Create custom annotation

[Poornima Sharma]

Hi Friends,

How can we create a custom or our own annotation? What all we need to install? Are there any plugins in eclipse for the same? Please guide.

Thanks,

[Jesper de Jong]

You can create your own annotations like this:

@interface MyAnnotation {
  String text();
}

See this tutorial for more information.

You don't need to install anything special to create your own annotations. They are a standard part of the Java language since Java version 5.

[Rob Spoor]

Just don't expect the compiler to do anything special with them. There are only three annotations recognized by Oracle's compiler, and it's not a coincidence they are all located in the java.lang package; they are @Override, @Deprecated and @SuppressWarnings. Some IDEs add support for their own annotations to their own compilers. An example is the @NotNull annotation of IntelliJ IDEA.

[Poornima Sharma]

I went through the tutorial but it was not clear to me. Rob said that there is a @NotNull annotation by IntelliJ, how can we create such an annotation ourselves.

[Andrew Monkhouse]

You have not really described what you are trying to do. This makes it almost impossible to help you, as Annotations can cover chapters in books. But to give you a very simple example, I could create a simple annotation like:

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
@interface Todo {
    String value();
}

And then use it in my application as:

import java.lang.annotation.*;

@Todo("Hello world")
public class Example {
    public static void main(String[] args) {
        Todo task = Example.class.getAnnotation(Todo.class);
        System.out.println("Found annotation: " + task.value());
    }
}

[Poornima Sharma]

Hi Andrew,

Thanks for the reply, could you please let me know how can I learn annotations in deep, are there any books or any tutorials available? I am really curious to learn it but i could not find anything satisfactory on internet.

Thanks for your guidance.

[Poornima Sharma]

Please guide

[Andrew Monkhouse]

One of the best books (and not only for annotations - it is one of the best books a Java developer can have) is Effective Java (2nd Edition). It has several sections on annotations, and effectively shows how to create a framework such as JUnit using annotations.

Java (A Beginner's Tutorial) also has a chapter on annotations. However the link I gave you is for the Kindle edition, and while I know it is also available under Safari, I don't know if hard copy books are still available.