servlet之ServletContext基础使用

1.mapping的问题：

一个servlet可以指定一个映射路径。
一个servlet可以指定多个映射路径
一个servlet可以指定通用映射路径
指定一些后缀或者前缀
默认请求路径

2.servletcontext

在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servletcontex对象，它代表了当前web应用。

1.共享数据（作用于session一样）

  我们在这个servlet中保存的数据，可以再另外的servlet中可以使用。

package com.kuang.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 {
    @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();
        context.setAttribute("username","1234");
    }

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

另一个

package com.kuang.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 HelloServlet2 extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doGet(req, resp);
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String username = (String) context.getAttribute("username");
        resp.setContentType("text/htm1");
        resp.getWriter().print(username);
    }
}

web.xml

<!DOCTYPE web-app PUBLIC
        "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
        "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>
    <display-name>Archetype Created Web Application</display-name>
    <!--声明servlet映射类路径-->
    <servlet>
        <servlet-name>GetServlet</servlet-name>
        <servlet-class>com.kuang.Servlet.GetServlet</servlet-class>
    </servlet>
     <!--声明servlet映射web路径-->
    <servlet-mapping>
        <servlet-name>GetServlet</servlet-name>
        <url-pattern>/servlet</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>HelloServlet2</servlet-name>
        <servlet-class>com.kuang.Servlet.HelloServlet2</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>HelloServlet2</servlet-name>
        <url-pattern>/servlet2</url-pattern>
    </servlet-mapping>
</web-app>

2.获取初始化参数

在web.xml中

    <!--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/ssm</param-value>
    </context-param>

在servlet中写入：

package com.kuang.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 ServletDmeo3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);
    }

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

然后再web.xml中加入映射

    <servlet>
        <servlet-name>gp</servlet-name>
        <servlet-class>com.kuang.Servlet.ServletDmeo3</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>gp</servlet-name>
        <url-pattern>/gp</url-pattern>
    </servlet-mapping>

3.请求转发

使用ServletContext 进行请求转发，请求转发路径不发生变化

package com.kuang.Servlet;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class ServletDemo04 extends Helloserlvt {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doGet(req, resp);
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");//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);
    }
}

//web.xml

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

4.读取资源文件

Properties
在Java目录下新建properties
在resources目录下新建properties
发现：都被打爆到同一个路径下：classes，我们俗称这个路径为classpath
思路：需要一个文件流

package com.kuang.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 ServletDemo05 extends Helloserlvt {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop = new Properties();
        prop.load(is);
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        resp.getWriter().print(username+"    "+password);
    }

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