Java Tutorials - Herong's Tutorial Examples - v8.22, by Herong Yang
What Is Annotation
This section describes the annotation programming facility that allows you to associates additional information with a program construct.
What Is Annotation? - Annotation is a programming facility that allows you to associates additional information with a program construct like class, method, variable, etc.
Information provided by an annotation has no impact on the behavior of the program construct. But Java compiler and other tools can make use of the annotated information.
Annotation facility is designed in Java with 4 components:
1. Annotation Type - A special kind of interface type that defined by a "@interface" declaration statement (also called annotation declaration statement).
2. Annotation Declaration - A "@interface" declaration statement that defines an annotation type with or without annotation elements to capture annotated information.
3. Annotation Element - An special method with no parameters declared in an annotation type to capture a piece of annotated information.
4. Annotation Invocation - A "@annotation_type_name(annotated_information)" statement to invoke a given annotation type with specified annotated information. Annotation invocation statements can only be placed immediately before declaration statements of targeted program constructs.
Here is an example of "enum" declaration statement:
/* Header.java
* Copyright (c) HerongYang.com. All Rights Reserved.
*/
// Annotation declaration
@interface Header {
String usage(); // Usage of the targeted construct
}
Now we can use this annotation type to associate information with different types of program constructs using annotation invocation statements.
/* HeaderTest.java
* Copyright (c) HerongYang.com. All Rights Reserved.
*/
// Annotation invocation targeting a static variable
@Header(usage="Testing the @Header annotation.")
class HeaderTest {
// Annotation invocation targeting a static variable
@Header(usage="To whom you want to greet.")
private static String name = "Herong";
// Annotation invocation targeting a method
@Header(usage="The execution entry point.")
public static void main(String[] arg) {
// Annotation invocation targeting a local variable
@Header(usage="The execution entry point.")
String message = "How are you?";
System.out.println(name+", "+message);
}
}
If you compile and run HeaderTest.java, you will get:
herong> javac Header.java herong> javac HeaderTest.java herong> java Header Herong, How are you?
As you can see, those annotation invocation statements have no impact on the compilation and execution.
So annotations are useless from the execution point of view. But they do offer you a standard way of adding structured comments to your Java program.
Table of Contents
Execution Process, Entry Point, Input and Output
Primitive Data Types and Literals
Bits, Bytes, Bitwise and Shift Operations
Managing Bit Strings in Byte Arrays
Reference Data Types and Variables
StringBuffer - The String Buffer Class
System Properties and Runtime Object Methods
Generic Classes and Parameterized Types
Generic Methods and Type Inference
Lambda Expressions and Method References
Java Modules - Java Package Aggregation
Execution Threads and Multi-Threading Java Programs
ThreadGroup Class and "system" ThreadGroup Tree
Synchronization Technique and Synchronized Code Blocks
Deadlock Condition Example Programs
Garbage Collection and the gc() Method
Assert Statements and -ea" Option
►Annotation Statements and Declarations
Default Values for Annotation Elements
Single-Element Annotation Invocation
No-Element (Marker) Annotation Invocation