一、ServletConfig

  在Servlet的配置文件中，可以使用init-param标签为Servlet配置一些初始化参数，这些参数可以包括字符集编码，数据库连接信息等。这些配置信息我们可以使用ServletConfig对象来获取。

获取ServletConfig的方法：
  （1）在类中重写方法init，并创建一个ServletConfig类型的成员变量，将init方法得到的ServletConfig对象赋值给成员变量

    private ServletConfig config;
    @Override
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
    }

  （2）直接在doGet方法中调用getServletConfig方法获得ServletConfig的对象

    ServletConfig config = getServletConfig();

示例：
  web.xml配置文件中的设置

//init-param标签是Servlet标签的子标签
 <servlet>
    <description></description>
    <display-name>Demo1</display-name>
    <servlet-name>Demo1</servlet-name>
    <servlet-class>com.demo.Demo1</servlet-class>
    <init-param>
    <param-name>username</param-name>
    <param-value>zhangsan</param-value>
    </init-param>
    <init-param>
    <param-name>password</param-name>
    <param-value>123456</param-value>
    </init-param>
  </servlet>

  通过ServletConfig对象获得配置信息

import java.io.IOException;

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

public class Demo1 extends HttpServlet {
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletConfig config = getServletConfig();
        String username = config.getInitParameter("username");
        String password = config.getInitParameter("password");
        
        response.getWriter().println(username);
        response.getWriter().println(password);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

二、ServletContext

1、概述

  当WEB服务器启动的时候，会为每个WEB应用程序创建一个对应的ServletContext对象，他代表了当前的WEB应用，也称之为是WEB应用的上下文，被整个WEB程序所共享，是WEB应用的域对象。
  域对象：有范围的Map，让数据在整个应用所共享。

2、功能

（1）获取WEB应用的初始化参数

  ServletContext获取WEB应用的初始化参数的方法与ServletConfig相似，配置信息都在web.xml文件中，通过context-param便签设置WEB应用的初始化信息，context-param便签是web-app的子标签，获取方法是getServletContext。

示例：
  配置信息：

<context-param>
  <param-name>username</param-name>
  <param-value>lisi</param-value>
  </context-param>

  获取初始化参数

    ServletContext context = getServletContext();
    String username = context.getInitParameter("username");
    response.getWriter().println(username);

（2）实现数据的共享

  由于ServletContext对象是WEB应用的域对象，能够被所有的Servlet所共享，所以能够实现数据的共享。

示例：
  首先我们在Demo1中设置WEB应用的属性值，并获取输出

import java.io.IOException;

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

public class Demo2 extends HttpServlet {

    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = getServletContext();
        context.setAttribute("username", "wangwu");
        Object username = context.getAttribute("username");
        response.getWriter().println(username);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }
}

  然后我们在Demo3中获取属性值，我们发现可以获得相同的属性值

import java.io.IOException;

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

public class Demo3 extends HttpServlet {
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = getServletContext();
        Object username = context.getAttribute("username");
        response.getWriter().println(username);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

（3）获取当前WEB应用的绝对路径

示例：

import java.io.IOException;

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

public class Demo4 extends HttpServlet {
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = getServletContext();
        String path = context.getRealPath("/");//"/"代表当前web应用的根路径
        response.getWriter().println(path);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

（4）请求转发和包含

  Servlet的service()方法是Servlet容器来调用的，用户不能在一个Servlet对象中直接调用另外一个Servlet的service方法,所以需要多个Servlet协同处理。Servlet规范提供了2个途径：

• 请求转发
    Servlet1先对用户的请求做一些预处理,然后把请求转发给Servlet2完成响应

示例：
  将Demo5的请求转发到Demo6中

import java.io.IOException;

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

public class Demo5 extends HttpServlet {
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        response.getWriter().println("hello");
        
        ServletContext context = getServletContext();
        RequestDispatcher rd = context.getRequestDispatcher("/Demo6");
        rd.forward(request, response);
        
        response.getWriter().println("hello");
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

import java.io.IOException;

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

public class Demo6 extends HttpServlet {
    private static final long serialVersionUID = 1L;
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = getServletContext();
        context.setAttribute("username", "zhaoliu");
        Object username = context.getAttribute("username");
        response.getWriter().println(username);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

  在浏览器中访问Demo5，得到结果：

请求转发

• 请求响应
    Servlet1把Servlet2生成的响应结果包含到自身的响应结果里面,完成响应

示例：
  修改Demo5的代码，将Demo5的请求包含到Demo6中

import java.io.IOException;

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

public class Demo5 extends HttpServlet {
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        response.getWriter().println("hello");
        
        ServletContext context = getServletContext();
        RequestDispatcher rd = context.getRequestDispatcher("/Demo6");
        rd.include(request, response);
        
        response.getWriter().println("hello");
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

请求包含

  对比两个结果，我们发现在转发之后响应的内容会被取消，包含前后可以继续响应其他内容，同时地址栏中的路径都不会发生变化，二者共享一个请求响应对象，实现时需要依赖RequestDispatcher接口(请求分发器)。
  注意：
  1、forward()方法先清空用于存放响应正文数据的缓冲区,然后在把请求转发给Servlet2完成响应，只有Servlet2响应的结果才会发送到浏览器
  2、这两个必须在服务器内部进行操作,并且以绝对路径开头