【正文】 followed by a server name and possibly a port number, as in the previous examples. I use URI as a generic term for any string that identifies a resource, where the location can be deduced from the context and isn39。t allow a web server to differentiate between an explicit request caused by clicking a link or submitting a form and an implicit request caused by resizing the browser window or using the browser39。s how it works: a client, typically a web browser, sends a request for a resource to a server, and the server sends back a response corresponding to the resource (or a response with an error message if it can39。s exactly what the HyperText Transport Protocol (HTTP) is for. The munication model defined by HTTP forms the foundation for all web application design. A basic understanding of HTTP is key to developing applications that fit within the constraints of the protocol, no matter which serverside technology you use. In this chapter, we look at the most important details of HTTP you need to be aware of as a web application developer. One other item: this book is about using JSP as the serverside technology. JSP is based on the Java servlet technology. Both technologies share a lot of terminology and concepts, so knowing a bit about servlets will help you even when you develop pure JSP applications. To really understand and use the full power of JSP, you need to know a fair bit about servlets. Hence, we look at servlet fundamentals in the last section of this chapter. The HTTP Request/Response Model HTTP and all extended protocols based on HTTP are based on a very simple munications model. Here39。t distinguish between various methods of triggering the request on the client. For example, HTTP doesn39。s address field. To send a request, the browser needs to know which server to talk to and which resource to ask for. This information is specified by an HTTP Uniform Resource Locator (URL): The first part of the URL shown specifies that the request is made using the HTTP protocol. This is followed by the name of the server, in this case . The web server waits for requests to e in on a specific TCP/IP port. Port number 80 is the standard port for HTTP requests. If the web server uses another port, the URL must specify the port number in addition to the server name. For example: This request is sent to a server that uses port 8080 instead of 80. The last part of the URL, /, identifies the resource that the client is requesting. A URL is actually a specialization of a Uniform Resource Identifier (URI, defined in the RFC2396 specification). A URL identifies a resource partly by its location, for instance the server that contains the resource. Another type of URI is a Uniform Resource Name (URN), which is a globally unique identifier that is valid no matter where the resource is located. HTTP deals only with the URL variety. The terms URI and URL are often used interchangeable, and unfortunately, they have slightly different definitions in different specifications. I39。 enUS。s just a name that means something to the server. The web server is configured to map these unique names to the real resources. Responses in Detail When the web server receives the request, it looks at the URI and decides, based on configuration information, how to handle it. It may handle it internally by simply reading an HTML file from the filesystem, or it can forward the request to some ponent that is responsible for the resource corresponding to the URI. This can be a program that uses database information, for instance, to dynamically generate an appropriate response. To the browser it makes no difference how the request is handled。 for instance, a question mark is encoded as %3F. When parameters are sent as part of the request body, they follow the same syntax。state=CA Due to the differences in how parameters are sent by GET and POST requests, as well as the differences in their intended purpose, browsers handle the requests in different ways. A GET request, parameters and all, can easily be saved as a bookmark, hardcoded as a link, and the response cached by the browser. Also, the browser knows that no damage is done if it needs to send a GET request again automatically, for instance if the user clicks the Reload button. A POST request, on the other hand, can39。t have the overhead of creating the process and loading the CGI script and possibly its interpreter. But another timesaver is that servlets can also access resources that remain loaded in the process memory between requests, such as database connections and persistent state. Scalability By virtue of being written in Java and the broad support for servlets, a servletbased application is extremely scalable. You can develop and test the application on a Windows PC using the standalone servlet reference implementation, and deploy it on anything from a more powerful server running Linux and Apache to a cluster of highend servers with an application server that supports loadbalancing and failover. Robustness and security Java is a strongly typed programming language. This means that you catch a lot of mistakes in the pilation phase that you would only catch during runtime if you used a script language such as Perl. Java39。我們都看到過(guò)一般的客戶端應(yīng)用軟件，但怎么樣才是一個(gè)真正的 Web 應(yīng)用程序？然而，它可以被定義為一個(gè)用于用戶接入的運(yùn)行在服務(wù)器上的程序，通過(guò)一個(gè)簡(jiǎn)單，一般用途的客戶。在本章中，我們看一下最重要的細(xì)節(jié)的 HTTP 您需要了解作為一個(gè) Web 應(yīng)用程序開(kāi)發(fā)。 A資源是一些事情的數(shù)據(jù)，如一個(gè)簡(jiǎn)單的 HTML 文件逐字返回到瀏覽器或程序，動(dòng)態(tài)生成的響應(yīng)。例如，不允許 HTTP Web 服務(wù)器來(lái)區(qū)分一個(gè)明確的要求所造成的點(diǎn)擊一個(gè)鏈接或提交表單和一個(gè)隱含的要求所造成的調(diào)整瀏覽器窗口或使用瀏覽器的后退按鈕。發(fā)送請(qǐng)求后，瀏覽器需要知道與哪些服務(wù)器交換數(shù)據(jù)，并要求得到資源。 網(wǎng)址實(shí)際上是一個(gè)專業(yè)化的統(tǒng)一資源標(biāo)識(shí)符（ URI ，所界定的符合 RFC 2396 規(guī)格） 。例如，當(dāng)請(qǐng)求已被交付給服務(wù)器，位置已經(jīng)定確，只有資源標(biāo)識(shí)符是很重要的。 下面是一個(gè)例子，一個(gè)有效的 HTTP 請(qǐng)求消息： GET / HTTP/ Host： 用戶代理： Mozilla/ (Windows。 user agent 標(biāo)題包含有關(guān)類型的瀏覽器提出請(qǐng)求。它可以識(shí)別一個(gè)可執(zhí)行的程序，記錄在一個(gè)數(shù)據(jù)庫(kù)中，或差不多任何 Web 服務(wù)器知道。一切關(guān)心的就得到了答復(fù)。下一次用戶要求這一資源，他可以向服務(wù)器發(fā)送它只要當(dāng)它被更新，因?yàn)檫@是最后一次