getOutputStream() has already been called for this response的解決方法

《輕量級(jí)J2EE企業(yè)應(yīng)用實(shí)戰(zhàn)》一書的第2章有一個(gè)使用SerlvetResponse輸出圖像的例子，代碼如下：

BufferedImage image = new BufferedImage(400, 400, BufferedImage.TYPE_INT_RGB);

Graphics g = image.getGraphics();

g.fillRect(0,0,400,400);

g.setColor(new Color(255,0,0));

g.fillArc(20,20,100,100,30,120);

g.setColor(new Color(0,255,0));

g.fillArc(20,20,100,100,150,20);

g.setColor(new Color(0,0,255));

g.fillArc(20,20,100,100,270,120);

g.setColor(new Color(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下運(yùn)行時(shí)拋出如下異常：

at org.apache.catalina.connector.Response.getWriter(Response.java:601)

at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:196)

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:185)

at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:116)

at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:76)

at org.apache.jsp.pages.drawImage_jsp._jspService(drawImage_jsp.java:84)

at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:328)

at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:315)

at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:210)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151)

at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:870)

at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)

at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)

at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)

at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:685)

at java.lang.Thread.run(Thread.java:595)

查看轉(zhuǎn)換后的JSP代碼，發(fā)現(xiàn)第84行如下(綠色高亮處)：

public void _jspService(HttpServletRequest request, HttpServletResponse response)

throws java.io.IOException, ServletException {

JspFactory _jspxFactory = null;

PageContext pageContext = null;

HttpSession session = null;

ServletContext application = null;

ServletConfig config = null;

JspWriter out = null;

Object page = 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("<!DOCTYPE html PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">\n");

out.write("<html>\n");

out.write("<head>\n");

out.write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=UTF-8\">\n");

out.write("<title>Draw Image</title>\n");

out.write("</head>\n");

out.write("<body>\n");

out.write("\r\n");

BufferedImage image = new BufferedImage(400, 400, BufferedImage.TYPE_INT_RGB);

Graphics g = image.getGraphics();

g.fillRect(0,0,400,400);

g.setColor(new Color(255,0,0));

g.fillArc(20,20,100,100,30,120);

g.setColor(new Color(0,255,0));

g.fillArc(20,20,100,100,150,20);

g.setColor(new Color(0,0,255));

g.fillArc(20,20,100,100,270,120);

g.setColor(new Color(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 (Throwable t) {

if (!(t instanceof SkipPageException)){

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);

}

我們看到在JSP頁(yè)面釋放資源的時(shí)候，調(diào)用了ServetResponse.getWriter()方法，之后程序即拋出異常了，查看Servlet的API發(fā)現(xiàn)問(wèn)題：

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()方法和該方法都有可能被調(diào)用，來(lái)輸出JSP頁(yè)面的內(nèi)容，如果其中的一個(gè)方法被調(diào)用了，再調(diào)用另一個(gè)方法就會(huì)拋出異常。

解決方法如下：

將JSP頁(yè)面的最后兩行代碼的注釋去掉，這兩行代碼的作用如下：

out.clear()：清空緩存的內(nèi)容。

pageContext.pushBody()：參考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

·返回一個(gè)新的BodyContent(代表一個(gè)HTML頁(yè)面的BODY部分內(nèi)容）
·保存JspWriter實(shí)例的對(duì)象out
·更新PageContext的out屬性的內(nèi)容

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生活就像打牌，不是要抓一手好牌，而是要盡力打好一手爛牌。

posted on 2008-01-19 22:24 Paul Lin 閱讀(43362) 評(píng)論(10) 編輯收藏所屬分類: J2EE基礎(chǔ)

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