在JEEE开发文件下载中。有时候看到：
java 代码
- 严重: Servlet.service() for servlet jsp threw exception
- java.lang.IllegalStateException: getOutputStream() has already been called for this response
- at org.apache.catalina.connector.Response.getWriter(Response.java:604)
- at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
- 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:186)
这样的错误，常常让人摸不清问题的所在。一个偶然的操作,我把servelt中业务逻辑的代码,用try,catch包起来。如下所示：
java 代码
- try {
- ............
- } catch (RuntimeException e) {
- e.printStackTrace();
- }
发现以上异常消失。一个
RuntimeException 错误的堆栈被打印出来。这个
RuntimeException 错误被处理之后，一切正常。于是联想到以后如何处理服务器的这一怪异行为。
到网上搜索后。
java 代码
- 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。
-
- 然后当然是要提出解决的办法，其实挺简单的（并不是和某些朋友说的那样--
- 将jsp内的所有空格和回车符号所有都删除掉），
然后又看了一下我先前的错误堆栈：
java 代码
- java.lang.IllegalStateException: getOutputStream() has already been called for this response
- at org.apache.catalina.connector.Response.getWriter(Response.java:604)
- at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
- 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:186)
- at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:118)
- at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:77)
- at org.apache.jsp.unhandledException_jsp._jspService(unhandledException_jsp.java:120)
- at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
- at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
呵呵。org.apache.jasper.runtime.PageContextImpl.release
由上面的堆栈可以看出。在
javax.servlet.http.HttpServlet.service中出错。会执行org.apache.jasper.runtime.JspFactoryImpl.releasePageContext，然后会调用org.apache.catalina.connector.Response.getWriter(Response.java:604)，由于
getWriter，和您下载文件时用的response.getOutputStream()冲突。见以下doc
java 代码
- getOutputStream
-
- public ServletOutputStream getOutputStream()
- throws java.io.IOException
-
- Returns a ServletOutputStream suitable for writing binary data in the response. The servlet container does not encode the binary data.
-
- Calling flush() on the ServletOutputStream commits the response.
- Either this method or getWriter() may be called to write the body, not both.
-
- Returns:
- a ServletOutputStream for writing binary data
- Throws:
- IllegalStateException - if the getWriter method has been called on this response
- java.io.IOException - if an input or output exception occurred
- See Also:
- getWriter()
至此。一切都明白了。想到经常在做服务器开发的时候，经常出现莫名其妙的服务器错误。在重新启动服务器后又消失掉了。有可能是再您的程序中，出现了unchecked的异常。而服务器在处理异常是有一些预订的行为。有可能在处理异常时又出现新的错误。从而把您的视线移到服务器出现的那个错误上。而我们往往对这些错误没什么办法。在这个时候，您不妨跟踪到您代码出错的那一行。把异常捕获。
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