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 and destroy,
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 doXXX
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.
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 the
PrintWriter 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
javax.servlet.ServletResponse.setContentLength(int)
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.
req an HttpServletRequestresp an HttpServletResponsejava.io.IOException if an input or output error is
detected when the servlet handles
the GET requestjavax.servlet.ServletException if the request for the GET
could not be handledjavax.servlet.ServletResponse.setContentType(java.lang.String)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.
req the HttpServletRequest
object that is sent to the servletlong 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 knownservice 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 the PrintWriter 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
javax.servlet.ServletResponse.setContentLength(int)
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.
req an HttpServletRequestresp an HttpServletResponsejava.io.IOException if an input or output error is
detected when the servlet handles
the requestjavax.servlet.ServletException if the request for the POST
could not be handledjavax.servlet.ServletOutputStreamjavax.servlet.ServletResponse.setContentType(java.lang.String)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.
req the HttpServletRequestresp the HttpServletResponsejava.io.IOException if an input or output error occurs
while the servlet is handling the
PUT requestjavax.servlet.ServletException if the request for the PUT
cannot be handledservice 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.
req the HttpServletRequestresp the HttpServletResponsejava.io.IOException if an input or output error occurs
while the servlet is handling the
DELETE requestjavax.servlet.ServletException if the request for the
DELETE cannot be handledservice 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
overrides doGet, 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.
req the HttpServletRequestresp the HttpServletResponsejava.io.IOException if an input or output error occurs
while the servlet is handling the
OPTIONS requestjavax.servlet.ServletException if the request for the
OPTIONS cannot be handledservice 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.
req the HttpServletRequestresp the HttpServletResponsejava.io.IOException if an input or output error occurs
while the servlet is handling the
TRACE requestjavax.servlet.ServletException if the request for the
TRACE cannot be handledservice 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(javax.servlet.ServletRequest,javax.servlet.ServletResponse)req the HttpServletRequestresp the HttpServletResponsejava.io.IOException if an input or output error occurs
while the servlet is handling the
TRACE requestjavax.servlet.ServletException if the request for the
TRACE cannot be handledjavax.servlet.Servlet.service(javax.servlet.ServletRequest,javax.servlet.ServletResponse)service method. There's no need to
override this method.
req the HttpServletRequestresp the HttpServletResponsejava.io.IOException if an input or output error occurs
while the servlet is handling the
TRACE requestjavax.servlet.ServletException if the request for the
TRACE cannot be handledjavax.servlet.Servlet.service(javax.servlet.ServletRequest,javax.servlet.ServletResponse)