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4.5 Handling Relative URLs in Web Applications

Suppose you have an image that you want displayed in a JSP page. If the image is used only by that particular JSP page, you can place the image and the JSP page in the same directory; the JSP page can then use a simple relative URL to name the image, as below:

<IMG SRC="MyImage.gif" WIDTH=" " HEIGHT="..." ALT="...">

If there are lots of different images, it is usually convenient to group them in a subdirectory. But each URL remains simple:

<IMG SRC="images/MyImage.gif" ...>.

So far, so good. But what if the same image is used by JSP or HTML pages that are scattered throughout your application? Copying the image lots of places would be wasteful and would make updating the image difficult. And even that wouldn't solve all your problems. For example, what if you have a servlet that uses an image? After all, you can't just use

out.println("<IMG SRC=\"MyImage.gif\" ...>"); // Fails!

because the browser would treat the image location as relative to the servlet's URL. But the default URL of a servlet is http://host/webAppPrefix/servlet/ServletName. Thus, the browser would resolve the relative URL MyImage.gif to http://host/web-AppPrefix/servlet/MyImage.gif. That, of course, will fail since servlet is not really the name of a directory; it is just an artifact of the default URL mapping. You have precisely the same problem when using the MVC architecture (see Section 3.8) where a RequestDispatcher forwards the request from a servlet to a JSP page. The browser only knows about the URL of the original servlet and thus treats image URLs as relative to that location.

If you aren't using Web applications, you can solve all these problems the same way: by using a URL that is relative to the server's root directory, not relative to the location of the file that uses the image. For example, you could put the images in a directory called images that is in the root directory. Then, a JSP page could use

<IMG SRC="/images/MyImage.gif" ...> and a servlet could do out.println("<IMG SRC=\"/images/MyImage.gif\" ...>");

Unfortunately, however, this trick fails when you use Web applications. If a JSP page uses an image URL of

<IMG SRC="/images/MyImage.gif" ...> 

the browser will request the image from the main server root, not from the base location of the Web application.

Exactly the same problem occurs with style sheets, applet class files, and even simple hypertext links that use URLs that begin with slashes. Note, however, that this problem does not occur in situations where the server resolves the URLs, only in cases where the browser does so. So, for example, it is perfectly safe for a JSP page that is in a Web app to do

<%@ taglib uri="/tlds/SomeFile.tld" ... %>

The server will correctly treat that URL as referring to the tlds directory within the Web application. Similarly, there is no problem using URLs that begin with / in locations passed to the getRequestDispatcher method of ServletContext; the server resolves them with respect to the Web application's root directory, not the overall server root.

Core Approach

URLs that are returned to the browser need to be handled specially. URLs that are handled by the server need not be.

There are three possible solutions to this dilemma. The first is the most commonly used but the least flexible. I recommend option (2) or (3).

  1. Use the Web application name in the URL. For example, you could create a subdirectory called images within your Web application, and a JSP page could refer to an image with the following URL. <IMG SRC="/webAppPrefix/images/MyImage.gif" ...> This would work both for regular JSP pages and JSP pages that are invoked by means of a RequestDispatcher. Servlets could use the same basic strategy. However, this approach has one serious drawback: if you change the URL prefix of the Web application, you have to change a large number of JSP pages and servlets. This restriction is unacceptable in many situations; you want to be able to change the Web application's URL prefix without changing any of the files within the Web app.

  2. Assign URLs that are at the top level of the Web application. For example, suppose you had a servlet named WithdrawServlet that was in the bankingpackage of a Web application named financial. The default URL to invoke that servlet would be http://host/financial/servlet/banking.WithdrawServlet. Thus, the serv-let would suffer from the problems just discussed when using images, style sheets, and so forth. But, there is no requirement that you use the default URL. In fact, many people feel that you should avoid default URLs in deployment scenarios. Instead, you can use the web.xml file to assign a URL that does not contain the servlet "subdirectory" (see Section 5.3, "Assigning Names and Custom URLs"). For example, Listing 4.4 shows a web.xml file that could be used to change the URL from http://host/financial/servlet/banking.WithdrawServlet to http://host/financial/Withdraw. Now, since the URL does not contain a "subdirectory" named servlet, the servlet can use simple relative URLs that contain only the filename or the subdirectory and the file, but without using a /. For instance, if the Web application contained an image called Cash.jpg, you could place it in the Web app's images directory and the servlet could use

    out.println("<IMG SRC=\"images/Cash.jpg\" ...>"); 

    Listing 4.4 web.xml that assigns top-level URL

    <?xml version="1.0" encoding="ISO-8859-1"?>
    <!DOCTYPE web-app PUBLIC
    "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
    "http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">
    <web-app>
    <servlet>
    <servlet-name>WithdrawServlet</servlet-name>
    <servlet-class>banking.WithdrawServlet</servlet-class>
    </servlet>
    <servlet-mapping>
    <servlet-name>WithdrawServlet</servlet-name>
    <url-pattern>/Withdraw</url-pattern>
    </servlet-mapping>
    </web-app>
  3. Use getContextPath. The most general solution is to explicitly add the Web application name to the front of each URL that begins with /. However, instead of hardcoding the name, you can use the getContextPathmethod of HttpServletRequestto determine the name at execution time. For example, a JSP page could do the following.

    <% String prefix = request.getContextPath();
       String url = prefix + "/images/MyImage.jpg"; %>
    <IMG SRC="<%= url %>" ...>

    If you have a number of URLs of this nature, you can make use of the BASE element to standardize the location to which relative URLs are resolved. For example:

    <HEAD>
    <BASE HREF="<%= request.getContextPath() %>">
    <TITLE>...</TITLE>
    </HEAD>

    The use of getContextPath is so generally applicable that it is worth capturing some of this functionality in a reusable utility. Listing 4.5 presents one such utility that not only modifies regular URLs, but also handles URLs that are to be used for session tracking that is based on URL rewriting.

    Listing 4.5 AppUtils.java

    package moreservlets;
    
    import javax.servlet.http.*;
    
    /** A small set of utilities to simplify the use of URLs in
     * Web applications.
    */
    
    public class AppUtils {
    /** For use in URLs referenced by JSP pages or servlets, where
     * you want to avoid hardcoding the Web app name. Replace
     * <PRE><XMP>
     * <IMG SRC="/images/foo.gif" ...>
     * with the following two lines:
     * <% String imageURL = webAppURL("/images/foo.gif",
     * request); %>
     * <IMG SRC="<%= imageURL %>"...>
     *
     * </XMP></PRE>
    public static String webAppURL(String origURL,
                               HttpServletRequest request) {
     return(request.getContextPath() + origURL);
    }
    
    /** For use when you want to support session tracking with
     * URL encoding and you are putting a URL
     * beginning with a slash into a page from a Web app.
     */
    
    public static String encodeURL(String origURL,
                               HttpServletRequest request,
                               HttpServletResponse response) {
     return(response.encodeURL (webAppURL(origURL, request)));
    }
    /** For use when you want to support session tracking with
     * URL encoding and you are using sendRedirect to send a URL
     * beginning with a slash to the client.
     */
    
    public static String encodeRedirectURL
                               (String origURL,
                               HttpServletRequest request,
                               HttpServletResponse response) {
     return(response.encodeRedirectURL
                       (webAppURL(origURL, request)));
     }
    }
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