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Interface javax.servlet.Servlet

public interface Servlet
This interface is for developing servlets. A servlet is a body of Java code that is loaded into and runs inside a network service, such as a web server. It receives and responds to requests from clients. For example, a client may need information from a database; a servlet can be written that receives the request, gets and processes the data as needed by the client, and then returns it to the client.

All servlets implement this interface. Servlet writers typically do this by subclassing either GenericServlet, which implements the Servlet interface, or by subclassing GenericServlet's descendent, HttpServlet. Developers need to directly implement this interface only if their servlets cannot (or choose not to) inherit from GenericServlet or HttpServlet. For example, RMI or CORBA objects that act as servlets will directly implement this interface.

The Servlet interface defines methods to initialize a servlet, to receive and respond to client requests, and to destroy a servlet and its resources. These are known as life-cycle methods, and are called by the network service in the following manner:

  1. Servlet is created then initialized.
  2. Zero or more service calls from clients are handled
  3. Servlet is destroyed then garbage collected and finalized
Initializing a servlet involves doing any expensive one-time setup, such as loading configuration data from files or starting helper threads. Service calls from clients are handled using a request and response paradigm. They rely on the underlying network transport to provide quality of service guarantees, such as reordering, duplication, message integrity, privacy, etc. Destroying a servlet involves undoing any initialization work and synchronizing persistent state with the current in-memory state.

In addition to the life-cycle methods, the Servlet interface provides for a method for the servlet to use to get any startup information, and a method that allows the servlet to return basic information about itself, such as its author, version and copyright.

See Also:
GenericServlet, HttpServlet

Method Index

o destroy()
Cleans up whatever resources are being held (e.g., memory, file handles, threads) and makes sure that any persistent state is synchronized with the servlet's current in-memory state.
o getServletConfig()
Returns a servlet config object, which contains any initialization parameters and startup configuration for this servlet.
o getServletInfo()
Returns a string containing information about the servlet, such as its author, version, and copyright.
o init(ServletConfig)
Initializes the servlet.
o service(ServletRequest, ServletResponse)
Carries out a single request from the client.

Methods

o init 
 public abstract void init(ServletConfig config) throws ServletException
Initializes the servlet. The method is called once, automatically, by the network service when it loads the servlet. It is guaranteed to finish before any service requests are accepted. After initialization, the network service does not call the init method again unless it reloads the servlet after it has unloaded and destroyed it.

The init method should save the ServletConfig object so that it can be returned by the getServletConfig method. If a fatal initialization error occurs, the init method should throw an appropriate "UnavailableException" exception. It should never call the method System.exit.

Parameters:
config - object containing the servlet's startup configuration and initialization parameters
Throws: ServletException
if a servlet exception has occurred
See Also:
UnavailableException, getServletConfig
o getServletConfig 
 public abstract ServletConfig getServletConfig()
Returns a servlet config object, which contains any initialization parameters and startup configuration for this servlet. This is the ServletConfig object passed to the init method; the init method should have stored this object so that this method could return it.

See Also:
init
o service 
 public abstract void service(ServletRequest req,
                              ServletResponse res) throws ServletException, IOException
Carries out a single request from the client. The method implements a request and response paradigm. The request object contains information about the service request, including parameters provided by the client. The response object is used to return information to the client. The request and response objects rely on the underlying network transport for quality of service guarantees, such as reordering, duplication, privacy, and authentication.

Service requests are not handled until servlet initialization has completed. Any requests for service that are received during initialization block until it is complete. Note that servlets typically run inside multi-threaded servers; servers can handle multiple service requests simultaneously. It is the servlet writer's responsibility to synchronize access to any shared resources, such as network connections or the servlet's class and instance variables. Information on multi-threaded programming in Java can be found in the Java tutorial on multi-threaded programming.

Parameters:
req - the client's request of the servlet
res - the servlet's response to the client
Throws: ServletException
if a servlet exception has occurred
Throws: IOException
if an I/O exception has occurred
o getServletInfo 
 public abstract String getServletInfo()
Returns a string containing information about the servlet, such as its author, version, and copyright.

o destroy 
 public abstract void destroy()
Cleans up whatever resources are being held (e.g., memory, file handles, threads) and makes sure that any persistent state is synchronized with the servlet's current in-memory state. The method is called once, automatically, by the network service when it unloads the servlet. After destroy is run, it cannot be called again until the network service reloads the servlet.

When the network service removes a servlet, it calls destroy after all service calls have been completed, or a service-specific number of seconds have passed, whichever comes first. In the case of long-running operations, there could be other threads running service requests when destroy is called. The servlet writer is responsible for making sure that any threads still in the service method complete. 

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