14_Annotation.md

Annotation

Characteristics of Annotations

• Annotation originally means comment, and it is a syntax based on interfaces
  • Although its role differs from comments, like comments, it can be attached to code to give classes special meaning or inject functionality
• Using annotations, you can directly specify validation conditions for data using annotations, making it easy to understand the validation conditions and keeping the code clean
• Annotations are broadly used for documentation, compiler checking, and code analysis
  • The documentation aspect is not widely used because JavaDoc exists
  • The essential purpose of annotations is to express metadata in source code
• The timing of interpretation can also be specified (Retention Policy)

Types of Annotations

There are broadly three types of annotations

1. Built-in annotations built into the JDK

• Mainly for the compiler, providing useful information to the compiler
• ex)

  • @Override

    The @Override annotation that appears when overriding a method through inheritance

    • Can only be placed before a method, and it indicates to the compiler that the current method is a method that overrides a super class method
    • Typos can occur in method names when overriding, and from the compiler's perspective, it cannot tell whether a new method is being created or overriding is being done
      • In such cases, the annotation can catch parts where typos may occur

  • @Deprecated

    • Indicates a method that should no longer be used because it may not be supported in future versions.

  • @SupressWarning

    • Conveys the programmer's intent to the compiler to remove warnings

  • @FunctionalInterface

    • Informs the compiler that the following interface is a functional interface
    • Used to prevent mistakes in advance for the same reason as the override annotation

2. Meta annotations that are annotations for representing information about annotations

• Annotations used on annotations that determine the behavior target of the annotation
  • Mainly used when defining new annotations
• ex)
  • @Target
    • Used to specify the targets to which the annotation can be applied
    • When specifying multiple values, braces { } must be used like in arrays
  • @Retention
    • An annotation mainly used when creating custom annotations, which allows you to set how long the annotation's memory should be retained
      • It is about setting the annotation's life cycle, i.e., determining how long the annotation will live!
    • Properties
      • RetentionPolicy
        • What is Retention Policy?
          • A policy that defines the lifecycle of an annotation
            • It is used to specify whether annotation information should be retained in the compiled class file
        • Types of Retention Policy
          • RetentionPolicy.SOURCE
            • Remains until the source code (.java)
              • Annotation information disappears during the compilation process
            • Mainly used to generate compiler warnings or error messages
          • RetentionPolicy.CLASS
            • Remains until the class file (.class) (=bytecode)
              • Not retained at runtime
            • Mainly used for code generation at compile time or for compiler processing
          • RetentionPolicy.RUNTIME
            • Remains until runtime (=effectively never disappears)
            • Mainly used to read or modify annotation information through reflection at runtime
              • ex) Providing functionality to change or add class behavior at runtime using specific annotations
  • @Inherited
    • An annotation that allows the annotation to be inherited by child classes
    • If this annotation is placed on a parent class, the child class is recognized as if it also has this annotation
  • @Native
    • An annotation placed on constant fields referenced by native methods
      • A native method refers to a method of the OS where the JVM is installed
      • Native methods are usually written in C, and in Java, only the method declaration is defined without implementation
      • Most methods of the Object class are native methods
      • We have been calling OS methods through the Java language
      • The process of connecting native methods with methods defined in Java is called JNI (Java Native Interface)

3. Custom Annotations created directly by developers