6 談一談Servlet與HttpServlet

1 創建一個類實現Servlet

package com.duowei.servlet;

import javax.servlet.*;
import javax.swing.*;
import java.io.IOException;

public class TestServlet implements Servlet {

    public void init(ServletConfig servletConfig) throws ServletException {
        System.out.println("init......");
    }

    public ServletConfig getServletConfig() {
        return null;
    }

    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        System.out.println("service......");
    }

    public String getServletInfo() {
        return null;
    }

    public void destroy() {
        System.out.println("destroy......");
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                  http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

  <servlet>
    <servlet-name>helloServlet</servlet-name>
    <servlet-class>com.duowei.servlet.TestServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>helloServlet</servlet-name>
    <url-pattern>/helloServlet</url-pattern>
  </servlet-mapping>

  <servlet>
    <servlet-name>helloHttpServlet</servlet-name>
    <servlet-class>com.duowei.servlet.TestHttpServlet</servlet-class>
  </servlet>

</web-app >

啓動Tomcat看一下init、service、destroy什麼時候執行
我們發現 init 在tomcat 啓動時候執行，destroy在tomcat結束運行時執行，service在每次訪問servlet時候執行。
試一下，多次訪問servlet，只執行service，不會再執行init了，可見Servlet是單例模式

2 創建一個類繼承HttpServlet

package com.duowei.servlet;

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

public class TestHttpServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doGet(req, resp);
        System.out.println("TestHttpServlet.................doGet");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
        System.out.println("TestHttpServlet.................doPost");
    }

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.service(req, resp);
        System.out.println("TestHttpServlet.................service");
    }
}

註冊servlet

  <servlet>
    <servlet-name>helloHttpServlet</servlet-name>
    <servlet-class>com.duowei.servlet.TestHttpServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>helloHttpServlet</servlet-name>
    <url-pattern>/helloHttpServlet</url-pattern>
  </servlet-mapping>

訪問以下TestHttpServlet

發現輸出
TestHttpServlet…doGet
TestHttpServlet…service
爲什麼先輸出doGet再輸出service

• 因爲我這裏重寫了service方法，所以訪問TestHttpServlet時候會執行我重寫的service方法
• 另外，我在service中調用了super.service(req, resp);那麼在訪問TestHttpServlet的時候，輸出System.out.println(“TestHttpServlet…service”);前會執行父類的service方法
• 我們看一下父類的service方法，也就是HttpServlet會幫我們判斷是GET請求方式的時候，調用doGet方法，所以先打印了TestHttpServlet…doGet；後打印了TestHttpServlet…service
  

上述所說是爲了讓我們瞭解

1. 瀏覽器通過URL訪問Tomcat；
2. 創建request請求對象，response響應對象；
3. Tomcat根據URL找到HttpServlet的類路徑，創建HttpServlet實例；
4. HttpServlet實例調用service方法，並判斷GET和POST去執行doGet或者doPost，以參數形式傳入request，response；
5. 返回response對象