Springboot使用了ResponseBodyAdvice处

为了统一接口响应的报文，现实了ResponseBodyAdvice接口，通过这个接口的实现类来统一处理报文

public class ResponseResultInterceptor implements HandlerInterceptor {
    //标记名称
    public static final String RESPONSE_RESULT_ANN = "RESPONSE-RESULT-ANN";

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
        if (handler instanceof HandlerMethod) {
            final HandlerMethod handlerMethod = (HandlerMethod) handler;
            final Class<?> clazz = handlerMethod.getBeanType();
            // 判断是否在类对象上添加了注解
            if(clazz.isAnnotationPresent(RestController.class) || clazz.isAnnotationPresent(ResponseBody.class)){
                // 设置此请求返回体，需要包装，往下传递，在ResponseBodyAdvice接口进行判断
                request.setAttribute(RESPONSE_RESULT_ANN, true);
            }
        }
        return true;
    }
}

/**
 * 使用 @ControllerAdvice & ResponseBodyAdvice
 * 拦截Controller方法默认返回参数，统一处理返回值/响应体
 */
@ControllerAdvice
public class ResponseResultHandler implements ResponseBodyAdvice<Object> {

    /**
     * 需要忽略的地址
     */
    private static String[] ignores = new String[]{
            //过滤swagger相关的请求的接口，不然swagger会提示base-url被拦截
            "/swagger-resources",
            "/v2/api-docs"
    };

    /**
     * 判断url是否需要拦截
     * @param uri
     * @return
     */
    private boolean ignoring(String uri) {
        for (String string : ignores) {
            if (uri.contains(string)) {
                return true;
            }
        }
        return false;
    }


    /**
     *  判断是否要执行 beforeBodyWrite 方法，true为执行，false不执行
     */
    @Override
    public boolean supports(MethodParameter method, Class<? extends HttpMessageConverter<?>> arg1) {
        ServletRequestAttributes sra = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = sra.getRequest();
        // 判断请求是否有包装标记
        return request.getAttribute(RESPONSE_RESULT_ANN) != null;
    }

    /**
     *  对返回值做包装处理
     */
    @Override
    public Object beforeBodyWrite(Object body, MethodParameter arg1, MediaType arg2,
                                  Class<? extends HttpMessageConverter<?>> arg3, ServerHttpRequest request, ServerHttpResponse response) {
        //判断url是否需要拦截
        if (this.ignoring(request.getURI().toString())) {
            return body;
        }
        if (body instanceof Result) {
            return body;
        }
        return Result.success(body);
    }
}

然而在测试返回值是String类型的时候，程序抛出一个类转换的异常：

java.lang.ClassCastException: cn.huolala.bme.spi.common.model.Result cannot be cast to java.lang.String
    at org.springframework.http.converter.StringHttpMessageConverter.addDefaultHeaders(StringHttpMessageConverter.java:44)
    at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:211)
    at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:290)
    at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:181)
    at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:82)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:123)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:879)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:793)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at cn.huolala.bme.spi.web.config.CorsFilter.doFilter(CorsFilter.java:51)
    at cn.huolala.bme.spi.web.config.CorsFilter$$FastClassBySpringCGLIB$$104bd17c.invoke(<generated>)
    at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218)
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:771)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:749)
    at cn.huolala.bme.plugin.core.aspect.SpiInterceptorInvokerImpl.invoke(SpiInterceptorInvokerImpl.java:16)
    at sun.reflect.GeneratedMethodAccessor124.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at cn.huolala.bme.plugin.core.aop.SpiInvocation.process(SpiInvocation.java:43)
    at cn.huolala.bme.plugin.log.LogPlugin.intercept(LogPlugin.java:56)
    at cn.huolala.bme.plugin.core.aop.SpiPluginHandler.invoke(SpiPluginHandler.java:30)
    at com.sun.proxy.$Proxy201.invoke(Unknown Source)
    at cn.huolala.bme.plugin.core.aspect.SpiInterceptorPointcut.invoke(SpiInterceptorPointcut.java:54)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:749)
    at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:691)
    at cn.huolala.bme.spi.web.config.CorsFilter$$EnhancerBySpringCGLIB$$ddd3bac8.doFilter(<generated>)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:109)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
    at org.apache.catalina.core.StandardHostValve.invoke$original$qNsflBfP(StandardHostValve.java:139)
    at org.apache.catalina.core.StandardHostValve.invoke$original$qNsflBfP$accessor$sPsJTqrz(StandardHostValve.java)
    at org.apache.catalina.core.StandardHostValve$auxiliary$b0wAX8bs.call(Unknown Source)
    at org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.InstMethodsInter.intercept(InstMethodsInter.java:88)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1594)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at com.alibaba.ttl.TtlRunnable.run(TtlRunnable.java:59)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)

奇怪的是使用swagger请求时候没有问题，在浏览器中直接请求就发生类型转换异常。

发现这个跟MessageConverter有关系。Springmvc内部定义了九个不同的MessageConverter用来处理不同的返回值。在AbstractMessageConverterMethodProcessor类下面的writeWithMessageConverters方法可以看出来，每个MessageConverer是根据返回类型和媒体类型来选择处理的MessageConverter的，下面是代码片段：

if (selectedMediaType != null) {
   selectedMediaType = selectedMediaType.removeQualityValue();
   for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
      if (messageConverter instanceof GenericHttpMessageConverter) {
         if (((GenericHttpMessageConverter) messageConverter).canWrite(
               declaredType, valueType, selectedMediaType)) {
            outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
                  (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
                  inputMessage, outputMessage);
            if (outputValue != null) {
               addContentDispositionHeader(inputMessage, outputMessage);
               ((GenericHttpMessageConverter) messageConverter).write(
                     outputValue, declaredType, selectedMediaType, outputMessage);
               if (logger.isDebugEnabled()) {
                  logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
                        "\" using [" + messageConverter + "]");
               }
            }
            return;
         }
      }
      else if (messageConverter.canWrite(valueType, selectedMediaType)) {
         outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
               (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
               inputMessage, outputMessage);
         if (outputValue != null) {
            addContentDispositionHeader(inputMessage, outputMessage);
            ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);
            if (logger.isDebugEnabled()) {
               logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
                     "\" using [" + messageConverter + "]");
            }
         }
         return;
      }
   }
}

终于找到原因：swagger请求时类型是application/json, 浏览器请求时类型是text/xml，使用StringMessageConverter。

解决方式&参考文章：https://blog.csdn.net/weixin_33961829/article/details/92143322