It is important to note the last clause in this definition: Annotations have no effect on a program at runtime. Stated more formally, the Java Language Specification (JLS), Section 9.7, provides the following definition:Īnnotation is a marker which associates information with a program construct, but has no effect at run time. These decorators are benign and do not execute any code in-and-of-themselves, but can be used by runtime frameworks or the compiler to perform certain actions. What Are Annotations?Īnnotations are decorators that are applied to Java constructs, such as classes, methods, or fields, that associate metadata with the construct. The interested reader can find the source code for the completed JSON serializer on GitHub. Along the way, we will cover many of the common stumbling blocks of annotations, including the quirks of the Java reflection framework and visibility concerns for annotation consumers. In order to demonstrate how annotations work in practice, we will create a Javascript Object Notation (JSON) serializer that processes annotated objects and produces a JSON string representing each object. In this article, we will cover the basics of annotations, including what annotations are, how they are useful in large-than-academic examples, and how to process them. While custom annotations are an often-overlooked aspect of the Java language, they can be a very useful asset in developing readable code and just as importantly, useful in understanding how many common frameworks, such as Spring or Hibernate, succinctly accomplish their goals. For example, it is not difficult to find Java source code that includes the annotation processed by the Java compiler, the annotation used by the Spring framework, or the used by the Hibernate framework, but rarely do we see custom annotations. Annotations are a powerful part of Java, but most times we tend to be the users rather than the creators of annotations.
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