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Servlet

1.Servlet简介

• Servlet就是sun公司开发动态web的一门技术。

• Sun在这些API中提供一个接口叫做：Servlet，如果你想开发一个Servlet程序，只需要完成两个小步骤：

  • 编写一个类，实现Servlet接口。
  • 把开发好的Java类部署到web服务器中。

• 把实现了Servlet接口的Java程序叫做，Servlet

2.HelloServlet

Serlvet接口Sun公司有两个默认的实现类：HttpServlet，GenericServlet。
接下来我们编写一个Servlet程序：

1. 编写一个普通类。

2. 实现Servlet接口，这里我们直接继承HttpServlet

package com.hh.servlet;import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
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 {// 由于get或者Post只是请求实现的不同方式，可以相互调用，业务逻辑都一样@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {System.out.println("进入doGet方法");PrintWriter writer = resp.getWriter();   //响应流
//        ServletOutputStream outputStream = resp.getOutputStream();writer.print("hello,servlet");}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {}
}

然后编写servlet映射：

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaeehttp://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"version="4.0"metadata-complete="true">
<!--    注册Servlet--><servlet><servlet-name>hello</servlet-name><servlet-class>com.hh.servlet.helloServlet</servlet-class></servlet><!-- Servlet请求路径-->
<!--    localhost:8080/s3/hello/hello--><servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello/hello</url-pattern></servlet-mapping><servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello</url-pattern></servlet-mapping><!--    注册404-->
<!--    <servlet>-->
<!--        <servlet-name>error</servlet-name>-->
<!--        <servlet-class>com.hh.servlet.errorServlet</servlet-class>-->
<!--    </servlet>-->
<!--    <servlet-mapping>-->
<!--        <servlet-name>error</servlet-name>-->
<!--        <url-pattern>/*</url-pattern>-->
<!--    </servlet-mapping>--></web-app>

运行Tomcat：

Servlet原理

首先浏览器发送Http请求给web容器（可以理解为Tomcat）然后Requset通过Servlet去调用Service方法，产生一个Response，这个请求赋给响应体，然后在给Web容器。其中Servlet这里是我们自己取编写的实现类，并且重写这些方法，比如接受并处理请求，且给出响应的信息。

3.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

在这个Servlet中保存的数据，可以在另外一个servlet中拿到；

package com.hh.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 {@Overrideprotected 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// Web容器启动的时候，会为每个WEB程序创建一个对应的ServletContext对象，代表当前WEB应用ServletContext context = this.getServletContext();String username = "哈哈哈哈哈";context.setAttribute("username",username); // 将一个数据保存在context中System.out.println("hello");}}

首先先通过setAttribute把数据保存在context中

接着从另外一个Servlet中取数据：

package com.hh.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 {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = this.getServletContext();String username = (String) servletContext.getAttribute("username");//resp.setCharacterEncoding("utf-8");resp.setContentType("text/html");resp.getWriter().print("名字"+username);}
}

结果:

拿到初始化信息

package com.hh.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.util.Properties;public class servlet_test3 extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = this.getServletContext();String url = servletContext.getInitParameter("url");resp.getWriter().print(url);Properties properties = new Properties();}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {super.doPost(req, resp);}
}

结果：

请求转发

package com.hh.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 servlet_test4 extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {System.out.println("进入了test4");ServletContext servletContext = this.getServletContext();servletContext.getRequestDispatcher("/gp").forward(req,resp);  //转发的请求路径}
}

结果：

读取资源文件

Properties

• 在java目录下新建properties
• 在resources目录下新建properties

package com.hh.servlet;import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;public class PropertiesServlet extends HelloServlet{@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {InputStream resourceAsStream = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/hh/servlet/aa.properties");// /代表当前项目位置Properties properties = new Properties();properties.load(resourceAsStream);String username = properties.getProperty("username");String password= properties.getProperty("password");resp.getWriter().print(username+":"+password);}}

结果：