Servlet
Packages
Java
Servlets are Java classes run by a web server that has an interpreter that
supports the Java Servlet specification. Servlets can be created using
the javax.servlet
and javax.servlet.http packages,
which are a standard part of the Java's enterprise edition, an expanded version
of the Java class library that supports large-scale development projects.
These classes implement the Java Servlet
and JSP specifications. At the time
of writing this tutorial,
the versions are Java Servlet 2.5 and JSP 2.1.
Java servlets have been created and compiled just like any other Java class. After
you install the servlet packages and add them to your computer's Class
path, you can compile servlets with the JDK's Java compiler
or any other current compiler.
Servlet API: There are two packages that are required to build
servlets.
1. javax.servlet 2.javax.servlet.http.
In general javax.servlet package has all the classes/interfaces not directly tied
to HTTP, and
javax.servlet.http package has the classes/interfaces that deal with the http
protocol.
1. javax.servlet package:
There are number
of interfaces and classes present
in this package, they are described below.
Interfaces:
Servlet:
Declares life cycle methods for a servlet ServletConfig:
Allows servlets to get initialization parameters ServletContext: Enables
servlets to log events ServletRequest: Used to read data from a
Classs:
GenericServlet: Implements the
Servlet and ServletConfig
ServletInputStream: Provides
an input stream
for reading requests
from a client ServletOutputStream: Provides
an output stream
for writing responses to a client. ServletException: Indicates that
a servlet error occurred.
Following
are the interfaces and their methods:
Servlet Interface:
void init(): Called when the
servlet is initialized
void service(): Called to process
a request from a client void destroy(): Called when the servlet
is unloaded
ServletConfig getServletConfig():
Returns a ServletConfig object that contains any initialization parameters
String
getServletInfo: Returns a string describing the servlet
ServletConfig Interface:
ServletContext getServletContext:
Returns the context for this servlet.
String getInitParmeter(String param):
Returns the value
of the initialization parameter
name param.
getInitParameterNames():
Returns all initialization parameter names
ServletContext interface:
getAtrribute(String attr):
Returns the value
of the server attribute named
attr. String
getServiceInfo(): Returns
information about the server.
Servlet getServlet(String sname): Returns the
servlet named sname. getServletNames(): Returns the names of servlets in
the server
ServletRequest Interface:
String getParameter(String pname): Returns the value
of the parameter named pname getParameterNames(): Returns the parameter
names for this request
String[ ] getParameterValues(): Returns the
parameter values for this request String getProtocol(): Returns a
description of the protocol
String getServerName(): Returns the name of the
serverIntgetServerPort():Returns the port number.
ServletResponse Interface:
PrinterWriter
getWriter(): Returns a PrintWriter that can be used to write character data to the response.
ServletOutputStream getOutputStream(): Returns a ServletOutputStream that
can be used to write binary
data to the response.
Following are the classes and their methods:
GenericServlet class implements Servlet and
ServletConfig interface.
Methods:
public void init(), public void destroy(), public abstract void
service()
ServletInputStream class
extends InputStream. It is implemented by the server and provides an input
stream that a servlet developer can use to read the data from
a client request. In addition
to this, one more method is added which returns the actual number of bytes read Int readLine(byte[ ] buffer, int offset, int size)
ServletOutputStream class extends OutputStream. It defines the print() and println()
methods, which output data to the stream.
ServletException class indicates that a
servlet problem has occurred.
The class has the following contructor ServletException( )
& ServletException(String s)
2. javax.servlet.http package:
There are number
of classes and interfaces present in this package, they are as follows:
Interface
HttpServletRequest: Enables
servlets to read
data from an HTTP request HttpServletResponse: Enables
servlets to write
data to an HTTP response. HttpSession: Allows session
data to be read and written HttpSessionContext: Allows sessions to be managed
Class
HttpServlet: Provides methods to handle
HTTP requests and responses
Cookie: Allows state information to be stored
on a client machine
Following
are the interfaces and their methods description HttpServletRequest Interface
Cookie[
] getCookies: Returns an array of the cookies in this request
String getMethod(): Returns the HTTP method for this request String getQueryString(): Returns any query
string in the URL
String getRemoteUser(): Returns the name
of the user
who issued this
request. String getRequestedSessionId(): Returns
the ID of the session
String getServletPath(): Returns the part of the URL that
identifies the servlet
HttpServletResponse Interface
Void addCookie(Cookie cookie): Adds cookie to the
HTTP response. Void sendError(int c): Send the error code c to the
client
Void sendError(int c , String s): Send the error code c and the message
s Void sendRedirect(String url): Redirects the client to url
