ServletContext vs ServletConfig

SerlvetConfig-
ServletConfig is a interface which is implemented by GenericServlet, and this Servlet or HttpServlet (sub class of GenericServlet) can be used to create your own Servlet class. Single servlet configuration passed by Servlet container to init() method to initialize some parameter on Servlet.

These parameter is defined in descriptor file (web.xml).<init-param> tag will be used under <servlet> tag.This Config object is public to a servlet and scope will be, as long as a servlet is executing, ServletConfig object will be available, it will be destroyed once the servlet execution is completed.

For example-

Web.xml file-

<servlet>
 <servlet-name>servlet-config</servlet-name>
 <servlet-class>com.test.MyServlet</servlet-class>
 <init-param>
  <param-name>name</param-name>
  <param-value>XXXXXX</param-value>
 </init-param>
</servlet>

MyServlet.java file- 

public void init(ServletConfig config) throws ServletException
{
    ServletContext sc = getServletContext();

    getServletContext().getInitParameter("name"); 
}

Now the question is when is difference between int() method and int(ServletConfig) method.

public void init(ServletConfig config)throws ServletException
This method Called by the servlet container to indicate to a servlet that the servlet is being placed into service.See Servlet#init. This implementation stores the ServletConfig object it receives from the servlet container for later use. When overriding this form of the method, call super.init(config).

public void init() throws ServletException
A convenience method which can be overridden so that there's no need to call super.init(config).Instead of overriding init(ServletConfig), simply override this method and it will be called by GenericServlet.init(ServletConfig config). The ServletConfig object can still be retrieved via getServletConfig().

ServletContext-
ServletContext is implemented by the servlet container for all servlet to communicate with its servlet container.This object is contained within the ServletConfig object. And to access the ServletContext  use ServletConfig object within a servlet. This again is specified in descriptor file (web.xml) under <context-param> tags, as this is not specific to specific servlet so, present globally i.e. outside of <servlet> tag.Mainly This is used to access common information among servlet, which is store in <context-param> tag.

ServletContext object is global to entire web application. Object of ServletContext will be created at the time of web application deployment.And scope will be, as long as web application is executing, ServletContext object will be available, and it will be destroyed once the application is removed from the server.ServletContext object will be available even before giving the first request.

For examplet-
Web.xml file

<context-param>
    <param-name>globalVariable</param-name>
    <param-value>Hello To all servlet</param-value>
</context-param> 

MyServlet.java file

public void doGet(HttpServletRequest request, HttpServletResponse response)
 throws IOException{
  PrintWriter pw = response.getWriter();
        pw.println(getServletContext().getInitParameter("globalVariable"));
}