java.lang.Object | +--javax.servlet.GenericServlet | +--javax.servlet.http.HttpServlet
Provides an abstract class to be subclassed to create an HTTP servlet suitable for a Web site. A subclass ofHttpServlet must override at least one method, usually one of these:
doGet, if the servlet supports HTTP GET requestsdoPost, for HTTP POST requestsdoPut, for HTTP PUT requestsdoDelete, for HTTP DELETE requestsinit anddestroy, to manage resources that are held for the life of the servletgetServletInfo, which the servlet uses to provide information about itselfThere's almost no reason to override theservice method.service handles standard HTTP requests by dispatching them to the handler methods for each HTTP request type (thedoXXX methods listed above).
Likewise, there's almost no reason to override thedoOptions anddoTrace 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.
HttpServlet()Does nothing, because this is an abstract class. | |
doDelete(HttpServletRequest req,HttpServletResponse resp)Called by the server (via the service method) to allow a servlet to handle a DELETE request. | |
doGet(HttpServletRequest req,HttpServletResponse resp)Called by the server (via the service method) to allow a servlet to handle a GET request. | |
doHead(HttpServletRequest req,HttpServletResponse resp)Receives an HTTP HEAD request from the protected service method and handles the request. | |
doOptions(HttpServletRequest req,HttpServletResponse resp)Called by the server (via the service method) to allow a servlet to handle a OPTIONS request. | |
doPost(HttpServletRequest req,HttpServletResponse resp)Called by the server (via the service method) to allow a servlet to handle a POST request. | |
doPut(HttpServletRequest req,HttpServletResponse resp)Called by the server (via the service method) to allow a servlet to handle a PUT request. | |
doTrace(HttpServletRequest req,HttpServletResponse resp)Called by the server (via the service method) to allow a servlet to handle a TRACE request. | |
getLastModified(HttpServletRequest req)Returns the time the HttpServletRequest object was last modified, in milliseconds since midnight January 1, 1970 GMT. | |
service(HttpServletRequest req,HttpServletResponse resp)Receives standard HTTP requests from the public service method and dispatches them to thedoXXX methods defined in this class. | |
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 |
publicHttpServlet()
protected voiddoGet(HttpServletRequest req,HttpServletResponse resp) throwsServletException, java.io.IOException
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 aPrintWriter 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 theServletResponse.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.
req - anHttpServletRequest object thatcontains the request the client has madeof the servletresp - anHttpServletResponse object thatcontains the response the servlet sendsto the clientjava.io.IOException - if an input or output error is detected when the servlet handlesthe GET requestServletException - if the request for the GETcould not be handledServletResponse.setContentType(java.lang.String)protected longgetLastModified(HttpServletRequest req)
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 - theHttpServletRequest object that is sent to the servletlong integer specifyingthe time theHttpServletRequestobject was last modified, in millisecondssince midnight, January 1, 1970 GMT, or-1 if the time is not knownprotected voiddoHead(HttpServletRequest req,HttpServletResponse resp) throwsServletException, java.io.IOException
Receives an HTTP HEAD request from the protectedservice 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 thedoHead 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.
req - the request object that is passedto the servletresp - the response object that the servletuses to return the headers to the clienjava.io.IOException - if an input or output error occursServletException - if the request for the HEADcould not be handledprotected voiddoPost(HttpServletRequest req,HttpServletResponse resp) throwsServletException, java.io.IOException
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 aPrintWriter 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 theServletResponse.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.
req - anHttpServletRequest object thatcontains the request the client has madeof the servletresp - anHttpServletResponse object thatcontains the response the servlet sendsto the clientjava.io.IOException - if an input or output error is detected when the servlet handlesthe requestServletException - if the request for the POSTcould not be handledServletOutputStream,ServletResponse.setContentType(java.lang.String)protected voiddoPut(HttpServletRequest req,HttpServletResponse resp) throwsServletException, java.io.IOException
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, seeRFC 2068.
This method does not need to be either safe or idempotent. Operations thatdoPut 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 - theHttpServletRequest object thatcontains the request the client made ofthe servletresp - theHttpServletResponse object thatcontains the response the servlet returnsto the clientjava.io.IOException - if an input or output error occurswhile the servlet is handling thePUT requestServletException - if the request for the PUTcannot be handledprotected voiddoDelete(HttpServletRequest req,HttpServletResponse resp) throwsServletException, java.io.IOException
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.
req - theHttpServletRequest object thatcontains the request the client made ofthe servletresp - theHttpServletResponse object thatcontains the response the servlet returnsto the clientjava.io.IOException - if an input or output error occurswhile the servlet is handling theDELETE requestServletException - if the request for theDELETE cannot be handledprotected voiddoOptions(HttpServletRequest req,HttpServletResponse resp) throwsServletException, java.io.IOException
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.
req - theHttpServletRequest object thatcontains the request the client made ofthe servletresp - theHttpServletResponse object thatcontains the response the servlet returnsto the clientjava.io.IOException - if an input or output error occurswhile the servlet is handling theOPTIONS requestServletException - if the request for theOPTIONS cannot be handledprotected voiddoTrace(HttpServletRequest req,HttpServletResponse resp) throwsServletException, java.io.IOException
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.req - theHttpServletRequest object thatcontains the request the client made ofthe servletresp - theHttpServletResponse object thatcontains the response the servlet returnsto the clientjava.io.IOException - if an input or output error occurswhile the servlet is handling theTRACE requestServletException - if the request for theTRACE cannot be handledprotected voidservice(HttpServletRequest req,HttpServletResponse resp) throwsServletException, java.io.IOException
service method and dispatches them to thedoXXX 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.req - theHttpServletRequest object thatcontains the request the client made ofthe servletresp - theHttpServletResponse object thatcontains the response the servlet returnsto the clientjava.io.IOException - if an input or output error occurswhile the servlet is handling theTRACE requestServletException - if the request for theTRACE cannot be handledServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)public voidservice(ServletRequest req,ServletResponse res) throwsServletException, java.io.IOException
service method. There's no need to override this method.service in classGenericServletreq - theHttpServletRequest object thatcontains the request the client made ofthe servletresp - theHttpServletResponse object thatcontains the response the servlet returnsto the clientjava.io.IOException - if an input or output error occurswhile the servlet is handling theTRACE requestServletException - if the request for theTRACE cannot be handledServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)