The Anatomy of a Servlet

As you just saw in HelloWorldServlet, a servlet is a Java class that implements a few important methods. You can choose to implement these methods yourself or to create a subclass of an existing servlet class that already implements them. The Servlet interface defines the methods that are required for a Java class to become a servlet. The interface definition is shown in Listing 2.3.

Listing 2.3 The Definition of the Servlet Interface

package javax.servlet;

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

Most of the time, you will create a servlet by subclassing either GenericServlet or HttpServlet. Both of these classes implement the Servlet interface, but they provide a few handy features that make them preferable to implementing the Servlet interface yourself.

The service Method

The heart of any servlet is the service method. As you just learned, the servlet container calls the service method to handle each request from a browser. By using HttpServlet, we avoided having to implement the service method because HttpServlet did it for us. However, this method is one you should know about. When calling the service method, the container passes in references to two objects: ServletRequest contains information about the request that invoked the servlet, and ServletResponse provides the mechanisms for sending back a response.

The init Method

Many times, a servlet needs to perform some initialization one time before it begins to handle requests. The init method in a servlet is called just after the servlet is first loaded, but before it begins to handle requests. Listing 2.4 shows a simple init method that initializes a database connection.

Listing 2.4 init Method from JDBCServlet.java

protected Connection conn;

public void init()
{
    try
    {
// Make sure the JdbcOdbcDriver class is loaded.
        Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

// Try to connect to a database via ODBC.
        conn = DriverManager.getConnection(
            "jdbc:odbc:examples");
    }
    catch (Exception exc)
    {
// If there's an error, use the servlet logging API.
        getServletContext().log(
            "Error making JDBC connection: ", exc);
    }
}

Notice that the init method in Listing 2.4, unlike the init method in the Servlet interface, does not take a parameter. One of the convenient features of the GenericServlet and HttpServlet classes is that they have an alternate version of init that doesn't take any parameters. In case you're wondering why it even matters, the init method in the Servlet interface takes a ServletConfig object as a parameter. The servlet is then responsible for keeping track of the ServletConfig object. The GenericServlet and HttpServlet classes perform this housekeeping chore and then provide the parameterless init method for you to do any servlet-specific initialization.

The destroy Method

Sometimes, the servlet container decides that it doesn't need to keep your servlet loaded anymore. This could happen automatically, or as the result of you deactivating the servlet with an administration tool. Before the servlet container unloads your servlet, it calls the destroy method to enable the servlet to perform any necessary cleanup. The cleanup usually involves closing database connections, open files, or network connections. Listing 2.5 shows the destroy method that is a companion to the init method in Listing 2.4.

Listing 2.5 destroy Method from JDBCServlet.java

public void destroy()
{
    try
    {
// Try to close the connection only if it's non-null.
        if (conn != null)
        {
            conn.close();
        }
    }
    catch (SQLException exc)
    {
// If there's an error, use the servlet logging API.
        getServletContext().log(
            "Error closing JDBC connection: ", exc);
    }
}

The getServletInfo and getServletConfig Methods

If you are subclassing GenericServlet or HttpServlet, you probably don't need to override the getServletInfo or getServletConfig methods. The Servlet API documentation recommends that you return information such as the author, version, and copyright from the getServletInfo method. Although there is no specific format for the string returned by the method, you should return only plain text, without any HTML or XML tags embedded within it.

The getServletConfig method returns the ServletConfig object that was passed to the servlet in the init method. Unless you are keeping track of the config object yourself, your best bet is to leave this method alone and let the superclass handle it.