I made an annotation in Java.


I was working and realized that I didn't understand annotations. I understand that @Override, @Duplicate, @Test are given, but I don't know how it works, so I looked it up.


What are annotations in Java?

The following is from wikipedia.

Java annotation is a function to enter additional information as metadata for classes, methods, and packages, and was added in Java SE 5. Annotations can also be created by implementing the java.lang.annotation.Annotation interface.

Types of annotations in Java

The types of annotation are classified into the following three types.

--Marker annotation – Annotation with no data and only a name. Example: @Override, @Duplicate --Single value annotation – Annotation that has only one data. It looks like a method call. Example: @SuppressWarnings --Full annotation – Annotation with multiple data.

Make it more familiar than learning

Create annotation class


import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

public @interface Info {
	String value();

To define the annotation class, add the following annotations.

CLASS: Recorded in the class file at compile time. Not loaded into the VM at run time. RUNTIME: Loaded into the VM at run time SOURCE: Discard at compile time

ʻANNOTATION_TYPE: Applicable to annotations CONSTRUCTOR: Applicable to constructor FIELD: Applicable to fields LOCAL_VARIABLE: Applicable locally METHOD: Applicable to methods PACKAGE: Applicable to packages PARAMETER`: Applicable to arguments

This time, I want to create a process to get the annotation value when executing the method, so Specify @Retention as RUNTIME and @Target as METHOD.

Create annotation class and call class

It will be the class to call. Add the created Info annotation to the method.


public class AnnotationInstance {
	public void execute() {
		System.out.println("Hello World!!");

@Info("hoge") The value is set in the @Info annotation. hoge is set in value.

It will be the calling main class.


import java.lang.reflect.Method;

public class AnnotationMain {

	public static void main(String[] args) {
		try {
			Class<?> clazz = Class.forName("AnnotationInstance");
			Method method1 = clazz.getMethod("execute", new Class<?>[] {});
			Info info1 = method1.getAnnotation(Info.class);
		} catch (ClassNotFoundException | NoSuchMethodException | SecurityException e) {

Get the method by reflection, and then get the annotation class.

I will try to run

Executing the ʻAnnotationMain.java` class created above will output the following result.

Execution result

It was confirmed that the value of the @Info annotation specified in the ʻexecute () method of ʻAnnotationInstance was acquired.

at the end

It was surprisingly easy to use. In the process of writing this article, I revisited some implementations of annotation classes. I don't usually touch it, but I think it's used because it's a framework and package development. I haven't been able to read it until now, so I'll read it without feeling weak in the future. I also touched the reflection for the first time in a while. I don't really understand that either, so I'll take a closer look.

Referenced site

Annotation-Wikipedia https://ja.wikipedia.org/wiki/%E3%82%A2%E3%83%8E%E3%83%86%E3%83%BC%E3%82%B7%E3%83%A7%E3%83%B3#Java%E3%81%AB%E3%81%8A%E3%81%91%E3%82%8B%E3%82%A2%E3%83%8E%E3%83%86%E3%83%BC%E3%82%B7%E3%83%A7%E3%83%B3 A little special Java annotation-Qiita https://qiita.com/kashira2339/items/6450714e42c37b441514 Sample to create your own annotation and get the value | ITSakura https://itsakura.com/java-annotation-make

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