javax.servlet.Servlet 回顾

The javax.servlet.Servlet Interface

　　Servlet programming is made possible through the classes and interfaces in two packages: javax.servlet and javax.servlet.http. Of those classes and interfaces, the javax.servlet.Servlet interface is of the utmost importance. All servlets must implement this interface or extend a class that does.

　　The Servlet interface has five methods whose signatures are as follows.

public void init(ServletConfig config) throws ServletException;
public void service(ServletRequest request, ServletResponse response) throws ServletException, java.io.IOException;
public void destroy();
public ServletConfig getServletConfig(); 
public java.lang.String getServletInfo();

　　Of the five methods in Servlet, the init, service, and destroy methods are the servlet's life cycle methods. The init method is called by the servlet container after the servlet class has been instantiated. The servlet container calls this method exactly once to indicate to the servlet that the servlet is being placed into service. The init method must complete successfully before the servlet can receive any requests. A servlet programmer can override this method to write initialization code that needs to run only once, such as loading a database driver, initializing values, and so on. In other cases, this method is normally left blank.

　　The servlet container calls the service method of a servlet whenever there is a request for the servlet. The servlet container passes a javax.servlet.ServletRequest object and a javax.servlet.ServletResponse object. The ServletRequest object contains the client's HTTP request information and the ServletResponse object encapsulates the servlet's response. The service method is invoked many times during the life of the servlet.

　　The servlet container calls the destroy method before removing a servlet instance from service. This normally happens when the servlet container is shut down or the servlet container needs some free memory. This method is called only after all threads within the servlet's service method have exited or after a timeout period has passed. After the servlet container has called the destroy method, it will not call the service method again on the same servlet. The destroy method gives the servlet an opportunity to clean up any resources that are being held, such as memory, file handles, and threads, and make sure that any persistent state is synchronized with the servlet's current state in memory.