Day40-Servlet原理、Mapping问题、ServletContext

最新推荐文章于 2023-06-26 11:03:42 发布

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#servlet #java #前端

本文介绍Servlet的工作原理，包括Servlet的调用流程、映射配置、ServletContext的使用及数据共享方式，并演示了请求转发和资源文件读取的具体实现。

Day40-Servlet原理、Mapping问题、ServletContext

Servlet原理

Servlet是由Web服务器调用，web服务器在收到浏览器请求之后

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Mapping问题

一个Servlet可以指定一个映射路径

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

查看

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一个Servlet可以指定多个映射路径

  <servlet-mapping>
    <servlet-name>helloxiaozhao</servlet-name>
    <url-pattern>/helloxiaozhao</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>helloxiaozhao</servlet-name>
    <url-pattern>/helloxiaozhao1</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>helloxiaozhao</servlet-name>
    <url-pattern>/helloxiaozhao2</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>helloxiaozhao</servlet-name>
    <url-pattern>/helloxiaozhao3</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>helloxiaozhao</servlet-name>
    <url-pattern>/helloxiaozhao4</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>helloxiaozhao</servlet-name>
    <url-pattern>/helloxiaozhao5</url-pattern>
  </servlet-mapping>

查看

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一个Servlet可以指定通用映射路径

<!--默认请求路径-->  
<servlet-mapping>
    <servlet-name>helloxiaozhao</servlet-name>
    <url-pattern>/*</url-pattern>
</servlet-mapping>

查看

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指定一些后缀或者前缀等等…

  <!--可以自定义后缀实现请求映射-->
  <servlet-mapping>
    <servlet-name>helloxiaozhao</servlet-name>
    <url-pattern>*.xz</url-pattern>
  </servlet-mapping>

查看

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优先级问题

  <!--注册Servlet-->
  <servlet>
    <servlet-name>helloxiaozhao</servlet-name>
    <servlet-class>com.xz.servlet.HelloServlet</servlet-class>
  </servlet>
  <!--Servlet的请求路径-->
  <servlet-mapping>
    <servlet-name>helloxiaozhao</servlet-name>
    <url-pattern>/helloxiaozhao</url-pattern>
  </servlet-mapping>
  <!--404页面-->
  <servlet>
    <servlet-name>error</servlet-name>
    <servlet-class>com.xz.servlet.ErrorServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>error</servlet-name>
    <url-pattern>/*</url-pattern>
  </servlet-mapping>

指定了固有的映射路径优先级最高，如果找不到就会走默认的处理请求

查看

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默认处理请求（自定义404）

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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。

共享数据

我在这个Servlet中保存的数据，可以在另一个servlet中拿到

package com.xz.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;

/**
 * @author 小龚
 * @create 2022-08-30 18:11
 */
public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //this.getInitParameter()初始化参数
        //this.getServletConfig()Servlet配置
        //this.getServletContext()Servlet上下文
        ServletContext context = this.getServletContext();

        String username = "小赵";//数据
        context.setAttribute("username",username);//将一个数据保存在了ServletContext中，名字为username，值为username（object）
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        PrintWriter out = resp.getWriter();
        out.print("已存放用户名");
    }

获取数据的Servlet

package com.xz.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;

/**
 * @author 小龚
 * @create 2022-08-30 22:11
 */
public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String username = (String) context.getAttribute("username");//需要强转

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().print("用户名："+username);
    }

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

查看

未拿到数据

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存放数据

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再次运行

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获取初始化参数

ServletDemo03

package com.xz.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;

/**
 * @author 小龚
 * @create 2022-08-30 22:50
 */
public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().print(url);
    }

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

web.xml配置

    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql//localhost:3306/mybatis</param-value>
    </context-param>
    <servlet>
        <servlet-name>gp</servlet-name>
        <servlet-class>com.xz.servlet.ServletDemo03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>gp</servlet-name>
        <url-pattern>/gp</url-pattern>
    </servlet-mapping>

测试

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请求转发

ServletDemo04

package com.xz.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;
import java.io.PrintWriter;

/**
 * @author 小龚
 * @create 2022-08-30 23:18
 */
public class ServletDemo04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //请求转发
        //RequestDispatcher rc = context.getRequestDispatcher("/gp");//转发的请求路径
        //rc.forward(req,resp);//调用forward方法实现请求转发
        //以上两句合并成一句
        context.getRequestDispatcher("/gp").forward(req,resp);

    }

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

web.xml

    <servlet>
        <servlet-name>sd4</servlet-name>
        <servlet-class>com.xz.servlet.ServletDemo04</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>sd4</servlet-name>
        <url-pattern>/sd4</url-pattern>
    </servlet-mapping>

测试

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路径没有变化

读取资源文件

Properties类

在java目录下新建properties（需要在注意资源导出失败问题）
在resources目录下新建properties

发现：都被打包到了同一个路径下：classes，我们俗称这个路径为classPath

读取配置文件

ServletDemo05

package com.xz.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;

/**
 * @author 小龚
 * @create 2022-08-30 23:55
 */
public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties pp= new Properties();
        pp.load(is);
        String username = pp.getProperty("username");
        String password = pp.getProperty("password");
        resp.setContentType("text/html");
        resp.getWriter().print(username+";"+password);
    }

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

db.properties

username=root
password=123456

web.xml

    <servlet>
        <servlet-name>sd5</servlet-name>
        <servlet-class>com.xz.servlet.ServletDemo05</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>sd5</servlet-name>
        <url-pattern>/sd5</url-pattern>
    </servlet-mapping>