Servlet 03 ----【javaweb-06】

SERVLET

ServletContext

　　web容器在启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用。

接下来，让我们来看看ServletContext对象有什么用处吧。

1. 数据传输

　　我们可以在保存一个类中的servlet的同时，在另一个类获取该servlet数据。

代码展示：

类1：

package com.charels.servlet;

import javax.servlet.ServletContext;
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 HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
　　　　　//this.getServletContext()  Servlet上下文
        ServletContext context = this.getServletContext();

        String name = "小明";
        context.setAttribute("name",name);

    }
}

类2.

package com.charels.servlet;

import javax.servlet.ServletContext;
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 GetServlet extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            ServletContext context = this.getServletContext();

            String name = (String)context.getAttribute("name");
            resp.setContentType("text/html");
            resp.setCharacterEncoding("GBK");
            resp.getWriter().print("名字"+name);
        }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

在web.xml注册，两个servlet。

  <!--  注册servlet-->
  <servlet>
    <servlet-name>hello</servlet-name>
    <servlet-class>com.charels.servlet.HelloServlet</servlet-class>
  </servlet>

  <!--  servlet的请求路径-->
  <servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>

  <servlet>
    <servlet-name>getc</servlet-name>
    <servlet-class>com.charels.servlet.GetServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>getc</servlet-name>
    <url-pattern>/getc</url-pattern>
  </servlet-mapping>

接下来，让我们康康运行Tomcat后的展示结果：

在这里要注意，根据代码的意思，要获取上下文数据，则需要先访问第一个类的servlet，再访问第二个才能获取数据。

第一个类的网页：

第二个类的网页：

2. 获取初始化参数

代码展示：

首先在web.xml中配置初始化数据：

    <!--配置一些web应用初始化参数-->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

接下来编写获取初始化数据的代码：

package com.charels.servlet;

import javax.servlet.ServletContext;
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 GetUrl extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");

        resp.getWriter().print(url);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

编写完毕后，注册servlet，然后运行Tomcat：

3. 请求与转发

　　我们可以通过一个servlet的类来实现页面的转发。

代码展示：

package com.charels.servlet;

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

public class Dri extends GetUrl{
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("进入Dri");

        ServletContext context = this.getServletContext();
        context.getRequestDispatcher("/g1").forward(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

结果展示：

　　控制台：

　　　　

　　页面展示：

　　　　

4. 读取资源文件

　　首先，在properties的包中创建一个db.properties并编写数据

　　

　　在servlet类中编写读取资源的代码：

package com.charels.servlet;

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

public class GetPro extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setCharacterEncoding("gbk");

        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");

        Properties pro = new Properties();
        pro.load(is);
        String username = pro.getProperty("username");
        String passsword= pro.getProperty("password");

        resp.getWriter().print(username + '：' + passsword);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

编写完毕后，运行Tomcat，展示结果：