Software Engineering Glossary

Java Annotation

Also known as: Annotation Custom Annotation @interface Java Annotations

Definition

A Java annotation is metadata attached to source code with an @Name tag. You declare a custom annotation with @interface, then control where it may appear with @Target and how long it is kept with @Retention. Annotations do not run by themselves. A compiler, an annotation processor, or runtime reflection has to read them and act. That is how JUnit finds @Test methods and how Spring honors @Autowired.

Key Takeaways

  • Annotations are labels, not executable logic. Something else must consume them.
  • @Retention(RetentionPolicy.RUNTIME) is required if you will read the annotation with reflection. The default is CLASS, which is invisible at runtime.
  • @Target limits the annotation to types, methods, fields, parameters, and so on.
  • Members look like methods and may return primitives, String, Class, enums, annotations, or arrays of those types.

How It Works

  1. You declare @interface MyTest and add meta-annotations for retention and target.
  2. You put @MyTest on a method, class, or field.
  3. At compile time a processor may generate code, or at runtime reflection calls isAnnotationPresent and getAnnotation.
  4. The JVM stores RUNTIME annotations with class metadata that the class loader built from bytecode.

Where It Is Used

  • JUnit uses @Test to discover test methods without a hardcoded list.
  • Spring, Jackson, Hibernate, and Bean Validation all drive behavior from annotations.
  • Lombok and MapStruct use SOURCE retention so the annotation disappears after code generation.

Related glossary terms