)) {
                DirtyCheck annotation = m.getAnnotation(DirtyCheck.class);
                String newVal = annotation.newValue();
                String oldVal = annotation.oldValue();
                return newVal.equals(oldVal);
            }
        }
        return false;
    }
}

Cheers, Michal