近期,查看公司应用日志系统error错误信息时,发现了大量的
nested exception is java.lang.IllegalStateException: getWriter() has already been called for this response异常。这个错误以前见到过,也解决过。于是想着趁有点空闲,总结下该错误。如有不对,望各位多包容,欢迎交流。
一、应用日志文件中的错误信息
ERROR 977 --- [io- 8686 -exec- 10 ] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.IllegalStateException: getWriter() has already been called for this response] with root cause java.lang.IllegalStateException: getWriter() has already been called for this response at org.apache.catalina.connector.Response.getOutputStream(Response.java: 575 ) ~[tomcat-embed-core- 8.5 . 5 .jar: 8.5 . 5 ] |
IllegalStateException: getWriter() has already been called for this response 从字面意思不难得出错误原因:
HttpServletResponse中的
PrintWriter已经被手动调用过了。所以当
servlet执行到方法结果处理逻辑时,需要将返回值输出到
writer中去,这时发现
PrintWriter已经被调用过。于是
servlet认为这是使用混乱的逻辑错误,于是抛出错误。
根本原因:在
Controller接口方法中,既手动调用PrintWriter向客户端输出内容,又设置了方法返回值。导致servlet需要两次将结果通过PrintWriter输出到客户端,结果报错。
/**1.反例:验证接口既手动调用PrintWriter输出流flush,又return 返回值. * 验证结果:客户端ajax请求正常返回,但同时,服务端出现异常,org.springframework.web.util.NestedServletException: Request processing failed; * nested exception is java.lang.IllegalStateException: getWriter() has already been called for this response * 结论:方法内已经执行过response输出流write动作. * servlet再次通过response输出流将返回值向客户端发送时,发现输出流已经被调用过,也就是getWriter()过。于是认为这是使用逻辑混乱,果断报错。 * */ @RequestMapping (value= "/checkGetWriterError" , produces = MediaType.APPLICATION_JSON_UTF8_VALUE) @ResponseBody public ServiceResponse<String> checkGetWriterError(HttpServletResponse response) throws Exception { log.info( "验证接口既手动调用输出流flush,又return 返回值.造成异常!" ); PrintWriter printWriter = response.getWriter(); printWriter.write(ServiceResponse.ok( "来自printWriter的返回值" ).toString()); printWriter.flush(); //没有该句也是报一样错. return ServiceResponse.ok( "成功了,恭喜你." ); } |
结果:程序正常返回结果,控制台出现异常。 (返回结果中编码问题,不在本文讨论范围,请忽略)

二、扩展验证附验证结果
验证PrintWriter输出流flush,无返回值
/**2.正例:验证接口仅手动调用PrintWriter输出流flush,无返回值.*/ @RequestMapping (value= "/checkGetWriterError2" , produces = MediaType.APPLICATION_JSON_UTF8_VALUE) @ResponseBody public void checkGetWriterError2(HttpServletResponse response) throws Exception { log.info( "验证接口手动调用PrintWriter输出流, 无return 返回值. 接口正常执行无异常!" ); PrintWriter printWriter = response.getWriter(); printWriter.write(ServiceResponse.ok( "来自printWriter的返回值" ).toString()); printWriter.flush(); } |
结果:程序正常返回结果,控制台无异常。 (返回结果中编码问题,不在本文讨论范围,请忽略) 
调用ServletOutputStream输出流flush, 无返回值
/**3.正例:验证接口既手动调用ServletOutputStream输出流flush, 无返回值.*/ @RequestMapping (value= "/checkGetWriterError3" , produces = MediaType.APPLICATION_JSON_UTF8_VALUE) @ResponseBody public void checkGetWriterError3(HttpServletResponse response) throws Exception { log.info( "验证接口手动调用ServletOutputStream输出流, 无return 返回值. 接口正常执行无异常!" ); ServletOutputStream output = response.getOutputStream(); output.write(ServiceResponse.ok( "来自ServletOutputStream的返回值" ).toString().getBytes()); output.flush(); } |
结果:程序正常返回结果,控制台无异常。

调用ServletOutputStream输出流flush,又return 返回值
/**4.反例:验证接口既手动调用ServletOutputStream输出流flush,又return 返回值.*/ @RequestMapping (value= "/checkGetWriterError4" , produces = MediaType.APPLICATION_JSON_UTF8_VALUE) @ResponseBody public ServiceResponse<String> checkGetWriterError4(HttpServletResponse response) throws Exception { log.info( "验证接口手动调用ServletOutputStream输出流, 又执行return 返回值. 接口返回异常!" ); ServletOutputStream output = response.getOutputStream(); output.write(ServiceResponse.ok( "来自ServletOutputStream的返回值" ).toString().getBytes()); output.flush(); return ServiceResponse.ok( "来自return的返回值." ); } |
结果:程序返回结果不正常,控制台无异常。 不仅ServletOutputStream输出流中的内容被发送到客户端,而且方法返回结果也输出到客户端。
