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Application Design and Development Practically all use of databases occurs from within application programs. Correspondingly, almost all user interaction with databases is indirect, via application programs. Not surprisingly, therefore, database systems have long supported tools such as form and GUI builders, which help in rapid development of applications that interface with users. In recent years, the Web has become the most widely used user interface to databases.50594
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Although many people interact with databases, very few people use a query language to interact with a database system directly. The most common way in which users interact with databases is through an application program that provides a user interface at the front end, and interfaces with a database at the back end. Such applications take input from users, typically through a forms based interface, and either enter data into a database or extract information from a database based on the user input, and generate output, which is displayed to the user.
Application programs may be used even when it is not apparent that they are being used. For example, a news site may provide a page that is transparently customized to inpidual users, even if the user does not explicitly fill any forms when interacting with the site. To do so, it actually runs an application program that generates a customized page for each user; customization can, for example, be based on the history of articles browsed by the user.
In the past 15 years, Web browsers have become the universal front end to database applications, connecting to the back end through the Internet. Browsers use a standardized syntax, the Hyper Text Markup Language (HTML) standard, which supports both formatted display of information, and creation of forms-based interfaces. The HTML standard is independent of the operating system or browser, and pretty much every computer today has a Web browser installed. Thus a Web-based application can be accessed from any computer that is connected to the Internet.
Many application servers provide built-in support for servlets. One of the most popular is the Tomcat Server from the Apache Jakarta Project. Other application servers that support servlets include Glassfish, JBoss, BEA Web logic Application Server, Oracle Application Server, and IBM’s WebSphere Application Server. The best way to develop servlet applications is by using an IDE such as Eclipse or NetBeans, which come with Tomcat or Glassfish servers built in.
Java Server Pages (JSP), a scripting language that allows HTML programmers to mix static HTML with dynamically generated HTML. The motivation is that, for many dynamic Web pages, most of their content is still static (that is, the same content is present whenever the page is generated). The dynamic content of the Web pages (which are generated, for example, on the basis of form parameters) is often a small part of the page. Creating such pages by writing servlet code results in a large amount of HTML being coded as Java strings. JSP instead allows Java code to be embedded in static HTML; the embedded Java code generates the dynamic part of the page. JSP scripts are actually translated into servlet code
that is then compiled, but the application programmer is saved the trouble of writing much of the Java code to create the servlet.
One of the reasons that the Java language became very popular is that it provides a safe mode for executing programs on users’ computers. Java code can be compiled into platform-independent “byte-code” that can be executed on any browser that supports Java. Unlike local programs, Java programs (applets) downloaded as part of a Web page have no authority to perform any actions that could be destructive. They are permitted to display data on the screen, or to make a network connection to the server from which the Web page was downloaded, in order to fetch more information. However, they are not permitted to access local files, to execute any system programs, or to make network connections to any other computers.