Interface Servlet

  • All Known Subinterfaces:
    HttpJspPage, JspPage
    All Known Implementing Classes:
    FacesServlet, GenericServlet, HttpServlet

    public interface Servlet
    Defines methods that all servlets must implement.

    A servlet is a small Java program that runs within a Web server. Servlets receive and respond to requests from Web clients, usually across HTTP, the HyperText Transfer Protocol.

    To implement this interface, you can write a generic servlet that extends jakarta.servlet.GenericServlet or an HTTP servlet that extends jakarta.servlet.http.HttpServlet.

    This interface defines methods to initialize a servlet, to service requests, and to remove a servlet from the server. These are known as life-cycle methods and are called in the following sequence:

    1. The servlet is constructed, then initialized with the init method.
    2. Any calls from clients to the service method are handled.
    3. The servlet is taken out of service, then destroyed with the destroy method, then garbage collected and finalized.

    In addition to the life-cycle methods, this interface provides the getServletConfig method, which the servlet can use to get any startup information, and the getServletInfo method, which allows the servlet to return basic information about itself, such as author, version, and copyright.

    See Also:
    GenericServlet, HttpServlet
    • Method Detail

      • init

        void init​(ServletConfig config)
           throws ServletException
        Called by the servlet container to indicate to a servlet that the servlet is being placed into service.

        The servlet container calls the init method exactly once after instantiating the servlet. The init method must complete successfully before the servlet can receive any requests. The container will ensure that actions performed in the init method will be visible to any threads that subsequently call the service method according to the rules in JSR-133 (i.e. there is a 'happens before' relationship between init and service).

        The servlet container cannot place the servlet into service if the init method

        1. Throws a ServletException
        2. Does not return within a time period defined by the Web server
        config - a ServletConfig object containing the servlet's configuration and initialization parameters
        ServletException - if an exception has occurred that interferes with the servlet's normal operation
        See Also:
        UnavailableException, getServletConfig()
      • getServletConfig

        ServletConfig getServletConfig()
        Returns a ServletConfig object, which contains initialization and startup parameters for this servlet. The ServletConfig object returned is the one passed to the init method.

        Implementations of this interface are responsible for storing the ServletConfig object so that this method can return it. The GenericServlet class, which implements this interface, already does this.

        the ServletConfig object that initializes this servlet
        See Also:
      • service

        void service​(ServletRequest req,
                     ServletResponse res)
              throws ServletException,
        Called by the servlet container to allow the servlet to respond to a request.

        This method is only called after the servlet's init() method has completed successfully.

        The status code of the response always should be set for a servlet that throws or sends an error.

        Servlets typically run inside multithreaded servlet containers that can handle multiple requests concurrently. Developers must be aware to synchronize access to any shared resources such as files, network connections, and as well as the servlet's class and instance variables.

        req - the ServletRequest object that contains the client's request
        res - the ServletResponse object that contains the servlet's response
        ServletException - if an exception occurs that interferes with the servlet's normal operation
        IOException - if an input or output exception occurs
      • getServletInfo

        String getServletInfo()
        Returns information about the servlet, such as author, version, and copyright.

        The string that this method returns should be plain text and not markup of any kind (such as HTML, XML, etc.).

        a String containing servlet information
      • destroy

        void destroy()
        Called by the servlet container to indicate to a servlet that the servlet is being taken out of service. This method is only called once all threads within the servlet's service method have exited or after a timeout period has passed. After the servlet container calls this method, it will not call the service method again on this servlet.

        This method gives the servlet an opportunity to clean up any resources that are being held (for example, memory, file handles, threads) and make sure that any persistent state is synchronized with the servlet's current state in memory.