HttpServlet

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HttpServlet

什么是 HttpServlet

HttpServlet 是 Servlet 接口的一个实现类

Servlet 框架是由两个 Java 包组成:javax.servlet 和 javax.servlet.http。在 javax.servlet 包中定义了所有的 Servlet 类都必须实现或扩展的的通用接口和类，在 javax.servlet.http 包中定义了采用 HTTP 通信协议的 HttpServlet 类

HttpServlet 的请求响应流程

Web 客户向 Servlet 容器发出 Http 请求
Servlet 容器解析 Web 客户的 Http 请求
Servlet 容器创建一个 HttpRequest 对象，在这个对象中封装 Http 请求信息
Servlet 容器创建一个 HttpResponse 对象
Servlet 容器调用 HttpServlet 的 service 方法，把 HttpRequest 和 HttpResponse 对象作为 service 方法的参数传给 HttpServlet 对象
HttpServlet 调用 HttpRequest 的有关方法，获取 HTTP 请求信息
HttpServlet 调用 HttpResponse 的有关方法，生成响应数据
Servlet 容器把 HttpServlet 的响应结果传给 Web 客户

如何创建 HttpServlet，实现步骤

继承 HttpServlet 抽象类
Override HttpServlet 的部分方法，常见的 doGet(), doPost() 方法
获取 HTTP 请求信息。通过 HttpServletRequest 对象来检索 HTML 表单所提交的数据或 URL 上的查询字符串
生成 HTTP 响应结果。通过 HttpServletResponse 对象生成响应结果

创建测试项目 HttpServletDemo，实现 HttpServlet

在 src/main 目录下，新建一个 package 名字随便取, 然后在 package 下新建一个 java(Source Root) 和 resources（Resources Root）两个文件夹
打开 HttpServletDemo/pom.xml 配置加载 javax.servlet、javax.servlet.jsp 资源包

打开 webapp/WEB-INF/web.xml，复制以下内容替换 web.xml 中的代码

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="https://jakarta.ee/xml/ns/jakartaee"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="https://jakarta.ee/xml/ns/jakartaee
                  https://jakarta.ee/xml/ns/jakartaee/web-app_5_0.xsd"
     version="5.0">
</web-app>

ServletContext

什么是 ServletContext？

ServletContext 是一个 web 应用的上下文对象

ServletContext 对象的生命周期？

ServletContext 中属性的生命周期从创建开始，到服务器关闭结束

如何获得 ServletContext ?

# 继承 HttpServlet
ServletContext servletContext = this.getServletContext();

1.1 域对象的通用的方法：

setAtrribute(String name,Object obj) # 键值对

getAttribute(String name) # 根据健取值，取到的值为 Object 类型，需要向下转型

removeAttribute(String name)
创建 com.test.servlet/TestHttpServletDemo.java

package com.test.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 TestHttpServletDemo extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = this.getServletContext();

        String name = "HttpServlet -> getServletContext -> setAttribute && getAttribute";
        servletContext.setAttribute("name", name);
    }

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

创建 com.test.servlet/TestHttpServletDemo2.java

package com.test.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 TestHttpServletDemo2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = this.getServletContext();
        String name = (String) servletContext.getAttribute("name");

        resp.setContentType("text/html; charset=UTF-8");
        resp.getWriter().write(name);
    }

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

webapp/WEB-INF/web.xml

<servlet>
    <servlet-name>demo</servlet-name>
    <servlet-class>com.test.servlet.TestHttpServletDemo</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>demo</servlet-name>
    <url-pattern>/demo</url-pattern>
  </servlet-mapping>

  <servlet>
    <servlet-name>demo2</servlet-name>
    <servlet-class>com.test.servlet.TestHttpServletDemo2</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>demo2</servlet-name>
    <url-pattern>/demo2</url-pattern>
  </servlet-mapping>

先访问 http://localhost:8080/demo/demo
在访问 http://localhost:8080/demo/demo2
HttpServlet

1.2 获取 web 初始化参数
创建 com.test.servlet/TestHttpServletDemo3.java

package com.test.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 TestHttpServletDemo3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = this.getServletContext();

        // 获取 web 初始化参数
        String db = servletContext.getInitParameter("db-mysql");
        resp.getWriter().println(db);
    }

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

webapp/WEB-INF/web.xml

  <!--  配置 web 应用的初始化参数  -->
  <context-param>
    <param-name>db-mysql</param-name>
    <param-value>jdbc:mysql://localhost:3306/DatabaseName</param-value>
  </context-param>

  ......

    <servlet>
    <servlet-name>demo3</servlet-name>
    <servlet-class>com.test.servlet.TestHttpServletDemo3</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>demo3</servlet-name>
    <url-pattern>/demo3</url-pattern>
  </servlet-mapping>

HttpServlet

1.3 请求转发（与重定向 1.5）
请求转发：客户端发起一次请求，一次响应。客户端向服务端 A 发起请求，A 接到请求后，调用 getRequestDispacther 把请求转发给指定的服务端 B。整个过程都在服务端完成，而且是在同一个请求里面。因此 A 和 B 共享一个 Request.

重定向：客户端发起两次请求，两次响应。客户发送一个请求到服务端 A，A 接到请求后，向客户端返回个响应，告诉客户端，你必须再发送一个请求到服务端 B。紧接着客服端再次发送一个请求到服务端 B。两次请求，相互独立，不共享 Request

getRequestDispatcher(String s) # s 转发路径
创建 com.test.servlet/TestHttpServletDemo4.java

package com.test.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;

public class TestHttpServletDemo4 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = this.getServletContext();

        RequestDispatcher requestDispatcher = servletContext.getRequestDispatcher("/demo3"); // 转发的请求路径
        requestDispatcher.forward(req, resp); // 调用 forward 实现请求转发
    }

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

webapp/WEB-INF/web.xml


  <servlet>
    <servlet-name>demo4</servlet-name>
    <servlet-class>com.test.servlet.TestHttpServletDemo4</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>demo4</servlet-name>
    <url-pattern>/demo4</url-pattern>
  </servlet-mapping>

HttpServlet

1.4 Properties
load() 从字节输入流中读取键值对
在 main 目录下创建 resources 文件夹, 并设置为 resources root。然后在 resources 文件夹下创建 test.properties

name=test

创建 com.test.servlet/TestHttpServletDemo5.java

package com.test.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 TestHttpServletDemo5 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {InputStream resourceAsStream = this.getServletContext().getResourceAsStream("/WEB-INF/classes/test.properties");

        Properties prop = new Properties();
        prop.load(resourceAsStream);
        String name = prop.getProperty("name");

        resp.getWriter().println(name);
    }

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

webapp/WEB-INF/web.xml

  <servlet>
    <servlet-name>demo5</servlet-name>
    <servlet-class>com.test.servlet.TestHttpServletDemo5</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>demo5</servlet-name>
    <url-pattern>/demo5</url-pattern>
  </servlet-mapping>

HttpServlet

1.5 重定向
sendRedirect(String s)
创建 com.test.servlet/TestHttpServletDemo6.java

package com.test.servlet;

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 TestHttpServletDemo6 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {resp.sendRedirect("/demo/demo5");
    }

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

webapp/WEB-INF/web.xml

<servlet>
    <servlet-name>demo6</servlet-name>
    <servlet-class>com.test.servlet.TestHttpServletDemo6</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>demo6</servlet-name>
    <url-pattern>/demo6</url-pattern>
  </servlet-mapping>

访问:http://localhost:8080/demo/demo6, 会重定向到 http://localhost:8080/demo/demo5

正文完

发表至： Java

2022-01-31

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