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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 to be subclassed to create
an HTTP servlet suitable for a Web site. A subclass of
HttpServlet
must override at least
one method, usually one of these:
-
doGet
, if the servlet supports HTTP GET requests -
doPost
, for HTTP POST requests -
doPut
, for HTTP PUT requests -
doDelete
, for HTTP DELETE requests -
init
anddestroy
, to manage resources that are held for the life of the servlet -
getServletInfo
, which the servlet uses to provide information about itself
There's almost no reason to override the service
method. service
handles standard HTTP
requests by dispatching them to the handler methods
for each HTTP request type (the do
XXX
methods listed above).
Likewise, there's almost no reason to override the
doOptions
and doTrace
methods.
Servlets typically run on multithreaded servers, so be aware that a servlet must handle concurrent requests and be careful to 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.
- See Also:
- Serialized Form
Constructor Summary | |
HttpServlet()
Does nothing, because this is an abstract class. |
Method Summary | |
protected void |
doDelete(HttpServletRequest req,
HttpServletResponse resp)
Called by the server (via the service method)
to allow a servlet to handle a DELETE request. |
protected void |
doGet(HttpServletRequest req,
HttpServletResponse resp)
Called by the server (via the service method) to
allow a servlet to handle a GET request. |
protected void |
doHead(HttpServletRequest req,
HttpServletResponse resp)
Receives an HTTP HEAD request from the protected service method and handles the
request. |
protected void |
doOptions(HttpServletRequest req,
HttpServletResponse resp)
Called by the server (via the service method)
to allow a servlet to handle a OPTIONS request. |
protected void |
doPost(HttpServletRequest req,
HttpServletResponse resp)
Called by the server (via the service method)
to allow a servlet to handle a POST request. |
protected void |
doPut(HttpServletRequest req,
HttpServletResponse resp)
Called by the server (via the service method)
to allow a servlet to handle a PUT request. |
protected void |
doTrace(HttpServletRequest req,
HttpServletResponse resp)
Called by the server (via the service method)
to allow a servlet to handle a TRACE 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 do XXX 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,
getServletName,
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
- Called by the server (via the
service
method) to allow a servlet to handle a GET request.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.
When overriding this method, read the request data, write the response headers, get the response's writer or output stream object, and finally, write the response data. It's best to include content type and encoding. When using a
PrintWriter
object to return the response, set the content type before accessing thePrintWriter
object.The servlet container must write the headers before committing the response, because in HTTP the headers must be sent before the response body.
Where possible, set the Content-Length header (with the
ServletResponse.setContentLength(int)
method), to allow the servlet container to use a persistent connection to return its response to the client, improving performance. The content length is automatically set if the entire response fits inside the response 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
- anHttpServletRequest
object that contains the request the client has made of the servletresp
- anHttpServletResponse
object that contains the response the servlet sends to the client- 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 (the default).Servlets that support HTTP GET requests and can quickly determine their last modification time should override this method. This makes browser and proxy caches work more effectively, reducing the load on server and network resources.
- Parameters:
req
- theHttpServletRequest
object that is sent to the servlet- Returns:
- a
long
integer specifying the time theHttpServletRequest
object was last modified, in milliseconds since midnight, January 1, 1970 GMT, or -1 if the time is not known
doHead
protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, java.io.IOException
Receives an HTTP HEAD request from the protected
service
method and handles the request. The client sends a HEAD request when it wants to see only the headers of a response, such as Content-Type or Content-Length. The HTTP HEAD method counts the output bytes in the response to set the Content-Length header accurately.If you override this method, you can avoid computing the response body and just set the response headers directly to improve performance. Make sure that the
doHead
method you write is both safe and idempotent (that is, protects itself from being called multiple times for one HTTP HEAD request).If the HTTP HEAD request is incorrectly formatted,
doHead
returns an HTTP "Bad Request" message.- Parameters:
req
- the request object that is passed to the servletresp
- the response object that the servlet uses to return the headers to the clien- Throws:
- java.io.IOException - if an input or output error occurs
- ServletException - if the request for the HEAD could not be handled
doPost
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, java.io.IOException
- Called by the server (via the
service
method) to allow a servlet to handle a POST request. The HTTP POST method allows the client to send data of unlimited length to the Web server a single time and is useful when posting information such as credit card numbers.When overriding this method, read the request data, write the response headers, get the response's writer or output stream object, and finally, write the response data. It's best to include content type and encoding. When using a
PrintWriter
object to return the response, set the content type before accessing thePrintWriter
object.The servlet container must write the headers before committing the response, because in HTTP the headers must be sent before the response body.
Where possible, set the Content-Length header (with the
ServletResponse.setContentLength(int)
method), to allow the servlet container to use a persistent connection to return its response to the client, improving performance. The content length is automatically set if the entire response fits inside the response buffer.When using HTTP 1.1 chunked encoding (which means that the response has a Transfer-Encoding header), do not set the Content-Length header.
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
- anHttpServletRequest
object that contains the request the client has made of the servletresp
- anHttpServletResponse
object that contains the response the servlet sends to the client- 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
- Called by the server (via the
service
method) to allow a servlet to handle a PUT request. The PUT operation allows a client to place a file on the server and is similar to sending a file by FTP.When overriding this method, 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. When using this method, it may be useful 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
- theHttpServletRequest
object that contains the request the client made of the servletresp
- theHttpServletResponse
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
- Called by the server (via the
service
method) to allow a servlet to handle a DELETE 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. When using this method, it may be useful to save a copy of the affected URL in temporary storage.
If the HTTP DELETE request is incorrectly formatted,
doDelete
returns an HTTP "Bad Request" message. - Parameters:
req
- theHttpServletRequest
object that contains the request the client made of the servletresp
- theHttpServletResponse
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
- Called by the server (via the
service
method) to allow a servlet to handle a OPTIONS request. The OPTIONS request determines which HTTP methods the server supports and returns an appropriate header. For example, if a servlet overridesdoGet
, this method returns the following header:Allow: GET, HEAD, TRACE, OPTIONS
There's no need to override this method unless the servlet implements new HTTP methods, beyond those implemented by HTTP 1.1.
- Parameters:
req
- theHttpServletRequest
object that contains the request the client made of the servletresp
- theHttpServletResponse
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
- Called by the server (via the
service
method) to allow a servlet to handle a TRACE request. A TRACE returns the headers sent with the TRACE request to the client, so that they can be used in debugging. There's no need to override this method. - Parameters:
req
- theHttpServletRequest
object that contains the request the client made of the servletresp
- theHttpServletResponse
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 thedo
XXX methods defined in this class. This method is an HTTP-specific version of theServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
method. There's no need to override this method. - Parameters:
req
- theHttpServletRequest
object that contains the request the client made of the servletresp
- theHttpServletResponse
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. There's no need to override this method. - Parameters:
req
- theHttpServletRequest
object that contains the request the client made of the servletresp
- theHttpServletResponse
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)
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