Repeating Annotations

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Until Java 8, two instances of the same annotation could not be applied to a single element. The standard workaround was to use a container annotation holding an array of some other annotation:

// Author.java
@Retention(RetentionPolicy.RUNTIME)
public @interface Author {
    String value();
}

// Authors.java
@Retention(RetentionPolicy.RUNTIME)
public @interface Authors {
    Author[] value();
}

// Test.java
@Authors({
    @Author("Mary"),
    @Author("Sam")
})
public class Test {
    public static void main(String[] args) {
        Author[] authors = Test.class.getAnnotation(Authors.class).value();
        for (Author author : authors) {
            System.out.println(author.value());
            // Output:
            // Mary
            // Sam
        }
    }
}

Java 8 provides a cleaner, more transparent way of using container annotations, using the @Repeatable annotation. First we add this to the Author class:

@Repeatable(Authors.class)

This tells Java to treat multiple @Author annotations as though they were surrounded by the @Authors container. We can also use Class.getAnnotationsByType() to access the @Author array by its own class, instead of through its container:

@Author("Mary")
@Author("Sam")
public class Test {
    public static void main(String[] args) {
        Author[] authors = Test.class.getAnnotationsByType(Author.class);
        for (Author author : authors) {
            System.out.println(author.value());
            // Output:
            // Mary
            // Sam
        }
    }
}

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