Chapter 2. The Structure and Deployment of Web Applications

Chapter 2. The Structure and Deployment of Web Applications
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Chapter 2. The Structure and Deployment of Web Applications

Construct the file and directory structure of a Web Application that may contain (a) static content, (b) JSP pages, (c) servlet classes, (d) the deployment descriptor, (e) tag libraries, (f) JAR files, and (g) Java class files; and describe how to protect resource files from HTTP access.

A Web application exists as a structured hierarchy of directories. The root of this hierarchy serves as the document root for files that are part of the application. For example, for a Web application with the context path /catalog in a Web container, the index.html file at the base of the Web application hierarchy can be served to satisfy a request from /catalog/index.html.

A special directory exists within the application hierarchy named "WEB-INF". This directory contains all things related to the application that aren't in the document root of the application. The WEB-INF node is NOT part of the public document tree of the application. NO file contained in the WEB-INF directory may be served directly to a client by the container. However, the contents of the WEB-INF directory are visible to servlet code using the getResource and getResourceAsStream method calls on the ServletContext, and may be exposed using the RequestDispatcher calls. Hence, if the Application Developer needs access, from servlet code, to application specific configuration information that he does not wish to be exposed directly to the Web client, he may place it under this directory. Since requests are matched to resource mappings in a case-sensitive manner, client requests for /WEB-INF/foo, /WEb-iNf/foo, for example, should not result in contents of the Web application located under /WEB-INF being returned, nor any form of directory listing thereof.

The contents of the WEB-INF directory are:

The /WEB-INF/web.xml deployment descriptor.
The /WEB-INF/classes/ directory for servlet and utility classes. The classes in this directory must be available to the application class loader.
The /WEB-INF/lib/*.jar area for Java ARchive files. These files contain servlets, beans, and other utility classes useful to the Web application. The Web application class loader must be able to load classes from any of these archive files.

The Web application class loader must load classes from the WEB-INF/classes directory first, and then from library JARs in the WEB-INF/lib directory. Also, any requests from the client to access the resources in WEB-INF/ directory MUST be returned with a SC_NOT_FOUND (404) response.

Web applications can be packaged and signed into a Web ARchive format (WAR) file using the standard Java archive tools. For example, an application for issue tracking might be distributed in an archive file called issuetrack.war.

When packaged into such a form, a META-INF directory will be present which contains information useful to Java archive tools. This directory MUST NOT be directly served as content by the container in response to a Web client's request, though its contents are visible to servlet code via the getResource and getResourceAsStream calls on the ServletContext. Also, any requests to access the resources in META-INF directory must be returned with a SC_NOT_FOUND (404) response.

Tag extensions written in JSP using tag files can be placed in one of two locations. The first possibility is in the /META-INF/tags/ directory (or a subdirectory of /META-INF/tags/) in a JAR file installed in the /WEB-INF/lib/ directory of the web application. Tags placed here are typically part of a reusable library of tags that can be easily dropped into any web application.

The second possibility is in the /WEB-INF/tags/ directory (or a subdirectory of /WEB-INF/tags/) of the web application. Tags placed here are within easy reach and require little packaging. Only files with a .tag or .tagx extension are recognized by the container to be tag files.

Tag files that appear in any other location are not considered tag extensions and must be ignored by the JSP container. For example, a tag file that appears in the root of a web application would be treated as content to be served.

The following is a listing of all the files in a sample Web application:

					
/index.html
/howto.jsp
/feedback.jsp
/images/banner.gif
/images/jumping.gif
/WEB-INF/web.xml
/WEB-INF/lib/jspbean.jar
/WEB-INF/lib/jstl.jar
/WEB-INF/jsp/example-taglib.tld 
/WEB-INF/jsp/debug-taglib.tld 
/WEB-INF/jsp2/jsp2-example-taglib.tld 
/WEB-INF/tags/displayProducts.tag
/WEB-INF/tags/helloWorld.tag     
/WEB-INF/tags/panel.tag          
/WEB-INF/tags/xhtmlbasic.tag     
/WEB-INF/classes/com/mycorp/servlets/MyServlet.class
/WEB-INF/classes/com/mycorp/util/MyUtils.class

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Describe the purpose and event sequence of the servlet life cycle: (1) servlet class loading, (2) servlet instantiation, (3) call the init method, (4) call the service method, and (5) call destroy method.	Home	Describe the purpose and semantics for each of the following deployment descriptor elements: error-page, init-param, mime-mapping, servlet, servlet-class, servlet-mapping, servlet-name, and welcome-file.