秒懂，Java 注解 （Annotation）你可以这样学
理解Java注解类型(@Annotation)
Annotation 简介
image.png
系统内建的
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public class Person {
public String getInfo() {
return "父类....";
}
}
class Son extends Person {
@Override // 此处明确指出方法复写操作
public String getInfo() {
return "子类!!!!";
}
}
class AnnotationDemo {
public static void main(String[] args) {
Person person = new Son();
System.out.println(person.getInfo());
// 子类!!!!
}
}
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public class Person {
@Deprecated // 此处声明不建议使用操作
public String getInfo() {
return "父类....";
}
}
class Son extends Person {
public String getInfo() { // 编译时,将出现警告信息
return "子类!!!!";
}
}
class AnnotationDemo {
public static void main(String[] args) {
Person person = new Son();
// 编译时,将出现警告信息
System.out.println(person.getInfo());
// 子类!!!!
}
}
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public class Demo<T> {
private T var;
public T getVar() {
return var;
}
public void setVar(T var) {
this.var = var;
}
}
class DemoTest {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
Demo demo = new Demo();
demo.setVar("tom");
System.out.println(demo.getVar());
}
}
image.png
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
@Deprecated
public class Demo<T> {
private T var;
public T getVar() {
return var;
}
public void setVar(T var) {
this.var = var;
}
}
class DemoTest {
@SuppressWarnings({"unchecked","Deprecated"}) // 此处压制两条警告
// @SuppressWarnings(value = {"unchecked","Deprecated"})
public static void main(String[] args) {
Demo demo = new Demo();
demo.setVar("tom");
System.out.println(demo.getVar());
}
}
自定义Annotation
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public @interface MyDefaultAnnotationNoneParam { // 定义Annotation
}
@MyDefaultAnnotationNoneParam // 使用自定义Annotation
class AnnotationDemos {
}
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public @interface MyDefaultAnnotationNoneParam { // 定义Annotation
public String value();
}
@MyDefaultAnnotationNoneParam("tom") // 使用自定义Annotation
class AnnotationDemos {
}
设置多个属性
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public @interface MyDefaultAnnotationNoneParam { // 定义Annotation
public String key();
public String value();
}
@MyDefaultAnnotationNoneParam(key="name",value = "tom") // 使用自定义Annotation
class AnnotationDemos {
}
传递多个可以定义一个数组
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public @interface MyDefaultAnnotationNoneParam { // 定义Annotation
public String[] value();
}
@MyDefaultAnnotationNoneParam(value = {"tom","jack"}) // 使用自定义Annotation
class AnnotationDemos {
}
设置默认值
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public @interface MyDefaultAnnotationNoneParam { // 定义Annotation
public String key() default "name";
public String value() default "tom";
}
@MyDefaultAnnotationNoneParam()
// 使用自定义Annotation
class AnnotationDemos {
}
使用枚举限制类型
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
enum MyName {
TOM, JACK;
}
public @interface MyDefaultAnnotationNoneParam { // 定义Annotation
public MyName name() default MyName.JACK; // 只能设置枚举中的取值
}
@MyDefaultAnnotationNoneParam(name = MyName.JACK)
// 使用自定义Annotation
class AnnotationDemos {
}
image.png
通过反射取出Annotation
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public class SimpleBeanOne {
@SuppressWarnings("unchecked")
@Deprecated
@Override
public String toString() {
return "Hello Word";
}
}
package com.kaishengit.wanggs;
import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public class ReflectDemo01 {
public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException {
Class<?> c = null;
// 取得class实例
c = Class.forName("com.kaishengit.wanggs.SimpleBeanOne");
// 获取头string方法
Method method = c.getMethod("toString");
// 获取全部Annotation
Annotation annotations[] = method.getDeclaredAnnotations();
for (Annotation annotation : annotations){
System.out.println(annotation);
}
}
/**
* @java.lang.Deprecated()
*
* toString方法虽然有3个Annotation注解,但是最后真正的只有一个,这是因为Deprecated 用的是RUNTIME的声明,所以只有它可以输出
*/
}
获取指定Annotation的内容
package com.kaishengit.wanggs;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
// * 此Annotation在类中执行依然有效
@Retention(value = RetentionPolicy.RUNTIME)
public @interface MyDefaultAnnotationNoneParam { // 定义Annotation
public String key() default "name";
public String value() default "tom";
}
package com.kaishengit.wanggs;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public class SimpleBeanOne {
@SuppressWarnings("unchecked")
@Deprecated
@Override
// 使用自定义Annotation 并设置值
@MyDefaultAnnotationNoneParam(key = "keys",value = "jack")
public String toString() {
return "Hello Word";
}
}
package com.kaishengit.wanggs;
import java.lang.reflect.Method;
/**
* @author Wgs
* @version 1.0
* @create：2018/05/07
*/
public class ReflectDemo01 {
public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException {
Class<?> c = null;
// 取得class实例
c = Class.forName("com.kaishengit.wanggs.SimpleBeanOne");
// 获取头string方法
Method method = c.getMethod("toString");
if (method.isAnnotationPresent(MyDefaultAnnotationNoneParam.class)) {
// 声明对象
MyDefaultAnnotationNoneParam mda = null;
// 取得自定义Annotation
mda = method.getAnnotation(MyDefaultAnnotationNoneParam.class);
String key = mda.key();
String value = mda.value();
System.out.println("key :" + key + "value:" + value);
}
}
/**
* key :keysvalue:jack
*
*/
}
@Targer
@Documented
总结
/** *自定义注解MyAnnotation */
@Target(ElementType.TYPE) //目标对象是类型
@Retention(RetentionPolicy.RUNTIME) //保存至运行时
@Documented //生成javadoc文档时，该注解内容一起生成文档
@Inherited //该注解被子类继承
public @interface MyAnnotation {
public String value() default ""; //当只有一个元素时，建议元素名定义为value(),这样使用时赋值可以省略"value="
String name() default "devin"; //String
int age() default 18; //int
boolean isStudent() default true; //boolean
String[] alias(); //数组
enum Color {GREEN, BLUE, RED,} //枚举类型
Color favoriteColor() default Color.GREEN; //枚举值
}
@MyAnnotation(
value = "info",
name = "myname",
age = 99,
isStudent = false,
alias = {"name1", "name2"},
favoriteColor = MyAnnotation.Color.RED
)
public class MyClass {
//使用MyAnnotation注解，该类生成的javadoc文档包含注解信息如下：
/* @MyAnnotation(value = "info", name = "myname", age = 99, isStudent = false, alias = {"name1","name2"}, favoriteColor = Color.RED) public class MyClass extends Object */
}
public class MySubClass extends MyClass{
//子类MySubClass继承了父类MyClass的注解
}
/***********注解声明***************/
/** * 水果名称注解 */
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitName {
String value() default " ";
}
/** * 水果颜色注解 */
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitColor {
/** * 颜色枚举 */
public enum Color{BLUE, RED, GREEN};
/** * 颜色属性 * @return */
Color fruitColor() default Color.GREEN;
}
/** * 水果供应商注解 */
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitProvider {
/** * 供应商编号 * @return */
public int id() default -1;
/** * 供应商名称 * @return */
public String name() default " ";
/** * 供应商地址 * @return */
public String address() default " ";
}
/***********注解使用***************/
public class Apple {
@FruitName("Apple")
private String appleName;
@FruitColor(fruitColor = FruitColor.Color.RED)
private String appleColor;
@FruitProvider(id = 1, name = "陕西红富士集团", address = "陕西红富士大厦")
private String appleProvider;
public String getAppleProvider() {
return appleProvider;
}
public void setAppleProvider(String appleProvider) {
this.appleProvider = appleProvider;
}
public String getAppleName() {
return appleName;
}
public void setAppleName(String appleName) {
this.appleName = appleName;
}
public String getAppleColor() {
return appleColor;
}
public void setAppleColor(String appleColor) {
this.appleColor = appleColor;
}
public void displayName(){
System.out.println(getAppleName());
}
}
/***********注解信息获取***************/
public class AnnotationParser {
public static void main(String[] args) {
Field[] fields = Apple.class.getDeclaredFields();
for (Field field : fields) {
//System.out.println(field.getName().toString());
if (field.isAnnotationPresent(FruitName.class)){
FruitName fruitName = field.getAnnotation(FruitName.class);
System.out.println("水果的名称：" + fruitName.value());
}else if (field.isAnnotationPresent(FruitColor.class)){
FruitColor fruitColor = field.getAnnotation(FruitColor.class);
System.out.println("水果的颜色："+fruitColor.fruitColor());
}else if (field.isAnnotationPresent(FruitProvider.class)){
FruitProvider fruitProvider = field.getAnnotation(FruitProvider.class);
System.out.println("水果供应商编号:" + fruitProvider.id() + " 名称:" + fruitProvider.name() + " 地址:" + fruitProvider.address());
}
}
}
}
/***********输出结果***************/
水果的名称：Apple
水果的颜色：RED
水果供应商编号:1 名称:陕西红富士集团 地址:陕西红富士大厦
