跟汤老师学Java笔记：注解的应用

完成：第一遍

1.注解一般怎么应用？

通过注解加载属性文件，为类中属性赋值

package annotation;import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Value {String value();
}

package annotation;import java.io.IOException;
import java.lang.reflect.Field;
import java.util.Properties;public class User {@Value("${user.username}")private String username;//如果是${user.pwd}，给password赋值的是配置文件里user.pwd对应的值//如果没有，那么就是将user.pwd赋值给password@Value("${user.pwd}")private String password;@Value("${user.email}")private String email;private String address;public String getUsername() {return username;}public User(){init();}// 初始化private void init() {/** 加载属性文件*/Properties p = new Properties();try {//Properties在src下的加载方法：p.load(User.class.getClassLoader().getResourceAsStream("info.properties"));} catch (IOException e) {e.printStackTrace();}/** 获取类中所有属性上的@Value注解，并解析处理*/// 获取所有属性Class<? extends User> cls = this.getClass();Field[] fields = cls.getDeclaredFields();// 循环获取每个属性上的@Value注解for (Field field : fields) {Value annotation = field.getAnnotation(Value.class);// 判断是否有该注解if(annotation!=null){// 获取注解上的值，并对值进行判断处理String value = annotation.value();if(value.startsWith("${")&&value.endsWith("}")){String key = value.substring(2, value.length()-1);value = p.getProperty(key);}// 为类中的属性赋值try {field.setAccessible(true);field.set(this, value);} catch (IllegalArgumentException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();}}}}public void setUsername(String username) {this.username = username;}public String getPassword() {return password;}public void setPassword(String password) {this.password = password;}public String getEmail() {return email;}public void setEmail(String email) {this.email = email;}@Overridepublic String toString() {return "User [username=" + username + ", password=" + password+ ", email=" + email + "]";}}

package annotation;public class Test {public static void main(String[] args) {System.out.println(new User());}
}

Properties一般放在src下

user.username=zs
user.pwd=123
user.email=zs@qq.com