Java @Annotations

If you have been using Java for about a week (probably even less) chances are you have already seen some annotations. Let’s review some stuff about them.

As explained on https://docs.oracle.com/javase/tutorial/java/annotations/ we can use annotations to:

• Give information to the compiler : so we can detect errors or even suppress warnings.
• Compile-time and deployment-time processing : we can generate code, xml files and so on.
• Runtime processing : some annotations can be reviewed at runtime (using reflection).

Annotations can be used anywhere you use a type.

One interesting fact is that annotations can potentially generate code, but they can not modify it. If you use tools like lombok you know that they actually modify the code, but they usually achieve this by accessing internal libraries.

Annotations are defined with the @interface keyword like so:

public @interface Version {
  int mayor();
  int minor();
}

They can receive any primitive types, enums, other annotations and arrays of those previously mentioned. Let’s define an enum:

public enum Usage {
  UTILITY,
  DTO,
  DAO,
  SERVICE
}

Then let’s create another annotation:

public @interface Info {
  String[] authors();
  Usage usage();
  Version version();
}

Now we use it:

@Info(
    authors = {"jsedano", "weirderror"},
    usage = Usage.UTILITY,
    version = @Version(mayor = 1, minor = 0))
public class VeryUtil {}

You can specify if the annotation needs to be retained by the virtual machine at run time or not; or even if they are to be discarded by the compiler. This is achieved by using a meta-annotation, meta-annotations are annotations that are applied to another annotations.

You can also define where the annotation can be used, for example in methods, in constructors and so on.

Let’s look at how the @Override annotation is defined (@Override is a predefined annotation that is use to mark methods that are using method overriding):

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

As you can see the meta-annotation @Target is used to say that this particular annotation can only be used above a method declaration and the @Retention annotation is saying that this annotation will not be recorded in the class file and will not be available at runtime.

In order to actually define functionality for an annotation type we need to implement an AbstractProcessor.