Ajax Basics Tutorial (1)-ajax Introduction 1.3 Web Application development process

Initially, all Web pages are static, the user requests a resource, and the server returns the resource. Nothing moves, nothing flashes. Frankly, for many web sites, this is also possible, the Web pages of these sites are only electronic text, in a generation, content fixed, and then published to many places. In the initial stages of browser development, the static nature of Web pages is not a problem, scientists simply use the Internet to exchange research papers, and universities are simply publishing course information online via the Internet. The business community has yet to discover what opportunities this new "channel" will offer. In fact, the previous company home page shows very little information, nothing more than some contact information or just a few documents. But before long, web users began to have new requirements, hoping to get a more dynamic online experience. Personal computers have become an indispensable resource for businesses, and more and more computers are starting to emerge from individual dormitories to home offices. With the advent of Windows 95, as people have learned the rich features of Corel WordPerfect and Microsoft Excel, users ' expectations are getting higher.

1.3.1 CGI

The first approach to making the Web more dynamic is the Public gateway Interface (Common Gateway interface,cgi). Unlike static Web Capture, a program can be created using CGI, which is executed when a user makes a request. If you want to display the items sold on a Web site, you can use CGI scripts to access the commodity database and display the results. By using simple HTML forms and CGI scripts, you can create simple online storefronts so that others can buy items from the browser. CGI scripts can be written in multiple languages, from Perl to Visual Basic, enabling people with different programming languages to write CGI scripts.

However, CGI is not the safest way to create a dynamic Web page. If you use CGI, you will allow others to execute programs on your system. In most cases this may not be a problem, but if a user has malicious intent, it is likely to take advantage of this to allow the system to run programs that you would otherwise not want to run. Despite this flaw, CGI is still in use today.

1.3.2 Applet

It is clear that CGI can be improved. In May 1995, Sun's John Gage and Andreessen (currently in Netscape Communications) announced the birth of a new programming language, which is java. Netscape Navigator support for this new language, initially to support set-top boxes. (You might think that the first companies to dabble in smart homes were Microsoft and Sony.) Just as all revolutions are coincidental, the advent of Java and the Internet is just as good as it was, at the right time, at the right place, when Java was released on the web for only a few months, and thousands of people downloaded it. As Netscape's Navigator supports Java, Dynamic Web pages have opened a new page: The applet era has arrived.

Applets allow developers to write small applications that can be embedded on a Web page. As long as the user is using a Java-enabled browser, the applet can be run in the browser's Java Virtual machine (Java MACHINE,JVM). Although the applet can do many things, it has some limitations: it is not normally allowed to read and write to the file system, it cannot load the local library, and it may not be able to start the program on the client. In addition to these restrictions, applets are run in a sandbox security model to help prevent users from running malicious code.

For many people, the initial exposure to the Java programming language began with applets, which were a great way to create dynamic Web applications. Applets allow you to create a FAT client application in the browser, but within the security limits of the platform. At that time, applets were widely used in many areas, but the web community was not completely "conquered" by applets [2]. Developers of fat customers are familiar with the problem that the appropriate Java version must be deployed on the client. Because the applet is running in the browser's virtual machine, the developer must ensure that the client has the appropriate version of Java installed. Although this problem can be solved, it does hinder the further promotion of applet technology. And if the applet is poorly written, it is likely to have an impact on the client host, which makes many customers hesitant about using an applet based solution. If you are not familiar with the applet, see Figure 1-1, which shows the clock applet provided by Sun.

Figure 1-1 Sun's Clock applet