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Trail: Servlets
Lesson: The Servlet Life Cycle

## Initializing a Servlet

> The default initialization of the `HttpServlet` class initializes the servlet and logs the initialization. To add initialization specific to your servlet, override the `init` method. When you override the `init` method, follow these rules:

> - If an initialization error occurs that renders the servlet incapable of handling client requests, throw an `UnavailableException`.

>   An example of this type of error is the inability to establish a required network connection.

> - Do not call the `System.exit` method

> Here is an example `init` method:

> >     public class BookDetailServlet ... {

> >         public void init() throws ServletException {
> >             BookDBFrontEnd bookDBFrontEnd =
> >                 (BookDBFrontEnd)getServletContext().getAttribute(
> >                     "examples.bookstore.database.BookDBFrontEnd");

> >             if (bookDBFrontEnd == null) {
> >                 getServletContext().setAttribute(
> >                     "examples.bookstore.database.BookDBFrontEnd",
> >                     BookDBFrontEnd.instance());
> >             }
> >             ...
> >         }

> > The `init` method is straightforward: it tries to get an attribute, and if the attribute does not yet have a value, it provides one.

> > There are other initialization tasks that a servlet might perform. If a servlet used a database, for example, the `init` method could try to open a connection and throw the `UnavailableException` if it was unsuccessful. Here is pseudo-code for what that `init` method might look like:

> >         public class DBServlet ... {

> >             Connection connection = null;

> >             public void init() throws ServletException {

> >                 // Open a database connection to prepare for requests
> >                 try {
> >                     databaseUrl = getInitParameter("databaseUrl");
> >                     ... // get user and password parameters the same way
> >                     connection = DriverManager.getConnection(databaseUrl,
> >                                                              user, password);
> >                 } catch(Exception e) {
> >                     throw new UnavailableException (this,
> >     		    "Could not open a connection to the database");
> >                 }
> >             }
> >             ...
> >         }

> ### Initialization Parameters

> The second version of the `init` method calls the `getInitParameter` method. This method takes the parameter name as an argument and returns a `String` representation of the parameter's value.

> The specification of initialization parameters is servlet engine specific. For example, the parameters are specified with the `init-param` element in a Java Servlet 2.2 Web application deployment descriptor when a servlet is run with Tomcat and a property when a servlet is run with the JSDK2.1 server. The Utilities for Running Servlets lesson contains a general explanation of how to configure servlet engines.

> If, for some reason, you need to get the parameter names, use the `getParameterNames` method.

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