介绍
尤其是我们在学习Spring时，都绕不开Annotation，而且使用的非常频繁，并且给我们带来很大的便利， 所以我们有必要了解JAVA Annotation。
在此，我们自定义两个Annotation：一个是Class相关的Annotation；另一个是Method相关的Annotation。然后，我们写一个类来使用那两个Annotation，这样我们就能比较快速的了解Annotation。
自定义一个Class相关的Annotation：Message.java
package shuai.study.annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Message {
String mess();
}
------------------------------------------------------------------------------------------------------------------------------------------------
对于上面的Annotation，你可能不清楚@Target(ElementType.TYPE) 和 @Retention(RetentionPolicy.RUNTIME) 是什么意思，那么我们就可以看一下JDK的源码：
public enum ElementType {
/** Class, interface (including annotation type), or enum declaration */
TYPE,
/** Field declaration (includes enum constants) */
FIELD,
/** Method declaration */
METHOD,
/** Parameter declaration */
PARAMETER,
/** Constructor declaration */
CONSTRUCTOR,
/** Local variable declaration */
LOCAL_VARIABLE,
/** Annotation type declaration */
ANNOTATION_TYPE,
/** Package declaration */
PACKAGE
}
public enum RetentionPolicy {
/**
* Annotations are to be discarded by the compiler.
*/
SOURCE,
/**
* Annotations are to be recorded in the class file by the compiler
* but need not be retained by the VM at run time. This is the default
* behavior.
*/
CLASS,
/**
* Annotations are to be recorded in the class file by the compiler and
* retained by the VM at run time, so they may be read reflectively.
*
* @see java.lang.reflect.AnnotatedElement
*/
RUNTIME
}
JDK的源码就是NB，写得非常清楚！
------------------------------------------------------------------------------------------------------------------------------------------------另一个Method相关的Annotation：Employee.java
package shuai.study.annotation;
import java.lang.annotation.Documented;
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)
@Documented
public @interface Employee {
String name() default "XXX";
int budget() default 0;
}
再写一个类Company.java，来使用上面的两个Annotation：
package shuai.study.annotation;
@Message(mess = "NB company publish month star, please pay attention to this message")
public class Company {
@Employee(name = "SB", budget = 6666)
public void monthStar() {
System.out.println("向钱看，向厚赚！");
}
}
最后再来一个测试启动类AnnotationTest.java：
package shuai.study.annotation;
import java.lang.reflect.Method;
public class AnnotationTest {
public static void main(String[] args) throws Exception {
Class<?> companyClass = Class.forName("shuai.study.annotation.Company");
if (companyClass.isAnnotationPresent(Message.class)) {
Message message = companyClass.getAnnotation(Message.class);
System.out.println("=== " + message.mess() + " ===");
}
Method[] methods = companyClass.getMethods();
int length = methods.length;
for (int index = 0; index < length; index++) {
if (methods[index].isAnnotationPresent(Employee.class)) {
Employee employee = methods[index].getAnnotation(Employee.class);
System.out.println("Employee Name: " + employee.name());
System.out.println("Employee Budget: " + employee.budget());
}
}
}
}
