Create your own Java annotations

What is an annotation?

Ability to add additional information to classes, methods, packages, etc.

Make your own annotation

import static java.lang.annotation.ElementType.TYPE;
import java.lang.annotation.Retention;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import java.lang.annotation.Target;

@Retention(RUNTIME)
@Target(TYPE)
public @interface TestAnnotation {
    String testValue();
}

@Retention Specifies the range in which annotation information is retained.

--SOURCE Discarded by the compiler. --CLASS Recorded in class files by the compiler, but not read at run time. (Default) --RUNTIME Recorded in class files by the compiler and read at run time.

@Target Specifies where annotations can be applied.

--TYPE class, interface or enum declaration --FIELD field declaration --METHOD method declaration --Declaration of argument of PARAMETER method --CONSTRUCTOR Constructor declaration --LOCAL_VARIABLE Local variable declaration --ʻANNOTATION_TYPE Annotation declaration --PACKAGE Package Declaration --TYPE_PARAMETERtype argument declaration --Where theTYPE_USE type is used --MODULE` module declaration

@interface Define the annotation.

Add your own annotation

@TestAnnotation(testValue = "test value")
public class AnnotatedClass {
}

This time, we are annotating the class.

Output annotation information

public class GetValue {
    public static void main(String[] args) {
        try {
            Class<?> targetClass = Class.forName("AnnotatedClass");
            var testAnnotation = (TestAnnotation) targetClass.getAnnotation(TestAnnotation.class);
            System.out.println(testAnnotation.testValue());
        } catch (Exception e) {
            //Handling of exceptions
        }
    }
}

When executed, test value is output.