Getting annotated variables from a class

Question

This question is a follow up to a question I found before
java: get all variable names in a class

What I want is to get variables from a class, but instead of getting them all, I want only the variables that have the annotation @isSearchable .

So basically I have 2 questions :

• How do I create an annotation ?

• How to filter my fields by only this annotation ?

And one more thing , if it is something I'm using frequently is it advisable (I'm guessing reflections should be slow).

Thank you

Answer 1

/** Annotation declaration */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface isSearchable{
    //...   
}

@isSearchable
public String anyField = "any value";

checking like:

//use MyClass.class.getDeclaredFields() if you want the fields only for this class.
//.getFields() returns the fields for all the class hierarchy
for(Field field : MyClass.class.getFields()){
    isSearchable s = field.getAnnotation(isSearchable.class);
    if (s != null) {
        //field has the annotation isSearchable
    } else {
        //field has not the annotation
    }
}

Answer 2

Here is an example

class Test {
    @IsSearchable
    String str1;
    String str2;

    @Target(ElementType.FIELD)
    @Retention(RetentionPolicy.RUNTIME)
    @interface IsSearchable {
    }

    public static void main(String[] args) throws Exception {
        for (Field f : Test.class.getDeclaredFields()) {
            if (f.getAnnotation(IsSearchable.class) != null) {
                System.out.println(f);
            }
        }
    }
}

prints

java.lang.String Test.str1