关于图片上传时自动生成缩略图的问题~~Can't read input file!错误－－－急~~

做图片上传的时候想自动生成缩略图，url打印出来什么都对~结果就是出现了这个错误啊~~~各位能人请帮忙~~~由于刚刚注册的账号~~实在是没有银子~~
似乎问题出现在这里
    java.io.File f = new java.io.File(url);        
    String newurl=request.getContextPath()+"/Simgs/"+filename+fileext;     
    Image src = javax.imageio.ImageIO.read(f);//问题在这里
错误状态   javax.imageio.IIOException: Can't read input file!
javax.imageio.ImageIO.read(Unknown Source)
org.apache.jsp.page.dealupload_jsp._jspService(org.apache.jsp.page.dealupload_jsp:152)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:322)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:291)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)