JAVA ANNOTATIONS HOW TO
JAVA ANNOTATIONS CODE

JAVA ANNOTATIONS CODE

While this may appear to be a nuanced distinction, it is a very important one that must be understood in order to grasp the usefulness of annotations.įor example, adding the annotation to an instance field does not in-and-of-itself change the runtime behavior of a program: The compiler simply includes the annotation at runtime, but the annotation does not execute any code or inject any logic that alters the normal behavior of the program (the behavior expected when the annotation is omitted). This is not to say that a framework may not change its behavior based on the presence of an annotation at runtime, but that the inclusion of an annotation does not itself change the runtime behavior of a program. 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.

JAVA ANNOTATIONS HOW TO

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. Now we're going to see how to take advantage of them by using Java's Reflection API.Annotations are a powerful part of Java, but most times we tend to be the users rather than the creators of annotations.

So far we've seen how to create custom annotations, and how to use them to decorate the Person class. Moreover, we want the initNames() method to be called before serialization.īy setting the key parameter of the annotation to “personAge,” we are indicating that we'll use this name as the identifier for the field in the JSON output.įor the sake of demonstration, we made initNames() private, so we can't initialize our object by calling it manually, and our constructors aren't using it either. In addition, the output should contain only the firstName, lastName, and age fields of that object. This.lastName = (0, 1).toUpperCase()īy using our custom annotations, we're indicating that we can serialize a Person object to a JSON string. Class Person String String = 'personAge')

Java annotations