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Short presentation about Java Servlets.
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Java ServletsSvetlin Nakov
Borislava Spasova
Contents
1. Java Servlets Technology Overview
• What is a Java Servlet?
• Servlet Services
• Why Use Servlets?
• Time Servlet – Example
• Deploying Servlets on Eclipse IDE
2. Servlets Architecture
• Servlets API
• Servlets Life-Cycle
Contents (2)
3. Servlet Examples• Processing Parameters – Hello Servlet
• Image Counter Servlet
4. Using Sessions• What is a Session?
• The Sessions API
• Session Timeout
5. Session Examples• Login / Logout Application
• The Browser's Cache Problems
Java ServletsTechnology Overview
What is a Java Servlet?
• Java Servlets are:
• Technology for generating dynamic Web pages (like PHP, ASP, ASP.NET, ...)
• Protocol and platform-independent server side components, written in Java, which extend the standard Web servers
• Java programs that serve HTTP requests
• The HttpServlet class
• Provides dynamic Web content generation (HTML, XML, …)
What is a Java Servlet? (2)
• Servlets
• Provide a general framework for services built on the request-response paradigm
• Portable to any Java application server
• Have access to the entire family of Java and Java EE APIs
• JDBC, Persistence, EJB, JMS, JAX-WS, JTA, JTS, RMI, JNDI, JAXP, ...
• Fundamental part of all Java Web application technologies (JSP, JSF, ...)
Servlet Services
• Java Servlets provide many useful services
• Provides low-level API for building Internet services
• Serves as foundation to JavaServer Pages (JSP) and JavaServer Faces (JSF) technologies
• Can deliver multiple types of data to any client
• XML, HTML, WML, GIF, etc...
• Can serve as “Controller” of JSP/Servlet application
Why Use Servlets?
• Portability
• Write once, serve everywhere
• Power
• Can take advantage of all Java APIs
• Elegance
• Simplicity due to abstraction
• Efficiency & Endurance
• Highly scalable
Why Use Servlets? (2)
• Safety
• Strong type-checking
• Memory management
• Integration
• Servlets tightly coupled with server
• Extensibility & Flexibility
• Servlets designed to be easily extensible, though currently optimized for HTTP uses
• Flexible invocation of servlet (SSI, servlet-chaining, filters, etc.)
Time Servlet – Example
import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class TimeServlet extends HttpServlet { public void doGet(HttpServletRequest aRequest, HttpServletResponse aResponse) throws ServletException, IOException { PrintWriter out = aResponse.getWriter(); out.println("<HTML>"); out.println("The time is: " + new java.util.Date()); out.println("</HTML>"); } }
Deploying Servlets on Eclipse IDE
• First create new Web application
Deploying Servlets on Eclipse IDE (2)
• Add new servlet to the Web application
Deploying Servlets on Eclipse IDE (3)
• Run the servlet
Deploying Servlets on Eclipse IDE (4)
• The servlet in action
Java ServletsTechnical Architecture
Servlets Architecture
• The HttpServlet class
• Serves client's HTTP requests
• For each of the HTTP methods, GET, POST, and others, there is corresponding method:
• doGet(…) – serves HTTP GET requests
• doPost(…) – serves HTTP POST requests
• doPut(…), doHead(…), doDelete(…), doTrace(…), doOptions(…)
• The Servlet usually must implement one of the first two methods or the service(…) method
Servlets Architecture (2)
• The HttpServletRequest object
• Contains the request data from the client
• HTTP request headers
• Form data and query parameters
• Other client data (cookies, path, etc.)
• The HttpServletResponse object
• Encapsulates data sent back to client
• HTTP response headers (content type, cookies, etc.)
• Response body (as OutputStream)
Servlets Architecture (3)
• The HTTP GET method is used when:• The processing of the request does not change the
state of the server
• The amount of form data is small
• You want to allow the request to be bookmarked
• The HTTP POST method is used when:• The processing of the request changes the state of
the server, e.g. storing data in a DB
• The amount of form data is large
• The contents of the data should not be visible in the URL (for example, passwords)
Servlets API
• The most important servlet functionality:
• Retrieve the HTML form parameters from the request (both GET and POST parameters)
• Retrieve a servlet initialization parameter
• Retrieve HTTP request header information
HttpServletRequest.getParameter(HttpServletRequest.getParameter(StringString))
ServletConfig.getInitParameterServletConfig.getInitParameter()()
HttpServletRequest.getHeader(HttpServletRequest.getHeader(StringString))
Servlets API (2)
• Set an HTTP response header / content type
• Acquire a text stream for the response
• Acquire a binary stream for the response
• Redirect an HTTP request to another URL
HttpServletResponse.setHeaderHttpServletResponse.setHeader(<name>, <value>) /(<name>, <value>) / HttpServletResponse.setContentType(HttpServletResponse.setContentType(StringString))
HttpServletResponse.getWriter()HttpServletResponse.getWriter()
HttpServletResponseHttpServletResponse..getOutputStream()getOutputStream()
HttpServletResponse.sendRedirect()HttpServletResponse.sendRedirect()
Servlets Life-Cycle
• You can provide an implementation of these methods in HttpServlet descendent classes to manipulate the servlet instance and the resources it depends on
• The Web container manages the life cycle of servlet instances
• The life-cycle methods should not be called by your code
• The Web container manages the life cycle of servlet instances
• The life-cycle methods should not be called by your code
init()
...()service()
doGet()
doPost()
doDelete()
destroy()
doPut()
New Destroyed
Running
The init() Method
• Called by the Web container when the servlet instance is first created
• The Servlets specification guarantees that no requests will be processed by this servlet until the init method has completed
• Override the init() method when:
• You need to create or open any servlet-specific resources that you need for processing user requests
• You need to initialize the state of the servlet
The service() Method
• Called by the Web container to process a user request
• Dispatches the HTTP requests to doGet(…), doPost(…), etc. depending on the HTTP request method (GET, POST, and so on)
• Sends the result as HTTP response
• Usually we do not need to override this method
The destroy() Method
• Called by the Web container when the servlet instance is being eliminated
• The Servlet specification guarantees that all requests will be completely processed before this method is called
• Override the destroy method when:• You need to release any servlet-specific
resources that you had opened in the init() method
• You need to persist the state of the servlet
Java ServletsExamples
Processing Parameters – Hello Servlet
• We want to create a servlet that takes an user name as a parameter and says "Hello, <user_name>"
• We need HTML form with a text field
• The servlet can later retrieve the value entered in the form field
<form method="GET or POST" action="the servlet"> <input type="text" name="user_name"></form>
String name = request.getParameter("user_name");
Hello Servlet – Example
<html><body> <form method="GET" action="HelloServlet"> Please enter your name: <input type="text" name="user_name"> <input type="submit" value="OK"> </form> </body></html>
HelloForm.htmlHelloForm.html
import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloServlet extends HttpServlet {
HelloHelloServletServlet..javajava
Hello Servlet – Example
public void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException, IOException {
response.setContentType("text/html"); ServletOutputStream out = response.getOutputStream(); String userName = request.getParameter("user_name"); out.println("<html><head>"); out.println("\t<title>Hello Servlet</title>"); out.println("</head><body>"); out.println("\t<h1>Hello, " + userName + "</h1>"); out.println("</body></html>");
}
HelloHelloServletServlet..javajava
Creating The Form in Eclipse IDE
• Create new HTML form
Creating New Servlet in Eclipse IDE
• Create new Servlet
Hello Servlet in Action
Hello Servlet –HTTP Request
GET /FirstWebApp/HelloServlet?user_name=Nakov HTTP/1.1Accept: image/gif, image/x-xbitmap, image/jpeg,image/pjpeg, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, application/x-shockwave-flash, */*Accept-Language: bgAccept-Encoding: gzip, deflateUser-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; Q312461)Host: nakov:8084Connection: Keep-Alive
• What happens when the user enters his name?
• Internet Explorer (IE) sends the following HTTP request to Tomcat
• What happens when the user enters his name?
• Internet Explorer (IE) sends the following HTTP request to Tomcat
Hello Servlet –HTTP Response
HTTP/1.1 200 OKContent-Length: 100Date: Fri, 26 Mar 2006 10:06:28 GMTServer: Apache-Coyote/1.1 <html><head>
<title>Hello Servlet</title></head><body>
<h1>Hello, Nakov</h1></body></html>
• What happens when Tomcat receive and process the HTTP request
• Tomcat sends the following HTTP response to Internet Explorer
• What happens when Tomcat receive and process the HTTP request
• Tomcat sends the following HTTP response to Internet Explorer
Image Counter Servlet
• We want to create a servlet that displays an image counter (as JPEG image)
• The servlet should maintain an internal counter
• Can be initialized in the init() method and incremented in the doGet() method
• It should produce binary output (the JPEG) image
• The content type should be set to "image/jpeg"
Image Counter Servlet (2)
import javax.servlet.*;import javax.servlet.http.*;... public class ImageCounterServlet extends HttpServlet { private String mStartDate; private int mVisitCounter; public void init() { mStartDate = (new Date()).toString(); mVisitCounter = 0; }
public BufferedImage createImage(String msg) { ... }
Image Counter Servlet (3)
public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { String msg; synchronized(this) { mVisitCounter++; msg = "" + mVisitCounter + " visits since " + mStartDate; } BufferedImage image = createImage(msg); response.setContentType("image/jpeg"); OutputStream out = response.getOutputStream(); // Encode the image in JPEG format and // write the image to the output stream } }
Image Counter Servlet in Action
Using Sessions
What is a Session?
• A session is a state associated with particular user that is maintained at the server side
• Sessions persist between the HTTP requests
• Sessions enable creating applications that depend on individual user data. For example:
• Login / logout functionality
• Wizard pages
• Shopping carts
• Personalization services
• Maintaining state about the user’s preferences
Sessions in Servlets
• Servlets include a built-in Sessions API
• Sessions are maintained automatically, with no additional coding
• The Web container associates an unique HttpSession object to each different client
• Different clients have different session objects at the server
• Requests from the same client have the same session object
• Sessions can store various data
The Sessions API
• The sessions API allows
• To get the HttpSession object from the HTTPServletRequest object
• Extract data from the user’s session object
• Append data to the user’s session object
• Extract meta-information about the session object, e.g. when was the session created
Getting The Session Object
• To get the session object use the method HttpServletRequest.getSession()
• Example:
• If the user already has a session, the existing session is returned
• If no session still exists, a new one is created and returned
• If you want to know if this is a new session, call the isNew() method
HttpSession session = request.getSession();HttpSession session = request.getSession();
Behind The Scenes
• When you call getSession() each user is automatically assigned a unique Session ID
• How does this Session ID get to the user?
• Option 1: If the browser supports cookies, the servlet will automatically create a session cookie, and store the session ID within the cookie
• In Tomcat, the cookie is called JSESSIONID
• Option 2: If the browser does not support cookies, the servlet will try to extract the session ID from the URL
Extracting Data From The Session
• The session object works like a HashMap
• Enables storing any type of Java object
• Objects are stored by key (like in hash tables)
• Extracting existing object:
• Getting a list of all “keys” associated with the session
Integer accessCount =Integer accessCount = (Integer)(Integer) session.getAttribute("accessCount");session.getAttribute("accessCount");
Enumeration attributes =Enumeration attributes = request.getAttributeNames();request.getAttributeNames();
Storing Data In The Session
• We can store data in the session object for using it later
• Objects in the session can be removed when not needed more
HttpSession session = request.getSession();HttpSession session = request.getSession();session.setAttribute("name", "Svetlin Nakov");session.setAttribute("name", "Svetlin Nakov");
session.session.removeremoveAttribute("name");Attribute("name");
Getting Additional Session Information
• Getting the unique session ID associated with this user, e.g. gj9xswvw9p
• Checking if the session was just created
• Checking when the session was first created
• Checking when the session was last active
public boolean isNew();public boolean isNew();
public String getId();public String getId();
public long getLastAccessedTime();public long getLastAccessedTime();
public long getCreationTime();public long getCreationTime();
Session Timeout
• We can get the maximal session validity interval(in seconds)
• After such interval of inactivity the session is automatically invalidated
• We can modify the maximal inactivity interval
• A negative value specifies that the session should never time out
public int getMaxInactiveInterval();public int getMaxInactiveInterval();
public void setMaxInactiveInterval (int seconds)public void setMaxInactiveInterval (int seconds);;
Terminating Sessions
• To terminate session manually use the method:
• Typically done during the "user logout"
• The session can become invalid not only manually
• Sessions can expire automatically due to inactivity
public void invalidate()public void invalidate();;
Login / Logout – Example
• We want to create a simple Web application that restricts the access by login form
• We will use sessions to store information about the authenticated users
• We will use the key "username"
• When it present, there is a logged in user
• During the login we will add the user name in the session
• Logout will invalidate the session
• The main servlet will check the current user
Login Form
<html> <head><title>Login</title></head> <body> <form method="POST" action="LoginServlet"> Please login:<br> Username: <input type="text" name="username"><br> Password: <input type="password" name="password"><br> <input type="submit" value="Login"> </form> </body></html>
LoginForm.htmlLoginForm.html
Login Servlet
public class LoginServlet extends HttpServlet { public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException, ServletException { String username = req.getParameter("username"); String password = req.getParameter("password"); PrintWriter out = resp.getWriter(); if (isLoginValid(username, password)) { HttpSession session = req.getSession(); session.setAttribute("USER", username); response.sendRedirect("MainServlet"); } else { response.sendRedirect("InvalidLogin.html"); } }}
LoginLoginServletServlet..javajava
Main Servlet
public class MainServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse resp) throws ServletException, IOException { HttpSession session = request.getSession(); String userName = (String)
session.getAttribute("USER"); if (userName != null) { response.setContentType("text/html"); ServletOutputStream out = resp.getOutputStream(); out.println("<html><body><h1>"); out.println("Hello, " + userName + "! "); out.println("</h1></body></html>"); } else { response.sendRedirect("LoginForm.html"); } }}
MainServlet.javaMainServlet.java
Logout Servlet
public class LogoutServlet extends HttpServlet { protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { HttpSession session = request.getSession(); session.invalidate(); response.setContentType("text/html"); ServletOutputStream out = response.getOutputStream(); out.println("<html><head>"); out.println("<title>Logout</title></head>"); out.println("<body>"); out.println("<h1>Logout successfull.</h1>"); out.println("</body></html>"); }}
LogoutServlet.javaLogoutServlet.java
Invalid Login Page
<html> <head> <title>Error</title> </head> <body> <h1>Invalid login!</h1> Please <a href="LoginForm.html">try again</a>. </body></html>
InvalidLogin.htmlInvalidLogin.html
The Browser's Cache Problems
• Most Web browsers use caching of the displayed pages and images
• This can cause the user to see old state of the pages
• Seems like a bug in the application
• To prevent showing the old state we need to disable the browser cache:
response.setHeader("Pragma", "No-cache");response.setDateHeader("Expires", 0);response.setHeader("Cache-Control", "no-cache");
Problems
1. Create a servlet that prints in a table the numbers from 1 to 1000 and their square root.
2. Create a servlet that takes as parameters two integer numbers and calculates their sum.
Create a HTML form that invokes the servlet. Try to use GET and POST methods.
3. Implement a servlet that plays the "Number guess game". When the client first invoke the servlet it generates a random number in the range [1..100]. The user is asked to guess this number. At each guess the servlet says only "greater" or "smaller". The game ends when the user tell the number.
Homework
1. Create a servlet that takes as a parameter a number and displays it as image that is hard to be recognized by OCR software. The image should have intentionally inserted defects.
2. Create an HTML form and a servlet for performing conversions of distances from one metric to another. The metrics that should be supported are: meter, centimeter, kilometer, foot, inch, yard, mile.1 cm = 0.01 meters 1 km = 1000 meters
1 foot = 0.3048 meters 1 inch = 0.0254 meters
1 yard = 0.9144 meters 1 mile = 1609.344 meters
1. Create a servlet that takes as a parameter a number and displays it as image that is hard to be recognized by OCR software. The image should have intentionally inserted defects.
2. Create an HTML form and a servlet for performing conversions of distances from one metric to another. The metrics that should be supported are: meter, centimeter, kilometer, foot, inch, yard, mile.1 cm = 0.01 meters 1 km = 1000 meters
1 foot = 0.3048 meters 1 inch = 0.0254 meters
1 yard = 0.9144 meters 1 mile = 1609.344 meters
Homework (2)
3. Create a sequence of HTML forms and servlets that allow entering information about a student. The information is entered in 3 steps in 3 separate forms:
Step 1: First name, last name, age
Step 2: Address (country, town, street)
Step 3: University, faculty, specialty
The data entered in the 3 steps should be stored in the session and finally displayed.
4. Create a servlet that reads an image (from WEB-INF\img\logo.gif) and returns it.
3. Create a sequence of HTML forms and servlets that allow entering information about a student. The information is entered in 3 steps in 3 separate forms:
Step 1: First name, last name, age
Step 2: Address (country, town, street)
Step 3: University, faculty, specialty
The data entered in the 3 steps should be stored in the session and finally displayed.
4. Create a servlet that reads an image (from WEB-INF\img\logo.gif) and returns it.
