Java Annotations

Annotations are metadata about a program. Annotations will not have a direct effect on the operation of code they annotate.

An annotation looks like the following 

@Test

@ symbol indicates the compiler that the text followed after @ is an annotation. In the following example Author is the annotation name.

@Author(
   name = "Thetestbook.com",
   date = "12/19/2015"
) 

class AnnotationTest() { ... } 

What are the possible ways of applying annotations.

Annotations can be applied in many places like class, method, field and other program elements.

eg:

 myString = (@NonNull String) refStr;

Here @NotNull is an annotation to make sure the typecast string will not be null.

@Readonly is another useful annotation to create read only lists.

class UnmodifiableList<T> implements
        @Readonly List<@Readonly T> { ... }

Another useful annotation is SuppressWarnings to supress warnings of a method

@SuppressWarnings("deprecation")
    void useDeprecatedMethod() {
   }

Repeating annotations:

There are certain annotation types which will allow you to repeat some events on a timely basis. @Schedule is one of those annotations which is very useful in scheduling some events.

@Schedule(dayOfMonth="last")
@Schedule(dayOfWeek="Fri", hour="23")
public void doPeriodicCleanup() { ... }

The above example will schedule the method run on last day of the month and also on Friday 11 PM every week. These kind of annotations are very useful in a production environment to cleanup / purge some files and unwanted data which will pile up during the processing.

@Alert(role="Manager")
@Alert(role="Administrator")
public class UnauthorizedAccessException extends SecurityException { ... }

The above code will alert the users with Manager role and Administrator role of the application when a securityException occurs. This will be useful to notify people when an unauthorized person try to access the application. They can take the necessary action on time because of the notifications.