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1、Combining JSP and ServletsThe technology of JSP and Servlet is the most important technology which use Java technology to exploit request of server, and it is also the standard which exploit business application .Java developers prefer to use it for a variety of reasons, one of which is already fami

2、liar with the Java language for the development of this technology are easy to learn Java to the other is a preparation, run everywhere to bring the concept of Web applications, To achieve a one-prepared everywhere realized. And more importantly, if followed some of the principles of good design, it

3、 can be said of separating and content to create high-quality, reusable, easy to maintain and modify the application. For example, if the document in HTML embedded Java code too much (script), will lead the developed application is extremely complex, difficult to read, it is not easy reuse, but also

4、 for future maintenance and modification will also cause difficulties. In fact, CSDN the JSP / Servlet forum, can often see some questions, the code is very long, can logic is not very clear, a large number of HTML and Java code mixed together. This is the random development of the defects. Early dy

5、namic pages mainly CGI (Common Gateway Interface, public Gateway Interface) technology, you can use different languages of the CGI programs, such as VB, C / C + + or Delphi, and so on. Though the technology of CGI is developed and powerful, because of difficulties in programming, and low efficiency,

6、 modify complex shortcomings, it is gradually being replaced by the trend. Of all the new technology, JSP / Servlet with more efficient and easy to program, more powerful, more secure and has a good portability, they have been many people believe that the future is the most dynamic site of the futur

7、e development of technology. Similar to CGI, Servlet support request / response model. When a customer submit a request to the server, the server presented the request Servlet, Servlet responsible for handling requests and generate a response, and then gave the server, and then from the server sent

8、to the customer. And the CGI is different, Servlet not generate a new process, but with HTTP Server at the same process. It threads through the use of technology, reduce the server costs. Servlet handling of the request process is this: When received from the clients request, calling service methods

9、, the method of Servlet arrival of the first judgement is what type of request (GET / POST / HEAD), then calls the appropriate treatment (DoGet / doPost / doHead) and generate a response. Although such a complex, in fact, simply said to Servlet is a Java class. And the general category of the differ

10、ence is that this type operating in a Servlet container, which can provide session management and targeted life-cycle management. So that when you use the Servlet, you can get all the benefits of the Java platform, including the safety of the management, use JDBC access the database and cross-platfo

11、rm capability. Moreover, Servlet using thread, and can develop more efficient Web applications. JSP technology is a key J2EE technology, it at a higher level of abstraction of a Servlet. It allows conventional static and dynamic HTML content generated by combining an HTML page looks like, but as a S

12、ervlet to run. There are many commercial application server support JSP technology, such as BEA WebLogic, IBM WebSphere, JRun, and so on. JSP and Servlet use more than simple. If you have a JSP support for Web servers, and a JSP document, you can put it Fangdao any static HTML files can be placed, d

13、o not have to compile, do not have to pack, do not have to ClassPath settings, you can visit as ordinary Web It did visit, the server will automatically help you to do other work. JSP document looks like an ordinary static HTML document, but inside contains a number of Java code. It uses. Jsp the su

14、ffix, used to tell the server this document in need of special treatment. When we visit a JSP page, the document will first be translated into a JSP engine Java source files, is actually a Servlet, and compiler, and then, like other Servlet, from Servlet engine to handle. Servlet engine of this type

15、 loading, handling requests from customers, and the results returned to the customer, as shown below: Figure 1: Calling the process of JSP pagesAfter another visit this page to the customer, as long as the paper there have been no changes, JSP engine has been loaded directly call the Servlet. If you

16、 have already been modified, it will be once again the implementation of the above process, translate, compile and load. In fact, this is the so-called first person to punishment. Because when the first visit to the implementation of a series of the above process, so will spend some time after such

17、a visit would not. Java servlets offer a powerful API that provides access to all the information about the request, the session, and the application. combining JSP with servlets lets you clearly separate the application logic from the presentation of the application; in other words, it lets you use

18、 the most appropriate component type for the roles of Model, View and Controller.Servlets, Filters, and Listeners A servlet is a Java class that extends a server with functionality for processing a request and producing a response. Its implemented using the classes and interfaces defined by the Serv

19、let API. The API consists of two packages: the javax.servlet package contains classes and interfaces that are protocol-independent, while the javax.servlet.忽略此处 package provides HTTP-specific extensions and utility classes.What makes a servlet a servlet is that the class implements an interface name

20、d javax.servlet.Servlet, either directly or by extending one of the support classes. This interface defines the methods used by the web container to manage and interact with the servlet. A servlet for processing HTTP requests typically extends the javax.servlet.忽略此处.HttpServlet class. This class imp

21、lements the Servlet interface and provides additional methods suitable for HTTP processing. Servlet LifecycleThe web container manages all aspects of the servlets lifecycle. It creates an instance of the servlet class when needed, passes requests to the instance for processing, and eventually remove

22、s the instance. For an HttpServlet, the container calls the following methods at the appropriate times in the servlet lifecycle.Besides the doGet( ) and doPost( ) methods, there are methods corresponding to the other HTTP methods: doDelete( ), doHead( ), doOptions( ), doPut( ), and doTrace( ). Typic

23、ally you dont implement these methods; the HttpServlet class already takes care of HEAD, OPTIONS, and TRACE requests in a way thats suitable for most servlets, and the DELETE and PUT HTTP methods are rarely used in a web application. Its important to realize that the container creates only one insta

24、nce of each servlet. This means that the servlet must be thread safe - able to handle multiple requests at the same time, each executing as a separate thread through the servlet code. Without getting lost in details, you satisfy this requirement with regards to instance variables if you modify the r

25、eferenced objects only in the init( ) and destroy( ) methods, and just read them in the request processing methods.Compiling and Installing a ServletTo compile a servlet, you must first ensure that you have the JAR file containing all Servlet API classes in the CLASSPATH environment variable. The JA

26、R file is distributed with all web containers. Tomcat includes it in a file called servlet.jar, located in the common/lib directory. On a Windows platform, you include the JAR file in the CLASSPATH. Reading a RequestOne of the arguments passed to the doGet( ) and doPost( ) methods is an object that

27、implements the HttpServletRequest interface. This interface defines methods that provide access to a wealth of information about the request.Generating a ResponseBesides the request object, the container passes an object that implements the HttpServletResponse interface as an argument to the doGet(

28、) and doPost( ) methods. This interface defines methods for getting a writer or stream for the response body. It also defines methods for setting the response status code and headers.Using Filters and ListenersThe servlet specification defines two component types beside servlets: filters and listene

29、rs. These two types were introduced in the Servlet 2.3 specification, so if youre using a container that doesnt yet support this version of the specification, Im afraid youre out of luck.FiltersA filter is a component that can intercept a request targeted for a servlet, JSP page, or static page, as

30、well as the response before its sent to the client. This makes it easy to centralize tasks that apply to all requests, such as access control, logging, and charging for the content or the services offered by the application. A filter has full access to the body and headers of the request and respons

31、e, so it can also perform various transformations. One example is compressing the response body if the Accept-Language request header indicates that the client can handle a compressed response. A filter can be applied to either a specific servlet or to all requests matching a URL pattern, such as UR

32、Ls starting with the same path elements or having the same extension.Listeners Listeners allow your application to react to certain events. Prior to Servlet 2.3, you could handle only session attribute binding events (triggered when an object was added or removed from a session). You could do this b

33、y letting the object saved as a sessionattribute(using the HttpSession.setAttribute() method)implement the HttpSessionBindingListener interface. With the new interfaces introduced in the 2.3 version of the specification, you can create listeners for servlet context and session lifecycle events as we

34、ll as session activation and passivation events (used by a container that temporarily saves session state to disk or migrates a session to another server). A new session attribute event listener also makes it possible to deal with attribute binding events for all sessions in one place, instead of pl

35、acing individual listener objects in each session.The new types of listeners follow the standard Java event model. In other words, a listener is a class that implements one or more of the listener interfaces. The interfaces define methods that correspond to events. The listener class is registered w

36、ith the container when the application starts, and the container then calls the event methods at the appropriate times.Initializing Shared Resources Using a ListenerBeans like this typically need to be initialized before they can be used. For instance, they may need a reference to a database or some

37、 other external data source and may create an initial information cache in memory to provide fast access even to the first request for data. You can include code for initialization of the shared resources in the servlet and JSP pages that need them, but a more modular approach is to place all this c

38、ode in one place and let the other parts of the application work on the assumption that the resources are already initialized and available. An application lifecycle listener is a perfect tool for this type of resource initialization. This type of listener implements the javax.servlet.ServletContext

39、Listener interface, with methods called by the container when the application starts and when it shuts down.Picking the Right Component Type for Each TaskThe Project Billboard application introduced is a fairly complex application. Half the pages are pure controller and business logic processing, it

40、 accesses a database to authenticate users, and most pages require access control. In real life, it would likely contain even more pages, for instance, pages for access to a shared document archive, time schedules, and a set of pages for administration. As the application evolves, it may become hard

41、 to maintain as a pure JSP application. Its easy to forget to include the access control code in new pages. This is clearly an application that can benefit from using a combination of JSP pages and the component types defined by the servlet specification for the MVC roles. Lets look at the main requ

42、irements and see how we can map them to appropriate component types:l Database access should be abstracted, to avoid knowledge of a specific data schema or database engine in more than one part of the application: beans in the role of Model can be used to accomplish this.l The database access beans

43、must be made available to all other parts of the application when it starts: an application lifecycle event listener is the perfect component type for this task. l Only authenticated users must be allowed to use the application: a filter can perform access control to satisfy this requirement. l Requ

44、est processing is best done with Java code: a servlet, acting as the Controller, fits the bill. l It must be easy to change the presentation: this is where JSP shines, acting as the View. Adding servlets, listeners, and filters to the mix minimizes the need for complex logic in the JSP pages. Placin

45、g all this code in Java classes instead makes it possible to use a regular Java compiler and debugger to fix potential problems.Centralized Request Processing Using a ServletWith a servlet as the common entry point for all application requests, you gain control over the page flow of the application.

46、 The servlet can decide which type of response to generate depending on the outcome of the requested action, such as returning a common error page for all requests that fail, or different responses depending on the type of client making the request. With the help from some utility classes, it can al

47、so provide services such as input validation, I18N preparations, and in general, encourage a more streamlined approach to request handling. When you use a servlet as a Controller, you must deal with the following basic requirements: l All requests for processing must be passed to the single Controll

48、er servlet.l The servlet must be able to distinguish requests for different types of processing. Here are other features you will want support for, even though they may not be requirements for all applications: l A strategy for extending the application to support new types of processing requests in

49、 a flexible manner.l A mechanism for changing the page flow of the application without modifying code.Mapping Application Requests to the ServletThe first requirement for using a Controller servlet is that all requests must pass through it. This can be satisfied in many ways. If you have played around a bit with servlets previously, youre probably used to invoking a servlet with a URI that starts with /myApp/servlet. This is a convention introduced by Su