【Java EE】统一功能返回

一、拦截器

1.1 拦截器的作用

在对于数据库进行增删查改的时候，如果当前页面不检查用户是否登录，然后就能操作成功是不合理的，解决方法有两个：

对于已经写好的每个接口都加上一个判断，从Session中获取用户信息，看UserInfo是不是空的，如果是空的，后端就将返回值设为一个错误的状态，然后前端根据这个状态强制跳转到用户登录界面。

这个状态最好是一个类，里面装上：

1. 状态码（code）
2. 状态信息（message）
3. 返回的数据（data）
可以使用拦截器，对于所有需要用户登录的界面进行拦截。代码如下:

@Slf4j
@Component
public class LoginInterceptor implements HandlerInterceptor {@Overridepublic boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {HttpSession session = request.getSession();UserInfo userInfo = (UserInfo) session.getAttribute(Constants.SESSION_USER_INFO);if (userInfo == null || userInfo.getId() <= 0) {log.info("用户未登录！");response.setStatus(401);return false;}return true;}
}
/*** 拦截器的配置信息*/
//@Configuration
public class WebConfig implements WebMvcConfigurer {@AutowiredLoginInterceptor loginInterceptor;private List<String> excludePaths = Arrays.asList("/user/login","/**/*.js","/**/*.css","/**/*.png","/**/*.html");@Overridepublic void addInterceptors(InterceptorRegistry registry) {registry.addInterceptor(loginInterceptor).addPathPatterns("/**").excludePathPatterns(excludePaths);}
}

1.2 使用拦截器的注意事项

拦截器的代码很短，主要把握好两个部分：
1. 配置拦截器（招保安）
2. 定义拦截器（招到以后安排哪部分的安保工作）

添加前置、后置、视图渲染的拦截工作

拦截器要加@component注解注册为Bean，方便Spring扫描得到

1.3 @DispatcherServlet源码分析

1. 继承关系

DispatcherServlet继承 FrameworkServlet
FrameworkServlet 继承 HttpServletBean
HttpServletBean 继承 HttpServlet
HttpServlet继承 GenericServlet
enericServlet实现 Servlet（是一个接口）
Servlet的声明周期有三部分：
1. init()
2. service()
3. destroy()

2. DispatcherServlet

是从日志中发现了DispatcherServlet这个类。

3. initStrategies

	protected void initStrategies(ApplicationContext context) {initMultipartResolver(context);initLocaleResolver(context);initThemeResolver(context);initHandlerMappings(context);initHandlerAdapters(context);initHandlerExceptionResolvers(context);initRequestToViewNameTranslator(context);initViewResolvers(context);initFlashMapManager(context);}

Multipart是和文件相关的，Resolver是解析器
本地初始化解析器
主体初始化解析器
处理器初始化映射
处理器初始化适配器
处理器初始化

4. doDispatch

// 主要代码
// 获得处理器
mappedHandler = getHandler(processedRequest);
// 获得处理器适配器
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// 执行拦截器（预处理阶段），如果返回false就立即停止（被拦截）
if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;
}
// 使用处理器适配器对于处理器进行调度，返回ModelAndView
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());// 拦截器的后处理
mappedHandler.applyPostHandle(processedRequest, response, mv);// 对于所有资源进行整理
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

4.1. doDispatch-applyPreHandle

	/*** Apply preHandle methods of registered interceptors.* @return {@code true} if the execution chain should proceed with the* next interceptor or the handler itself. Else, DispatcherServlet assumes* that this interceptor has already dealt with the response itself.*/boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {for (int i = 0; i < this.interceptorList.size(); i++) {HandlerInterceptor interceptor = this.interceptorList.get(i);if (!interceptor.preHandle(request, response, this.handler)) {triggerAfterCompletion(request, response, null);return false;}this.interceptorIndex = i;}return true;}

这是一个拦截器链，然后遍历每一个拦截器，对于各个拦截器进行拦截的前置处理。

4.2. doDispatch-applyPostHandle

	/*** Apply postHandle methods of registered interceptors.*/void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv)throws Exception {for (int i = this.interceptorList.size() - 1; i >= 0; i--) {HandlerInterceptor interceptor = this.interceptorList.get(i);interceptor.postHandle(request, response, this.handler, mv);}}

遍历拦截器链上的每一个拦截器，对于其进行后置拦截处理

二、统一数据返回格式

2.1 快速入门

对于每个接口，如果都是一样的返回格式，那么对于前后端交流是非常友好的。

实现代码：

@ControllerAdvice
public class ResponseAdvice implements ResponseBodyAdvice {@Overridepublic boolean supports(MethodParameter returnType, Class converterType) {return true;}@Overridepublic Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {return Result.success(body);}
}

supports方法是决定要不要使用统一返回格式：
1. true——使用
2. false——不使用
beforeBodyWrite方法：在写入响应正文前需要做的事情。
1. 此处就是对于所有的返回正文进行了一次封装，Result就是一个常见的返回类，有code，msg，data等信息

2.2 存在问题

当返回String的时候，会报500的错误。

原因？源代码整起

先根据错误堆栈信息打个断点试试

子类：

追根溯源，

就是因为write方法会传进去一个Result类型的参数给addDefaultHeaders，

而addDefaultHeaders方法是被子类重写过的，

所以会调用子类重写后的方法，（父类中接收这个的参数是一个泛型，对应到这个情况中正好是Result类型）

重写的方法中定死了这个参数是一个String类型，所以会发生类型转换异常。

所以需要给String做一个单独的处理!

处理方法就是使用SpringBoot内置的Jackson对于信息进行序列化，防止源码中的这种错误。

代码实现：

@ControllerAdvice
public class ResponseAdvice implements ResponseBodyAdvice {@Autowiredprivate ObjectMapper objectMapper;@Overridepublic boolean supports(MethodParameter returnType, Class converterType) {return true;}@SneakyThrows@Overridepublic Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {// 对于String类型的响应正文进行序列化if (body instanceof String) {return objectMapper.writeValueAsString(body);}return Result.success(body);}
}

三、统一异常处理

统一异常也是一个面向切面编程思想的实现，其对于所有发生异常的接口进行捕获。

主要使用到的注解有三个：

@ResponseBody

类注解，因为返回的仍然是数据，是响应正文。

@ControllerAdvice

类注解，统一功能返回都是用这个注解。

@ExceptionHandler

方法注解。

实现代码:

@ControllerAdvice
@ResponseBody
public class ExceptionAdvice {@ExceptionHandlerpublic Object handler(Exception e) {return Result.failed(e.getMessage());}@ExceptionHandlerpublic Object handler(ArithmeticException e) {return Result.failed(e.getMessage());}@ExceptionHandlerpublic Object handler(NullPointerException e) {return Result.failed(e.getMessage());}
}

3.1 `@ControlelrAdvice`为什么能够对代码做到不侵入？源码整起

查看@ControlelrAdvice注解的源代码：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface ControllerAdvice {// ......
}

@ControlelrAdvice派生于@Compoent，所以可以不加五大注解也能够被Spring找到。

再看DispatcherServlet的源码，能够找到异常处理器的初始化，工作过程：

	protected void initStrategies(ApplicationContext context) {initMultipartResolver(context);initLocaleResolver(context);initThemeResolver(context);initHandlerMappings(context);initHandlerAdapters(context);initHandlerExceptionResolvers(context);initRequestToViewNameTranslator(context);initViewResolvers(context);initFlashMapManager(context);}

其中重点关注initHandlerAdapters(context)、initHandlerExceptionResolvers(context)这两个方法。

private void initHandlerAdapters(ApplicationContext context) {this.handlerAdapters = null;if (this.detectAllHandlerAdapters) {// Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.Map<String, HandlerAdapter> matchingBeans =BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);if (!matchingBeans.isEmpty()) {this.handlerAdapters = new ArrayList<>(matchingBeans.values());// We keep HandlerAdapters in sorted order.AnnotationAwareOrderComparator.sort(this.handlerAdapters);}}else {try {HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);this.handlerAdapters = Collections.singletonList(ha);}catch (NoSuchBeanDefinitionException ex) {// Ignore, we'll add a default HandlerAdapter later.}}// Ensure we have at least some HandlerAdapters, by registering// default HandlerAdapters if no other adapters are found.if (this.handlerAdapters == null) {this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);if (logger.isTraceEnabled()) {logger.trace("No HandlerAdapters declared for servlet '" + getServletName() +"': using default strategies from DispatcherServlet.properties");}}}
private void initHandlerExceptionResolvers(ApplicationContext context) {this.handlerExceptionResolvers = null;if (this.detectAllHandlerExceptionResolvers) {// Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);if (!matchingBeans.isEmpty()) {this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());// We keep HandlerExceptionResolvers in sorted order.AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);}}else {try {HandlerExceptionResolver her =context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class);this.handlerExceptionResolvers = Collections.singletonList(her);}catch (NoSuchBeanDefinitionException ex) {// Ignore, no HandlerExceptionResolver is fine too.}}// Ensure we have at least some HandlerExceptionResolvers, by registering// default HandlerExceptionResolvers if no other resolvers are found.if (this.handlerExceptionResolvers == null) {this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class);if (logger.isTraceEnabled()) {logger.trace("No HandlerExceptionResolvers declared in servlet '" + getServletName() +"': using default strategies from DispatcherServlet.properties");}}}

可以看到这两个方法是极其相似的，都是从Bean工厂中提取Bean，如果没有提取到，然后又使用其他手段去获得Bean。

从Bean工厂获得Bean的类型各不相同，分别是:

HandlerAdapter.class
HandlerExceptionResolver.class

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapterimplements BeanFactoryAware, InitializingBean {@Overridepublic void afterPropertiesSet() {// Do this first, it may add ResponseBody advice beansinitControllerAdviceCache();initMessageConverters();// ...}private void initControllerAdviceCache() {// 如果这个Bean是空，直接返回if (getApplicationContext() == null) {return;}// 获得所有被@ControllerAdvice标注的BeanList<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());List<Object> requestResponseBodyAdviceBeans = new ArrayList<>();//...}}
public class ExceptionHandlerExceptionResolver extends AbstractHandlerMethodExceptionResolverimplements ApplicationContextAware, InitializingBean {@Overridepublic void afterPropertiesSet() {// Do this first, it may add ResponseBodyAdvice beansinitExceptionHandlerAdviceCache();initMessageConverters();}// 如果这个Bean是空，直接返回private void initExceptionHandlerAdviceCache() {if (getApplicationContext() == null) {return;}// 获得所有被@ControllerAdvice标注的BeanList<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());for (ControllerAdviceBean adviceBean : adviceBeans) {Class<?> beanType = adviceBean.getBeanType();if (beanType == null) {throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);}// 获得异常处理方法的处理器ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);if (resolver.hasExceptionMappings()) {this.exceptionHandlerAdviceCache.put(adviceBean, resolver);}if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {this.responseBodyAdvice.add(adviceBean);}}// ...}
}
public class ExceptionHandlerMethodResolver {@Nullableprivate Method getMappedMethod(Class<? extends Throwable> exceptionType) {List<Class<? extends Throwable>> matches = new ArrayList<>();for (Class<? extends Throwable> mappedException : this.mappedMethods.keySet()) {if (mappedException.isAssignableFrom(exceptionType)) {matches.add(mappedException);}}if (!matches.isEmpty()) {if (matches.size() > 1) {// 如果匹配到多个异常会进行排序，取第一个matches.sort(new ExceptionDepthComparator(exceptionType));}return this.mappedMethods.get(matches.get(0));}else {return NO_MATCHING_EXCEPTION_HANDLER_METHOD;}}
}

他们两个都实现了InitializingBean接口，所以都有afterPropertiesSet这个方法，能够执行一些额外的初始化。

public interface InitializingBean {void afterPropertiesSet() throws Exception;
}

这个方法是在bean的依赖属性被注入之后（此时Spring Application Context已经有了一些Bean），执行一些额外的初始化