Tomasz Dziurko

Today I am going to present the most important methods, classes and interfaces in the Servlet API.

public interface Servlet

Main interface from which all starts. It defines key methods for the life cycle of servlet:

public void init(ServletConfig config) throws ServletException;
public ServletConfig getServletConfig();
public void service(ServletRequest req, ServletResponse res) throws ServletException, java.io.IOException;

public void destroy();

and one additional method

public String getServletInfo();

which should return information about servlet (version, author, resposibility, etc.)

public abstract class GenericServlet implements Servlet, ServletConfig

Abstract class implementing all Servlet interface methods, except method service (), which is implemented by a class one step  lower in the hierarchy: HttpServlet. Additionally GenericServlet also provides methods from ServletConfig interface allowing developer to read init parameters of servlet

public String getInitParameter(java.lang.String name) { }
public Enumeration getInitParameterNames() { }

These parameters are defined in the web.xml file by using the elements <init-param> nested in the declaration of a servlet:

<web-app>
  <servlet>
    <servlet-name>FirstToLoadServlet</servlet-name>
    <servlet-class>pl.tdziurko.scwcd.servlets.FirstToLoadServlet</servlet-class>
    <init-param>
      <param-name>initParameter</param-name>
      <param-value>initValue</param-value>
    </init-param>
    <init-param>
      <param-name>anotherInitParameter</param-name>
      <param-value>anotherInitValue</param-value>
    </init-param>
  </servlet>
  <servlet-mapping>
    <servlet-name>FirstToLoadServlet</servlet-name>
    <url-pattern>/FirstToLoadServlet</url-pattern>
  </servlet-mapping>
</web-app>

Another method of the interface ServletConfig

public ServletContext getServletContext () {}

returns the context in which servlet is running. From this context we can get information about server version and many other things which I don’t cover in this episode of “Preparing to SCWCD” series

The final method in class GenericServlet worth mentioning is

public void init () throws ServletException {}

This method is added for the developers convenience. Rather than overloading the init (ServletConfig config) and remember to call super.init (config) , we can put our init logic in servlet initialization method init (), which is called by the init (ServletConfig config).

public abstract class HttpServlet extends GenericServlet

It’s the class, which will be extended by all our servlets. It has methods for handling HTTP requests:

protected void doGet (HttpServletRequest req, HttpServletResponse resp) throws ServletException, java.io.IOException {}

and its brothers working with other HTTP protocol requests: doPost, doHead, doOptions, doPut, doDelete and doTrace.

The two most important methods in this class are:

public void service (ServletRequest req, ServletResponse res) throws ServletException, java.io.IOException {}
protected void service (HttpServletRequest req, HttpServletResponse resp) {}

The first one (public one) is called by the container and redirects call to second one (protected), which, depending on the type of request calls one of doXXX() method.

Be Sociable, Share!

•