Java annotations are a powerful feature in Java that allow you to add metadata to your code in a declarative way. They provide additional information about classes, methods, and fields, which can be processed at compile-time or runtime. Annotations are defined using the @ symbol followed by the name of the annotation.

Annotations are widely used in Java frameworks and libraries, including Spring, Hibernate, and JUnit, to simplify configuration, enhance functionality, and support customizations.

Let's take a look at an example of a user-defined annotation:

1// Define a user-defined annotation
2@interface MyAnnotation {
3    String value();
4    int number() default 0;
5}
6
7// Annotate a class using the user-defined annotation
8@MyAnnotation(value = "Hello", number = 42)
9public class MyClass {
10    // ...
11}

In this example, we define a user-defined annotation MyAnnotation with two elements value and number. We then annotate a class MyClass using MyAnnotation and provide values for the elements.

Annotations can be used to control program behavior, generate boilerplate code, enforce coding standards, and provide additional documentation. They are a powerful tool for extending the capabilities of the Java language and enabling framework-specific functionality.