getOutputStream() has already been called for this response异常的原因

Caused by: java.lang.IllegalStateException: getOutputStream() has already been called for this response
	at org.apache.catalina.connector.Response.getWriter(Response.java:611)
	at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
	at javax.servlet.ServletResponseWrapper.getWriter(ServletResponseWrapper.java:112)
	at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125)
	at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118)
	at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:180)
	at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:118)
	at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:77)
	at org.apache.jsp.error._500_jsp._jspService(_500_jsp.java:84)
	at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
	at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:377)
	... 18 more

tomcat5下jsp出现getOutputStream() has already been called for this response异常的原因和解决方法

在tomcat5下jsp中出现此错误一般都是在jsp中使用了输出流（如输出图片验证码，文件下载等），
没有妥善处理好的原因。
具体的原因就是
在tomcat中jsp编译成servlet之后在函数_jspService(HttpServletRequest request, HttpServletResponse response)的最后
有一段这样的代码
finally {
       if (_jspxFactory != null) _jspxFactory.releasePageContext(_jspx_page_context);
     }
这里是在释放在jsp中使用的对象，会调用response.getWriter(),因为这个方法是和
response.getOutputStream()相冲突的！所以会出现以上这个异常。

采用方法很简单.在使用OutputStream输出流完成后,调用下面2个方法即可解决该问题:
out.clear();
out = pageContext.pushBody();

示例代码:

OutputStream os=response.getOutputStream();
os.write(new String("true   "+"nowNum=" + nowNum+"===").getBytes());
os.flush();
os.close();

out.clear();
out = pageContext.pushBody();



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在jsp向页面输出图片的时候,使用response.getOutputStream()会有这样的提示：java.lang.IllegalStateException:getOutputStream() has already been called for this response,会抛出Exception
原因一:
JSP默认的输出流为PrintWriter ,即<% %>以外的东西所默认的输出方式,如果你尝试在JSP中使用ServletOutputStream就会引起错误.要嘛直接改用Servlet输出(复写service方法),要嘛删除除%><%中的任何东西(包括HTML标签,空格,回车等东西)应该就可以。
对于这样的情况应该这样来解决，删除%><%之间的所有内容包括空格和换行符，最后也要消除空格和换行符，最好再加上一句response.reset()。
原因二:
    
在J2EE的API参考里有这么个:

ServletResponse的getWriter()方法里会抛出这个异常,

IllegalStateException - if the getOutputStream method has already been called
for this response object

而它的getOutputStream()方法里会抛出这个异常.

IllegalStateException - if the getOutputStream method has already been called for this response object

并且两者的函数申明里都有这么样的一句
Either this method or getOutputStream() may be called to write the body, not both.
Either this method or getWriter() may be called to write the body, not both.
以上说明也解释了为什么在往页面中写入图片的时候要使用如下循环格式
OutputStream output=response.getOutputStream();
while((len=in.read(b)) >0)
    {
    output.write(b,0,len);
  
    }
output.flush();
而不是把response.getOutputStream().write()放到循环体内