servlet API

Servlet API:

 Two packages contain the classes and interfaces that are required to build servlets. These  are javax.servlet and javax.servlet.http. They constitute the Servlet API.

·         These packages are not part of the Java core packages. Instead, they are standard extensions. Therefore, they are not included in the Java Software Development Kit. You must download Tomcat to obtain their functionality.  The Servlet API is supported by most Web servers, such as those from Sun,Microsoft, and others.

javax.servlet Package

·         The javax.servlet package contains a number of interfaces and classes that establish the framework in which servlets operate.  The most significant of these is Servlet. All servlets must implement this interface or extend a class that implements the interface.  The ServletRequest and ServletResponse interfaces are also very important.

Interface	Description
Servlet	Declares life cycle methods for a servlet.
ServletConfig	Allows servlets to get initialization parameters.
ServletContext	Enables servlets to log events and access information about their environment.
ServletRequest	Used to read data from a client request.
ServletResponse	Used to write data to a client response.
SingleThreadModel	Indicates that the servlet is thread safe.

Class	Description
GenericServlet	Implements the Servlet and ServletConfig interfaces.
ServletInputStream	Provides an input stream for reading requests from a client.
ServletOutputStream	Provides an output stream for writing responses to a client.
ServletException	Indicates a servlet error occurred.
UnavailableException	Indicates a servlet is unavailable.

Servlet Interface:

       Method                                Description

void destroy( )                                                           Called when the servlet is unloaded.

ServletConfig getServletConfig( )               Returns a ServletConfig object that containsany initialization parameters.

String getServletInfo( )                               Returns a string describing the servlet.

void init(ServletConfig sc)

throws ServletException                  Called when the servlet is initialized.Initialization parameters for the   servlet can be obtained from sc. An UnavailableException should be thrown if the servlet cannot be initialized.

void service(ServletRequest req,

ServletResponse res)

throws ServletException,

IOException                                       Called to process a request from a client. The request from the client can be read from req. The response to the client can be written to res. An exception is generated if a servlet or IO problem occurs.

The init( ), service( ), and destroy( ) methods are the life cycle methods of the servlet. These are invoked by the server. The getServletConfig( ) method is called by the servlet to obtain initialization parameters. A servlet developer overrides the getServletInfo( ) method to provide a string with useful information (for example, author, version, date, copyright). This method is also invoked by the server.

ServletConfig Interface:

The ServletConfig interface is implemented by the server. It allows a servlet to obtain configuration data when it is loaded.

           

            Method                                                      Description

ServletContext  getServletContext( )                     Returns the context for this servlet.

String   getInitParameter(String param)               Returns the value of the initialization                                                                                                               Parameter named param.

Enumeration   getInitParameterNames( )                         Returns an enumeration of all initialization parameter names.

String   getServletName( )                                       Returns the name of the invoking servlet.

ServletContext Interface

The ServletContext interface is implemented by the server. It enables servlets to obtain information about  their environment

Method                                                      Description

Object getAttribute(String attr)                  Returns the value of the server attribute named attr.

String getMimeType(String file)                  Returns the MIME type of file.

String getRealPath(String vpath)               Returns the real path that corresponds to the virtual path vpath.

String getServerInfo( )                                Returns information about the server.

void log(String s)                                          Writes s to the servlet log.

void log(String s, Throwable e)                    Write s and the stack trace for e to the servlet log.

void setAttribute(String attr, Object val)   Sets the attribute specified by attr to the value passed in val.

ServletRequest Interface

The ServletRequest interface is implemented by the server. It enables a servlet to obtain information about a client request.

Method                                   Description

Object getAttribute(String attr)                              Returns the value of the attribute named attr.

String getCharacterEncoding( )                               Returns the character encoding of the request.

int getContentLength( )                                           Returns the size of the request. The value –1 is returned if the size is unavailable.

String getContentType( )    

Returns the type of the request. A null value is returned if the typecannot be determined.

ServletInputStream getInputStream( )

throws IOException

Returns a ServletInputStream that can be used to read binary data from the request. An IllegalStateException is thrown if getReader( ) has already been invoked for this request.

String getParameter(String pname)                       Returns the value of the parameter named pname.

Enumeration getParameterNames( )                      Returns an enumeration of the parameter names for                                                      this request.

String[ ] getParameterValues(String name )        Returns an array containing values associated with the parameter specified by name.

String getProtocol( )                                                 Returns a description of the protocol.

BufferedReader getReader( )

throws IOException                                                  Returns a buffered reader that can be used to read text from the request. An IllegalStateException is thrown if getInputStream( ) has already been invoked for this request.

String getRemoteAddr( )                                           Returns the string equivalent of the client IP address.

String getRemoteHost( )                                          Returns the string equivalent of the client host name.

String getScheme( )

Returns the transmission scheme of the URL used or  the request (for example, “http”, “ftp”).

String getServerName( )                                          Returns the name of the server.

int getServerPort( )                                                  Returns the port number.

ServletResponse Interface

The ServletResponse interface is implemented by the server. It enables a servlet to formulate a response for a client.

Method                                                      Description

String getCharacterEncoding( )                               Returns the character encoding for the response.

ServletOutputStream getOutputStream( )

throws IOException                                                  Returns a ServletOutputStream that can beused to write binary data to the response. An IllegalStateException is thrown if getWriter( ) has already been invoked for this request.

PrintWriter getWriter( )

throws IOException                                                  Returns a PrintWriter that can be used to write character data to the response.An llegalStateException is thrown if getOutputStream( ) has already been invoked for this request.

void setContentLength(int size)                             Sets the content length for the response to

 size.

void setContentType(String type)                           Sets the content type for the response to type.

SingleThreadModel Interface

This interface is used to indicate that only a single thread will execute the service( ) method of a servlet at a given time. It defines no constants and declares no methods. If a servlet implements this interface, the server has two options. First, it can create several instances of the servlet. When a client request arrives, it is sent to an available

instance of the servlet. Second, it can synchronize access to the servlet.

GenericServlet Class

The GenericServlet class provides implementations of the basic life cycle methods for a servlet and is typically subclassed by servlet developers. GenericServlet implements the Servlet and ServletConfig interfaces. In addition, a method to append a string to the server log file is available. The signatures of this method are shown here:

void log(String s)

void log(String s, Throwable e)

Here, s is the string to be appended to the log, and e is an exception that occurred.

ServletInputStream Class

The 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. It defines the default constructor. In addition, a method is provided to read bytes from the stream. Its signature is shown here:

int readLine(byte[ ] buffer, int offset, int size) throws IOException

Here, buffer is the array into which size bytes are placed starting at offset. The method returns the actual number of bytes read or –1 if an end-of-stream condition is encountered.

ServletOutputStream Class

The ServletOutputStream class extends OutputStream. It is implemented by the server and provides an output stream that a servlet developer can use to write data to a client response. A default constructor is defined. It also defines the print( ) and println( ) methods, which output data to the stream.

Servlet Exception Classes

javax.servlet defines two exceptions. The first is ServletException, which indicates that a servlet problem has occurred. The second is UnavailableException, which extends ServletException. It indicates that a servlet is unavailable.