Servlet简介和ServletContext

0x01: 什么是Servlet？

• 是sun公司开发动态web的技术
• 实现了servlet接口的Java程序

0x02: Servlet的实现类有哪些？

Servlet接口默认有两个实现类

• HttpServlet
• GenericServlet
  
  HttpServlet 继承自 GenericServlet, 一般我们自己写类只需要继承HttpServlet，重写方法就可以了

public class HelloServlet extends HttpServlet {

    //由于get或者post只是请求实现的不同的方式，可以相互调用，业务逻辑都一样；
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //ServletOutputStream outputStream = resp.getOutputStream();
        PrintWriter writer = resp.getWriter(); //响应流
        writer.print("Hello,Serlvet");
    }

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

0x03：Servlet的原理

浏览器发送http请求给web容器，web容器产生Request和Response对象，这两个对象调用servlet接口的Service方法，Service方法再将返回的Responce信息返回给Responce对象，web容器从Responce对象读取将其响应给客户端。（注意，如果web容器是首次被访问，需要先把我们的java类变成class字节码文件）

什么是ServletContext

Web容器在启动时，为每个web程序创建一个对应的ServletContext，它代表当前web应用

ServletContext的一些用法和特性

在同一个web应用中，不同的servlet可以共享数据

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 = "黎星澄"; //数据
        context.setAttribute("username",username); //将一个数据保存在了ServletContext中，名字为：username 。值 username

    }

}

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");

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().print("名字"+username);

    }

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

获取初始化参数

    <!--在web.xml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>

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    ServletContext context = this.getServletContext();
    String url = context.getInitParameter("url"); //得到name为url的参数
    resp.getWriter().print(url);
}

请求转发

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    ServletContext context = this.getServletContext();
    System.out.println("进入了ServletDemo04");
    //RequestDispatcher requestDispatcher = context.getRequestDispatcher("/xxx");           //转发的请求路径
    //requestDispatcher.forward(req,resp);      //调用forward实现请求转发；
    context.getRequestDispatcher("/gp").forward(req,resp);
}
//请求转发和重定向的区别？
请求转发：像当于在服务器内部将请求转给请求的资源，然后由服务器相应给客户端
重定向：服务器将请求资源作为响应信息响应给客户端，客户端根据这个信息对服务器发起请求

读取资源文件

在java目录下新建properties

在resources目录下新建properties

都被打包到同一路径:classes

username=admin
password=123456

public class Servlet_Properties extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/hu/servlet/test.properties"); //输入流

        Properties prop = new Properties();                                                                               //创建一个Properties对象
        prop.load(is);                                                                                                    //将读取到的加载到prop里
        String user = prop.getProperty("username");
        String pwd = prop.getProperty("password");

        resp.getWriter().print(user+":"+pwd);

    }

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