servlet 的 init() 方法是做什么用的？

Question

When I decompile the GenericServlet and check the init() , I see the following code .

public void init(ServletConfig servletconfig)
    throws ServletException
{
    config = servletconfig;
    init();
}

public void init()
    throws ServletException
{
}

What is the init method actually doing here ? Am I missing something ?

Answer 1

Yes, it does nothing. It could have been abstract, but then each servlet would be forced to implement it. This way, by default, nothing happens on init() , and each servlet can override this behaviour. For example, you have two servlets:

public PropertiesServlet extends HttpServlet {

   private Properties properties;

   @Override
   public void init() {
       // load properties from disk, do be used by subsequent doGet() calls
   }
}

and

public AnotherServlet extends HttpServlet {

   // you don't need any initialization here, 
   // so you don't override the init method.
}

Answer 2

From the javadoc :

/**
 *
 * A convenience method which can be overridden so that there's no need
 * to call <code>super.init(config)</code>.
 *
 * <p>Instead of overriding {@link #init(ServletConfig)}, simply override
 * this method and it will be called by
 * <code>GenericServlet.init(ServletConfig config)</code>.
 * The <code>ServletConfig</code> object can still be retrieved via {@link
 * #getServletConfig}. 
 *
 * @exception ServletException  if an exception occurs that
 *                  interrupts the servlet's
 *                  normal operation
 *
 */

So it does nothing and is just a convenience.

Answer 3

Constructor might not have access to ServletConfig since container have not called init(ServletConfig config) method.

init() method is inherited from GenericServlet which has a ServletConfig as its property. thats how HttpServlet and what ever custom servlet you write by extending HttpServlet gets ServletConfig .

and GenericServlet implements ServletConfig which has getServletContext method. so your custom servlets init method will have access to both of those.

Answer 4

Servlet container calls servlet init() method before handling client requests. It is called just one times after servlet is created. By default it does nothing. You can override this method and it is also good for performing one-time activities. Such as database connection or reading configuration data etc.

public void init(ServletConfig config) throws ServletException {
     super.init(config);

     // You can define your initial parameter in web.xml file.
     String initialParameter = config.getInitParameter("initialParameter");
     // Do some stuff with initial parameters

}

What happens if init() method throw an exception?

The servlet destroy() will not be called because it is unsuccesfull initialization. Servlet container may try to instantiate and initialize a new instance of this failed servlet later.

Answer 5

Who said init method do nothing. It's use for one time servlet initialization of variable etc. For example you can put PreparedStatement initialization in there. So that the next call of the servlet, there is no need anymore for PS initialization. You must know that init is part of Servlet lifecycle.

The first time the servlet loaded in Glassfish or Tomcat, the init method will be called and servlet stays in memory of the application server. The subsequent call will execute servlet and the init method will not be called. However the variable initialization is there, already initialized.

The init() method simply creates (once only) and loads some data that will be used throughout the life of the servlet.