javax.servlet.http
Class HttpServlet

java.lang.Object
  |
  +--javax.servlet.GenericServlet
        |
        +--javax.servlet.http.HttpServlet
public abstract class HttpServlet
extends GenericServlet
implements java.io.Serializable

Provides an abstract class that you can subclass to create an HTTP servlet, which receives requests from and sends responses to a Web site. When you subclass HttpServlet, you must override at least one method, usually one of these:

You usually will not override the service method. service handles standard HTTP requests by dispatching them to the handler methods for each HTTP request type (the doxxx methods listed above).

Likewise, you usually will not override the doOptions and doTrace methods. The service method supports HTTP 1.1 TRACE and OPTIONS requests by dispatching them to doTrace and doOptions.

Servlets typically run on multithreaded servers, so you must write your servlet to handle concurrent requests and synchronize access to shared resources. Shared resources include in-memory data such as instance or class variables and external objects such as files, database connections, and network connections. See the Java Tutorial on Multithreaded Programming for more information on handling multiple threads in a Java program.

Version:
$Version$
Author:
Various
See Also:
Serialized Form

Constructor Summary
HttpServlet()
          Does nothing, because this is an abstract class.
 
Method Summary
protected  void doDelete(HttpServletRequest req, HttpServletResponse resp)
          Receives an HTTP DELETE request from the protected service method and handles the request.
protected  void doGet(HttpServletRequest req, HttpServletResponse resp)
          Receives an HTTP GET request from the protected service method and handles the request.
protected  void doOptions(HttpServletRequest req, HttpServletResponse resp)
          Receives an HTTP OPTIONS request from the protected service method and handles the request.
protected  void doPost(HttpServletRequest req, HttpServletResponse resp)
          Receives an HTTP POST request from the protected service method and handles the request.
protected  void doPut(HttpServletRequest req, HttpServletResponse resp)
          Receives an HTTP PUT request from the protected service method and handles the request.
protected  void doTrace(HttpServletRequest req, HttpServletResponse resp)
          Receives an HTTP TRACE request from the protected service method and handles the request.
protected  long getLastModified(HttpServletRequest req)
          Returns the time the HttpServletRequest object was last modified, in milliseconds since midnight January 1, 1970 GMT.
protected  void service(HttpServletRequest req, HttpServletResponse resp)
          Receives standard HTTP requests from the public service method and dispatches them to the doxxx methods defined in this class.
 void service(ServletRequest req, ServletResponse res)
          Dispatches client requests to the protected service method.
 
Methods inherited from class javax.servlet.GenericServlet
destroy, getInitParameter, getInitParameterNames, getServletConfig, getServletContext, getServletInfo, init, init, log, log
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

HttpServlet

public HttpServlet()
Does nothing, because this is an abstract class.
Method Detail

doGet

protected void doGet(HttpServletRequest req,
                     HttpServletResponse resp)
              throws ServletException,
                     java.io.IOException
Receives an HTTP GET request from the protected service method and handles the request. The GET method allows a client to read information from the Web server, passing a query string appended to an URL to tell the server what information to return.

Overriding this method to support a GET request also automatically supports an HTTP HEAD request. A HEAD request is a GET request that returns no body in the response, only the request header fields.

If you override this method, you should read data from the request, set entity headers in the response, access the writer or output stream object, and finally, write the response data. When you set headers, be sure to include content type and encoding. If you use a PrintWriter object to return the response, you must set the content type before you access the PrintWriter object.

The servlet engine must write the headers before the response data, because the headers can be flushed at any time after the data is written.

If you can set the Content-Length header (with the javax.servlet.ServletResponse.#contentType method), the servlet can use a persistent connection to return its response to the client, improving performance dramatically. If you cannot set Content-Length, you can sometimes avoid the performance penalty if the response fits in an internal buffer.

The GET method should be safe, that is, without any side effects for which users are held responsible. For example, most form queries have no side effects. If a client request is intended to change stored data, the request should use some other HTTP method.

The GET method should also be idempotent, meaning that it can be safely repeated. Sometimes making a method safe also makes it idempotent. For example, repeating queries is both safe and idempotent, but buying a product online or modifying data is neither safe nor idempotent.

If the request is incorrectly formatted, doGet returns an HTTP BAD_REQUEST message.

Parameters:
req - an HttpServletRequest object that contains the request the client has made of the servlet
resp - an HttpServletResponse object that contains the response the servlet sends to the object
Throws:
java.io.IOException - if an input or output error is detected when the servlet handles the GET request
ServletException - if the request for the GET could not be handled
See Also:
ServletResponse.setContentType(java.lang.String)

getLastModified

protected long getLastModified(HttpServletRequest req)
Returns the time the HttpServletRequest object was last modified, in milliseconds since midnight January 1, 1970 GMT. If the time is unknown, this method returns a negative number.

Servlet engines that support HTTP GET requests should override getLastModified to provide an accurate object modification time. This makes browser and proxy caches work more effectively, reducing the load on server and network resources.

Parameters:
req - the HttpServletRequest object that is sent to the servlet
Returns:
a long integer specifying the time the HttpServletRequest object was last modified, in milliseconds since midnight, January 1, 1970 GMT, or -1 if the time is not known

doPost

protected void doPost(HttpServletRequest req,
                      HttpServletResponse resp)
               throws ServletException,
                      java.io.IOException
Receives an HTTP POST request from the protected service method and handles the request. The HTTP POST method allows the client to send data of unlimited length to the Web server once and is useful when posting information such as credit card numbers.

If you override this method, you should read data from the HttpServletRequest object, set headers for the response (including Content-Type and Content-Encoding), access a PrintWriter or output stream object, and then write any response data using a ServletOutputStream object.

If you use a PrintWriter object to write response data, set the Content-Type header before you access the PrintWriter object. The servlet engine must write the headers before the the response data, because the headers can be flushed at any time after the servlet engine begins to write the body of the response.

If you use HTTP 1.1 chunked encoding (which means that the response has a Transfer-Encoding header), do not set the Content-Length header. If you do not use chunked encoding, set the content length to allow the servlet to take advantage of the HTTP "connection keep alive" feature, If you cannot set the content length and therefore cannot use "keep alive," you may be able to avoid the performance penalty if the response fits in an internal buffer.

This method does not need to be either safe or idempotent. Operations requested through POST can have side effects for which the user can be held accountable, for example, updating stored data or buying items online.

If the HTTP POST request is incorrectly formatted, doPost returns an HTTP BAD_REQUEST message.

Parameters:
req - an HttpServletRequest object that contains the request the client has made of the servlet
resp - an HttpServletResponse object that contains the response the servlet sends to the object
Throws:
java.io.IOException - if an input or output error is detected when the servlet handles the request
ServletException - if the request for the POST could not be handled
See Also:
ServletOutputStream, ServletResponse.setContentType(java.lang.String)

doPut

protected void doPut(HttpServletRequest req,
                     HttpServletResponse resp)
              throws ServletException,
                     java.io.IOException
Receives an HTTP PUT request from the protected service method and handles the request. The PUT operation allows a client to place a file on the server and is similar to sending a file by FTP.

If you override this method, your method must leave intact any content headers sent with the request (including Content-Length, Content-Type, Content-Transfer-Encoding, Content-Encoding, Content-Base, Content-Language, Content-Location, Content-MD5, and Content-Range). If your method cannot handle a content header, it must issue an error message (HTTP 501 - Not Implemented) and discard the request. For more information on HTTP 1.1, see RFC 2068 .

This method does not need to be either safe or idempotent. Operations that doPut performs can have side effects for which the user can be held accountable. If you override this method, you may want to save a copy of the affected URL in temporary storage.

If the HTTP PUT request is incorrectly formatted, doPut returns an HTTP BAD_REQUEST message.

Parameters:
req - the HttpServletRequest object that contains the request the client made of the servlet
resp - the HttpServletResponse object that contains the response the servlet returns to the client
Throws:
java.io.IOException - if an input or output error occurs while the servlet is handling the PUT request
ServletException - if the request for the PUT cannot be handled

doDelete

protected void doDelete(HttpServletRequest req,
                        HttpServletResponse resp)
                 throws ServletException,
                        java.io.IOException
Receives an HTTP DELETE request from the protected service method and handles the request. The DELETE operation allows a client to remove a document or Web page from the server.

This method does not need to be either safe or idempotent. Operations requested through DELETE can have side effects for which users can be held accountable. If you override this method, you may want to save a copy of the affected page in temporary storage.

If the HTTP DELETE request is incorrectly formatted, doDelete returns an HTTP BAD_REQUEST message.

Parameters:
req - the HttpServletRequest object that contains the request the client made of the servlet
resp - the HttpServletResponse object that contains the response the servlet returns to the client
Throws:
java.io.IOException - if an input or output error occurs while the servlet is handling the DELETE request
ServletException - if the request for the DELETE cannot be handled

doOptions

protected void doOptions(HttpServletRequest req,
                         HttpServletResponse resp)
                  throws ServletException,
                         java.io.IOException
Receives an HTTP OPTIONS request from the protected service method and handles the request. The OPTIONS request determines which HTTP methods the server supports and returns an appropriate header. For example, if a servlet overrides doGet, this method returns the following header:

Allow: GET, HEAD, TRACE, OPTIONS

You do not need to override this method unless your servlet implements new HTTP methods, beyond those implemented by HTTP 1.1.

Parameters:
req - the HttpServletRequest object that contains the request the client made of the servlet
resp - the HttpServletResponse object that contains the response the servlet returns to the client
Throws:
java.io.IOException - if an input or output error occurs while the servlet is handling the OPTIONS request
ServletException - if the request for the OPTIONS cannot be handled

doTrace

protected void doTrace(HttpServletRequest req,
                       HttpServletResponse resp)
                throws ServletException,
                       java.io.IOException
Receives an HTTP TRACE request from the protected service method and handles the request. HTTP TRACE returns the headers sent with the TRACE request to the client, so that they can be used in debugging. You do not usually need to override this method.
Parameters:
req - the HttpServletRequest object that contains the request the client made of the servlet
resp - the HttpServletResponse object that contains the response the servlet returns to the client
Throws:
java.io.IOException - if an input or output error occurs while the servlet is handling the TRACE request
ServletException - if the request for the TRACE cannot be handled

service

protected void service(HttpServletRequest req,
                       HttpServletResponse resp)
                throws ServletException,
                       java.io.IOException
Receives standard HTTP requests from the public service method and dispatches them to the doxxx methods defined in this class. This method is an HTTP-specific version of the javax.servlet.Servlet.service method. You will probably not need to override this method.
Parameters:
req - the HttpServletRequest object that contains the request the client made of the servlet
resp - the HttpServletResponse object that contains the response the servlet returns to the client
Throws:
java.io.IOException - if an input or output error occurs while the servlet is handling the TRACE request
ServletException - if the request for the TRACE cannot be handled
See Also:
Servlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)

service

public void service(ServletRequest req,
                    ServletResponse res)
             throws ServletException,
                    java.io.IOException
Dispatches client requests to the protected service method. You will usually not override this method.
Parameters:
req - the HttpServletRequest object that contains the request the client made of the servlet
resp - the HttpServletResponse object that contains the response the servlet returns to the client
Throws:
java.io.IOException - if an input or output error occurs while the servlet is handling the TRACE request
ServletException - if the request for the TRACE cannot be handled
Overrides:
service in class GenericServlet
See Also:
Servlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)