Interface Servlet,JspPage,HttpJspPage

javax.servlet
Interface Servlet

All Known Implementing Classes:
GenericServlet, HttpServlet

public interface Servlet

Defines methods that all servlets must implement.

A servlet is a small Java program that runs within a Web server. Servlets receive and respond to requests from Web clients, usually across HTTP, the HyperText Transfer Protocol.

To implement this interface, you can write a generic servlet that extends javax.servlet.GenericServlet or an HTTP servlet that extends javax.servlet.http.HttpServlet.

This interface defines methods to initialize a servlet, to service requests, and to remove a servlet from the server. These are known as life-cycle methods and are called in the following sequence:

1. The servlet is constructed, then initialized with the init method.
2. Any calls from clients to the service method are handled.
3. The servlet is taken out of service, then destroyed with the destroy method, then garbage collected and finalized.

In addition to the life-cycle methods, this interface provides the getServletConfig method, which the servlet can use to get any startup information, and the getServletInfo method, which allows the servlet to return basic information about itself, such as author, version, and copyright.




Method Summary
void destroy()
Called by the servlet container to indicate to a servlet that the servlet is being taken out of service.
ServletConfig getServletConfig()
Returns a ServletConfig object, which contains initialization and startup parameters for this servlet.
java.lang.String getServletInfo()
Returns information about the servlet, such as author, version, and copyright.
void init(ServletConfig config)
Called by the servlet container to indicate to a servlet that the servlet is being placed into service.
void service(ServletRequest req, ServletResponse res)
Called by the servlet container to allow the servlet to respond to a request.


Method Detail
init

public void init(ServletConfig config)
throws ServletException

Called by the servlet container to indicate to a servlet that the servlet is being placed into service.

The servlet container calls the init method exactly once after instantiating the servlet. The init method must complete successfully before the servlet can receive any requests.

The servlet container cannot place the servlet into service if the init method

1. Throws a ServletException
2. Does not return within a time period defined by the Web server

Parameters:
config - a ServletConfig object containing the servlet's configuration and initialization parameters
Throws:
ServletException - if an exception has occurred that interferes with the servlet's normal operation
See Also:
UnavailableException, getServletConfig()

getServletConfig

public ServletConfig getServletConfig()

Returns a ServletConfig object, which contains initialization and startup parameters for this servlet. The ServletConfig object returned is the one passed to the init method.

Implementations of this interface are responsible for storing the ServletConfig object so that this method can return it. The GenericServlet class, which implements this interface, already does this.

Returns:
the ServletConfig object that initializes this servlet
See Also:
init(javax.servlet.ServletConfig)

service

public void service(ServletRequest req,
ServletResponse res)
throws ServletException,
java.io.IOException

Called by the servlet container to allow the servlet to respond to a request.

This method is only called after the servlet's init() method has completed successfully.

The status code of the response always should be set for a servlet that throws or sends an error.

Servlets typically run inside multithreaded servlet containers that can handle multiple requests concurrently. Developers must be aware to synchronize access to any shared resources such as files, network connections, and as well as the servlet's class and instance variables. More information on multithreaded programming in Java is available in the Java tutorial on multi-threaded programming.

Parameters:
req - the ServletRequest object that contains the client's request
res - the ServletResponse object that contains the servlet's response
Throws:
ServletException - if an exception occurs that interferes with the servlet's normal operation
java.io.IOException - if an input or output exception occurs

getServletInfo

public java.lang.String getServletInfo()

Returns information about the servlet, such as author, version, and copyright.

The string that this method returns should be plain text and not markup of any kind (such as HTML, XML, etc.).

Returns:
a String containing servlet information

destroy

public void destroy()

Called by the servlet container to indicate to a servlet that the servlet is being taken out of service. This method is only called once all threads within the servlet's service method have exited or after a timeout period has passed. After the servlet container calls this method, it will not call the service method again on this servlet.

This method gives the servlet an opportunity to clean up any resources that are being held (for example, memory, file handles, threads) and make sure that any persistent state is synchronized with the servlet's current state in memory.


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javax.servlet.jsp
Interface JspPage

All Superinterfaces:
Servlet

All Known Subinterfaces:
HttpJspPage

public interface JspPage
extends Servlet

The JspPage interface describes the generic interaction that a JSP Page Implementation class must satisfy; pages that use the HTTP protocol are described by the HttpJspPage interface.

Two plus One Methods

The interface defines a protocol with 3 methods; only two of them: jspInit() and jspDestroy() are part of this interface as the signature of the third method: _jspService() depends on the specific protocol used and cannot be expressed in a generic way in Java.

A class implementing this interface is responsible for invoking the above methods at the appropriate time based on the corresponding Servlet-based method invocations.

The jspInit() and jspDestroy() methods can be defined by a JSP author, but the _jspService() method is defined automatically by the JSP processor based on the contents of the JSP page.

_jspService()

The _jspService()method corresponds to the body of the JSP page. This method is defined automatically by the JSP container and should never be defined by the JSP page author.

If a superclass is specified using the extends attribute, that superclass may choose to perform some actions in its service() method before or after calling the _jspService() method. See using the extends attribute in the JSP_Engine chapter of the JSP specification.

The specific signature depends on the protocol supported by the JSP page.

public void _jspService(ServletRequestSubtype request,
ServletResponseSubtype response)
throws ServletException, IOException;


Method Summary
void jspDestroy()
The jspDestroy() method is invoked when the JSP page is about to be destroyed.
void jspInit()
The jspInit() method is invoked when the JSP page is initialized.

Methods inherited from interface javax.servlet.Servlet
destroy, getServletConfig, getServletInfo, init, service


Method Detail
jspInit

public void jspInit()

The jspInit() method is invoked when the JSP page is initialized. It is the responsibility of the JSP implementation (and of the class mentioned by the extends attribute, if present) that at this point invocations to the getServletConfig() method will return the desired value. A JSP page can override this method by including a definition for it in a declaration element. A JSP page should redefine the init() method from Servlet.

jspDestroy

public void jspDestroy()

The jspDestroy() method is invoked when the JSP page is about to be destroyed. A JSP page can override this method by including a definition for it in a declaration element. A JSP page should redefine the destroy() method from Servlet.

javax.servlet.jsp
Interface HttpJspPage

All Superinterfaces:
JspPage, Servlet

public interface HttpJspPage extends JspPage

The HttpJspPage interface describes the interaction that a JSP Page Implementation Class must satisfy when using the HTTP protocol.

The behaviour is identical to that of the JspPage, except for the signature of the _jspService method, which is now expressible in the Java type system and included explicitly in the interface.



Method Summary
void _jspService(HttpServletRequest request, HttpServletResponse response)
The _jspService()method corresponds to the body of the JSP page.

Methods inherited from interface javax.servlet.jsp.JspPage
jspDestroy, jspInit

Methods inherited from interface javax.servlet.Servlet
destroy, getServletConfig, getServletInfo, init, service


Method Detail
_jspService

public void _jspService(HttpServletRequest request,
HttpServletResponse response)
throws ServletException,
IOException

The _jspService()method corresponds to the body of the JSP page. This method is defined automatically by the JSP container and should never be defined by the JSP page author.

If a superclass is specified using the extends attribute, that superclass may choose to perform some actions in its service() method before or after calling the _jspService() method. See using the extends attribute in the JSP_Engine chapter of the JSP specification.

Parameters:
request - Provides client request information to the JSP.
response - Assists the JSP in sending a response to the client.
Throws:
ServletException - Thrown if an error occurred during the processing of the JSP and that the container should take appropriate action to clean up the request.
IOException - Thrown if an error occurred while writing the response for this page.