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Basic foilset Overview of JDBC: Java Database Connectivity

Given by Nancy McCracken at Java Servlets on Spring Semester 1999. Foils prepared May 19 99
Outside Index Summary of Material

A servlet is a Java program that is run by a Web server.
Servlets follow a standard servlet API defining the interface between the server and the servlet, and are designed to work within a request/response model. The servlet API is part of the standard extensions of JDK.
Servlets can provide all the services of standard CGI, but are platform-independent, i.e. they can be used with any server implementing the API.
More generally, servlets play the role of providing middle-tier services between the client and the back-end applications.
References:

Table of Contents for full HTML of Overview of JDBC: Java Database Connectivity

 Denote Foils where Image has important information
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1 Java Servlets
2 Servlet Resources for Foil 2 Overview of Java Servlets
3 Architectural Roles for Servlets
4 Temporary and Permanent Servlets
5 Using Servlets
6 The Servlet Life Cycle
7 The init method
8 The service method
9 The destroy method
10 Example Servlet (from MageLang)
11 Service method of the Example
12 Servlet Initialization Information
13 Servlet Context Information
14 The Servlet Request parameter
15 Utility classes
16 Generic Servlets
17 HTTPServlet
18 Processing HTTP requests
19 Security Issues
20 Threading Issues
21 Additional Servlet Resources for Foil 21 Additional Resources

Outside Index Summary of Material

HTML version of Basic Foils prepared May 19 99

Foil 1 Java Servlets

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
2/4/99
Nancy McCracken
NPAC
HTML version of Basic Foils prepared May 19 99

Foil 2 Overview of Java Servlets

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index Servlet Resources for Foil 2
A servlet is a Java program that is run by a Web server.
Servlets follow a standard servlet API defining the interface between the server and the servlet, and are designed to work within a request/response model. The servlet API is part of the standard extensions of JDK.
Servlets can provide all the services of standard CGI, but are platform-independent, i.e. they can be used with any server implementing the API.
More generally, servlets play the role of providing middle-tier services between the client and the back-end applications.
References:
HTML version of Basic Foils prepared May 19 99

Foil 3 Architectural Roles for Servlets

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
Middle-tier process:
  • Connection management to multiple clients
  • Transaction management
  • Business Rule Enforcement
  • Supporting different types of clients, such as HTML and Java applet
Proxy server for applets to connect to other hosts.
Protocol support other protocols: SMTP, POP, FTP, . . .
Some servers allow the <servlet> tag in HTML, and the server will call the servlet to insert text into the outgoing HTML stream
CGI scripts
HTML version of Basic Foils prepared May 19 99

Foil 4 Temporary and Permanent Servlets

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
A temporary servlet is started when a request arrives and shut down after the response is generated.
A permanent servlet is loaded when the server is started and lives until the server is shut down.
  • This is useful when startup costs are high, such as a servlet that establishes a connection to a database.
  • Also useful for permanent server-side service, such as an RMI server.
  • Provides faster response to client requests when this is crucial.
Being temporary or permanent is part of the server configuration.
HTML version of Basic Foils prepared May 19 99

Foil 5 Using Servlets

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
Install Java Servlet Development Kit (JSDK). This works with both JDK1.1 and JDK1.2 platforms.
Servlets can be tested with the utility program servletrunner.
Servlet API is a standard extension to the JDK under javax:
  • Package javax.servlet
  • Package javax.servlet.http
The servlet API provides support in 4 categories:
  • Servlet life cycle
  • Access to servlet context
  • Utility classes
  • HTTP-specific support classes
HTML version of Basic Foils prepared May 19 99

Foil 6 The Servlet Life Cycle

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
On the web server host machine, servlets run under the same process as the web server.
The web server is responsible for creating an instance of the servlet and invoking standard methods from the interface (in a similar way to a browser invoking methods of an applet). There are three main methods:
  • init ( )
  • service ( )
  • destroy ( )
and two helper methods:
  • getServletConfig ( )
  • getServletInfo ( )
HTML version of Basic Foils prepared May 19 99

Foil 7 The init method

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
public void init (ServletConfig config)
The init method is invoked when the servlet is first started. For permanent servlets, this is when the server is started.
The init method is called only once and is guaranteed to finish before any calls to the service method.
The ServletConfig object passed as the parameter has a method getServletContext ( ) that returns a ServletContext with information about the servlet environment.
HTML version of Basic Foils prepared May 19 99

Foil 8 The service method

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
public void service (ServletRequest req, ServletResponse res)
Each request message from the client results in invoking the service method.
There are two ways the client can send information:
  • parameter name/value pairs
  • streams - the servlet can invoke getInputStream(), which returns a ServletInputStream.
Similarly, the servlet can invoke getOutputStream () to generate the response. Other methods, such as setContentType can set information that the web server will use in generating the output to the client.
More than one instance of the service method can be invoked at one time to respond to multiple requests.
HTML version of Basic Foils prepared May 19 99

Foil 9 The destroy method

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
Called when the servlet in unloaded to clean up any open resources.
Although the server normally waits until all service calls are terminated to invoke destroy, it may not be possible, and your destroy method should make sure that resources are not being used.
HTML version of Basic Foils prepared May 19 99

Foil 10 Example Servlet (from MageLang)

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
This servlet returns a page to the web browser:
import java.io.*;
import javax.servlet.*;
public SampleServlet implements Servlet
{ private ServletConfig config;
public void init (ServletConfig config) throws ServletException
{ this.config = config; }
public void destroy ( ) { } // do nothing
public ServletConfig getServletConfig ( )
{ return config; }
public String getServletInfo()
{ return "A simple servlet"; }
HTML version of Basic Foils prepared May 19 99

Foil 11 Service method of the Example

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
public void service (ServletRequest req, ServletResponse res)
throws ServletException, IOException
{ res.setContentType ( "text/html" );
PrintWriter out = res.getWriter ( );
out.println ( "<html>");
out.println ( "<head>");
out.println ( "<title> A Sample Servlet </title>");
out.println ( "</head>");
out.println ( "<body>");
out.println ( "<h1> A Sample Servlet </h1>");
out.println ( "</body>");
out.println ( "</html>");
}
}
HTML version of Basic Foils prepared May 19 99

Foil 12 Servlet Initialization Information

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
This information is passed to the servlet via the ServletConfig object parameter of the init method.
Each web server has its own information tags defined.
The getInitParameterNames ( ) returns an Enumeration of all the parameters available to the servlet on that server.
A particular parameter is obtained by config.getInitParameter ( "timezone");
HTML version of Basic Foils prepared May 19 99

Foil 13 Servlet Context Information

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
This information is available at any time through the ServerConfig object. (This is why you should save the ServerConfig parameter from the init method.): Servlet Context c = config.getServletContext ( )
The ServletContext object has methods:
  • getAttribute
  • getMimeType
  • getRealPath
  • getServerInfo
  • getServlet (String name )
  • getServletNames
  • log (String message )
HTML version of Basic Foils prepared May 19 99

Foil 14 The Servlet Request parameter

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
The servlet gets information from the ServletRequest parameter either by a Stream:
  • req.getInputStream (or req.getReader for ASCII chars )
or by getting individual parameter names:
  • req.getParameter ( "firstname" );
  • req.getParameter ( "secondname" );
There are additional methods for getting request information, such as:
  • getRemoteAddr
  • getProtocol
  • getServerName
  • . . .
HTML version of Basic Foils prepared May 19 99

Foil 15 Utility classes

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
There are two exception classes in the servlet API.
  • javax.servlet.ServletException, the servlet should throw this exception to the server in the event of general failure.
  • javax.servlet.UnavailableException, servlets can report this at any time to the server. The servlet may write a log entry so that an administrator can take some action.
HTML version of Basic Foils prepared May 19 99

Foil 16 Generic Servlets

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
This subclass of Servlet provides an init method that takes care of saving the ServletConfig parameter.
It provides several methods that use the ServletConfig object to return information.
It also provides a dummy destroy method.
HTML version of Basic Foils prepared May 19 99

Foil 17 HTTPServlet

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
This subclass of GenericServlet provides automatic separation of requests according to the HTTP type of request:
  • GET, HEAD, POST, PUT, DELETE, TRACE, CONNECT, OPTIONS
The service method of HTTPLservlet checks the type of request and calls one of several special methods if they are present:
  • doGet, doHead, doDelete, doOptions, doPost, doTrace
  • Note that if you supply a doGet method, the service method can use if for doHead, doTrace and doOptions as well by extracting header information.
doGet and doPost methods should read request data, set response headers and write response data.
HTML version of Basic Foils prepared May 19 99

Foil 18 Processing HTTP requests

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
The doPost method is required to open an InputStream to get the parameters, however, it can call a method parsePostData, that returns a Hashtable with the names and values of the parameters.
To create a cookie on the browser, you create an object of type Cookie and use res.addCookie( c) to send it to the browser. Cookies are retrieved by using methods getCookies, and getName and getValue for individual cookies.
There is also a special HPPTSession class that implicitly uses cookies to allow you to save session information as name/value pairs in between requests.
HTML version of Basic Foils prepared May 19 99

Foil 19 Security Issues

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
Most servers allow the server administrator to associate different levels of "trust" to servlets, usually depending on where they came from. The levels usually control the access to the file system and networking.
  • Note that a fully trusted servlet can even issue System.exit(), stopping the web server.
Servers may also allow Access Control Lists to be created which can allow different users of groups of users access to some web pages (or not).
HTML version of Basic Foils prepared May 19 99

Foil 20 Threading Issues

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index
A web server may call the applet's service method for more than one request at a time by creating multiple threads.
  • You can disallow this by using javax.servlet.SingleThreadModel, which makes it possible to write simple servlets where only one instance of the service method is ever running at one time.
If your service method uses outside resources, such as instance data from the servlet object, files, or databases, you must carefully consider what might happen under multiple calls to the service method, and use synchronization where appropriate.
HTML version of Basic Foils prepared May 19 99

Foil 21 Additional Resources

From Overview of JDBC: Java Database Connectivity Java Servlets -- Spring Semester 1999. *
Full HTML Index Additional Servlet Resources for Foil 21
Main servlet support page at Sun: http://java.sun.com/products/servlet
List of environments supporting servlets at http://java.sun.com/products/servlet/runners.html
Information on the HTTP protocol at the World Wide Web Consortium's web site http://www.w3.org/Protocols
Servlet Central web magazine http://www.servletcentral.com
Jrun Magazine http://www.jrunmag.com
Books:
  • Java Servlets, Karl Moss, McGraw-Hill.
  • Java Servlet Programming, Jason Hunter and William Crawford, O'Reilly.
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