JSP：getOutputStream() has already been called for this response

JSP页面，用小脚本显示一张图片

<%@page import="java.io.OutputStream"%>
<%@page import="java.io.FileInputStream"%>
<%@page import="java.io.InputStream"%>
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%
String path=request.getRealPath("/");
response.setContentType("image/png");
InputStream is=new FileInputStream(path+"img/2.png");
byte[] content=new byte[is.available()];
is.read(content);
OutputStream os=response.getOutputStream();
os.write(content,0,content.length);
%>

浏览器正常显示了图片，但是控制台报错:

一月 12, 2018 11:45:53 上午 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet [jsp] in context with path [/jsp2] threw exception [java.lang.IllegalStateException: getOutputStream() has already been called for this response] with root cause
java.lang.IllegalStateException: getOutputStream() has already been called for this response
    at org.apache.catalina.connector.Response.getWriter(Response.java:624)
    at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:211)
    at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:118)
    at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:111)
    at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:184)
    at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:120)
    at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:75)
    at org.apache.jsp.重新定向_jsp._jspService(重新定向_jsp.java:143)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:443)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
    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 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:504)
    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:803)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)

错误提示：

java.lang.IllegalStateException: getOutputStream() has already been called for this response

getOutputStream 被重复调用的。但是我自己之调用了一次，到底在什么地方被调用了呢？

查找原因：

查找两处，浏览器代码，以及jsp翻译后的java文件

 /*
  * Generated by the Jasper component of Apache Tomcat
  * Version: Apache Tomcat/8.5.24
  * Generated at: 2018-01-12 04:05:33 UTC
  * Note: The last modified time of this file was set to
  *       the last modified time of the source file after
  *       generation to assist with modification tracking.
  */
 package org.apache.jsp;

 import javax.servlet.*;
 import javax.servlet.http.*;
 import javax.servlet.jsp.*;
 import java.io.OutputStream;
 import java.io.FileInputStream;
 import java.io.InputStream;

 public final class 重新定向_jsp extends org.apache.jasper.runtime.HttpJspBase
     implements org.apache.jasper.runtime.JspSourceDependent,
                  org.apache.jasper.runtime.JspSourceImports {

   private static final javax.servlet.jsp.JspFactory _jspxFactory =
           javax.servlet.jsp.JspFactory.getDefaultFactory();

   private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;

   private static final java.util.Set<java.lang.String> _jspx_imports_packages;

   private static final java.util.Set<java.lang.String> _jspx_imports_classes;

   static {
     _jspx_imports_packages = new java.util.HashSet<>();
     _jspx_imports_packages.add("javax.servlet");
     _jspx_imports_packages.add("javax.servlet.http");
     _jspx_imports_packages.add("javax.servlet.jsp");
     _jspx_imports_classes = new java.util.HashSet<>();
     _jspx_imports_classes.add("java.io.OutputStream");
     _jspx_imports_classes.add("java.io.FileInputStream");
     _jspx_imports_classes.add("java.io.InputStream");
   }

   private volatile javax.el.ExpressionFactory _el_expressionfactory;
   private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;

   public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
     return _jspx_dependants;
   }

   public java.util.Set<java.lang.String> getPackageImports() {
     return _jspx_imports_packages;
   }

   public java.util.Set<java.lang.String> getClassImports() {
     return _jspx_imports_classes;
   }

   public javax.el.ExpressionFactory _jsp_getExpressionFactory() {
     if (_el_expressionfactory == null) {
       synchronized (this) {
         if (_el_expressionfactory == null) {
           _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
         }
       }
     }
     return _el_expressionfactory;
   }

   public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() {
     if (_jsp_instancemanager == null) {
       synchronized (this) {
         if (_jsp_instancemanager == null) {
           _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
         }
       }
     }
     return _jsp_instancemanager;
   }

   public void _jspInit() {
   }

   public void _jspDestroy() {
   }

   public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
       throws java.io.IOException, javax.servlet.ServletException {

     final java.lang.String _jspx_method = request.getMethod();
     if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
       response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD");
       return;
     }

     final javax.servlet.jsp.PageContext pageContext;
     javax.servlet.http.HttpSession session = null;
     final javax.servlet.ServletContext application;
     final javax.servlet.ServletConfig config;
     javax.servlet.jsp.JspWriter out = null;
     final java.lang.Object page = this;
     javax.servlet.jsp.JspWriter _jspx_out = null;
     javax.servlet.jsp.PageContext _jspx_page_context = null;

     try {
       response.setContentType("text/html; charset=UTF-8");
       pageContext = _jspxFactory.getPageContext(this, request, response,
                   null, true, 8192, true);
       _jspx_page_context = pageContext;
       application = pageContext.getServletContext();
       config = pageContext.getServletConfig();
       session = pageContext.getSession();
       out = pageContext.getOut();
       _jspx_out = out;

       out.write("\r\n");//注意此处out被调用
       out.write("\r\n");
       out.write("\r\n");
       out.write("\r\n");

 String path=request.getRealPath("/");
 response.setContentType("image/png");
 InputStream is=new FileInputStream(path+"img/2.png");
 byte[] content=new byte[is.available()];
 is.read(content);
 OutputStream os=response.getOutputStream();
 os.write(content,0,content.length);

     } catch (java.lang.Throwable t) {
       if (!(t instanceof javax.servlet.jsp.SkipPageException)){
         out = _jspx_out;
         if (out != null && out.getBufferSize() != 0)
           try {
             if (response.isCommitted()) {
               out.flush();
             } else {
               out.clearBuffer();
             }
           } catch (java.io.IOException e) {}
         if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
         else throw new ServletException(t);
       }
     } finally {
       _jspxFactory.releasePageContext(_jspx_page_context);
     }
   }
 }

为什么会被调用输入换行呢？ 查找 jsp：

标出红线的四个地方有换行，正好4个，和翻译后的java文件中符合。

缩进掉这几个回车，就没有问题了，图片正常显示，控制台没有报错。