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！