[Servlet/서블릿] What is a Servlet? / Methods in the Servlet lifecycle / Servlet operation process

1. FirstServlet.java

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class FirstServlet extends HttpServlet {
	@Override
	public void init() throws ServletException{
		System.out.println("메시지 호출");
	}
	@Override
	public void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		System.out.println("메소드 호출.");
	}
	@Override
	
	public void destroy() {
		System.out.println("destroy 메소드 호출.");
	}
}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="https://jakarta.ee/xml/ns/jakartaee" xsi:schemaLocation="https://jakarta.ee/xml/ns/jakartaee https://jakarta.ee/xml/ns/jakartaee/web-app_5_0.xsd" id="WebApp_ID" version="5.0">
  <servlet>
  <servlet-name>aaa</servlet-name>
  <servlet-class>sec01.ex01.FirstServlet</servlet-class>
  </servlet>
  <servlet-mapping>
  <servlet-name>aaa</servlet-name>
  <url-pattern>/first</url-pattern>
  </servlet-mapping>
</web-app>

 

Output

 

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2. SecondServlet.java

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class SecondServlet extends HttpServlet {

	@Override
	public void init() throws ServletException{
		System.out.println("메시지 호출");
	}
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
	throws ServletException, IOException{
		System.out.println("doGet 메소드 호출>>>");
	}
	
	@Override
	public void destroy() {
		System.out.println("destroy 메소드 호출>>>");
	}
}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="https://jakarta.ee/xml/ns/jakartaee" xsi:schemaLocation="https://jakarta.ee/xml/ns/jakartaee https://jakarta.ee/xml/ns/jakartaee/web-app_5_0.xsd" id="WebApp_ID" version="5.0">
  <servlet>
  <servlet-name>aaa</servlet-name>
  <servlet-class>sec01.ex01.FirstServlet</servlet-class>
  </servlet>
  <servlet>
  <servlet-name>bbb</servlet-name>
  <servlet-class>sec01.ex01.SecondServlet</servlet-class>  
  </servlet>
  
  <servlet-mapping>
  <servlet-name>aaa</servlet-name>
  <url-pattern>/first</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
  <servlet-name>bbb</servlet-name>
  <url-pattern>/second</url-pattern>
  </servlet-mapping>
</web-app>

When a request is made with the mapping name "/second"

Output

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3. Life Cycle of a Servlet

There are three life cycle methods of a Servlet :

• init()
  • The Servlet.init() method is called by the Servlet container to indicate that this Servlet instance is instantiated successfully and is about to put into service.

  @Override
  	public void init() throws ServletException{
  		System.out.println("메시지 호출");
  	}
  

• service()
  • The service() method of the Servlet is invoked to inform the Servlet about the client requests.
  • This method uses ServletRequest object to collect the data requested by the client.
  • This method uses ServletResponse object to generate the output content.

  @Override
  	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
  	throws ServletException, IOException{
  		System.out.println("doGet 메소드 호출>>>");
  	}
  

  • req is the ServletRequest object which encapsulates the connection from client to server
  • resp is the ServletResponse object which encapsulates the connection from server back to the client
• destroy()
  • The destroy() method runs only once during the lifetime of a Servlet and signals the end of the Servlet instance.

  @Override
  	public void destroy() {
  		System.out.println("destroy 메소드 호출>>>");
  	}
  }
  

---

4. @WebServlet

@WebServlet annotation is used to map the servlet with the specified name.

import java.io.IOException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ThirdServlet
 */
@WebServlet("/third")
public class ThirdServlet extends HttpServlet {
	

	/**
	 * @see Servlet#init(ServletConfig)
	 */
	public void init(ServletConfig config) throws ServletException {
		System.out.println("ThridServlet init 메소드 호출.");
	}

	/**
	 * @see Servlet#destroy()
	 */
	public void destroy() {
		// TODO Auto-generated method stub
	}

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		System.out.println("ThirdServlet doGet 메소드 호출.");
		}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		System.out.println("ThirdServlet destroy 메소드 호출.");
	}
}

Example of simple servlet by annotation

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
		out.print("<html><body>");
		out.print("<h3>Hello Servlet</h3>");
		out.print("</body></html>");
		System.out.println("ThirdServlet doGet 메소드 호출.");
		}

Output

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5. Requesting a servlet using the <form> tag

The following picture depicts that workflow with Java servlet on the server side:

 

Reference

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https://www.codejava.net/java-ee/servlet/handling-html-form-data-with-java-servlet