What is a Web Servlet?

What is a Servlet? #

Servlet

A Servlet is a Java-based web application programming technology used to create Dynamic Web Pages.

• When building a web application, there are various requests and responses, and these requests and responses follow certain rules.
• Handling these requests and responses individually would be very difficult.
• A Servlet can be understood as a technology that allows you to systematically manage the flow of web requests and responses through simple method calls.

Servlet Container

• Servlets are Java classes used to write web applications, which are then executed by a web container within a web server.
• The web container creates a Servlet instance and runs it on the server. When a web browser sends a request to the server, the Servlet performs the appropriate action for the request and responds to the web browser in HTTP format.

Key Characteristics of Servlets #

1. A web application component that operates dynamically in response to client requests.
2. Responds using HTML.
3. Operates using Java threads.
4. Used as a controller in the MVC pattern.
5. Inherits from the javax.servlet.http.HttpServlet class, which supports HTTP protocol services.
6. Slower than UDP.
7. Has the disadvantage of requiring Servlet recompilation when HTML changes.

What is a Servlet Container? #

• As the name suggests, it's a container that holds and manages Servlets.
• A Servlet container manages Servlets according to the rules of the implemented Servlet class. When a client sends a request, the container creates two objects, HttpServletRequest and HttpServletResponse.
• It then generates a dynamic page based on whether it's a POST or GET request and sends the response.

HttpServletRequest #

Used to deliver HTTP protocol request information to the Servlet. It has methods to read header information, parameters, cookies, URI, URL, and methods to read the Body's Stream.

HttpServletResponse #

The WAS knows which client sent the request and creates an HttpServletResponse object to send a response to that client, passing it to the Servlet. This object is then used to send content type, response codes, response messages, and more.

Key Functions of a Servlet Container #

1. Lifecycle Management

   • Manages the lifecycle of Servlets.
   • The moment the Servlet container starts, it loads the Servlet class, instantiates it,
   • calls the initialization method. When a request comes in, it finds and executes the appropriate Servlet method.
   • When the Servlet's life ends, it is removed from memory through garbage collection.

2. Communication Support

   • Creates sockets and communicates with the web server to receive client requests and send responses.
   • When we think about communication, we know that we create sockets, listen on specific ports, and when a connection request comes in, we create a stream to receive the request.
   • The Servlet container handles this entire process for us.
   • The Servlet container provides APIs for functions like creating sockets, listening, and accepting, simplifying complex processes.
   • This allows developers to focus more on business logic.

3. Multithreading Management

   • When a request comes in for a Servlet, the Servlet container creates a thread to perform the task.
   • Therefore, even if multiple requests come in simultaneously, it can manage concurrent tasks in a multithreading environment.
   • Additionally, once a thread is loaded into memory, there's no need to create it again, making memory management efficient.

4. Declarative Security Management

   • The Servlet container supports security-related features.
   • Therefore, there's no need to implement security-related methods within the Servlet or Java class.
   • Security management is generally recorded in the XML deployment descriptor, so even if a security issue requires source modification, there's no need to modify the Java source code and recompile.

Servlet Operation Process #

Servlet Operation Process

1. Creates Servlet Request and Response objects.
2. Refers to the configuration file to identify the Servlet to map.
3. Checks for the existence of the Servlet instance; if it doesn't exist, calls the init() method to create it.
4. Creates a thread in the Servlet Container and executes the service method.
5. Once the response is processed, executes the destroy() method to destroy the Servlet Request and Response objects.

Example Code #

public class myServlet extends HttpServlet {

    @Override
    public void init(ServletConfig config) throws ServletException {
        System.out.println("init method 호출!");
    }
    
    @Override
    public void destroy() {
        System.out.println("destroy method 호출!");
    }
    
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException
    {
        System.out.println("doGet service method 호출!");		
    }
    
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException
    {
        System.out.println("doPost service method 호출!");		
    }
	
}

• init()

  • This method initializes the Servlet when it is first requested.
  • Just as you new a class to use it, a Servlet class also needs to be initialized before it can be used.
  • An initialized Servlet is managed as a singleton, so if the same Servlet class is called again, it won't re-initialize but will instead call the existing Servlet class.

• service()

  • This is the Servlet's service method required when the Servlet container receives a request and sends a response. Methods like doGet and doPost of the HttpServlet class, which implements the Servlet interface, are called.

• destroy()

  • Servlet classes that are no longer in use are periodically removed by the Servlet container calling this method.
  • When a Servlet is removed, all threads corresponding to its service methods are terminated, or
  • if it has timed out due to long usage, it needs to be re-initialized to be used again.