Chapter 5. URL Actions

Chapter 5. URL Actions Topics in This Chapter Overview The <c:import> Action The <c:redirect> Action The <c:url> Action The <c:param> Action Accessing External Resources Accessing Resources in Foreign Contexts Redirecting a Response If you've developed Web applications with JavaServer Pages (JSP), you have probably found many uses for <jsp:include> and <jsp:forward>. The former includes the contents of a resource and the latter forwards control to a Web component, such as a servlet or another JSP page. On the other hand, you may have found that those actions have limited capabilities; for example, the URLs that you specify for those actions must be relative URLs, so you cannot use them to access URLs outside your Web application. JSTL provides a set of URL actions that augment the capabilities provided by <jsp:include> and <jsp:forward>; those actions are the subject of this chapter. Before we discuss the JSTL URL actions, let's review some Web application basics and define a few terms used throughout this chapter. Java-based Web applications are stored in a directory on your filesystem; for example, Figure 5-1 illustrates a Web application that resides under the C:\core-jstl\webapp directory. Figure 5-1. A Simple Java-Based Web Application Java-based Web applications reside in a directory, but they are defined by a context; for example, the Web application depicted in Figure 5-1 could be defined in Tomcat's configuration file with a Context element, like this:[1] [1] Different JSP containers use different terms for context paths; for example, Resin calls them web-app ids. <Context path="/core-jstl" docBase="C:/core-jstl/webapp"/> The path attribute of the Context element defines a URL that you use to access a Web application that resides in a directory specified by the docBase attribute; for example, to access the Web application shown in Figure 5-1 you would use the URL $SCHEME$HOSTNAME/core-jstl, where $SCHEME$HOSTNAME represents a scheme and a host name. For example, if the scheme is http:// and the host name is localhost, the URL for the Web application defined above would be http://localhost/core-jstl. As websites grow, it is not uncommon for them to contain more than one Web application. From the perspective of a single Web application, the other Web applications in the same website are referred to as foreign contexts. For example, if your website has a registration Web application and a shopping application, the registration application is a foreign context relative to the shopping application, and vice versa. When you access resources with <jsp:include>, you can specify either a context-relative path or a page-relative path; the former is relative to the top-level directory in a context (a Web application), and the latter is relative to the JSP page in which the <jsp:include> action resides. Context-relative paths always start with a forward slash, whereas page-relative paths do not. For example, for the application shown in Figure 5-1, you can: Access test_2.jsp from test_1.jsp with a context-relative path, like this: <jsp:include page='/jsp/test_2.jsp'/> or with a page-relative path, like this: <jsp:include page='jsp/test_2.jsp'/>. Access test_1.jsp from test_2.jsp with a context-relative path, like this: <jsp:include page='/test_1.jsp'/> or with a page-relative path, like this: <jsp:include page='../test_1.jsp'/>. Now that we have established some common vocabulary, let's take a look at the JSTL URL actions.