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 javax.servlet.GenericServlet or an HTTP servlet that extends javax.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.

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 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

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.

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. More information on multithreaded programming in Java is available in the Java tutorial on multi-threaded programming.

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

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.