Is it meaningful to define a constructor for a servlet, and what is the best way to initialize data?

The constructor can be defined in a servlet, but it will not be called by the servlet container during initialization.

Initialization of data in a servlet should be done using the init() method. This method is called once by the servlet container after the servlet instance is created and before processing requests. You can obtain initialization parameters from the deployment descriptor (web.xml) or using annotations.

Example of initialization:

// Example servlet
public class MyServlet extends HttpServlet {

    private String initialData;

    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config); // Mandatory call to parent init method

        // Getting init parameter from web.xml
        initialData = config.getInitParameter("myInitialData");

        // Or obtaining servlet context for global parameters
        // ServletContext context = config.getServletContext();
        // String globalParameter = context.getInitParameter("globalParam");

        System.out.println("Servlet initialized with data: " + initialData);
    }

    // ... doGet, doPost methods, etc.
}

Corresponding fragment of web.xml:

<!-- web.xml fragment -->
<servlet>
    <servlet-name>MyServlet</servlet-name>
    <servlet-class>com.example.MyServlet</servlet-class>
    <init-param>
        <param-name>myInitialData</param-name>
        <param-value>Some important configuration</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup> <!-- Load servlet at application startup -->
</servlet>

Using annotations for initialization (since Servlet 3.0):

// Example servlet with annotations
@WebServlet(
    name = "MyServletAnnotated",
    urlPatterns = {"/myannotatedservlet"},
    initParams = {
        @WebInitParam(name = "myInitialDataAnnotated", value = "Annotated config data")
    },
    loadOnStartup = 1
)
public class MyServletAnnotated extends HttpServlet {

    private String initialDataAnnotated;

    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config); // Mandatory call to parent init

        // Getting init parameter from annotation
        initialDataAnnotated = config.getInitParameter("myInitialDataAnnotated");

        System.out.println("Servlet initialized with annotated data: " + initialDataAnnotated);
    }

    // ... doGet, doPost methods, etc.
}

Thus, for initializing data in a servlet, you should use the init() method, not the constructor.