ServletContext

一、简介

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。

二、作用

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。

三、实例 3.1、通过ServletContext对象实现不同的Servlet的数据存取

1.步骤一：编写Store类实现数据的存放

package com.massimo.charpter11;

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 Get extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext context = this.getServletContext();
        //取出数据
        String userName = (String)context.getAttribute("userName");
        System.out.println(userName);
    }

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

2.步骤二：编写Get类取出Store类存放的数据

package com.massimo.charpter11;

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 Get extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext context = this.getServletContext();
        //取出数据
        String userName = (String)context.getAttribute("userName");
        System.out.println(userName);
    }

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

3.步骤三：在web.xml中注册Servlet

4.步骤四：配置Tomcat，运行程序

3.2、使用ServletContext对象获取初始化参数

1.步骤一：在web.xml中配置初始化参数

  <context-param>
    <param-name>urlparam-name>
    <param-value>localhost:8080/massimoparam-value>
  context-param>

2.步骤二：编写GetInitParameter类获取初始化参数

package com.massimo.charpter11;

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.io.PrintWriter;

public class GetInitParameter extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext context = this.getServletContext();
        //获取初始化参数
        String url = context.getInitParameter("url");
        PrintWriter writer = resp.getWriter();
        writer.println(url);

    }

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

3.步骤三：注册Servlet，运行程序

3.3、实现请求转发

1.步骤一：编写RequestForwarding类，实现转发

package com.massimo.charpter11;

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 RequestForwarding extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("进入了RequestForwarding");
        ServletContext context = this.getServletContext();
        //  /page是转发的路径，然后调用forward实现请求转发
        context.getRequestDispatcher("/page").forward(req , resp);
    }

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

2.步骤二：编写Page类，作为转发的目的地

package com.massimo.charpter11;

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.PrintWriter;

public class Page extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setCharacterEncoding("UTF-8");
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter writer = resp.getWriter();
        writer.println("转发成功！");
    }

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

3.步骤三：注册Servlet

<servlet>
    <servlet-name>RequestForwardingservlet-name>
    <servlet-class>com.massimo.charpter11.RequestForwardingservlet-class>
  servlet>
  <servlet-mapping>
    <servlet-name>RequestForwardingservlet-name>
    <url-pattern>/forwardurl-pattern>
  servlet-mapping>
  
  <servlet>
    <servlet-name>Pageservlet-name>
    <servlet-class>com.massimo.charpter11.Pageservlet-class>
  servlet>
  <servlet-mapping>
    <servlet-name>Pageservlet-name>
    <url-pattern>/pageurl-pattern>
  servlet-mapping>

4.步骤四：运行程序

3.4、读取资源文件

1.步骤一：建立properties文件，要求不能超出main目录，应为代码就在main目录下

2.步骤二：创建GetPropertiesFile类读取资源文件

package com.massimo.charpter11;

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.io.InputStream;
import java.util.Properties;

public class GetPropertiesFile extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext context = this.getServletContext();
        //通过context获取输入流对象
        InputStream in = context.getResourceAsStream("/WEB-INF/classes/bb.properties");
        //创建Properties对象
        Properties prop = new Properties();
        //通过prop对象加载输入流
        prop.load(in);
        //获取Properties文件数据
        String userName = prop.getProperty("userName");
        String password = prop.getProperty("password");
        resp.getWriter().println(userName + ":" + password);
    }

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

注意：getResourceAsStream()的参数是文件的相对路径，具体路径参照下图。

可能出现的问题： 项目src下的配置文件没有同步至target。

解决方法：在文件都标明的情况下，在pom.xml中添加如下标签。

<build>
  <resources>
    <resource>
      <directory>src/main/javadirectory>
      <includes>
        <include>**/*.ymlinclude>
        <include>**/*.propertiesinclude>
        <include>**/*.xmlinclude>
      includes>
      <filtering>falsefiltering>
    resource>
    <resource>
      <directory>src/main/resourcesdirectory>
      <includes>
        <include>**/*.ymlinclude>
        <include>**/*.propertiesinclude>
        <include>**/*.xmlinclude>
        <include>**/*.confinclude>
      includes>
      <filtering>falsefiltering>
    resource>
    <resource>
      <directory>libdirectory>
      <includes>
        <include>**/*.jarinclude>
      includes>
    resource>
  resources>
build>

3.步骤三：注册Servlet，运行程序