Java Applets and Servlets

1
Java Applets and Servlets

• ?????? ???
• ? ? ?

2
Concepts

• Java has been successful
• Object-orientation, simplicity, portability,
  safety, dynamicity
• There were many other programming language that
  had similar characteristics like Java but never
  had much success
• Java was so special
• Java came out with a killer application in the
  right place at the right time
• applets, HotJava browser that demonstrated
  the utility of applets
• Servlets and Applets
• Servlets
• are designed for Web servers
• are either faceless or have a straight face(HTML)
• could be heavy, doing all sorts of mundane work
  behind the scene
• Applets
• are designed for Web clients
• can have all sorts of pretty faces(GUIs)
• are light, particularly for use in network
  computers or devices
• Both can be mobile mobile code, not mobile
  objects

3
Java Applets(1)

• The benefit of using applets
• The Web browser provides a universal client
  environment
• The independent and platform-independent
  client-side components
• A common API (the Applet API that we will discuss
  later)
• The dynamic loading over the net
• Security (the applet security model)
• Applet programming mode
• An applet must extend the java.applet.Applet
  class
• ltAPPLET gt tag allows you to embed the applet in
  an HTML document
• ltAPPLETgt tags major attributes
• codebase, code, name, width, height, archive
• You can use the PARAM element to supply initial
  values to the applet

4
Java Applets(2)

• Applet life cycle
• Once the class file of an applet is downloaded by
  the Web browser
• An instance of the applet is automatically
  created
• The applet will go through a life cycle,
  supported by its life-cycle methods, and these
  methods are automatically called by the Web
  browser
• Life-cycle methods
• init()
• is called when the applet is first loaded or
  reloaded
• start()
• is called after initialization or stopping to
  re-start
• stop()
• is called when the user leaves the Web page where
  the applet is embedded
• destroy()
• is called after stopping
• Another method (particularly for display)
• paint(Graphics g) is for painting the screen

5
The Applet API(1)

• The classes and interfaces in the Applet API are
  all part of the java.applet package
• Applet class
• The Applet class provides a standard interface
  for applets
• All applets are subclasses of the Applet class
• The Applet class extends the java.awt.Panel class
• Constructors
• Applet()
• Methods
• Life-cycle methods
• init(), start(), stop(), destory(), isActive()
• Methods to get and manipulate its data
• getParameter(String name), getParameterInfo(),
  getAudioClip(java.net.URL url),
  getAudioClip(java.net.URL url, String name),
  play(java.net.URL url), play(java.net.URL url,
  String name), getImage(java.net.URL url),
  getImage(java.net.URL url, String name)

6
The Applet API(2)

• Applet class
• Methods (cont)
• Methods to obtain information about this applet
  and to manipulate it
• getAppletContext(), getAppletInfo(),
  getCodeBase(), getDocumentBase(), getLocale(),
  resize(java.awt.Dimension d), resize(int width,
  int height), setStub(AppletStub stub),
  showStatus(String msg)
• AppletContext interface
• The AppletContext interface gives applets access
  to information about their environment, the
  document containing the applets, and the other
  applets in the same document
• Methods
• getApplet(String name), getApplets(),
  getAudioClip(java.net.URL url),
  getImage(java.net.URL url), showDocument(java.net.
  URL url), showDocument(java.net.URL url, String
  target), showStatus(String msg)

7
The Applet API(3)

• AppletStub interface
• The stub serves as the interface between the
  applet and its browser environment
• Methods
• isActive(), getParameter(String name),
  getAppletContext(), getCodeBase(),
  getDocumentBase(), appletResize(int height, int
  weight)
• AudioClip interface
• The AudioClip interface is a simple abstraction
  for playing an audio clip
• Methods
• loop(), play(), stop()

8
Sample Data Access Applets(1)

• Fg 12-1. JDBC Usage Scenario Java Applet(1)

9
Sample Data Access Applets(2)
10
Sample Data Access Applets(3)
11
Java Servlets(1)

• The benefit of using servlets
• The Web browser can provide a universal client
  environment
• The independent and platform-independent
  server-side components
• A common API
• The dynamic loading, either locally or over the
  net
• Security (the servlet security model)

12
Java Servlets(2)

• Servlets can be used to generate dynamic HTML
  content
• Faster than CGI
• Servlets use only lightweight threads to handle
  client requests
• Safe than CGI
• Automatic memory management and the servlet
  security model
• Fg 12-5. Dynamic HTML Generation Using HTTP
  Servlets

13
Java Servlets(3)

• Servlet programming model
• A servlet
• must implement the javax.servlet.Servlet
  interface
• or extend a class that implements Servlet
  interface, such as javax.servlet.GenericServlet
  or javax.servlet.http.HttpServlet
• Servlets follow this request-service-response
  programming model
• Request- represented by a java.servlet.ServletRequ
  est object
• Service- performed by invoking the service()
  method
• Response- represented by a java.servlet.ServletRes
  ponse object

14
Java Servlets(4)

• Servlet life cycle
• Once invoked, a servlet will go through a life
  cycle, supported by its life-cycle methods
• These methods are automatically called by the Web
  server
• Life-cycle methods
• init() is called by the network service when it
  loads this servlet
• service() is called after initialization
• destroy() is called before unloading and is
  called only once
• Two additional methods for HTTP servlets
• doGet() for processing the HTTP GET method
• doPost() for processing the HTTP POST method
• These methods are called after initialization

15
Java Servlets(3)

• Loading and invoking servlets
• From the /servlets/directory
• http//hostport/servlet/ltservlet_classgt
• From a remote location
• http//hostport/servlet/ltservlet_URLgt
• By setting up Server Side Includes
• By configuring the Admin Tool
• By defining a Filter Chain
• Server side includes
• ltSERVLETgt tag allows you to embed a servlet in an
  HTML document, which must have the special file
  extension .shtml
• ltSERVLETgt tags major attributes
• codebase, code
• You can use the PARAM element to supply initial
  values to the servlet

16
The Servlet API(1)

• The classes and interfaces in the Servlet API are
  all part of the javax.servlet package
• Servlet interface
• All servlets implement the Servlet interface
• Methods
• Life-cycle methods
• init(ServletConf config), service(ServletRequest
  req, ServletResponse resp), destroy()
• Methods to obtain information about this servlet
  
• getServletConfig(), getServletInfo()
• ServletConfig interface
• The ServletConfig interface is implemented by
  network services or servlets to pass
  configuration information to a servlet when it is
  first loaded
• Methods
• getInitParameter(String name), getInitParameterNam
  es(), getServletContext()

17
The Servlet API(2)

• ServletContext interface
• The ServletContext interface is implemented by
  network services
• It gives servlets access to information about
  their environment and allows servlets to log
  significant events
• Methods
• getServlet(String name), getServlets(),
  getServerInfo(), getAttribute(String name),
  getRealPath(String path), getMimeType(String
  file), log(String msg)
• ServletRequest interface
• The servletRequest interface provides the means
  for getting information from the client to the
  servlet for a service request
• Methods
• Methods to obtain information on this request
• getProtocol(), getScheme(), getParameterNames(),
  getParameter(String name), getParameterValues(Stri
  ng name), getAttribute(String name),
  getContentLength(), getContentType(),
  getInputStream()
• Methods to obtain information on the sender and
  receiver
• getRemoteAddr(), getRemoteHost(),
  getServerName(), getServerPort(),
  getRealPath(String path)

18
The Servlet API(3)

• ServletResponse interface
• The ServletResponse interface provides the means
  for sending response data from the servlet to the
  client
• Methods
• setContentLength(int len), setContentType(Striing
  type), getOutputStream()
• GenericServlet class
• The GenericServlet class is an abstract class
  that simplifies writing servlets
• It implements the Servlet and ServletConfig
  interfaces
• Constructors
• GenericServlet()
• Methods
• Life-cycle methods
• init(ServletConf config), service(ServletRequest
  req, ServletResponse resp), destroy()
• Methods to obtain information about this servlet
  
• getInitParameter(String name), getInitParameterNam
  es(), getServletContext(), getServletConfig(),
  getServletInfo()
• Additional methods
• log(String msg)

19
The Servlet API(4)

• ServletInputStream class
• The ServletInputStream class is an abstract class
  for reading servlet request data
• It extends the java.io.InputStream class
• Constructors
• ServletInputStream()
• Methods
• readLine(byte b, int off, int len)
• ServletOutputStream class
• The StervletOutputStream class is an abstract
  class for writing servlet response data
• It extends the java.io.OutputStream class
• Constructor
• ServletOutputStream()
• Methods
• print(int i), print(long l), print(float f),
  print(double d), print(char c), print(String s),
  print(boolean b), println(), println(int i),
  println(long l), println(float f), println(double
  d), println(char c), println(String s),
  println(boolean b)

20
The HTTP Servlet API(1)

• The classes and interfaces in the HTTP servlet
  API are all part of the javax.servlet.http
  package
• HttpServlet class
• The HttpServlet class is an abstract class that
  simplifies writing HTTP 1.0 servlets
• It extends the GenericServlet class and provides
  a framework for handling the HTTP protocol
• Constructors
• HttpServlet()
• Methods
• service(HttpServletRequest req,
  HttpServletResponse resp), service(ServletReqeust
  req, ServletResponse resp), doGet(HttpSErvletReque
  st req, HttpServletResponse resp),
  doPost(HttpServletRequest req, HttpServletResponse
  resp), getLastModified(HttpServletRequest req)
• HttpUtils class
• The HttpUtils class contains utility methods
  useful to HTTP servlets
• Methods
• getRequestURL(HttpServletRequest req),
  parsePostData(int ServletInputStream),
  parseQueryString(String s)
• Constructors
• HttpUtils()

21
The HTTP Servlet API(2)

• HttpServletRequest interface
• The HttpServletRequest intreface extends the
  ServletRequest interface and represents an HTTP
  servlet request
• Methods to obtain information on this request
• getAuthType(), getHeaderNames(), getHeader(String
  name), getDateHeader(String name),
  getIntHeader(String name), getMethod(),
  getQueryString()
• Methods to obtain information on the sender and
  receiver
• getRemoteUser(), getRequestURI(),
  getServletPath(), getPathInfo(),
  getPathTranslated()
• HttpServletResponse interface
• The HttpServletResponse interface extends the
  ServletResponse interface and represents an HTTP
  servlet response
• Class constants
• SC_OK(int), SC_CREATED(int), SC_ACCEPTED(int),
  SC_NO_CONTENT(int), SC_MOVED_PERMANENTLY(int),
  SC_MOVED_TEMPORARILY(int),.
• Methods
• containsHeader(String name), setHeader(String
  name, String value), setDateHeader(String name,
  long value), setIntHeader(String name, int
  value), setStatus(int sc), setStatus(int sc,
  String msg), sendError(int sc), sendError(int sc,
  String msg), sendRedirect(String location)

22
Sample Data Access Servlets(1)
23
Sample Data Access Servlets(2)
24
Conclusion

• Java applets
• Easy to use
• More sophisticated GUIs than HTML pages for data
  presentation
• More programming effort, a fat client, and
  performance problems
• The importance of Java applets for data access on
  the Web client is diminishing
• The lack of uniform and in-time support among Web
  browser vendors
• The restricted communication channel between
  applets and HTML pages and performance issues
• Java servlets
• Easy to use
• HTML pages for display or only limited GUI
  support
• Much less programming effort, only a thin client,
  and fewer performance problems
• The importance of Java servlets for client/server
  data access is increasing
• Java servlets will also be indispensable when
  using reusable server components from the Web