有一个使用SerlvetResponse输出图像的例子,代码如下:
<%
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下运行时抛出如下异常:
at org.apache.catalina.connector.Response.getWriter(Response.java:601)
at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:196)
..................
at org.apache.jsp.pages.drawImage_jsp._jspService(drawImage_jsp.java:84)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98).
...............
查看转换后的JSP代码,发现第84行如下(绿色高亮处):
finally {
if (_jspxFactory != null) _jspxFactory.releasePageContext(_jspx_page_context);
}
}
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
- 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:
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
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