【正文】 ludes web server functionality to provide full support for HTTP in addition to the servlet runtime environment. Containers can also be embedded in other servers, such as a climatecontrol system, to offer a webbased interface to the system. A container bundled as part of an application server can distribute the execution of servlets over multiple hosts. The server can balance the load evenly over all containers, and some servers can even provide failover capabilities in case a host crashes. No matter what type it is, the servlet container is responsible for mapping an ining request to a servlet registered to handle the resource identified by the URI and passing the request message to that servlet. After the request is processed, it39。s error handling is also much more robust than C/C++, where an error such as division by zero typically brings down the whole server. In addition, servlets use specialized interfaces to server resources that aren39。t tie you to one specific vendor. Also, servlets are written in the Java programming language, so they can be used on any operating system with a Java runtime environment. Integration Servlets are developed in Java and can therefore take advantage of all other Java technologies, such as JDBC for database access, JNDI for directory access, RMI for remote resource access, etc. Starting with Version , the servlet specification is part of the Java 2 Enterprise Edition (J2EE), making servlets an important ingredient of any largescale enterprise application, with formalized relationships to other serverside technologies such as Enterprise JavaBeans. Efficiency Servlets execute in a process that is running until the servletbased application is shut down. Each servlet request is executed as a separate thread in this permanent process. This is far more efficient that the CGI model, where a new process is created for each request. First of all (and most obvious), a servlet doesn39。ll take a closer look at how JSP pages are actually turned into servlets automatically. If you39。s okay to send the request again Besides the GET and POST methods, HTTP specifies the following methods: OPTIONS The OPTIONS method is used to find out what options (., methods) a server or a resource offers. HEAD The HEAD method is used to get a response with all headers generated by a GET request but without the body. It can make sure a link is valid or to see when a resource was last modified. PUT The PUT method is used to store the message body content on the server as a resource identified by the URI. DELETE The DELETE method is used to delete the resource identified by the URI. TRACE The TRACE method is used for testing the munication between the client and the server. The server sends back the request message, exactly as it received it, as the body of the response. These methods aren39。t be bookmarked as easily。 rv: ) Accept: image/gif, image/jpeg, image/pjpeg, image/png, */* Acceptlanguage: enUS Acceptcharset: iso88591,*,utf8 city=Hermosa+Beachamp。 Win 9x 。state=CA Hermosa Beach weather forecast /a When you use a form to send user input to the server, you can specify whether to use the GET or POST method with the method attribute, as shown here: form action=/forecast method=POST City: input name=city type=text State: input name=state type=text p input type=SUBMIT /form If the user enters Hermosa Beach and CA in the form fields and clicks on the Submit button, the browser sends a request message like this to the server: POST /forecast HTTP/ Host: UserAgent: Mozilla/ (Windows。 URL encoded name/value pairs separated by ampersands. Request Methods As described earlier, GET is the most monly used request method, intended to retrieve a resource without causing anything else to happen on the server. The POST method is almost as mon as GET。t get confused with characters used to separate name/value pairs and other parts of the URI. In this example, the space between Hermosa and Beach is encoded as a plus sign. Other special characters are encoded as their corresponding hexadecimal ASCII value。s customer number and the list of items to be purchased transferred as parameters. Parameters can be sent in one of two ways: tacked on to the URI in the form of a query string or sent as part of the request message body. This is an example of a URL with a query string: The query string starts with a question mark (?) and consists of name/value pairs separated by ampersands (amp。 the next time the user asks for this resource, he can ask the server to send it only if it39。 all it cares about is getting a response. The response message looks similar to the request message. It consists of three things: a status line, response headers, and an optional response body. Here39。 it39。s why the generic term resource is used. In fact, there39。s preferences, such as use a supported image format and the preferred language. These are just a few of the headers that can be included in a request message. The resource identifier (URI) doesn39。 rv: ) Accept: image/gif, image/jpeg, image/pjpeg, image/png, */* AcceptLanguage : en AcceptCharset : iso88591,*,utf8 The request line specifies the GET method and asks for the resource named / to be returned using the HTTP/ protocol version. The various headers provide additional information. The Host header tells the server the hostname used in the URL. A server may have multiple names, so this information is used to distinguish between multiple virtual web servers sharing the same web server process. The UserAgent header contains information about the type of browser making the request. The server can use this to send different types of responses to different types of browsers. For instance, if the server knows whether Inter Explorer or Netscap