Servlets1. What are Servlets? Where are they? Their job. Servlet container. Only Http?

Size: px

Start display at page:

Download "Servlets1. What are Servlets? Where are they? Their job. Servlet container. Only Http?"

Joella Rogers
6 years ago
Views:

1 What are Servlets? Servlets1 Fatemeh Abbasinejad A program that runs on a web server acting as middle layer between requests coming from a web browser and databases or applications on the Http server 2 Where are they? Their job Client HTTP Server Servlets Read any data sent by the user Generate results (from db, ) Format the result inside a document (embedding inside HTML) Send the document back to the client Thin clients applications that require minimal client side support 3 4 Only Http? Servlet container Note: servlets are not restricted to the web application servers that handle http requests They can be embedded in mail or ftp servers to extend their functionality The server that executes a servlet often is referred to as the servlet container Example: Apache Tomcat 5 6 1

2 Servlet Interface A servlet extends the functionality of a server 2 packages provide the classes and interfaces to define servlets javax.servlet.* javax.servlet.http.* Servlet Interface All servlets implement the servlet interface from javax.servlet This interface has 5 methods Void init(servletconfig config) servletconfig getservletconfig() String getservletinfo() Void Service(ServletRequest,ServletResponse) Void destroy() 7 8 Interfaces Defines a protocol behavior that can be implemented by any class anywhere It defines a set of methods But. does not implement them A collection of methods without implementation Interfaces A class that implements the interface agrees to implement all the methods Then what were abstract classes?! Interface can not implement any methods, abstract class can implement any A class can implement many interfaces but can only have one super class 9 10 Servlet s Lifecycle Servlet container loads the servlet into memory in response to a request Servlet container invokes the servlet s init() method When init() finishes servlet responds with its service() method service() receives request, processes it, sends response Servlet Lifecycle service() is only called once per request New requests results in a new thread When servlet container terminates destroy() is called to release resources

3 Servlet Package Servlet package has 2 abstract classes that implement the interface servlet Servlet Servlet Package The classes have default implementations of all servlet s methods Why all?? implements implements Most servlets override these methods by extending them GenericServlet HttpServlet an abstract class an abstract class HttpServlet Class HttpServlet Class Web based servlets typically extend this class This class overrides the service() method Defines doget() and dopost() Get request HttpServlet s Service() doget() Then calls the appropriate method Post request Determines the request type dopost() HttpServletClass service() determines who (doget or dopost) to call Your Class doget() { dopost() { extends HttpServlet service() doget() dopost() dopost() and doget() Receive arguments: HttpServletRequst object HttpServletResponse object HttpServletRequest : access data as part of the request HttpServletResponse : make it easy to return results Return: void You work with these

Exceptions thrown If doget() and dopost() are unable to handle a client s request it throws an exception of type ServletException If doget() and dopost() encounter an error during system processing

4 Exceptions thrown If doget() and dopost() are unable to handle a client s request it throws an exception of type ServletException If doget() and dopost() encounter an error during system processing (reading/writing) it throws a IOException Get and Post Overview The most common request types (request methods) <form method = Post action = /someplace > Post appends form data to the browser request then a script on the server can access the form Browsers cache web pages but not the server s response to a post request Because: the next post might not return the same result Example: surveys ; each response changes the overall survey Get appends the data directly to the end of the URL Such pages are often cached Example: search engines; in case you perform the search again! Action : specifies the URL of a script on the server Simple Example <head> <title> Handling an HTTP Get Request </title> </head> Simple Example <body> <form action = "welcome1" method = "get"> <p> <label> Click the button to invoke the servlet <input type = "submit" value = "Get HTML Document" /> </label> </p> </form> </body> </html>

5 Simple Example The form invokes the servlet you have defined How? The form s action specifies a path that invokes the servlet A Simple Servlet Import required packages Define a class that extends HttpServlet (discussed before) Remember to throw exceptions Override doget()/dopost() <form action = "welcome1" method = "get"> import javax.servlet.*; import required packages public class WelcomeServlet extends HttpServlet { define a class that extends HttpServlet override doget()/dopost() response.setcontenttype( "text/html" ); PrintWriter out = response.getwriter(); // head section of document remember that it might throw exceptions out.println( "<title>a Simple Servlet Example</title>" ); out.println( "<h1>welcome to Servlets!</h1>" ); out.close(); // close stream to complete the page 27 What and How do we send? The response (in this example) is an XHTML document containing a simple text. Response is sent via PrintWriter object obtained from HttpServletResponse PrintWriter out=response.getwriter(); 28 import javax.servlet.*; public class WelcomeServlet extends HttpServlet { PrintWriter out = response.getwriter(); object obtained from response.setcontenttype( "text/html" ); HttpServletResponse // head section of document out.println( "<title>a Simple Servlet Example</title>" ); out.println( "<h1>welcome to Servlets!</h1>" ); out.close(); // close stream to complete the page 29 What and How do we send? Use response s setcontenttype() to specify the content of the data to be sent In this example we use txt/html response.setcontenttype("text/html"); 30 5

6 import javax.servlet.*; public class WelcomeServlet extends HttpServlet { PrintWriter out = response.getwriter(); response.setcontenttype( "text/html" ); // head section of document Specify content to be sent out.println( "<title>a Simple Servlet Example</title>" ); out.println( "<h1>welcome to Servlets!</h1>" ); out.close(); // close stream to complete the page 31 What and How do we send? With the out object s println() method create an HTML document out.println ( <html_tag> somethig <html_tag> ) out.close() Flushes the output buffer and sends the informstion to the client 32 import javax.servlet.*; public class WelcomeServlet extends HttpServlet { PrintWriter out = response.getwriter(); response.setcontenttype( "text/html" ); // head section of document declare head section out.println( "<title>a Simple Servlet Example</title>" ); declare body section out.println( "<h1>welcome to Servlets!</h1>" ); out.close(); close stream 33 Deployment Descriptor stored in web.xml specifies various config. parameter such as: the name used to invoke the servlet description of the servlet the class name of the servlet class 34 <?xml version="1.0" encoding="iso "?> <web-app xmlns=" xmlns:xsi=" xsi:schemalocation= " version="2.4"> <web-app> <description> CS160 Example </description> <display-name> My Example </display-name> <servlet> declares servlet <servlet-name> welcome1 </servlet-name> <servlet-class> WelcomeServlet </servlet-class> </servlet> name chosen for servlet compiled,fully qualified class s name <servlet-mapping> <servlet-name> welcome1 </servlet-name> <url-pattern> /welcome1 </url-pattern> </servlet-mapping> maps servlet to URL </web-app> Deploying Setup your Tomcat Web applications are deployed in the webapps subdirectory of Tomcat context root The top level directory for an entire web application

Directory Structure Create your web applications here Create a directory myapp for your web application Create WEB-INF under myapp Create classes under WEB-INF Directory Structure Static content

7 Directory Structure Create your web applications here Create a directory myapp for your web application Create WEB-INF under myapp Create classes under WEB-INF Directory Structure Static content (JSP, HTML, Images) in myapp dynamic content in WEB-INF web.xml in WEB-INF servlets in classes Another Example <head> <title> Handling an HTTP Post Request </title> </head> invokes the servlet <body> <form action = "welcome2" method = get > <p> <label> Type your first name and press the Submit button <input type = text" name = firstname" /> <input type = "submit" value = Submit" /> </label> </p> </form> </body> </html> Another Example Requests containing data parameters are passed as name/value pairs in get request request object s getparameter() receives parameter name as argument String firstname = request.getparameter( firstname ); 41 import javax.servlet.*; public class WelcomeServlet2 extends HttpServlet { receive parameter from request object String firstname = request.getparameter( firstname ); PrintWriter out = response.getwriter(); response.setcontenttype( "text/html" ); // head section of document out.println( "<title>another Servlet Example</title>" ); out.println( "<h1>hello + firstname +!</h1>" ); out.close(); // close stream to complete the page 42 7

8 <?xml version="1.0" encoding="iso "?> <web-app xmlns=" xmlns:xsi=" xsi:schemalocation= " version="2.4"> <web-app> <description> CS160 Example </description> <display-name> My Example2 </display-name> <servlet> <servlet-name> welcome2 </servlet-name> <servlet-class> WelcomeServlet2 </servlet-class> </servlet> name chosen for servlet class s name <servlet-mapping> <servlet-name> welcome2 </servlet-name> <url-pattern> /welcome2 </url-pattern> </servlet-mapping> </web-app> map servlet to URL 43 get Requests with Data Note: the browser appends?firstame=myname at the end of the action URL /WelcomeServlet2?firstname=myname What happens if you type in the above URL? more data: separated by & 44 post Requests dopost() :same arguments, same exceptions Same procedures for the get request HTML form <form action = "welcome3" method = post > 45 8

Java Enterprise Edition. Java EE Oct Dec 2016 EFREI/M1 Jacques André Augustin Page 1

Java Enterprise Edition. Java EE Oct Dec 2016 EFREI/M1 Jacques André Augustin Page 1 Java Enterprise Edition Java EE Oct Dec 2016 EFREI/M1 Jacques André Augustin Page 1 Java Beans Java EE Oct Dec 2016 EFREI/M1 Jacques André Augustin Page 2 Java Bean POJO class : private Attributes public