Servlets

1. Servlets CIS 285 25 January 2005 Mary Robinson Robinson_CIS285Winter2005

2. What is a Servlet • Pure Java alternative to CGI • Java technology that runs in the server tier • Dynamic • Exists in the “layer” between browser and database applications Robinson_CIS285Winter2005

3. Servlets • A web component written in java, whose purpose is the generation of dynamic content • Its execution and lifecycle are managed by a Web container within an application server. • Can interact with Web clients indirectly via the container, using HTTP request-response protocol. • Client, via web browser, can invoke a servlet: • Entering a URL in the browser • Clicking a hypertext link • Submitting an HTML form Robinson_CIS285Winter2005

4. How Do I Run Servlets ? • Need a Web container (servlet container is no longer required • Tomcat (Apache), WebSphere, BEA Web Logic, JRUN • Some servlet reference implementations are still free • Can be run standalone • Can be integrated with Web server: • Apache • IIS, IES(nee NES) and so forth Robinson_CIS285Winter2005

5. Why Use Servlets? • Easier to program • Servlets all share the same structure and basic method calls – easier to code • Servlet classes provide parsing and decoding of input data • Support for session tracking • Support for cookies • Little training is required to write servlets. • J2EE compliant – just need to follow the J2EE specifications and servlets should run correctly. Robinson_CIS285Winter2005

6. Why Use Servlets? • Better use of system resources • CGI starts a new process for each HTTP request • For each Java Servlet, each new request for a specific servlet starts a new thread of execution • Java Portability • Write once, run anywhere • Supported by many application servers • Part of Java 2 Platform (J2EE) Robinson_CIS285Winter2005

7. Why Use Servlets? • Type checking • Java is a typed language • Exception handling allows for robust error handling • JVM protect the system from Java coding issues • For each Java Servlet, each new request for a specific servlet starts a new thread of execution • Security • Runs in the Web container • Can restrict servlet access • Can be part of a Single (global) Sign On security architecture. Robinson_CIS285Winter2005

8. Using Servlets • Servlets extend function of server to include server-side programming as a java class. • May do anything the Web server cannot inherently do: • Query a database • Perform calculations such as tax on purchases • Servlets are portable and runtime environment is standardized, therefore the application server’s handling of requests is also standardized. Robinson_CIS285Winter2005

9. HTTP Transaction • HTTP is a protocol that allows Web browsers to talk to servers and exchange information • HTTP provides a standard way of communicating between browsers and servers • HTTP expects the client to initiate a request and the server to respond. Robinson_CIS285Winter2005

10. HTTP Flows: Forms / POST Request GET (request) HTML document RequestParamExample.html Return (response) document Submit form (POST action) (request) Server returns the HTML document Generated by the java servlet – based on the input from the form (response) Client / Browser Web Server / Application Server Robinson_CIS285Winter2005

11. Initiating an HTTP Response • Request line. This line contains a request method, the document location, and the protocol version. • Header section. This series of lines contains HTTP headers that are used to pass other information about the request, and about the client itself, to the server. A blank line then separates the header section from the entity body. • Entity body. This section contains other data to be passed to the server. There is usually information here only when a form is submitted. Robinson_CIS285Winter2005

12. Transaction Example If we typed the URL into Netscape: http://webmaster.merrimack.edu/simple.html The browser would issue an HTTP request similar to the following: GET /simple.html HTTP/1.0 User-Agent: Mozilla/4.5 [en] (X11; SunOS 5.5.1 sun4m) Accept: image/gif, image/x-xbitmap, image/jpeg, */* Robinson_CIS285Winter2005

13. Transaction Example The server then responds to the request in a similar fashion: • Status line. This line contains the protocol version, a status code, and a reason phrase. • Header section. This series of lines contains HTTP headers that are used to pass other information about the response, and about the server itself, to the client. A blank line then separates the header section from the entity body. • Entity body. This section, if present, contains the document (or object) requested. Robinson_CIS285Winter2005

14. Server Response May Look Like This: HTTP/1.1 200 OK Date: Mon, 24 Jan 2005 23:33:10 GMT Server Apache/1.3.1 (Unix) Last-Modified: Tue, 20 Dec 2004 21:00:15 GMT Content-Length: 49 Content-Type: text/html <HTML> Welcome to the webmaster server… <HTML> Robinson_CIS285Winter2005

15. Request Methods • Request line of a client request that contains an HTTP command called a request method • Defines what server does with the request • Currently several methods are defined by HTTP 1.1 standard • Only a few are widely supported by HTTP servers Robinson_CIS285Winter2005

16. Methods HEAD Method • The HEAD method is identical to the GET method except that the server does not return a document; it returns only the header section for the request. The HEAD method is useful for verifying that a document exists for checking links or to get information about the file type and modification time only. POST Method • The POST method allows the server to receive data from the client. It is most commonly used to send the data in HTML forms to the server for processing. This method passes data to the server in the entity body of the request. Robinson_CIS285Winter2005

17. Other Methods • PUT method is becoming more widely supported. It is used for publishing documents to the Web server from a client. Many of the latest HTML authoring packages support posting documents to a Web server via the PUT method. • DELETE method is used to remove a document from a Web server. Robinson_CIS285Winter2005

18. Server Response • Informational. The request was received and is being processed. • Success. The client request was successful. • Redirection. The client request was not performed; further action must be taken by the client. • Client error. The client’s request was incomplete or incorrect and cannot be fulfilled. • Server error. The request was not fulfilled, due to a server problem Robinson_CIS285Winter2005

19. Server Response • Informational 1XX • 100 Continue • Success 2XX • 200 OK • Redirection 3XX • 301 Moved permanently • 302 Found (Moved Temporarily • 304 Not Modified • Client error 4XX • 400 Bad Request • 403 Forbidden • 404 Not Found • Server error 5XX • 500 Internal Server Error Robinson_CIS285Winter2005

20. HTTP Flows: Forms / POST Request GET (request) HTML document RequestParamExample.html Return (response) document Submit form (POST action) (request) Server returns the HTML document Generated by the java servlet – based on the input from the form (response) Client / Browser Web Server / Application Server Robinson_CIS285Winter2005

21. HTTP Flows - HTML Robinson_CIS285Winter2005

22. HTTP Protocol Request • Conversation between a browser and a server • Request phase • Request (POST in this example) • Header values • A blank line • Posted data (when request method is POST) Robinson_CIS285Winter2005

23. HTTP Protocol Request POST /example/servlets/RequestParamExample HTTP 1.0 Referrer: http:// Connection: Keep-Alive User-Agent: Mozilla/4.72 [en (WinNT 5.0; U)] Host: localhost:8080 Cookie: USERID=spot Accept: image/gif, image/x-xbitmap, image/jpeg, */* Accept-Language: en Accept-Charset: iso-8859-1,*,utf-8 Content-type: application/x-www-form-urlencoded Content-length: 80 Robinson_CIS285Winter2005

24. HTTP Protocol - Response Response phase • Status information (200 in example) • Header values • A blank line • Output document (HTML) Robinson_CIS285Winter2005

25. HTTP Protocol - Request HTTP/1.1 200 ok Content-Type: text/html Set-Cookie: sessionid=5H2HXGYAAAAEWAAAAZJCI;Path=/ Cache-Control: no-cache=“set-cookie,setcookie2” Expires: Thu, 26 Dec 1994 16:00:00 GMT Set-Cookie: USERID=spot; Expires=Fri, 24-Jul-2005 21:30:37 GMT Accept: image/gif, image/x-xbitmap, image/jpeg, */* Content-Language: en <HTML> <BODY> <H1>Mary Robinson</H1> </BODY> </HTML> Robinson_CIS285Winter2005

26. Servlet Process Flow • Client makes a request naming a servlet as part of the URL • Web server forward request to Servlet engine (Web container • Servlet engine locates instance of a Servlet class • Servlet engine calls Servlet’s service method • Servlet builds response dynamically and passes it to Web server • Web server sends the response back to the client Robinson_CIS285Winter2005

27. Servlet Process Flow URL request Servlet Instance response Application Server Browser (client) Web Server Robinson_CIS285Winter2005

28. What is a Servlet ? • Servlets, as the name implies, are service requests • The servlet API is a set of java classes which define a standard interface between a Web client and a Web server • Servlet API includes two packages • javax.servlet* • javax.servlet.http* Robinson_CIS285Winter2005

29. Java Servlet API • The servlet API is a set of Java classes that implement one of the interfaces defined in the javax.servlet* or javax.servlet.http* javax.servlet.Servlet Implements javax.servlet.GenericServlet javax.servlet.http.HttpServlet Robinson_CIS285Winter2005

30. Servlet Process • Servlets are deployed or loaded into a servlet container • The container invokes method callbacks • On servlets methods during different stages of its lifecycle • When a client requests service Robinson_CIS285Winter2005

31. Process Flow • Client (Web browser) initiates call to the servlet • Web browser sends HTTP request to Web server • Web server determines content – dynamic for a servlet • Web server forward request to application server, again using HTTP, to an embedded HTTP server Robinson_CIS285Winter2005

32. Process Flow Continued • The Web container within the application server handles communication with the servlet, passing the request and response as objects • Servlet is Java code with standard Java interfaces. Example: • Servlet may collaborate with other classes and objects to accomplish work such as database queries • Servlet has answer for client and formats the answer in HTML dynamically • Directly • Indirectly – JSP’s Robinson_CIS285Winter2005

33. Java Servlet Lifecycle Methods • Individual servlet lifecycle Create Initialize Unavailable For service Available For service Destroy Servicing requests Unload Robinson_CIS285Winter2005

34. Servlet Lifecycle Methods • Basic interface, javax.servlet.Servlet, defines a set of five methods that have to be implemented to provide a service and for the lifecycle methods • init • service • doGet • doPost • destroy • Individual servlet lifecycle Robinson_CIS285Winter2005

35. Servlet Lifecycle • The lifecycle of a servlet is expressed in the Java servlet API: • in the init • service (doGet or doPost) • destroy methods of the Servlet interface. Robinson_CIS285Winter2005

36. Life Cycle • Web server passes request to application server • Application server uses a JVM to execute the servlet • JVM allows a servlet to handle multiple requests simultaneously on separate lightweight threads of execution • Over head of loading and starting servlet is significantly reduced to one occurrence • Application server keeps servlet in memory waiting for another request. Robinson_CIS285Winter2005

37. Java Servlet Lifecycle (Initialize failed) Create Initialize Unavailable For service Available For service Destroy (Unavailable Exception thrown) Servicing requests Unload Robinson_CIS285Winter2005

38. Servlet Creation • Servlets are loaded and instantiated • When the container is started • When the container determines the servlets are needed to service requests(first requests) Robinson_CIS285Winter2005

39. Servlet Initialization • Before servlet can handle requests from clients • Perform one-time activities and initialize costly resources • Init() method performs servlet initialization such as loading default data parameters • Two init() methods • One takes no input parameters • Other takes a ServletConfig reference as a parameter Robinson_CIS285Winter2005

40. Servlet Request Handling • Accept client requests and send responses back via the Web server. • Service(), doGet(), doPost()..methods are called in response to clients (each HTTP request is on a different thread). • Default service() method calls the doGet() method whenever an HTTP GET request is sent by the Web client (usually a URL). • The doPost() method is called whenever an HTTP POST request is sent by the Web browser client(usually action in HTML form). Robinson_CIS285Winter2005

41. Java Servlet API • The servlet API is a set of Java classes that implement one of the interfaces defined in the javax.servlet* or javax.servlet.http* javax.servlet.Servlet Implements javax.servlet.GenericServlet javax.servlet.http.HttpServlet Robinson_CIS285Winter2005

42. Servlets and HTTP • Most servlets are used in a Web environment where HTTP is used between a Web client (browser) and the Web server/application server • Much of the standard processing must take place in this HTTP-based environment is managed by javax.servlet.http.HttpServlet • Typical servlet will extend this class and override one or more of the standard methods • Ex. doGet() or doPost() to supply specific servlet behavior Robinson_CIS285Winter2005

43. ServletConfig & Initialization Parameters • The ServletConfig is used by the servlet container to pass information to the servlet during initialization • Accessed via the GenericServlet method • getServletConfig() • ServletConfig: • Contains initialization parameters as a set of name/value pairs. • public String getInitParameter (String name) • public Enumeration getInitParameterNames() • Maintains a reference to the servletContext object which gives the servlet information about the serve • Public ServletContext getServletContext() Robinson_CIS285Winter2005

44. Servlet Definition web.xml File <servlet> <servlet-name>RegistrationServlet</servlet-name> <display-name>RegistrationServlet</display-name> <servlet-class>com.ibm.ils.exam.servlet.RegistrationServlet <servlet-class> <init-param> <param-name>MaxTries</param-name> <param-value>4</param-value> </init-param> <init-param> <param-name>MaxTries</param-name> <param-value>4</param-value> </init-param> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>RegistrationServlet</servlet-name> <url-pattern>Register</url-pattern> </servlet-mapping> Robinson_CIS285Winter2005

45. Example Servlet: With Init Parameters // servlets init method public void init() { String maxTries; string autoSave; maxTries = getInitParameter(“MaxTries”); autoSave = getInitParameter(“AutoSave”); // process the parameters …. } Robinson_CIS285Winter2005

46. Example Servlet: With Init Parameters // servlets init method public void init() { ServletConfig config = getServletConfig(); String maxTries; string autoSave; maxTries = config.getInitParameter(“MaxTries”); autoSave = config.getInitParameter(“AutoSave”); // process the parameters …. } Robinson_CIS285Winter2005

47. HTTPServlet • An HTTP-specific request handler • Adds HTTP specific methods • doGet() – handle a GET request (URL) • doPost() – handle a POST request(HTML form) • Subclasses override the doGET, doPost, and so forth, methods and may override init() and destroy() • doGet() and doPost() do the work and are called by service() Robinson_CIS285Winter2005

48. Requests and Responses • The service(), doGet() and doPost() methods each have two parameters: • HttpServletRequest – provides access to request data (parameters), HttpSession information, and so forth. • HttpServletResponse – provides services to allow the servlet to supply a response to the requesting client. • Most servlet programming involves reading a request and writing a response. Robinson_CIS285Winter2005

49. HttpServlet Request • Represents client’s request • Getters for aspects of request, for example: • Request header, content type, length, method • Request URL as a String and request servlet “path” • Client security type • Access request parameters by name • Possible scope for object sharing among participants satisfying the request Robinson_CIS285Winter2005

50. Request Protocol • The request object encapsulates all information from the client request. The following methods are available to access parameters: • getParameterNames() • Returns an enumeration of parameters on the HTML page • getParameterValues(String name) • Returns the value of a multivalued parameter • getParameter(String name) • Returns the value of a specific named parameter • getReader() • Returns a BufferedReader to view input Robinson_CIS285Winter2005