Java Annotations
Java Annotations
"Annotations are only metadata and do not contain any business logic"
"Annotations only provide information about the attribute (class/method/package/field) on which it is defined. The consumer is a piece of code that reads this information and then performs the necessary logic."
@Override annotation in above code. Even if I don’t put @Override, code works properly without any issue.
@Override tells the compiler that this method is an overridden method (metadata about the method), and if any such method does not exist in a parent class, then throw a compiler error (method does not override a method from its super class).
if I would have made a typography mistake and used the method name as toStrring() {double r}, and if I wouldn’t have used @Override, my code would have compiled and executed successfully
注解需要三要素：定义、使用、读取并执行
Target
Retention - RetentionPolicy.RUNTIME
package AnnotationsSample;
public class BusinessLogic {
public BusinessLogic() {
super();
}
public void compltedMethod() {
System.out.println("This method is complete");
}
@Todo(priority = Todo.Priority.HIGH)
public void notYetStartedMethod() {
// No Code Written yet
}
@Todo(priority = Todo.Priority.MEDIUM, author = "Uday", status = Todo.Status.STARTED)
public void incompleteMethod1() {
//Some business logic is written
//But its not complete yet
}
@Todo(priority = Todo.Priority.LOW, status = Todo.Status.STARTED, first = "good william")
public void incompleteMethod2() {
//Some business logic is written
//But its not complete yet
}
public static void main(String[] args) {
System.out.println(BusinessLogic.class.getAnnotation(Todo.class).first());
}
}package AnnotationsSample;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@interface Todo {
String first() default "tianya";
public enum Priority {LOW, MEDIUM, HIGH}package AnnotationsSample;
public enum Status {STARTED, NOT_STARTED}
String author() default "Yash";
Priority priority() default Priority.LOW;
Status status() default Status.NOT_STARTED;
}import java.lang.reflect.Method;
public class TodoReport {
public TodoReport() {
super();
}
public static void main(String[] args) {
getTodoReportForBusinessLogic();
}
/**
* This method iterates through all messages of BusinessLogic class and fetches annotations defined on each of them.
* After that it displays the information from annotation accordingly.
*/
private static void getTodoReportForBusinessLogic() {
Class businessLogicClass = BusinessLogic.class;
for(Method method : businessLogicClass.getMethods()) {
Todo todoAnnotation = method.getAnnotation(Todo.class);
if(todoAnnotation != null) {
System.out.println(" Method Name : " + method.getName());
System.out.println(" Author : " + todoAnnotation.author());
System.out.println(" Priority : " + todoAnnotation.priority());
System.out.println(" Status : " + todoAnnotation.status());
System.out.println(" Status : " + todoAnnotation.first());
System.out.println(" --------------------------- ");
}
}
}