SpringBoot之自定义注解（基于BeanPostProcessor接口实现）

步骤
使用@interface 自定义注解
编写注解处理类，实现BeanPostProcessor接口
原理
实现BeanPostProcessor接口的类即为Bean后置处理器，Spring加载机制会在所有Bean初始化的时候遍历调用每个Bean后置处理器。
其顺序为：Bean实例化-》依赖注入-》Bean后置处理器-》@PostConstruct

缺陷
只有在会实例化成Bean的类中使用，注解才会生效。（因为是使用了Bean后置处理器实现的嘛）

代码示例
完整参考代码github

自定义注解接口

import org.springframework.stereotype.Component;
import java.lang.annotation.*;

@Target({ElementType.FIELD}) //声明应用在属性上
@Retention(RetentionPolicy.RUNTIME) //运行期生效
@Documented //Java Doc
@Component
public @interface Boy {
    String value() default "";
}

注解处理类

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.stereotype.Component;

import java.lang.reflect.Field;

@Component //注意：Bean后置处理器本身也是一个Bean
public class BoyAnnotationBeanPostProcessor implements BeanPostProcessor {
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        /**
         * 利用Java反射机制注入属性
         */
        Field[] declaredFields = bean.getClass().getDeclaredFields();
        for (Field declaredField : declaredFields) {
            Boy annotation = declaredField.getAnnotation(Boy.class);
            if (null == annotation) {
                continue;
            }
            declaredField.setAccessible(true);
            try {
                declaredField.set(bean, annotation.value());
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        }
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object o, String s) throws BeansException {
        return o; //这里要返回o，不然启动时会报错
    }
}

注解使用类

import org.springframework.stereotype.Service;
import javax.annotation.PostConstruct;

@Service
public class HelloBoy {

    @Boy("小明")
    String name = "world";

    public void sayHello() {
        System.out.println("hello, " + name);
    }
}

测试类

import com.markey.annotations.Boy.HelloBoy;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)
@SpringBootTest
public class HelloBoyTest {

    @Autowired
    HelloBoy helloBoy;

    @Test
    public void HelloBoyTest() {
        helloBoy.sayHello();
    }

}

注解无效示例
测试类

import com.markey.annotations.Boy.HelloBoy;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)
@SpringBootTest
public class HelloBoyTest {

    HelloBoy helloBoy = new HelloBoy(); //新建的对象，不是容器中的Bean
    @Test
    public void HelloBoyTest() {
        helloBoy.sayHello();
    }
}

Tips
其实Spring很多原生注解（例如@Autowired，其处理类是AutowiredAnnotationBeanPostProcessor）,也是通过实现BeanPostProcessor接口这种方式来实现的。