java自定义注解学习（三）_注解解析及应用

上篇文章已经介绍了注解的基本构成信息。这篇文章，主要介绍注解的解析。毕竟你只声明了注解，是没有用的。需要进行解析。主要就是利用反射机制在运行时进行查看和利用这些信息

常用方法汇总

在Class、Field、Method、Constructor中都有如下方法：

//获取所有的注解 public Annotation[] getAnnotations() //获取所有本元素上直接声明的注解，忽略inherited来的 public Annotation[] getDeclaredAnnotations() //获取指定类型的注解，没有返回null public <A extends Annotation> A getAnnotation(Class<A> annotationClass) //判断是否有指定类型的注解 public boolean isAnnotationPresent(Class<? extends Annotation> annotationClass)

Annotation 是一个借口，它表示注解，源码为：

public interface Annotation { boolean equals(Object obj); int hashCode(); String toString(); //返回真正的注解类型 Class<? extends Annotation> annotationType(); }

实际上，所有的注解类型、内部实现时，都是扩展的Annotation

对于Method和Contructor,他们都有方法参数

public Annotation[][] getParameterAnnotations()

应用注解

日常工作中，每个公司都会自定义注解进行记录日志的，我们就做一个简单的记录日志操作的注解，结合aop和springboot

1.建立springboot项目

这里不再赘述，主要需要引入aop

 <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-aop</artifactId> </dependency>

2.定义自定义注解

package com.kevin.anno.annotation; 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) public @interface KevinLog { String value() default ""; } 

3.定义aspect及解析注解

package com.kevin.anno.aspect; import com.kevin.anno.annotation.KevinLog; import org.aspectj.lang.ProceedingJoinPoint; import org.aspectj.lang.annotation.Around; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Pointcut; import org.aspectj.lang.reflect.MethodSignature; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.stereotype.Component; import org.springframework.web.context.request.RequestContextHolder; import org.springframework.web.context.request.ServletRequestAttributes; import javax.servlet.http.HttpServletRequest; import java.lang.reflect.Method; @Aspect @Component public class LogAscpect { private final static Logger logger = LoggerFactory.getLogger(LogAscpect.class); @Pointcut("@annotation(com.kevin.anno.annotation.KevinLog)") public void log() { } @Around("log()") public Object aroundAdvice(ProceedingJoinPoint point) throws Throwable { Object object = null; long start = System.currentTimeMillis(); Method method = ((MethodSignature) MethodSignature.class.cast(point.getSignature())).getMethod(); KevinLog kevinLog = method.getAnnotation(KevinLog.class); String operationName = kevinLog.value(); object = point.proceed(point.getArgs()); long end = System.currentTimeMillis(); Long excuteTime = end - start; print(operationName, excuteTime, point); return object; } private void print(String operationName, Long excuteTime, ProceedingJoinPoint point) { ServletRequestAttributes attributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes(); HttpServletRequest request = attributes.getRequest(); //operationName logger.info("operationName={}", operationName); //time logger.info("time={}", excuteTime); // url logger.info("url={}", request.getRequestURL()); //method logger.info("method = {}", request.getMethod()); //ip logger.info("ip = {}", request.getRemoteAddr()); //类方法 logger.info("class_method={}", point.getSignature().getDeclaringTypeName() + "." + point.getSignature().getName()); //参数 logger.info("args = {}", point.getArgs()); } } 

4. 在请求方法上加上自定义注解

package com.kevin.anno.controller; import com.kevin.anno.annotation.KevinLog; import org.springframework.web.bind.annotation.RequestMapping; import org.springframework.web.bind.annotation.RestController; @RestController public class HelloController { @RequestMapping(value = "/hello") @KevinLog("kevin test !") public String hello() { return "hello kevin"; } } 

5.启动测试

访问：http://localhost:8080/hello

页面出现：hello kevin

控制台打印信息如下：

2018-10-22 10:38:22.456 INFO 3916 --- [nio-8080-exec-2] com.kevin.anno : operationName=kevin test ! 2018-10-22 10:38:22.456 INFO 3916 --- [nio-8080-exec-2] com.kevin.anno.aspect.LogAscpect : time=7 2018-10-22 10:38:22.456 INFO 3916 --- [nio-8080-exec-2] com.kevin.anno.aspect.LogAscpect : url=http://localhost:8080/hello 2018-10-22 10:38:22.456 INFO 3916 --- [nio-8080-exec-2] com.kevin.anno.aspect.LogAscpect : method = GET 2018-10-22 10:38:22.457 INFO 3916 --- [nio-8080-exec-2] com.kevin.anno.aspect.LogAscpect : ip = 0:0:0:0:0:0:0:1 2018-10-22 10:38:22.457 INFO 3916 --- [nio-8080-exec-2] com.kevin.anno.aspect.LogAscpect : class_method=com.kevin.anno.controller.HelloController.hello 2018-10-22 10:38:22.457 INFO 3916 --- [nio-8080-exec-2] com.kevin.anno.aspect.LogAscpect : args = {}

总结

其实, 大家可以自己写这玩玩，比较这个demo还用到了aop，工作中很少接触到aop。以至于面试的时候，问你aop的时候，自己都没有实际的应用过。

好了。玩的开心！