Annotations

Introduction link

In Java, annotations can be used. These do not have an influence on the functionality of the code themselves. They allow to add additional metadata to an element. These can make the code more understandable for developers but can also be used by other parts of the program to work with. A good example for this is Spring Boot. In Spring Boot annotations are used to tell the framework which method of a controller should be called how. For example with @GetMapping it is defined that a method should be called when a GET request is sent to a specific URL.

Creating Annotations link

In Java, annotations can be created with the keyword @interface. For each annotation, a retention and target must be defined. The retention is set with the @Retention annotation and determines until when the annotation is retained. With SOURCE it is only present in the source code, with CLASS it is also written into the compiled .class file and with RUNTIME it is still present during the execution of the program. The target is set with @Target and determines on which elements the annotation can be applied. For example on methods, classes or variables.

Example link

CustomAnnotation.java

import java.lang.annotation.Retention;
import java.lang.annotation.Target;
 
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
 
@Retention(RUNTIME)
@Target(FIELD)
public @interface CustomAnnotation {
    public String name() default = "";
}

Ball.java

import CustomAnnotation;
 
public class Ball {
    @CustomAnnotation
    public Color farbe;
 
    @CusotmAnnotation(name = "Ballgrösse")
    public String grösse;
}