Class HttpServlet

  • All Implemented Interfaces:
    java.io.Serializable, Servlet, ServletConfig

    public abstract class HttpServlet
    extends GenericServlet
    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 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 doMethod 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 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 noBodyWriter 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 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.

        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 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 - 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
      • 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 servlet
        resp - the response object that the servlet uses to return the headers to the client
        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 noBodyWriter 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 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 - 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 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 2616 .

        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 - 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
        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 - 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
        Called by the server (via the service method) to allow a servlet to handle an 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.

        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
        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 - 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