package zzz;

import java.io.IOException;
import java.io.PrintWriter;

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

public class Hello extends HttpServlet {
 @Override
 public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
  // 设置响应消息编码
  response.setContentType("text/html;charset=utf-8");
  PrintWriter out = response.getWriter();
  out.println("Hello World");
 }
 
 @Override
 public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
  this.doGet(request, response);
 }
}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
 xmlns="http://xmlns.jcp.org/xml/ns/javaee" 
 xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
 <display-name>zzz</display-name>

 <servlet> <!-- 注册Servlet -->
  <servlet-name>Hello</servlet-name> <!-- 指定Servlet名称 -->
  <servlet-class>zzz.Hello</servlet-class> <!-- 指定Servlet完整类名 -->
 </servlet>
 <servlet-mapping> <!-- 映射Servlet对外访问路径 -->
  <servlet-name>Hello</servlet-name> <!-- 指定Servlet名称 -->
  <url-pattern>/hello</url-pattern> <!-- 指定访问Servlet的虚拟路径 -->
 </servlet-mapping>
</web-app>

<servlet>
  <servlet-name>Hello</servlet-name>
  <servlet-class>zzz.Hello</servlet-class>
 <!-- 自动加载servlet程序 -->
 <load-on-startup>1</load-on-startup> 
 </servlet>
 <servlet-mapping>
  <servlet-name>Hello</servlet-name>
  <url-pattern>/hello</url-pattern>
 </servlet-mapping>

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
 xmlns="http://xmlns.jcp.org/xml/ns/javaee" 
 xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
 <display-name>zzz</display-name>

 <servlet>
  <servlet-name>Hello</servlet-name>
  <servlet-class>zzz.Hello</servlet-class>
  <init-param>
   <param-name>company</param-name>
   <param-value>dahua</param-value>
  </init-param>
 </servlet>
 <servlet-mapping>
  <servlet-name>Hello</servlet-name>
  <url-pattern>/hello</url-pattern>
 </servlet-mapping>
</web-app>

package zzz;

import java.io.IOException;
import java.io.PrintWriter;

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

public class Hello extends HttpServlet {
 @Override
 public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
  // 设置响应消息编码
  response.setContentType("text/html;charset=utf-8");
  PrintWriter out = response.getWriter();
  
  ServletConfig config = this.getServletConfig();
  String param = config.getInitParameter("company");
  out.println("company: " + param);
 }
 
 @Override
 public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
  this.doGet(request, response);
 }
} 

<context-param>
 <param-name>name</param-name>
 <param-value>luoxn28</param-value>
</context-param>
<context-param>
 <param-name>company</param-name>
 <param-value>dahua</param-value>
</context-param>

package zzz;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;

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

public class Hello extends HttpServlet {
 @Override
 public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
  // 设置响应消息编码
  response.setContentType("text/html;charset=utf-8");
  PrintWriter out = response.getWriter();
  
  ServletContext context = this.getServletContext();
  Enumeration<String> paramNames = context.getInitParameterNames();
  while (paramNames.hasMoreElements()) {
   String name = paramNames.nextElement();
   String value = context.getInitParameter(name);
   out.println(name + ": " + value + "<br/>");
  }
 }
 
 @Override
 public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
  this.doGet(request, response);
 }
}