Servlet基础

定义：实现了servlet接口的Java程序叫做servlet

1、新建maven项目，导入servlet依赖

        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>4.0.1</version>
        </dependency>
        <!-- servlet依赖的jar包start -->
        <!-- jsp依赖jar包start -->
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>javax.servlet.jsp-api</artifactId>
            <version>2.3.3</version>
        </dependency>

2、新建HelloServlet类，实现servlet接口，直接继承HttpServlet

//实现Servlet接口
public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("进入doget方法");
　　　　 PrintWriter writer = resp.getWriter();
        writer.print("Hello,Servlet");
    }

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

3、编写Hello.jsp，在WEB-INF目录下新建一个jsp的文件夹，新建hello.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Alan</title>
</head>
<body>
    ${msg}
</body>
</html>

4、在web.xml中注册Servlet，

<?xml version="1.0" encoding="UTF-8"?>
<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/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>com.alan.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/user</url-pattern>
    </servlet-mapping>

</web-app>

5、配置Tomcat，并启动测试

新增Artifact，选择war，右侧可自定义项目运行路径

启动tomcat

小结：

servlet的映射过程，浏览器连接web服务，web服务通过已注册的信息找到对应的servlet进行访问

新增ErrorServlet类

public class ErrorServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");

        PrintWriter writer = resp.getWriter();
        writer.print("<h1>404</h1>");
    }

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

web.xml注册ErrorServlet

  <servlet>
    <servlet-name>ErrorServlet</servlet-name>
    <servlet-class>com.alan.servlet.ErrorServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ErrorServlet</servlet-name>
    <url-pattern>/*</url-pattern>
  </servlet-mapping>

启动tomcat测试

结论：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

ServletContext

web服务启动时都会创建一个ServletContext，代表当前web应用，是Servlet池

不同的Servlet可以通过ServletContext共享数据

设置一个set和一个get，测试实现结果

ServletContext context = this.getServletContext();
context.setAttribute("username","alan");

ServletContext context = this.getServletContext();
String username = (String)context.getAttribute("username");

获取web.xml中的初始化参数

  <!-- 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>

ServletContext context = this.getServletContext();
String url = context.getInitPatamter("url");

指定路径转发请求

//配置转发路径
RequestDispatcher requestDispatcher=context.getRequestDispatcher("/user");
//forward实现转发
requestDispatcher.forward(req,resp);

转发和重定向的概念

转发：A需要资源，只能去找B，B再去找C，再向A返回资源

重定向：A需要资源，A找到B，B告知A资源在C，A找C获得资源

异同：页面都实现跳转，请求转发URL不发生变化，重定向URL发生变化

读取资源文件

//db.properties
username=xxxx
password=xxxx

InputStream inputStream = this.getServletContext().getResourcesAsStream("/WEB-INF/classes/db.properties");
Properties prop = new Properties();
prop.load(inputStream);
String username = prop.getProperty("username");