How Java annotations work

How
Java annotations
work

My credentials

✅ EqualsVerifier
✅ AnnotationScript
⛔ “Parallel Java”

What is this magic?

@Entity
@Data
public class User {
    @Id
    private Long id;

    @NotNull
    @Valid
    private String name;

    @Autowired
    private Service service;

    @Override
    public String toString() {
        return "User: " + name;
    }
}

What is an annotation?

metadata for code
for code analysis
overpowered

Syntax of an annotation

@Target(ElementType.METHOD)
public @interface MyAnnotation {

    String value();
}

Allowed parameter types

primitive types (int, long, double, etc)
String, Class, Enum
Another annotation
1-dimensional arrays of these

Must be CONSTANT

Parameter syntax

public @interface MyAnnotation {

    String value();
    int count() default 0;
}

✅ @MyAnnotation("x")
✅ @MyAnnotation(value = "x")
✅ @MyAnnotation(value = "x", count = 42)

⛔ @MyAnnotation("x", count = 42)
⛔ @MyAnnotation(count = 42)

Meta-annotations


`@Target`	🟢
`@Retention`	🟢
`@Documented`	⭕
`@Repeatable`	⭕
`@Inherited`	⭕

Optional meta-annotations


`@Documented`	put it in Javadoc
`@Repeatable`	use it multiple times
`@Inherited`	also put it on subclasses

Targets

@Target({ ElementType.FIELD, ElementType.METHOD })

Type
Field
Method
Parameter
Constructor
Local variable

Annotation type
Package
Type parameter
Type use
Module
Record

Retention: runtime

@Retention(RetentionPolicy.RUNTIME)

Available through reflection

Spring
JPA

Reading RUNTIME annotations

@MyAnnotation("woohoo")
public class MyClass {}

Class<?> myClass = MyClass.class;

Annotation[] annotations = myClass.getDeclaredAnnotations();
                               // Annotation[1] = { MyAnnotation("woohoo") }

MyAnnotation ann = myClass.getAnnotation(MyAnnotation.class);
                               // MyAnnotation("woohoo")

ann.value()
                               // "woohoo"

Reading RUNTIME annotations

public class MyClass {
    @MyAnnotation("wheee")
    public String myField;
}

Class<?> myClass = MyClass.class;
Field field = myClass.getField("myField");

Annotation[] annotations = field.getDeclaredAnnotations();
                               // Annotation[1] = { MyAnnotation("wheee") }

MyAnnotation ann = field.getAnnotation(MyAnnotation.class);
                               // MyAnnotation("wheee")

ann.value()
                               // "wheee"