getOutputStream() has already been called for this

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java.lang.IllegalStateException: getOutputStream() has already been called for this response
tomcat有此现象,weblogic下没有
原因：
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responsegetOutputStream()相冲突的！所以会出现以上这个异常。
解决办法:
在使用完输出流以后调用以下两行代码即可：
out.clear();
out = pageContext.pushBody();

API:
public java.io.PrintWriter getWriter()
    throws java.io.IOException
Returns a PrintWriter object that can send character text to the client. The PrintWriter uses the character encoding returned by getCharacterEncoding(). If the response's character encoding has not been specified as described in getCharacterEncoding (i.e., the method just returns the default value ISO-8859-1), getWriter updates it to
ISO-8859-1. Calling flush() on the PrintWriter commits the response.
Either this method or getOutputStream() may be called to write the body, not both.
  Returns:
     a PrintWriter object that can return character data to the client
Throws:
    UnsupportedEncodingException - if the character encoding returned by getCharacterEncoding cannot be used
java.lang.IllegalStateException - if the getOutputStream method has already been called for this response object
   java.io.IOException - if an input or output exception occurred
See Also:
  getOutputStream(), setCharacterEncoding(java.lang.String)
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。
解决方法如下：
out.clear()：清空缓存的内容。
pageContext.pushBody()：参考API

public BodyContent pushBody()
Return a new BodyContent object, save the current "out" JspWriter, and update the value of the "out" attribute in the page scope attribute namespace of the PageContext.
Returns:  the new BodyContent
·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

This exception also may caused by there are some space character between <%..%> and <%..%> in jsp.