Servlet Basic Tutorial

Servlet Tutorial - Introduction to Servlets

Some Example Applications

A few of the many applications for servlets include,

• Processing data POSTed over HTTPS using an HTML form, including purchase order or credit card data. A servlet like this could be part of an order-entry and processing system, working with product and inventory databases, and perhaps an on-line payment system.
• Allowing collaboration between people. A servlet can handle multiple requests concurrently; they can synchronize requests to support systems such as on-line conferencing.
• Forwarding requests. Servlets can forward requests to other servers and servlets. This allows them to be used to balance load among several servers that mirror the same content. It also allows them to be used to partition a single logical service over several servers, according to task type or organizational boundaries.

The central abstraction in the Servlet API is the Servlet interface. All servlets implement this interface, either directly or, more commonly, by extending a class that implements it such as HttpServlet. The inheritance hierarchy looks as follows.

The Servlet interface provides the following methods that manage the servlet and its communications with clients.

• destroy()
  Cleans up whatever resources are being held and makes sure that any persistent state is synchronized with the servlet's current in-memory state.

• getServletConfig()
  Returns a servlet config object, which contains any initialization parameters and startup configuration for this servlet.

• getServletInfo()
  Returns a string containing information about the servlet, such as its author, version, and copyright.

• init(ServletConfig)
  Initializes the servlet. Run once before any requests can be serviced.

• service(ServletRequest, ServletResponse)
  Carries out a single request from the client.

When a servlet accepts a service call from a client, it receives two objects, ServletRequest and ServletResponse. The ServletRequest class encapsulates the communication from the client to the server, while the ServletResponse class encapsulates the communication from the servlet back to the client.

The ServletRequest interface allows the servlet access to information such as the names of the parameters passed in by the client, the protocol (scheme) being used by the client, and the names of the remote host that made the request and the server that received it. It also provides the servlet with access to the input stream, ServletInputStream, through which the servlet gets data from clients that are using application protocols such as the HTTP POST and PUT methods. Subclasses of ServletRequest allow the servlet to retrieve more protocol-specific data. For example, HttpServletRequest contains methods for accessing HTTP-specific header information.

The ServletResponse interface gives the servlet methods for replying to the client. It allows the servlet to set the content length and mime type of the reply, and provides an output stream, ServletOutputStream, and a Writer through which the servlet can send the reply data. Subclasses of ServletResponse give the servlet more protocol-specific capabilities. For example, HttpServletResponse contains methods that allow the servlet to manipulate HTTP-specific header information.

Servers load and run servlets, which then accept zero or more requests from clients and return data to them. They can also remove servlets. These are the steps of a servlets lifecycle. The next paragraphs describe each step in more detail, concentrating on concurrency issues.

When a server loads a servlet, it runs the servlet's init method. Even though most servlets are run in multi-threaded servers, there are no concurrency issues during servlet initialization. This is because the server calls the init method once, when it loads the servlet, and will not call it again unless it is reloading the servlet. The server can not reload a servlet until after it has removed the servlet by calling the destroy method. Initialization is allowed to complete before client requests are handled (that is, before the service method is called) or the servlet is destroyed.

After the server loads and initializes the servlet, the servlet is able to handle client requests. It processes them in its service method. Each client's request has its call to the service method run in its own servlet thread: the method receives the client's request, and sends the client its response.

Servlets can run multiple service methods at a time. It is important, therefore, that service methods be written in a thread-safe manner. If, for some reason, a server should not run multiple service methods concurrently, the servlet should implement the SingleThreadModel interface. This interface guarantees that no two threads will execute the servlet's service methods concurrently.

Servlets run until they are removed from the service. When a server removes a servlet, it runs the servlet's destroy method. The method is run once; the server will not run it again until after it reloads and reinitializes the servlet.

The diagram below shows the basic interactions between clients, a web server, and a servlet registered with the web server.