Caused by: java.lang.IllegalStateException: getWriter() has already been called for this response

异常堆栈：看着是一个响应流被再次使用

Caused by: java.lang.IllegalStateException: getWriter() has already been called for this response
	at org.apache.catalina.connector.Response.getOutputStream(Response.java:550) ~[tomcat-embed-core-9.0.53.jar:9.0.53]
	at org.apache.catalina.connector.ResponseFacade.getOutputStream(ResponseFacade.java:210) ~[tomcat-embed-core-9.0.53.jar:9.0.53]
	at javax.servlet.ServletResponseWrapper.getOutputStream(ServletResponseWrapper.java:105) ~[tomcat-embed-core-9.0.53.jar:4.0.FR]
	at org.springframework.http.server.ServletServerHttpResponse.getBody(ServletServerHttpResponse.java:97) ~[spring-web-5.3.10.jar:5.3.10]
	at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:422) ~[spring-web-5.3.10.jar:5.3.10]
	at org.springframework.http.converter.AbstractGenericHttpMessageConverter.write(AbstractGenericHttpMessageConverter.java:104) ~[spring-web-5.3.10.jar:5.3.10]
	at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:290) ~[spring-webmvc-5.3.10.jar:5.3.10]
	at org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor.handleReturnValue(HttpEntityMethodProcessor.java:219) ~[spring-webmvc-5.3.10.jar:5.3.10]
	at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:78) ~[spring-web-5.3.10.jar:5.3.10]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:135) ~[spring-webmvc-5.3.10.jar:5.3.10]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895) ~[spring-webmvc-5.3.10.jar:5.3.10]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.10.jar:5.3.10]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.10.jar:5.3.10]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1067) ~[spring-webmvc-5.3.10.jar:5.3.10]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963) ~[spring-webmvc-5.3.10.jar:5.3.10]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.3.10.jar:5.3.10]
	... 35 common frames omitted

找了半天检查了所有拦截器都没发现为什么，后来发现  统一异常处理 HandlerExceptionResolver，在用响应流写了异常信息jjson 以后， 方法返回值返回了一个null，返回null 的时候 spring会返回一个默认值。

处理办法发：return new ModelAndView();

    @Override
    public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {
        System.out.println( "异常：" + ex );
        try {
            response.setCharacterEncoding("UTF-8");
            response.setContentType("application/json; charset=utf-8");

            PrintWriter  writer = response.getWriter();
            writer.write("{\"=exMsg\":"+  ex.getMessage() +"}" );
            writer.flush();
        } catch (IOException e) {
            e.printStackTrace();
        }

        //不能返回null，不要spring或默认返回一个
        return new ModelAndView();
        //return null;
    }