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 handleRequestInternal(HttpServletRequest request, HttpServletResponse response) {
Map resultMap = new HashMap();
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()