解决getWriter() has already been called for this response异常

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]

/**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.");

}

/**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();

}

/**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();

}

/**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.");

}