前言

本文章会对getWriter() has already been called for this response异常在SpringBoot下出现的情况进行分析和解决。
如果你只是想看解决方案，请直接拖到文章最后。

问题分析

在把tomcat应用升级到SpringBoot后，部分http接口出现了getWriter() has already been called for this response异常。但是异常报出的很模糊，下面贴出异常：

java.lang.IllegalStateException: getWriter() has already been called for this response
    at org.apache.catalina.connector.Response.getOutputStream(Response.java:590)
    at org.apache.catalina.connector.ResponseFacade.getOutputStream(ResponseFacade.java:194)
    at javax.servlet.ServletResponseWrapper.getOutputStream(ServletResponseWrapper.java:100)
    at org.springframework.boot.web.servlet.support.ErrorPageFilter$ErrorWrapperResponse.getOutputStream(ErrorPageFilter.java:371)
    at javax.servlet.ServletResponseWrapper.getOutputStream(ServletResponseWrapper.java:100)
    at org.springframework.session.web.http.OnCommittedResponseWrapper.getOutputStream(OnCommittedResponseWrapper.java:139)
    at org.springframework.http.server.ServletServerHttpResponse.getBody(ServletServerHttpResponse.java:83)
    at com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter.writeInternal(FastJsonHttpMessageConverter.java:330)
    at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:227)
    at com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter.write(FastJsonHttpMessageConverter.java:244)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:661)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
    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:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at com.howbuy.cms.base.filter.CheckLoginFilter.doFilter(CheckLoginFilter.java:157)
    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:99)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
    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:92)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
    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.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.session.web.http.SessionRepositoryFilter.doFilterInternal(SessionRepositoryFilter.java:151)
    at org.springframework.session.web.http.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:86)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.boot.web.servlet.support.ErrorPageFilter.doFilter(ErrorPageFilter.java:128)
    at org.springframework.boot.web.servlet.support.ErrorPageFilter.access$000(ErrorPageFilter.java:66)
    at org.springframework.boot.web.servlet.support.ErrorPageFilter$1.doFilterInternal(ErrorPageFilter.java:103)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
    at org.springframework.boot.web.servlet.support.ErrorPageFilter.doFilter(ErrorPageFilter.java:121)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:650)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:800)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:806)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1498)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$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)

字面意思很明显，getWriter()已经被调用。 并且能看到的有用信息只是fastJson。(因为我的tomcat应用还包含sitemesh，urlwriter等，该异常也会出现在sitemesh与urlwriter上，并不一定是fastJson)

通过本地调试，却并没有复现出该问题(奇怪是产线一直出现)。起初怀疑是并发问题，但细一想应该不会。这个问题只有在升级后才出现。

进而怀疑是不是因为SpringMVC的版本问题(升级前该应用的springMvc版本为2.5.3，升级后为5.1.8)？ 将应用去掉SpringBoot, 采用SpringMVC方式 (Spring版本依然为5.1.8)，无法重现。

自此，该问题陷入泥潭（因为本地无法复现，无法追踪）

转机-发现问题

很幸运，在对一个文件上传接口进行测试时，该问题在本地终于出现。
下面贴出代码（已删除不必要的代码）：

@ResponseBody
@RequestMapping("/fileupload/new.htm")
protected Map<String, Object> handleRequestInternal(HttpServletRequest request, HttpServletResponse response) {
    Map<String, Object> resultMap = new HashMap<String, Object>();
    PrintWriter printWriter = null;
    try {
        printWriter = response.getWriter();
        ...
   } catch (IOException e) {
        LOGGER.error("[ fileUpload ][error] -> 上传失败");
        resultMap.put("success", false);
        resultMap.put("msg", "上传失败");
    } finally {
    }
    return resultMap;;
}

因为在升级后，以前的代码并没有过多的更改。 可以发现，这个接口本来是要返回json的，但因为历史原因，写法采用的是response.getWriter().write()的方式，并且升级过程中，没有完全删除response.getWriter()。而在升级后，增加了fastJson的配置。
下面贴出fastJson配置：

@ControllerAdvice
public class JsonpConverter extends FastJsonHttpMessageConverter implements ResponseBodyAdvice {
@Override
    public Object beforeBodyWrite(Object o, MethodParameter methodParameter, MediaType mediaType, Class aClass, ServerHttpRequest serverHttpRequest, ServerHttpResponse serverHttpResponse) {
        HttpServletRequest servletRequest = ((ServletServerHttpRequest) serverHttpRequest).getServletRequest();
        String callback = servletRequest.getParameter("callback");
        if(StringUtils.isNotBlank(callback)){
            JSONPObject jsonp = new JSONPObject(callback);
            jsonp.addParameter(o);

            HttpServletResponse response = ((ServletServerHttpResponse) serverHttpResponse).getServletResponse();
            PrintWriter pw = null;
            try {
                pw = response.getWriter();
                pw.write(jsonp.toJSONString());
            } catch (IOException e) {
                e.printStackTrace();
            }finally {
                if(pw != null){
                    pw.flush();
                    pw.close();
                }
            }
        }
        return o;
    }
}

这里采用的也是response.getWriter()。 而报错原因就是getWriter()应被调用。我通过去掉文件上传接口中的response.getWriter()后本地不在出现该异常。

问题再追踪

对于fastJson，这个配置是以前沿用下来的，为什么以前可以，现在不可以了呢？

通过源码追踪，我在tomcat-embed-core-9.0.24.jar下的Response类中的getOutputStream()方法中发现了这么一段代码：

public ServletOutputStream getOutputStream() throws IOException {
        if (this.usingWriter) {
            throw new IllegalStateException(sm.getString("coyoteResponse.getOutputStream.ise"));
        } else {
            this.usingOutputStream = true;
            if (this.outputStream == null) {
                this.outputStream = new CoyoteOutputStream(this.outputBuffer);
            }

            return this.outputStream;
        }
    }

我能肯定，我的业务代码和fastjson代码并没有改变(文件上传中的response.getWriter()已经删除，只保留fastjson配置中的response.getWriter())。
实际调用的response.getOutputSteam，并且注意，代码中有个if判断usingWriter, 而usingWriter被设置为true的地方只有一个：

public PrintWriter getWriter() throws IOException {
        if (this.usingOutputStream) {
            throw new IllegalStateException(sm.getString("coyoteResponse.getWriter.ise"));
        } else {
            if (ENFORCE_ENCODING_IN_GET_WRITER) {
                this.setCharacterEncoding(this.getCharacterEncoding());
            }

            this.usingWriter = true;
            this.outputBuffer.checkConverter();
            if (this.writer == null) {
                this.writer = new CoyoteWriter(this.outputBuffer);
            }

            return this.writer;
        }
    }

可以看到这两个方法，互相判断，应该是为了保证调用的resposne写方法一致，要不都用getWriter()，要不都用getOutputStream()。而我并没有使用getOutputStream()，那就只能说SpringBoot中底层代码用的getOutputStream。 那我理解就是新的Spring已经不希望你使用getWriter()了么？

解决方案

总之，无论现在的Spring处于什么原因，造成了这个问题，只要避免了代码中存在response.getWriter()写法就能避免该问题。尽可能的采用response.getOutputStream()来输出。
如果应用中包含的有fastJson, siteMesh, urlWriter等插件，都要更改response.getWriter()为response.getOutputStream()