The annotations on the code or decorators have become very common. They allow the programmer to add additional useful information about how to improve the code or change how to compile / run a particular class. They are a Java extension to allow aspect-oriented programming.
There are three types of annotations, according to the moment when they are used:
Information for the Compiler
These annotations allow the compiler to indicate whether or not to ignore errors and warnings or what to do with them. If you’ve worked with a Java IDE (like eclipse) probably you would have used this type of annotations, for example using @Override on a function to indicate that you are overwriting a method defined on a parent class. This annotation is completely optional, but allows both the compiler and the developer to check that they are indeed overwriting existing hierarchical functionality.
For example:
public class Parent {
public void do(){
System.out.println("Parent");
}
}
public class Son extends Parent{
@Override
public void do(){
System.out.println("Son");
}
}
Compiler-time and deployment-time processing
These annotations allow the compiler to add extra information about how to generate the code. It can serve to modify classes (adding or modifying functionality from those described in the source code), create new classes (based on a file descriptor), etc …
These annotations will only be visible at this point, they are not written on the .class files and therefore they cannot be available at runtime.
Runtime Annotations
This annotations can be used on runtime and they work on a very similar way as an interface.
Let’s see an example on how to create a Runtime Annotation and how can we use it. The annotation named MyAnnotation can be applied to elements oftype field:
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface MyAnnotation {
}
Now we can create an annotated class by this annotation:
public class myObject
{
@MyAnnotation
public String field;
}
This way, we can check by reflection if an object has an annotated field on any part of the code:
Class<?> res = objeto.getClass();
for (Field f : res.getFields()) {
if (f.isAnnotationPresent(MyAnnotation.class)) {
System.out.println("OK");
}
}
More Information:
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