Enhancing a Web Server with Servlets Servlets

Enhancing a Web Server with Servlets

Servlets • Here focus on both sides of a client-server relationship. • The client requests that some action be performed and the server performs the action and responds to the client. • This request-response model of communication is the foundation for the highest-level view of networking in Java—servlets.

Servlets • A common implementation of the requestresponse model is between World Wide Web browsers and World Wide Web servers. • When a user selects a Web site to browse through their browser (the client application), a request is sent to the appropriate Web server (the server application). • The server normally responds to the client by sending the appropriate HTML Web page.

Servlets • A servlet extends the functionality of a server. The javax. servlet package and the javax. servlet. http package provide the classes and interfaces to define servlets.

Servlets • Servlet technology today is primarily designed for use with the HTTP protocol of the Web, but servlets are being developed for other technologies. • Servlets are effective for developing Web-based solutions that help provide secure access to a Web site, that interact with databases on behalf of a client, . . .

Servlets • . . . that dynamically generate custom HTML documents to be displayed by browsers and that maintain unique session information for each client.

Servlets • Many developers feel that servlets are the right solution for database-intensive applications that communicate with socalled thin clients—applications that require minimal client-side support.

Servlets • The server is responsible for the database access. Clients connect to the server using standard protocols available on all client platforms. • Thus, the logic code can be written once and reside on the server for access by clients.

Servlets • Our servlet example will make use of JDBC (Java Database Connectivity) database facilities to build a multi-tier client-server application that access a database.

Servlets • The Servlet APIs are now developed by the Apache group (www. apache. org). • Before you can program with servlets, you must download and install the Apache group's implementation of servlets called Tomcat. • You may download Tomcat at no charge from Sun Microsystems at the Web site: java. sun. com/products/jsp/tomcat

Servlets • You may download Tomcat at no charge from Sun Microsystems at the Web site: java. sun. com/products/jsp/tomcat

Servlets • After downloading Tomcat, install it on your system and carefully read the readme file supplied in the doc directory. • It explains how to set up Tomcat and discusses how to start the server that can be used to test servlets if you do not have a Web server that supports servlets.

Servlets • To develop servlets, you also need to copy the servlet. jar file containing the servlet class files from the installation directory to your JDK extensions directory (the directory c: jdk 1. 3jrelibext on Windows or the directory ~/jdk 1. 3/jre/lib/ext on UNIX).

Overview of Servlet Technology • Servlets are the analog on the server side to applets on the client side. Servlets are normally executed as part of a Web server. • In fact, servlets have become so popular that they are now supported by most major Web servers, . . .

Overview of Servlet Technology • . . . including the Netscape Web servers, Microsoft's Internet Information Server (IIS), the World Wide Web Consortium's Jigsaw Web server and the popular Apache Web server.

Overview of Servlet Technology • The servlets in this chapter show the communication between clients and servers via the HTTP protocol of the World Wide Web. • A client sends an HTTP request to the server. The server receives the request and directs it to be processed by appropriate servlets.

Overview of Servlet Technology • The servlets do their processing (which often includes interacting with a database), then return their results to the client— normally in the form of HTML documents to display in a browser, but other data formats, such as images and binary data, can be returned.

The Servlet API

The Servlet API • We discuss at a high level the servlet-related classes, methods and exceptions. Architecturally, all servlets must implement the Servlet interface. • The methods of interface Servlet are invoked automatically (by the server on which the servlet is installed).

The Servlet API • The servlet packages define two abstract classes that implement the interface Servlet—class Generic. Servlet (from the package javax. servlet) and class Http. Servlet (from the package javax. servlet. http).

The Servlet API • These classes provide default implementations of all the Servlet methods. • Most servlets extend either Generic. Servlet or Http. Servlet and override some or all of their methods with appropriate customized behaviors.

Servlets Methods of Interface Servlet

Method Description • void init( Servlet. Config config ) • This method is automatically called once during a servlet's execution cycle to initialize the servlet. The Servlet. Config argument is supplied automatically by the server that executes the servlet.

Method Description • Servlet. Config get. Servlet. Config() • This method returns a reference to an object that implements interface Servlet. Config. This object provides access to the servlet's configuration information such as initialization parameters and the servlet's Servlet. Context, which provides the servlet with access to its environment (i. e. , the server in which the servlet is executing).

Method Description • void service( Servlet. Request request, Servlet. Response response ) • This is the first method called on every servlet to respond to a client request.

Method Description • String get. Servlet. Info() • This method is defined by a servlet programmer to return a String containing servlet information such as the servlet's author and version.

Method Description • void destroy() • This "cleanup" method is called when a servlet is terminated by the server on which it is executing. • This is a good method to use to deallocate a resource used by the servlet (such as an open file or an open database connection).

The Servlet API • The example we present extends class Http. Servlet, which defines enhanced processing capabilities for servlets that extend the functionality of a Web server. • The key method in every servlet is method service, which receives both a Servlet. Request object and a Servlet. Response object.

The Servlet API • These objects provide access to input and output streams that allow the servlet to read data from the client and. . . • . . . send data to the client. These streams can be either byte-based streams or characterbased streams.

The Servlet API • If problems occur during the execution of a servlet, either Servlet. Exceptions or IOExceptions are thrown to indicate the problem.

Http. Servlet Class

Http. Servlet Class • Web-based servlets typically extend the class Http. Servlet. • This class Http. Servlet overrides the method service to distinguish between the typical requests received from a client Web browser.

Http. Servlet Class • The two most common HTTP request types (also known as request methods) are get and post. • A get request gets (or retrieves) information from the server. • Common uses of get requests are to retrieve an HTML document or an image.

Http. Servlet Class • A post request posts (or sends) data to the server. • Common uses of post requests are to send the to Web-server, information from an HTML form in which the client enters data, . . . • . . . to send to the server, information so that it can search on the Internet, . . . • . . . or query a database for the client, . . . • . . . to send authentication information to the server, etc.

Http. Servlet Class • The class Http. Servlet defines the methods do. Get and do. Post, which respond to get and post requests from a client, respectively. • These methods are called by the Http. Servlet class's service method, which is called when a request arrives at the server. • The method service first determines the request type, then calls the appropriate method.

Http. Servlet Class • Methods do. Get and do. Post receive as arguments an Http. Servlet. Request object and an Http. Servlet. Response object that enable interaction between the client and the server.

Http. Servlet Class • The methods of Http. Servlet. Request make it easy to access the data supplied as part of the request. • The Http. Servlet. Response methods make it easy to return the servlet's results in HTML format to the Web client.

Http. Servlet Class • The interfaces Http. Servlet. Request and Http. Servlet. Response are discussed now.

Http. Servlet. Request Interface Important methods of interface Http. Servlet. Request Method Descriptions

Http. Servlet. Request Interface • Every call to do. Get or do. Post for an Http. Servlet receives an object that implements interface Http. Servlet. Request. • The Web server that executes the servlet creates an Http. Servlet. Request object and passes this to the servlet's service method (which, in turn, passes it to do. Get or do. Post).

Http. Servlet. Request Interface • This object contains the request from the client. A variety of methods are provided to enable the servlet to process the client's request. Some of these methods are from interface Servlet. Request—the interface that Http. Servlet. Request extends.

Http. Servlet. Request Interface • String get. Parameter( String name ) • Returns the value associated with a parameter sent to the servlet as part of a GET or POST request. The name argument represents the parameter name.

Http. Servlet. Request Interface • Enumeration get. Parameter. Names() • Returns the names of all the parameters sent to the servlet as part of a POST request

Http. Servlet. Request Interface • String[] get. Parameter. Values( String name ) • Returns a String array containing the values for a specified servlet parameter.

Http. Servlet. Request Interface • Cookie[] get. Cookies() • Returns an array of Cookie objects stored on the client by the server. Cookies can be used to uniquely identify clients to the servlet.

Http. Servlet. Request Interface • Http. Session get. Session( boolean create ) • Returns an Http. Session object associated with the client's current browsing session. • An Http. Session object can be created by this method (true argument) if an Http. Session object does not already exist for the client. • Http. Session objects can be used in similar ways to Cookies for uniquely identifying clients.

Http. Servlet. Response Interface Important methods of interface Http. Servlet. Response Method Descriptions

Http. Servlet. Response Interface • Every call to do. Get or do. Post for an Http. Servlet receives an object that implements interface Http. Servlet. Response. • The Web server that executes the servlet creates an Http. Servlet. Response object and passes this to the servlet's service method (which, in turn, passes it to do. Get or do. Post).

Http. Servlet. Response Interface • This object contains the response to the client. A variety of methods are provided to enable the servlet to formulate the response to the client. Some of these methods are from interface Servlet. Response—the interface that Http. Servlet. Response extends.

Http. Servlet. Response Interface • A few key methods used in this interface are presented as follow:

Http. Servlet. Response Interface • void add. Cookie( Cookie cookie ) • Used to add a Cookie to the header of the response to the client. The Cookie's maximum age and whether the client allows Cookies to be saved determine whether or not Cookies will be stored on the client.

Http. Servlet. Response Interface • Servlet. Output. Stream get. Output. Stream() • Obtains a byte-based output stream enabling binary data to be sent to the client. • Print. Writer get. Writer() • Obtains a character-based output stream enabling text data to be sent to the client.

Http. Servlet. Response Interface • void set. Content. Type( String type ) • Specifies the MIME type of the response to the browser. The MIME type helps the browser determine how to display the data (or possibly what other application to execute to process the data). • For example, MIME type "text/html" indicates that the response is an HTML document, so the browser displays the HTML page.

Multi-tier Client/Server Application with Servlets

Multi-tier Client/Server Application with Servlets • Servlets can communicate with databases via JDBC (Java Database Connectivity). • JDBC provides a uniform way for a Java program to connect with a variety of databases in a general manner without having to deal with the specifics of those database systems.

Multi-tier Client/Server Application with Servlets • Many of today's applications are three-tier distributed applications, consisting of a user interface, business logic and database access. • The user interface in such an application is often created using HTML (as shown in this chapter) or Dynamic HTML.

Multi-tier Client/Server Application with Servlets • In some cases, Java applets are also used for this tier. • HTML is the preferred mechanism for representing the user interface in systems where portability is a concern. • Because HTML is supported by all browsers, designing the user interface to be accessed through a Web browser guarantees portability across all platforms that have browsers.

Multi-tier Client/Server Application with Servlets • Using the networking provided automatically by the browser, the user interface can communicate with the middle-tier business logic. • The middle tier can then access the database to manipulate the data. All three tiers may reside on separate computers that are connected to a network.

Multi-tier Client/Server Application with Servlets • In multi-tier architectures, Web servers are increasingly used to build the middle tier. • They provide the business logic that manipulates data from databases and that communicates with client Web browsers.

Multi-tier Client/Server Application with Servlets • Servlets, through JDBC, can interact with popular database systems. • Developers do not need to be familiar with the specifics of each database system. • Rather, developers use SQL-based queries and the JDBC driver handles the specifics of interacting with each database system.

Multi-tier Client/Server Application with Servlets • The servlet of Fig 27. 32 (a servlet) and the HTML document of Fig 27. 33 (the client) show a three-tier distributed application that displays the user interface in a browser using HTML.

Multi-tier Client/Server Application with Servlets • The middle tier is a Java servlet that handles requests from the client browser and provides access to the third tier—a Microsoft Access database (set up as an ODBC data source) accessed via JDBC.

Multi-tier Client/Server Application with Servlets • The servlet in this example is a guest book servlet that allows the user to register for several different mailing lists. • When the servlet receives a post request from the HTML document of Fig 27. 33, it ensures that the required data fields are present, then stores the data in the database and sends a confirmation page to the client.

Multi-tier Client/Server Application with Servlets • The class Guest. Book. Servlet extends class Http. Servlet (line 9) so it is capable of responding to GET and POST requests. • Servlets are initialized by overriding method init (line 14). • Method init is called exactly once in a servlet's lifetime and is guaranteed to complete before any client requests are accepted.

Multi-tier Client/Server Application with Servlets • The method init takes a Servlet. Config argument and throws a Servlet. Exception. • The argument provides the servlet with information about its initialization parameters (i. e. , parameters not associated with a request, but passed to the servlet by the Web server for initializing servlet variables).

Multi-tier Client/Server Application with Servlets • In this example, the servlet's init method performs the connection to the Microsoft Access database. • The method loads the Jdbc. Odbc. Driver at line 20 with: Class. for. Name( "sun. jdbc. odbc. Jdbc. Odbc. Driver" );

Multi-tier Client/Server Application with Servlets • Lines 21 and 22 connection = Driver. Manager. get. Connection( URL, "" ); • attempt to open a connection to the Guests database.

Multi-tier Client/Server Application with Servlets • The string jdbc: odbc: Guests stored in URL specifies the database URL (Uniform Resource Locator) that helps the program locate the database (possibly on a network or in the local file system of the computer). • The URL specifies the protocol for communication (jdbc), the subprotocol for communication (odbc) and the name of the database (Guests).

Multi-tier Client/Server Application with Servlets • The URL specifies the protocol for communication (jdbc), the subprotocol for communication (odbc) and the name of the database (Guests). • The subprotocol odbc indicates that the program will be using jdbc to connect to a Microsoft ODBC data source (see appendix H for information on setting up an ODBC data source).

Multi-tier Client/Server Application with Servlets • ODBC is a technology developed by Microsoft to allow generic access to disparate database systems on the Windows platform (and some UNIX platforms). • The Java 2 Software Development Kit (J 2 SDK) comes with the JDBC-to-ODBC-bridge database driver to allow any Java program to access any ODBC data source.

Multi-tier Client/Server Application with Servlets • The driver is defined by class Jdbc. Odbc. Driver in package sun. jdbc. odbc. The second and third arguments to get. Connection represent the username and password (in this example the database does not have a username and password).

Multi-tier Client/Server Application with Servlets • Lines 23 -25 statement = connection. prepare. Statem ent( "INSERT INTO Guests values ( " + "? , ? , ? ); " ); • create a Prepared. Statement that will be used to insert a record into the database.

Multi-tier Client/Server Application with Servlets • The question mark characters in the string represent the placeholders for values that will be inserted. • These values are specified with Prepared. Statement set method calls before executing the insert operation. • The eight placeholders in this example represent the user's email address, first name, last name, company and the four mailing lists the user would like to register to receive.

Multi-tier Client/Server Application with Servlets • When a post request is received from the HTML document in Fig 27. 33, method do. Post (line 33) responds by reading the HTML form field values from the post request, setting the parameters for the INSERT INTO operation on the database (lines 54 -65) and executing the insert operation (line 66). • Line 67 closes the statement to ensure that the insert operation is committed to the database.

Multi-tier Client/Server Application with Servlets • The if structure at lines 44 -50 determines if the email, first name or last name parameters are empty Strings. If so, the servlet response asks the user to return to the HTML form and enter those fields. • Line 83 defines method destroy to ensure that the database connection is closed before the servlet terminates.

Multi-tier Client/Server Application with Servlets • Figure 27. 33 defines the HTML document that presents the guest book form to the user and POSTs the information to the servlet of Fig 27. 32.

Multi-tier Client/Server Application with Servlets • Lines 9 -11 specify that the form's action is to post information to the Guest. Book. Servlet. • The screen captures show the form filled with one set of information (the first screen) and the confirmation Web page that was sent back to the client as the response to the post request.

Servlets Server-Side Java Programming

Servlets and Server-Side Java Programming • java. sun. com/products/servlet/index. html • The servlet page at the Sun Microsystems, Inc. • Java Web site provides access to the latest servlet information, servlet resources and the Java Servlet Development Kit (JSDK).

Servlets and Server-Side Java Programming • theserverside. com/home/index. jsp • This Web site is an excellent resource for anyone doing server-side Java development and development with the Java 2 Enterprise Edition.

Servlets and Server-Side Java Programming • www. servlets. com This is the Web site for the book Java Servlet Programming published by O'Reilly. • The book provides a variety of resources. • This book is an excellent resource for programmers who are learning servlets.

Servlets and Server-Side Java Programming • www. servletcentral. com Servlet Central is an online magazine for serverside Java programmers. • This includes technical articles and columns, news and "Ask the Experts. " • Resources include: books, servlet documentation links on the Web, a servlet archive, a list of servlet-enabled applications and servers and servlet development tools.

Servlets and Server-Side Java Programming • http: //www. servletsource. com/ is a general servlet resource site containing code, tips, tutorials and links to many other Web sites with information on servlets. • http: //www. cookiecentral. com/ A good all-around resource site for cookies.

Servlets and Server-Side Java Programming • www. purpletech. com/java/servletfaq/ The Purple Servlet FAQ is a great resource with dozens of links to tutorials, other servlet FAQs, mailing lists and newsgroups, articles, web servers, whitepapers and Java e-mail resources.

Servlets and Server-Side Java Programming • http: //www. servletforum. com/ is an on-line newsgroup dedicated to Java Servlets. Post your own questions or check out the archived list of previously asked questions.

Servlets and Server-Side Java Programming • www. enhydra. org/ Enhydra is an open source Java/XML application server and development environment available for free download. • www. locomotive. org/locolink/disp? home The Locomotive Project is an open source, servlet-compatible, web application server available for free download.

Servlets and Server-Side Java Programming • www. servlet. com/srvpages/srvdev. h tml • The Servlet, Inc. Servlet Developer's Forum has links to numerous web resources, examples, products that use servlets and server-enabled web servers.

Summary About Servlets

Summary about Servlets • The request-response model of communication is the foundation for the highest-level view of networking in Java— servlets. • A servlet extends the functionality of a server.

Summary about Servlets • The javax. servlet package and the javax. servlet. http package provide the classes and interfaces to define servlets.

Summary about Servlets • A common implementation of the requestresponse model is between World Wide Web browsers and World Wide Web servers. • When a user selects a Web site to browse through their browser (the client application), a request is sent to the appropriate Web server (the server application). • The server normally responds to the client by sending the appropriate HTML Web page.

Summary about Servlets • All servlets must implement the Servlet interface. The methods of interface Servlet are invoked automatically (by the server on which the servlet is installed). • An Http. Servlet defines enhanced processing capabilities for servlets that extend the functionality of a Web server.

Summary about Servlets • Class Http. Servlet overrides method service to distinguish between the typical requests received from a client Web browser. • The two most common HTTP request types (also known as request methods) are get and post.

Summary about Servlets • Class Http. Servlet defines methods do. Get and do. Post to respond to get and post requests from a client, respectively. • Servlets can communicate with databases via JDBC (Java Database Connectivity). • JDBC provides a uniform way for a Java program to connect with a variety of databases in a general manner without having to deal with the specifics of those database systems.

Summary about Servlets • To connect to a database, you must first load the database driver, then connect to the database with class Driver. Manager's get. Connection method. • The URL to connect to a database specifies the protocol for communication, the subprotocol for communication and the name of the database.

Summary about Servlets • The subprotocol odbc indicates that the program will be using jdbc to connect to a Microsoft ODBC data source. • ODBC is a technology developed by Microsoft to allow generic access to disparate database systems on the Windows platform (and some UNIX platforms).

Summary about Servlets • A Prepared. Statement creates an SQL statement that will be executed at a later time. • The parameters to the Prepared. Statement are specified with set method calls before executing the Prepared. Statement.