ServletContext

1、簡介

web容器在啟動時會為每個web程式建立一個ServletContext物件，它代表了當前的web應用

2、應用（以下方法現在幾乎不用）

• 共享資料（setAttribute()，getAttribute()）

  在一個Servlet中儲存的資料，可以在另一個Servlet中拿到，例：

  放置類

  public class HelloServlet extends HttpServlet {
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          ServletContext context = this.getServletContext();
          String userName = "張晟偉";
          context.setAttribute("userName",userName);//將userName放進content物件
      }
  }
   
  

  獲取類

  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().println("名字："+userName);
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req, resp);
      }
  }
   
  

  xml

   <servlet>
          <servlet-name>hello</servlet-name>
          <servlet-class>com.zhang.servlet.HelloServlet</servlet-class>
      </servlet>
      <servlet-mapping>
          <servlet-name>hello</servlet-name>
          <url-pattern>/hello</url-pattern>
      </servlet-mapping>
      <servlet>
          <servlet-name>getcontext</servlet-name>
          <servlet-class>com.zhang.servlet.GetServlet</servlet-class>
      </servlet>
      <servlet-mapping>
          <servlet-name>getcontext</servlet-name>
          <url-pattern>/getc</url-pattern>
      </servlet-mapping>
   
  

  測試結果：

  原理圖：

• 獲取初始化引數（getInitParameter()）

  java程式碼

  public class ServletDemo_01 extends HttpServlet {
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          ServletContext context = this.getServletContext();
          String url = context.getInitParameter("url");
          resp.getWriter().println(url);
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req, resp);
      }
  }
  

  xml

   <!--
      context-param標籤為配置一些web應用的初始化引數
      -->
      <context-param>
          <param-name>url</param-name>
          <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
      </context-param>
      <servlet>
          <servlet-name>gp</servlet-name>
          <servlet-class>com.zhang.servlet.ServletDemo_01</servlet-class>
      </servlet>
      <servlet-mapping>
          <servlet-name>gp</servlet-name>
          <url-pattern>/gp</url-pattern>
      </servlet-mapping>
  

• 請求轉發（RequestDispatcher）

  原理圖：

  java程式碼

  public class ServletDemo_04 extends HelloServlet{
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          ServletContext context = this.getServletContext();
  //        RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");//請求引數物件，轉發請求路徑
  //        requestDispatcher.forward(req,resp);//呼叫方法實現請求轉發
          context.getRequestDispatcher("/gp").forward(req,resp);//將被註釋的兩行程式碼合併
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req, resp);
      }
  }
  

  xml

  <servlet>
          <servlet-name>respdispatcher</servlet-name>
          <servlet-class>com.zhang.servlet.ServletDemo_04</servlet-class>
      </servlet>
      <servlet-mapping>
          <servlet-name>respdispatcher</servlet-name>
          <url-pattern>/rc</url-pattern>
      </servlet-mapping>
  

  實現效果：

  !

• 讀取資原始檔（Properties）

  新建Properties

  • 在resources下新建properties檔案

    啟動伺服器propertis檔案匯出

  • 在java目錄下新建properties檔案

    注：由於在maven中約定大於配置，所以寫在java目錄下的propertis檔案可能無法被匯出或者生效

    啟動伺服器發現在target檔案相應路徑下並沒有aa.properties檔案

    解決方法

    在pom.xml檔案中配置resources

    <!--在build中配置resources，來防止我們資源匯出失敗問題-->
        <build>
            <resources>
                <resource>
                    <directory>src/main/resources</directory>
                    <includes>
                        <include>**/*.properties</include>
                        <include>**/*.xml</include>
                    </includes>
                    <filtering>true</filtering>
                </resource>
                <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
            </resources>
        </build>
    

    重啟伺服器，問題解決

  • 發現properties檔案都被打包到了同一個路徑下：classes，我們俗稱 這個路徑為 classpath（類路徑）

  讀取Properties

  需要一個流及properties中的內容

  java程式碼

  public class ServletDemo_05 extends HttpServlet {
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          ServletContext context = this.getServletContext();
          InputStream i = context.getResourceAsStream("/WEB-INF/classes/db.properties");
          Properties prop = new Properties();
          prop.load(i);
          String user = prop.getProperty("username");
          String pwd = prop.getProperty("password");
          resp.getWriter().println(user+":"+pwd);
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req, resp);
      }
  }
  

  xml

  <servlet>
      <servlet-name>a_05</servlet-name>
      <servlet-class>com.zhang.servlet.ServletDemo_05</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>a_05</servlet-name>
      <url-pattern>/rp</url-pattern>
  </servlet-mapping>
  

  實現效果