Hello，大家好，我是阿粉，Java 的注解相信大家天天都在用，但是關(guān)于注解的原理，大家都了解嗎？這篇文章通過(guò)意見(jiàn)簡(jiǎn)單的示例給大家演示一下注解的使用和原理。

Java 元注解

注解（Annotation）是一種可以放在 Java 類上，方法上，屬性上，參數(shù)前面的一種特殊的注釋，用來(lái)注釋注解的注解叫做元注解。元注解我們平常不會(huì)編寫(xiě)，只需要添加到我們自己編寫(xiě)的注解上即可，。

Java 自帶的常用的元注解有@Target，@Retention，@Documented，@Inherited 分別有如下含義

1. @Target：標(biāo)記這個(gè)注解使用的地方，取值范圍在枚舉 java.lang.annotation.ElementType：TYPE,FIELD,METHOD,PARAMETER,CONSTRUCTOR,LOCAL_VARIABLE,ANNOTATION_TYPE,PACKAGE,TYPE_PARAMETER,TYPE_USE。
2. @Retention ：標(biāo)識(shí)這個(gè)注解的生命周期，取值范圍在枚舉 java.lang.annotation.RetentionPolicy，SOURCE,CLASS,RUNTIME，一般定義的注解都是在運(yùn)行時(shí)使用，所有要用 @Retention(RetentionPolicy.RUNTIME);
3. @Documented：表示注解是否包含到文檔中。
4. @Inherited ：使用@Inherited定義子類是否可繼承父類定義的Annotation。@Inherited僅針對(duì)@Target(ElementType.TYPE)類型的annotation有效，并且僅針對(duì)class的繼承，對(duì)interface的繼承無(wú)效。

定義注解

上面介紹了幾個(gè)元注解，下面我們定義一個(gè)日志注解來(lái)演示一下，我們通過(guò)定義一個(gè)名為OperationLog ?的注解來(lái)記錄一些通用的操作日志，比如記錄什么時(shí)候什么人查詢的哪個(gè)表的數(shù)據(jù)或者新增了什么數(shù)據(jù)。編寫(xiě)注解我們用的是 @interface 關(guān)鍵字，相關(guān)代碼如下：

package?com.api.annotation;

import?java.lang.annotation.*;

/**
?*?

?*?Function：

?*?Author：@author?子悠

?*?Date：2020-11-17 22:10

?*?Desc：用于記錄操作日志

?*/
@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public?@interface?OperationLog?{

????/**
?????*?操作類型
?????*
?????*?@return
?????*/
????String?type()?default?OperationType.SELECT;

????/**
?????*?操作說(shuō)明
?????*
?????*?@return
?????*/
????String?desc()?default?"";

????/**
?????*?請(qǐng)求路徑
?????*
?????*?@return
?????*/
????String?path()?default?"";

????/**
?????*?是否記錄日志，默認(rèn)是
?????*
?????*?@return
?????*/
????boolean?write()?default?true;

????/**
?????*?是否需要登錄信息
?????*
?????*?@return
?????*/
????boolean?auth()?default?true;
???/**
?????*?當(dāng)?type?為?save?時(shí)必須
?????*
?????*?@return
?????*/
????String?primaryKey()?default?"";

????/**
?????*?對(duì)應(yīng)?service?的?Class
?????*
?????*?@return
?????*/
????Class?defaultServiceClass()?default?Object.class;
}

說(shuō)明

上面的注解，我們?cè)黾恿?code style="font-size: 14px;overflow-wrap: break-word;padding: 2px 4px;border-radius: 4px;margin-right: 2px;margin-left: 2px;background-color: rgba(27, 31, 35, 0.05);font-family: "Operator Mono", Consolas, Monaco, Menlo, monospace;word-break: break-all;color: rgb(40, 202, 113);">@Target({ElementType.METHOD}) , @Retention(RetentionPolicy.RUNTIME), @Documented 三個(gè)元注解，表示我們這個(gè)注解是使用在方法上的，并且生命周期是運(yùn)行時(shí)，而且可以記錄到文檔中。然后我們可以看到定義注解采用的u是@interface ?關(guān)鍵字，并且我們給這個(gè)注解定義了幾個(gè)屬性，同時(shí)設(shè)置了默認(rèn)值。主要注意的是平時(shí)我們編寫(xiě)的注解一般必須設(shè)置@Target和@Retention，而且 @Retention一般設(shè)置為RUNTIME，這是因?yàn)槲覀冏远x的注解通常要求在運(yùn)行期讀取，另外一般情況下，不必寫(xiě)@Inherited。

使用

上面的動(dòng)作只是把注解定義出來(lái)了，但是光光定義出來(lái)是沒(méi)有用的，必須有一個(gè)地方讀取解析，才能提現(xiàn)出注解的價(jià)值，我們就采用 Spring 的 AOP 攔截這個(gè)注解，將所有攜帶這個(gè)注解的方法所進(jìn)行的操作都記錄下來(lái)。

package?com.api.config;

import?lombok.extern.slf4j.Slf4j;
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.springframework.beans.factory.annotation.Autowired;
import?org.springframework.core.annotation.Order;
import?org.springframework.stereotype.Component;
import?org.springframework.web.bind.annotation.GetMapping;
import?org.springframework.web.bind.annotation.PostMapping;
import?org.springframework.web.bind.annotation.RequestMapping;

import?javax.servlet.http.HttpServletRequest;
import?java.lang.reflect.Field;
import?java.lang.reflect.Method;
import?java.util.*;

/**
?*?

?*?Function：

?*?Author：@author?子悠

?*?Date：2020-11-17 14:40

?*?Desc：aspect for operation log

?*/
@Aspect
@Component
@Order(-5)
@Slf4j
public?class?LogAspect?{
????/**
?????*?Pointcut?for?methods?which?need?to?record?operate?log
?????*/
????@Pointcut("within(com.xx.yy.controller..*)?&&?@annotation(com.api.annotation.OperationLog)")
????public?void?logAspect()?{
????}

????/**
?????*?record?log?for?Admin?and?DSP
?????*
?????*?@param?joinPoint?parameter
?????*?@return?result
?????*?@throws?Throwable
?????*/
????@Around("logAspect()")
????public?Object?around(ProceedingJoinPoint?joinPoint)?throws?Throwable?{
????????Object?proceed?=?null;
????????String?classType?=?joinPoint.getTarget().getClass().getName();
????????Class?targetCls?=?Class.forName(classType);
????????MethodSignature?ms?=?(MethodSignature)?joinPoint.getSignature();
????????Method?targetMethod?=?targetCls.getDeclaredMethod(ms.getName(),?ms.getParameterTypes());
????????OperationLog?operation?=?targetMethod.getAnnotation(OperationLog.class);
????????if?(null?!=?operation?&&?operation.write())?{
????????????SysMenuOpLogEntity?opLogEntity?=?new?SysMenuOpLogEntity();
????????????StringBuilder?change?=?new?StringBuilder();
????????????if?(StrUtil.isNotBlank(operation.type()))?{
????????????????switch?(operation.type())?{
????????????????????case?OperationType.ADD:
????????????????????????proceed?=?joinPoint.proceed();
????????????????????????String?addString?=?genAddData(targetCls,?operation.defaultServiceClass(),?joinPoint.getArgs());
????????????????????????opLogEntity.setAfterJson(addString);
????????????????????????change.append(OperationType.ADD);
????????????????????????break;
????????????????????case?OperationType.DELETE:
????????????????????????String?deleteString?=?autoQueryDeletedData(targetCls,?operation.primaryKey(),?operation.defaultServiceClass(),?joinPoint.getArgs());
????????????????????????opLogEntity.setBeforeJson(deleteString);
????????????????????????change.append(OperationType.DELETE);
????????????????????????proceed?=?joinPoint.proceed();
????????????????????????break;
????????????????????case?OperationType.EDIT:
????????????????????????change.append(OperationType.EDIT);
????????????????????????setOpLogEntity(opLogEntity,?targetCls,?operation.primaryKey(),?operation.defaultServiceClass(),?joinPoint.getArgs());
????????????????????????proceed?=?joinPoint.proceed();
????????????????????????break;
????????????????????case?OperationType.SELECT:
????????????????????????opLogEntity.setBeforeJson(getQueryString(targetCls,?operation.defaultServiceClass(),?joinPoint.getArgs()));
????????????????????????change.append(operation.type());
????????????????????????proceed?=?joinPoint.proceed();
????????????????????????break;
????????????????????case?OperationType.SAVE:
????????????????????????savedDataOpLog(opLogEntity,?targetCls,?operation.primaryKey(),?operation.defaultServiceClass(),?joinPoint.getArgs());
????????????????????????change.append(operation.type());
????????????????????????proceed?=?joinPoint.proceed();
????????????????????????break;
????????????????????case?OperationType.EXPORT:
????????????????????case?OperationType.DOWNLOAD:
????????????????????????change.append(operation.type());
????????????????????????proceed?=?joinPoint.proceed();
????????????????????????break;
????????????????????default:
????????????????}
????????????????opLogEntity.setExecType(operation.type());
????????????}
????????????StringBuilder?changing?=?new?StringBuilder();
????????????if?(StrUtil.isNotBlank(opLogEntity.getExecType()))?{
????????????????if?(operation.auth())?{
????????????????????LoginUserVO?loginUser?=?getLoginUser();
????????????????????if?(null?!=?loginUser)?{
????????????????????????opLogEntity.setUserId(loginUser.getUserId());
????????????????????????opLogEntity.setUserName(loginUser.getUserName());
????????????????????????changing.append(loginUser.getUserName()).append("-");
????????????????????}?else?{
????????????????????????log.error("用戶未登錄");
????????????????????}
????????????????}
????????????????opLogEntity.setCreateTime(DateUtils.getCurDate());
????????????????opLogEntity.setRemark(getOperateMenuName(targetMethod,?operation.desc()));
????????????????opLogEntity.setPath(getPath(targetMethod,?targetMethod.getName()));
????????????????opLogEntity.setChanging(changing.append(change).toString());
????????????????menuOpLogService.save(opLogEntity);
????????????}
????????}
????????return?proceed;
????}

????/**
?????*?query?data?by?userId
?????*
?????*?@param?targetCls???????????class
?????*?@param?defaultServiceClass?default?service?class
?????*?@return
?????*?@throws?Exception
?????*/
????private?String?queryByCurrentUserId(Class?targetCls,?Class?defaultServiceClass)?throws?Exception?{
????????BaseService?baseService?=?getBaseService(targetCls,?defaultServiceClass);
????????LoginUserVO?loginUser?=?dspBaseService.getLoginUser();
????????if?(null?!=?loginUser)?{
????????????Object?o?=?baseService.queryId(loginUser.getUserId());
????????????return?JsonUtils.obj2Json(o);
????????}
????????return?null;
????}

????/**
?????*?return?query?parameter
?????*
?????*?@param?targetCls???????????class
?????*?@param?args????????????????parameter
?????*?@param?defaultServiceClass?default?service?class
?????*?@return
?????*?@throws?Exception
?????*/
????private?String?getQueryString(Class?targetCls,?Class?defaultServiceClass,?Object[]?args)?{
????????if?(args.length?>?0)?{
????????????Class?entityClz?=?getEntityClz(targetCls,?defaultServiceClass);
????????????for?(Object?arg?:?args)?{
????????????????if?(arg.getClass().equals(entityClz)?||?arg?instanceof?BaseModel)?{
????????????????????return?JsonUtils.obj2Json(arg);
????????????????}
????????????}
????????}
????????return?null;
????}

????/**
?????*?save?record?log?while?OperatorType?is?SAVE
?????*
?????*?@param?opLogEntity?????????entity
?????*?@param?targetCls???????????class
?????*?@param?primaryKey??????????primaryKey
?????*?@param?defaultServiceClass?default?service?class
?????*?@param?args????????????????parameter
?????*?@throws?Exception
?????*/
????private?void?savedDataOpLog(SysMenuOpLogEntity?opLogEntity,?Class?targetCls,?String?primaryKey,?Class?defaultServiceClass,?Object[]?args)?throws?Exception?{
????????Class?entityClz?=?getEntityClz(targetCls,?defaultServiceClass);
????????BaseService?baseService?=?getBaseService(targetCls,?defaultServiceClass);
????????for?(Object?arg?:?args)?{
????????????if?(arg.getClass().equals(entityClz))?{
????????????????if?(StrUtil.isNotBlank(primaryKey))?{
????????????????????Field?declaredField?=?entityClz.getDeclaredField(primaryKey);
????????????????????declaredField.setAccessible(true);
????????????????????Object?primaryKeyValue?=?declaredField.get(arg);
????????????????????//if?primary?key?is?not?null?that?means?edit,?otherwise?is?add
????????????????????if?(null?!=?primaryKeyValue)?{
????????????????????????//query?data?by?primary?key
????????????????????????Object?o?=?baseService.queryId(primaryKeyValue);
????????????????????????opLogEntity.setBeforeJson(JsonUtils.obj2Json(o));
????????????????????}
????????????????}
????????????????opLogEntity.setAfterJson(JsonUtils.obj2Json(arg));
????????????}
????????}
????}

????/**
?????*?set?parameter?which?edit?data
?????*
?????*?@param?opLogEntity?????????entity
?????*?@param?targetCls???????????class
?????*?@param?primaryKey??????????primaryKey
?????*?@param?defaultServiceClass?default?service?class
?????*?@param?args????????????????parameter
?????*?@throws?Exception
?????*/
????private?void?setOpLogEntity(SysMenuOpLogEntity?opLogEntity,?Class?targetCls,?String?primaryKey,?Class?defaultServiceClass,?Object[]?args)?throws?Exception?{
????????Map?saveMap?=?autoQueryEditedData(targetCls,?primaryKey,?defaultServiceClass,?args);
????????if?(null?!=?saveMap)?{
????????????if?(saveMap.containsKey(ASPECT_LOG_OLD_DATA))?{
????????????????opLogEntity.setBeforeJson(saveMap.get(ASPECT_LOG_OLD_DATA));
????????????}
????????????if?(saveMap.containsKey(ASPECT_LOG_NEW_DATA))?{
????????????????opLogEntity.setBeforeJson(saveMap.get(ASPECT_LOG_NEW_DATA));
????????????}
????????}
????}

????/**
?????*?query?data?for?edit?and?after?edit?operate
?????*
?????*?@param?targetCls???????????class
?????*?@param?primaryKey??????????primaryKey
?????*?@param?defaultServiceClass?default?service?class
?????*?@param?args????????????????parameter
?????*?@return?map?which?data
?????*?@throws?Exception
?????*/
????private?Map?autoQueryEditedData(Class?targetCls,?String?primaryKey,?Class?defaultServiceClass,?Object[]?args)?throws?Exception?{
????????if?(StrUtil.isBlank(primaryKey))?{
????????????throw?new?Exception();
????????}
????????Map?map?=?new?HashMap<>(16);
????????Class?entityClz?=?getEntityClz(targetCls,?defaultServiceClass);
????????BaseService?baseService?=?getBaseService(targetCls,?defaultServiceClass);
????????for?(Object?arg?:?args)?{
????????????if?(arg.getClass().equals(entityClz))?{
????????????????Field?declaredField?=?entityClz.getDeclaredField(primaryKey);
????????????????declaredField.setAccessible(true);
????????????????Object?primaryKeyValue?=?declaredField.get(arg);
????????????????//query?the?data?before?edit
????????????????if?(null?!=?primaryKeyValue)?{
????????????????????//query?data?by?primary?key
????????????????????Object?o?=?baseService.queryId(primaryKeyValue);
????????????????????map.put(ASPECT_LOG_OLD_DATA,?JsonUtils.obj2Json(o));
????????????????????map.put(ASPECT_LOG_NEW_DATA,?JsonUtils.obj2Json(arg));
????????????????????return?map;
????????????????}
????????????}
????????}
????????return?null;
????}

????/**
?????*?return?JSON?data?which?add?operate
?????*
?????*?@param?targetCls???????????class
?????*?@param?args????????????????parameter
?????*?@param?defaultServiceClass?default?service?class
?????*?@return?add?data?which?will?be?added
?????*?@throws?Exception
?????*/
????private?String?genAddData(Class?targetCls,?Class?defaultServiceClass,?Object[]?args)?throws?Exception?{
????????List