JavaWeb——ServletContext对象及使用

「这是我参与11月更文挑战的第28天，活动详情查看：2021最后一次更文挑战」

什么是ServletContext

Web容器在启动时，它会为每个Web应用程序都创建一个对应的ServletContext对象，这个对象代表当前Web应用。并且它被所有客户端共享（比如你在淘宝登陆了，那你的登陆信息会被带到很多页面，就是跳到其他页面发现也是登陆上的）。

ServletContext的应用

共享数据

多个Servlet通过ServletContext对象实现数据共享

• 添加属性：setAttribute(String name, Object obj);
• 得到值：getAttribute(String name)，这个方法返回Object
• 删除属性：removeAttribute(String name)

测试：

首先创建一个放置数据的类

java复制代码package com.cheng.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;
import java.io.PrintWriter;

public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String username = "万里顾一程";//数据
        context.setAttribute("username","万里顾一程");//将一个数据保存在ServletContext中，username=万里顾一程
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

注册Servlet

xml复制代码<servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.cheng.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

然后创建一个接收数据的类

java复制代码package com.cheng.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 attribute = (String) context.getAttribute("username");//返回值，并类型强转为String
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().print("名字="+attribute);
    }

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

注册Servlet

xml复制代码<servlet>
        <servlet-name>hello1</servlet-name>
        <servlet-class>com.cheng.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello1</servlet-name>
        <url-pattern>/hello1</url-pattern>
    </servlet-mapping>

启动Tomcat开始测试：

1.启动HelloServlet放置数据到ServletContext中

2.启动GetServlet从ServletContext中获得数据

获取配置的初始化参数

在web.xml里配置的初始化参数可以用getInitParameter()方法获取

xml复制代码    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

写一个实现类

java复制代码package com.cheng.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 ServletDemon03 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);
    }
}

注册Servlet

xml复制代码 <servlet>
        <servlet-name>hello3</servlet-name>
        <servlet-class>com.cheng.servlet.ServletDemon03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello3</servlet-name>
        <url-pattern>/hello3</url-pattern>
    </servlet-mapping>

运行Tomcat，查看结果

请求转发

RequestDispatcher 代表请求的派发者。

Forward是指内部转发。当一个Servlet处理请求的时候，它可以决定自己不继续处理，而是转发给另一个Servlet处理。

例如，我们已经编写了一个能处理/hello3的ServletDemon03，继续编写一个能处理/hello4的ServletDemon04，ServletDemon04在收到浏览器的请求后，它并不自己发送响应，而是把请求和响应都转发给路径为/hello3的ServletDemon03，后续请求的处理实际上是由ServletDemon03完成的。这种处理方式称为转发（Forward）

流程图如下所示：

编些一个实现请求转发的实现类

java复制代码package com.cheng.servlet;

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;
import java.io.IOException;

/**
 * @User len
 * @Classname com.cheng.servlet.ServletDemon04
 * @Project javaweb-01-servlet
 * @Description TODO
 * @Author wpcheng
 * @Create 2021-02-06-18:36
 * @Version 1.0
 */
public class ServletDemon04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //RequestDispatcher 代表请求的派发者,                            只能使用绝对路径
        RequestDispatcher dispatcher = context.getRequestDispatcher("/hello3");//请求的是地址为/hello3的资源,因此该请求是由路径为/hello3的servlet实现的
        dispatcher.forward(req,resp);//内部转发
    }

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

注册路径为/hello4的servlet

xml复制代码    <servlet>
        <servlet-name>hello4</servlet-name>
        <servlet-class>com.cheng.servlet.ServletDemon04</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello4</servlet-name>
        <url-pattern>/hello4</url-pattern>
    </servlet-mapping>

注册路径为/hello3的servlet

xml复制代码<servlet>
        <servlet-name>hello3</servlet-name>
        <servlet-class>com.cheng.servlet.ServletDemon03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello3</servlet-name>
        <url-pattern>/hello3</url-pattern>
    </servlet-mapping>

启动Tomcat查看转发结果

==注：转发是在Web服务器内部完成的，对浏览器来说，它只发出了一个HTTP请求，浏览器的地址栏路径仍然是/hello4，浏览器并不知道该请求在Web服务器内部实际上做了一次转发。==

读取资源文件

类路径：即发布到Tomcat服务器后的ClassPath路径，在classes目录下

Properties可以用来保存属性集（类似Map, 以键值的形式保存数据，不同的是Properties都是String类型的）。这个类的优势是可以从流中获得属性集，或者把属性集报错到流中。

获取配置文件的信息

properties复制代码username=root
password=123456

实现类

java复制代码package com.cheng.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 ServletDemon05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //                                                                 “ / ”代表了当前工程的根目录
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/jdbc.properties");
        Properties prop = new Properties();
        prop.load(is);//下载文件流
        String user = prop.getProperty("username");//通过键获取属性
        String pwd = prop.getProperty("password");
        resp.getWriter().println(user);
        resp.getWriter().println(pwd);
    }

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

注册servlet

xml复制代码<servlet>
        <servlet-name>hello5</servlet-name>
        <servlet-class>com.cheng.servlet.ServletDemon05</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello5</servlet-name>
        <url-pattern>/hello5</url-pattern>
    </servlet-mapping>

实现

本文转载自: 掘金

开发者博客 – 和开发相关的 这里全都有