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。
