厚积薄发————getOutputStream() has already been called for this response的解决方法
2011-07-25 12:53
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《轻量级J2EE企业应用实战》一书的第2章有一个使用SerlvetResponse输出图像的例子，代码如下：
.java:76)atorg.apache.jsp.pages.drawImage_jsp._jspService(drawImage_jsp.java:84)atorg.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)atjavax.servlet.http.HttpServlet.service(HttpServlet.java:803)atorg.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:328)atorg.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:315)atorg.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)atjavax.servlet.http.HttpServlet.service(HttpServlet.java:803)atorg.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)atorg.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)atorg.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:210)atorg.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174)atorg.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)atorg.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)atorg.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)atorg.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151)atorg.apache.coyote.http11.Http11Processor.process(Http11Processor.java:870)atorg.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)atorg.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)atorg.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)atorg.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:685)atjava.lang.Thread.run(Thread.java:595)
我们看到在JSP页面释放资源的时候，调用了ServetResponse.getWriter()方法，之后程序即抛出异常了，查看Servlet的API发现问题：
[b][b][b]如API所言，由于ServletResponse.getOutputStream()方法和该方法都有可能被调用，来输出JSP页面的内容，如果其中的一个方法被调用了，再调用另一个方法就会抛出异常。[/b][/b][/b]
解决方法如下：
将JSP页面的最后两行代码的注释去掉，这两行代码的作用如下：
out.clear()：清空缓存的内容。
pageContext.pushBody()：参考API
<%
BufferedImageimage=newBufferedImage(400,400,BufferedImage.TYPE_INT_RGB);
Graphicsg=image.getGraphics();
g.fillRect(0,0,400,400);
g.setColor(newColor(255,0,0));
g.fillArc(20,20,100,100,30,120);
g.setColor(newColor(0,255,0));
g.fillArc(20,20,100,100,150,20);
g.setColor(newColor(0,0,255));
g.fillArc(20,20,100,100,270,120);
g.setColor(newColor(0,0,0));
g.drawString("red:climb",300,80);
g.drawString("green:swim",300,120);
g.drawString("blue:jump",300,160);
ImageIO.write(image,"bmp",response.getOutputStream());
//out.clear();
//out=pageContext.pushBody();
%>在Tomcat下运行时抛出如下异常：atorg.apache.catalina.connector.Response.getWriter(Response.java:601)
atorg.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:196)
atorg.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125)
atorg.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118)
atorg.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:185)
atorg.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:116)
atorg.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImplpublicvoid_jspService(HttpServletRequestrequest,HttpServletResponseresponse)
throwsjava.io.IOException,ServletException{
JspFactory_jspxFactory=null;
PageContextpageContext=null;
HttpSessionsession=null;
ServletContextapplication=null;
ServletConfigconfig=null;
JspWriterout=null;
Objectpage=this;
JspWriter_jspx_out=null;
PageContext_jspx_page_context=null;
try{
_jspxFactory=JspFactory.getDefaultFactory();
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");
out.write("\n");
out.write("<!DOCTYPEhtmlPUBLIC\"-//W3C//DTDHTML4.01Transitional//EN\"\"http://www.w3.org/TR/html4/loose.dtd\">\n");
out.write("<html>\n");
out.write("<head>\n");
out.write("<metahttp-equiv=\"Content-Type\"content=\"text/html;charset=UTF-8\">\n");
out.write("<title>DrawImage</title>\n");
out.write("</head>\n");
out.write("<body>\n");
out.write("\r\n");
BufferedImageimage=newBufferedImage(400,400,BufferedImage.TYPE_INT_RGB);
Graphicsg=image.getGraphics();
g.fillRect(0,0,400,400);
g.setColor(newColor(255,0,0));
g.fillArc(20,20,100,100,30,120);
g.setColor(newColor(0,255,0));
g.fillArc(20,20,100,100,150,20);
g.setColor(newColor(0,0,255));
g.fillArc(20,20,100,100,270,120);
g.setColor(newColor(0,0,0));
g.drawString("red:climb",300,80);
g.drawString("green:swim",300,120);
g.drawString("blue:jump",300,160);
ImageIO.write(image,"bmp",response.getOutputStream());
//out.clear();
//out=pageContext.pushBody();
out.write("\r\n");
out.write("</body>\n");
out.write("</html>");
}catch(Throwablet){
if(!(tinstanceofSkipPageException)){
out=_jspx_out;
if(out!=null&&out.getBufferSize()!=0)
out.clearBuffer();
if(_jspx_page_context!=null)_jspx_page_context.handlePageException(t);
}
}finally{
if(_jspxFactory!=null)_jspxFactory.releasePageContext(_jspx_page_context);
}
}.java:76)atorg.apache.jsp.pages.drawImage_jsp._jspService(drawImage_jsp.java:84)atorg.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)atjavax.servlet.http.HttpServlet.service(HttpServlet.java:803)atorg.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:328)atorg.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:315)atorg.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)atjavax.servlet.http.HttpServlet.service(HttpServlet.java:803)atorg.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)atorg.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)atorg.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:210)atorg.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174)atorg.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)atorg.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)atorg.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)atorg.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151)atorg.apache.coyote.http11.Http11Processor.process(Http11Processor.java:870)atorg.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)atorg.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)atorg.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)atorg.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:685)atjava.lang.Thread.run(Thread.java:595)
查看转换后的JSP代码，发现第84行如下
publicvoid_jspService(HttpServletRequestrequest,HttpServletResponseresponse)
throwsjava.io.IOException,ServletException{
JspFactory_jspxFactory=null;
PageContextpageContext=null;
HttpSessionsession=null;
ServletContextapplication=null;
ServletConfigconfig=null;
JspWriterout=null;
Objectpage=this;
JspWriter_jspx_out=null;
PageContext_jspx_page_context=null;
try{
_jspxFactory=JspFactory.getDefaultFactory();
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");
out.write("\n");
out.write("<!DOCTYPEhtmlPUBLIC\"-//W3C//DTDHTML4.01Transitional//EN\"\"http://www.w3.org/TR/html4/loose.dtd\">\n");
out.write("<html>\n");
out.write("<head>\n");
out.write("<metahttp-equiv=\"Content-Type\"content=\"text/html;charset=UTF-8\">\n");
out.write("<title>DrawImage</title>\n");
out.write("</head>\n");
out.write("<body>\n");
out.write("\r\n");
BufferedImageimage=newBufferedImage(400,400,BufferedImage.TYPE_INT_RGB);
Graphicsg=image.getGraphics();
g.fillRect(0,0,400,400);
g.setColor(newColor(255,0,0));
g.fillArc(20,20,100,100,30,120);
g.setColor(newColor(0,255,0));
g.fillArc(20,20,100,100,150,20);
g.setColor(newColor(0,0,255));
g.fillArc(20,20,100,100,270,120);
g.setColor(newColor(0,0,0));
g.drawString("red:climb",300,80);
g.drawString("green:swim",300,120);
g.drawString("blue:jump",300,160);
ImageIO.write(image,"bmp",response.getOutputStream());
//out.clear();
//out=pageContext.pushBody();
out.write("\r\n");
out.write("</body>\n");
out.write("</html>");
}catch(Throwablet){
if(!(tinstanceofSkipPageException)){
out=_jspx_out;
if(out!=null&&out.getBufferSize()!=0)
out.clearBuffer();
if(_jspx_page_context!=null)_jspx_page_context.handlePageException(t);
}
}finally{
if(_jspxFactory!=null)_jspxFactory.releasePageContext(_jspx_page_context);
}
}publicjava.io.PrintWritergetWriter() throwsjava.io.IOException ReturnsaPrintWriterobjectthatcansendcharactertexttotheclient.ThePrintWriterusesthecharacterencodingreturnedbygetCharacterEncoding().Iftheresponse'scharacterencodinghasnotbeenspecifiedasdescribedingetCharacterEncoding(i.e.,themethodjustreturnsthedefaultvalueISO-8859-1),getWriterupdatesittoISO-8859-1. Callingflush()onthePrintWritercommitstheresponse. EitherthismethodorgetOutputStream()maybecalledtowritethebody,notboth. Returns: aPrintWriterobjectthatcanreturncharacterdatatotheclient Throws: UnsupportedEncodingException-ifthecharacterencodingreturnedbygetCharacterEncodingcannotbeused java.lang.IllegalStateException-ifthegetOutputStreammethodhasalreadybeencalledforthisresponseobject java.io.IOException-ifaninputoroutputexceptionoccurred SeeAlso: getOutputStream(),setCharacterEncoding(java.lang.String)
我们看到在JSP页面释放资源的时候，调用了ServetResponse.getWriter()方法，之后程序即抛出异常了，查看Servlet的API发现问题：
publicjava.io.PrintWritergetWriter() throwsjava.io.IOException ReturnsaPrintWriterobjectthatcansendcharactertexttotheclient.ThePrintWriterusesthecharacterencodingreturnedbygetCharacterEncoding().Iftheresponse'scharacterencodinghasnotbeenspecifiedasdescribedingetCharacterEncoding(i.e.,themethodjustreturnsthedefaultvalueISO-8859-1),getWriterupdatesittoISO-8859-1. Callingflush()onthePrintWritercommitstheresponse. EitherthismethodorgetOutputStream()maybecalledtowritethebody,notboth. Returns: aPrintWriterobjectthatcanreturncharacterdatatotheclient Throws: UnsupportedEncodingException-ifthecharacterencodingreturnedbygetCharacterEncodingcannotbeused java.lang.IllegalStateException-ifthegetOutputStreammethodhasalreadybeencalledforthisresponseobject java.io.IOException-ifaninputoroutputexceptionoccurred SeeAlso: getOutputStream(),setCharacterEncoding(java.lang.String)
[b][b][b]如API所言，由于ServletResponse.getOutputStream()方法和该方法都有可能被调用，来输出JSP页面的内容，如果其中的一个方法被调用了，再调用另一个方法就会抛出异常。[/b][/b][/b]
解决方法如下：
将JSP页面的最后两行代码的注释去掉，这两行代码的作用如下：
out.clear()：清空缓存的内容。
pageContext.pushBody()：参考API
publicBodyContentpushBody()
ReturnanewBodyContentobject,savethecurrent"out"JspWriter,andupdatethevalueofthe"out"attributeinthepagescopeattributenamespaceofthePageContext.
Returns:
thenewBodyContent
·返回一个新的BodyContent(代表一个HTML页面的BODY部分内容）
·保存JspWriter实例的对象out
·更新PageContext的out属性的内容
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