getOutputStream() has already been called for this response的解决方法

喜欢ヅ旅行 2022-08-20 06:09 165阅读 0赞

《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：

![None.gif][]<%   
![None.gif][]  BufferedImage image  = new BufferedImage(400, 400 , BufferedImage.TYPE\_INT\_RGB);  
![None.gif][]  Graphics g =  image.getGraphics();  
![None.gif][]  g.fillRect(0,0,400,400 );  
![None.gif][]  g.setColor(new Color(255,0,0 ));    
![None.gif][]  g.fillArc(20,20,100,100,30,120 );  
![None.gif][]  g.setColor(new Color(0,255,0 ));  
![None.gif][]  g.fillArc(20,20,100,100,150,20 );  
![None.gif][]  g.setColor(new Color(0,0,255 ));  
![None.gif][]  g.fillArc(20,20,100,100,270,120 );  
![None.gif][]  g.setColor(new Color(0,0,0 ));  
![None.gif][]  g.drawString("red:climb" , 300, 80 );  
![None.gif][]  g.drawString("green:swim", 300, 120 );  
![None.gif][]  g.drawString("blue:jump", 300, 160 );  
![None.gif][]  ImageIO.write(image, "bmp" , response.getOutputStream());  
![None.gif][]  // out.clear();  
![None.gif][]  //out = pageContext.pushBody();   
![None.gif][] %>

在Tomcat下运行时抛出如下异常：

![None.gif][]   
![None.gif][]    at org.apache.catalina.connector.Response.getWriter(Response.java:601)  
 ![None.gif][]    at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:196 )  
![None.gif][]    at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125 )  
![None.gif][]    at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118 )  
![None.gif][]    at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:185 )  
![None.gif][]    at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:116 )  
![None.gif][]    at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:76 )  
![None.gif][]    at org.apache.jsp.pages.drawImage\_jsp.\_jspService(drawImage\_jsp.java:84)  
 ![None.gif][]    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98 )  
![None.gif][]    at javax.servlet.http.HttpServlet.service(HttpServlet.java:803 )  
![None.gif][]    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:328 )  
![None.gif][]    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:315 )  
![None.gif][]    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265 )  
![None.gif][]    at javax.servlet.http.HttpServlet.service(HttpServlet.java:803 )  
![None.gif][]    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269 )  
![None.gif][]    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188 )  
![None.gif][]    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:210 )  
![None.gif][]    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174 )  
![None.gif][]    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127 )  
![None.gif][]    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117 )  
![None.gif][]    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108 )  
![None.gif][]    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151 )  
![None.gif][]    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:870 )  
![None.gif][]    at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665 )  
![None.gif][]    at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528 )  
![None.gif][]    at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81 )  
![None.gif][]    at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:685 )  
![None.gif][]    at java.lang.Thread.run(Thread.java:595)

查看转换后的JSP代码，发现第84行如下(绿色高亮处)：

![None.gif][]  public void  \_jspService(HttpServletRequest request, HttpServletResponse response)  
![ExpandedBlockStart.gif][]        throws java.io.IOException, ServletException  \{  
![InBlock.gif][]  
![InBlock.gif][]    JspFactory \_jspxFactory = null;  
![InBlock.gif][]    PageContext pageContext = null;  
![InBlock.gif][]    HttpSession session = null;  
![InBlock.gif][]    ServletContext application = null;  
![InBlock.gif][]    ServletConfig config = null;  
![InBlock.gif][]    JspWriter out = null;  
![InBlock.gif][]    Object page = this;  
![InBlock.gif][]    JspWriter \_jspx\_out = null;  
![InBlock.gif][]    PageContext \_jspx\_page\_context = null;  
![InBlock.gif][]  
![InBlock.gif][]  
![ExpandedSubBlockStart.gif][]    try \{  
![InBlock.gif][]      \_jspxFactory = JspFactory.getDefaultFactory();  
![InBlock.gif][]      response.setContentType("text/html; charset=UTF-8");  
![InBlock.gif][]      pageContext = \_jspxFactory.getPageContext(this, request, response,  
![InBlock.gif][]                  null, true, 8192, true);  
![InBlock.gif][]      \_jspx\_page\_context = pageContext;  
![InBlock.gif][]      application = pageContext.getServletContext();  
![InBlock.gif][]      config = pageContext.getServletConfig();  
![InBlock.gif][]      session = pageContext.getSession();  
![InBlock.gif][]      out = pageContext.getOut();  
![InBlock.gif][]      \_jspx\_out = out;  
![InBlock.gif][]  
![InBlock.gif][]      out.write("\\r\\n");  
![InBlock.gif][]      out.write("\\n");  
![InBlock.gif][]      out.write("<!DOCTYPE html PUBLIC \\"\-//W3C//DTD HTML 4.01 Transitional//EN\\" \\"http://www.w3.org/TR/html4/loose.dtd\\">\\n");  
![InBlock.gif][]      out.write("<html>\\n");  
![InBlock.gif][]      out.write("<head>\\n");  
![InBlock.gif][]      out.write("<meta http-equiv=\\"Content\-Type\\" content=\\"text/html; charset=UTF\-8\\">\\n");  
![InBlock.gif][]      out.write("<title>Draw Image</title>\\n");  
![InBlock.gif][]      out.write("</head>\\n");  
![InBlock.gif][]      out.write("<body>\\n");  
![InBlock.gif][]      out.write("\\r\\n");  
![InBlock.gif][]  
![InBlock.gif][]  BufferedImage image = new BufferedImage(400, 400, BufferedImage.TYPE\_INT\_RGB);  
![InBlock.gif][]  Graphics g = image.getGraphics();  
![InBlock.gif][]  g.fillRect(0,0,400,400);  
![InBlock.gif][]  g.setColor(new Color(255,0,0));    
![InBlock.gif][]  g.fillArc(20,20,100,100,30,120);  
![InBlock.gif][]  g.setColor(new Color(0,255,0));  
![InBlock.gif][]  g.fillArc(20,20,100,100,150,20);  
![InBlock.gif][]  g.setColor(new Color(0,0,255));  
![InBlock.gif][]  g.fillArc(20,20,100,100,270,120);  
![InBlock.gif][]  g.setColor(new Color(0,0,0));  
![InBlock.gif][]  g.drawString("red:climb" , 300, 80);  
![InBlock.gif][]  g.drawString("green:swim", 300, 120);  
![InBlock.gif][]  g.drawString("blue:jump", 300, 160);  
![InBlock.gif][]  ImageIO.write(image, "bmp", response.getOutputStream());  
![InBlock.gif][]  //out.clear();  
![InBlock.gif][]  //out = pageContext.pushBody();  
![InBlock.gif][]  
![InBlock.gif][]      out.write("\\r\\n");  
![InBlock.gif][]      out.write("</body>\\n");  
![InBlock.gif][]      out.write("</html>");  
![ExpandedSubBlockStart.gif][]    \} catch (Throwable t) \{  
![ExpandedSubBlockStart.gif][]      if (!(t instanceof SkipPageException))\{  
![InBlock.gif][]        out = \_jspx\_out;  
![InBlock.gif][]        if (out != null && out.getBufferSize() != 0)  
![InBlock.gif][]          out.clearBuffer();  
![InBlock.gif][]        if (\_jspx\_page\_context != null) \_jspx\_page\_context.handlePageException(t);  
![ExpandedSubBlockEnd.gif][]      \}  
![ExpandedSubBlockStart.gif][]    \} finally \{  
![InBlock.gif][]      if (\_jspxFactory != null) \_jspxFactory.releasePageContext(\_jspx\_page\_context);  
![ExpandedSubBlockEnd.gif][]    \}  
![ExpandedBlockEnd.gif][]  \}

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：

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`

`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：  
  
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  
  
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

[None.gif]: /images/20220731/f5bb0f9fe0a5422aa2d55296928e1d58.png
[ExpandedBlockStart.gif]: /images/20220731/d10da28412fe418ebf8513c921a39e94.png
[InBlock.gif]: /images/20220731/145b8f7a80cf45afaf1a435fc5ef6159.png
[ExpandedSubBlockStart.gif]: /images/20220731/e1e7f6cd35e548a5bdd5c2bc1e53827c.png
[ExpandedSubBlockEnd.gif]: /images/20220731/907ce7fc2c414064ae6a111a670ff050.png
[ExpandedBlockEnd.gif]: /images/20220731/06b1a443f0a6461a86b94eda36148fff.png