Transcript CS441 Lecture 4

Introduction to Servlets
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Topics
• Web Applications and the Java Server.
• HTTP protocol.
• Servlets
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Web Applications
• Two Types of Web Applications:
– Presentation oriented: Interactive web applications with
dynamic content usually provided with DHTML, XML,
servlets or java server pages.
– Service oriented: Web applications that utilize
presentation oriented applications as clients. They
consist of web services, SOAP protocol services, RMI
etc.
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Web Applications
» For example an Aplication server or an RMI server that
processes Queries to the databse server. It receives the results
of the query formats it and passes it over to the presentation
oriented application so that it can be sent to the requestor
(client of the presentation application)
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Web Applications
• Web Components: Servlets, JSP (Java Server
Pages).
• Web Components are supported by a web
container.
• For instance: Tomcat has a servlet engine and a
JSP engine. Those engines are the web containers.
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Web Applications
• Web Containers provide to web components:
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Activation
Initialization
Configuration
Translation from http to a servlet request object and
Translation from a servlet response object to http.
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Web Applications
• Web Components can communicate directly
with a database (via JDBC) or
• They can communicate with a database via
Java Beans components or
• They can communicate with other service
oriented applications such as a RMI server,
or web services, or an application server.
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Web Applications
HTTPServletRequest
Object
Database
HTTP Request
Web
Client
Containers
HTTP Response
Components
HTTPServletResponse
Object
Java Bean
Components
Database
RMI
Server
Web
Services
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HTTP Protocol
• Request/ Response packets.
– Client initiates a request via a TCP connection to a port
(8080 or 80).
– Server sends a response packet to client
• Protocol provides methods such as:
– GET
– POST
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PUT
DELETE
HEAD
And others
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HTTP Protocol
• If GET is used by client then:
– Data can be part of the URL
http://www.cs.iit.edu/index.html?firtsname=“fm”&lastname=“ln”
– Data is not cached by the browser application.
– Data is URL encoded i.e. spaces are converted to +
signs, nonalphanumeric characters are converted to %
signs followed by the two hex digits representing the
character etc.
– Use it when the data is small.
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HTTP protocol
• If client uses POST:
– The data is part of the body of the HTTP
packet.
– Data is cached.
– Use it if data is larger and the interaction with
the server changes the state of the resource in
the server.
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HTTP Protocol
• Error messages can be returned by the
server in the response packet.
• Error messages are coded:
– 400 means bad request.
– 401 means unauthorized request.
– 500 Internal server error.
Etc.
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Servlets
• A servlet is a java program that executes on the
server side.
• It extends the capabilities of a server.
• Two types of servlets:
– Generic type
– HTTP type
Note: nevertheless other protocols can be supported such
as FTP, POP, SMTP
• We will concentrate on HTTP type.
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Servlets
• HTTP type servlets
– Extend the web server by executing requests
received in HTTP (from a client) protocol
packets and by returning responses that are
translated back to HTTP protocol packets.
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Servlets
• Clients to Servlets
– Client side programs that sent requests can be:
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•
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Html files (forms).
Applets.
JavaScript.
XML.
Java application programs.
Active Server pages (ASP) programs.
Other.
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Servlets
• Execution environment:
– Web Server needs the servlet engine
(container).
– The interface between the servlet engine and
the servlet program is defined by the Servlet
API.
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Servlets
– The servlet container is responsible for:
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Creating an instance of the servlet program.
Calling the servlet’ s init() method to initialize the servlet.
Configures the servlet according to preset properties.
Calling the servlet’ s service method.
Creating the stream to send to the servlet a request object that
encapsulates an y data the client sent as part of the request.
• Creating the stream to receive the response object from the servlet
that encapsulates the data generated by the servlet as a result of the
request.
• Can handle multiple service calls that be made simultineously
(multiple requests arriving at the same time).
• Destroys the servlet by calling destroy() method.
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Servlets
• Servlet applications provide:
– Processing of a Get or Post type of a request via individual
methods that need to be overridden.
• The servlet engine reads the request type off the arriving HTTP
packets, calls the service method of the servlet, the service method
will call either a doGet or doPost servlet method.
– Handle many requests at the same time.
– Provide session capability with a client so that multiple
request/response pairs can be serviced without loosingtrack of the
particular client.
– Can communicate with other servlets forming a chain.
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Servlets
– Can generate a response in Html tags. The servelt
engine will extract the html code from the java code
and form send it to the client.
• This facilitates the generation of dynamically formed
responses based on the request.
– Can execute JDBC code to connect to a database and
retrieve information. The response object will then
encapsulate the data.
– Can act as clients to a Remote Method Invocation
Servers in a distributed objects network.
– Can communicate with Java Beans.
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Servlets
• Servlet API
– javax.servlet
– javax.servlet.http
Implemented by abstract class
GenericServlet
Servlet interface
Implemented by abstract class
HttpServlet
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Servlets
• The abstract classes provide default implementation for methods:
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void init(ServletConfig config)
ServletConfig getServletConfig()
void service(ServletRequest requ, ServletResponse resp)
String getServletInfo()
void destroy()
In addition, the HttpServlet class has methods:
• doGet (HTTPServletRequest requ, HttpServletResponse resp)
• doPost (HTTPServletRequest requ, HttpServletResponse resp)
These methods have to be overridden by the servlet class that the developer provides.
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Servlets
• Example:
– Client receives an HTML form.
– The form in its action tag calls a servlet.
– The servlet creates html code and sends back
to the client for display by the browser.
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Servlets Example-File
helloworld,html (simplified)
<html>
<head>
<title>Untitled Document</title>
</head>
<body bgcolor="#FFFFFF">
<p><a href="servlet/HelloWorldExample"></p>
</body>
</head>
</html>
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Servlets Example – file
HelloWorld.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class HelloWorld extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) throws
IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>"); out.println("<head>");
out.println("<title>Hello World!</title>");
out.println("</head>"); out.println("<body>");
out.println("<h1>Hello World!</h1>");
out.println("</body>");
out.println("</html>"); } }
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Servlets Example – file
HelloWorld.java
• The Browser receives the html code:
<html>
<head>
<title>Hello World!</title>
</head>
<body>
<h1>Hello World!</h1>
</body>
</html>
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Servlets-Another example
<html>
<body>
<Form action=http://localhost:8080/ExampleSite/TechSupport”
method=“POST”>
<table><tr><td>
<Input types=“Text” Name=“firstName”></input></td></tr></table>
</form>
</body>
</html>
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Servlets-Another example
TechSupport.java file
import javax.servlet.*;
import javax.servlet.http.*
import java.io.*;
import java.util.*;
public class TechSupport extends HttpServlet
{ protected void doPost(HttpServletRequest req, HttpServletResponse res)
throws ServletException
{ res.setContextType(“text/html”);
String myparam=req.getParameter(“firstName”);
PrintWriter out = res.getWriter();
out.println(“<HTML>”);
out.println(“<HEAD>”);
out.println(“<BODY>”);
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Servlets-Another example
if(myparam.equals(“MyName”))
{
out.println(“<p”>);
out.println(“You can now login”);
out.println(“</p>”);
}
else
{
out.println(“<p”>);
out.println(“I don’t recognize you”);
out.println(“</p>”);
}
out.println(“</BODY>”);
out.println(“</HEAD>”);
out.println(“<HTML>);
out.close();
}
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