JavaWeb篇_12——ServletContext對(duì)象

ServletContext對(duì)象

ServletContext對(duì)象介紹

ServletContext官方叫Servlet上下文。服務(wù)器會(huì)為每一個(gè)Web應(yīng)用創(chuàng)建一個(gè)ServletContext對(duì)象。這個(gè)對(duì)象全局唯一，而且Web應(yīng)用中的所有Servlet都共享這個(gè)對(duì)象。所以叫全局應(yīng)用程序共享對(duì)象。

ServletContext對(duì)象的作用

• 相對(duì)路徑轉(zhuǎn)絕對(duì)路徑
• 獲取容器的附加信息
• 讀取配置信息
• 全局容器

相對(duì)路徑轉(zhuǎn)絕對(duì)路徑

context.getRealPath("path")

該方法可以將一個(gè)相對(duì)路徑轉(zhuǎn)換為絕對(duì)路徑，在文件上傳與下載時(shí)需要用到該方法做路徑的轉(zhuǎn)換。

   	//獲取ServletContext對(duì)象
       ServletContext servletContext = this.getServletContext();
       //轉(zhuǎn)換路徑
       String realPath = servletContext.getRealPath("image.PNG");
       File file=new File(realPath);

獲取容器的附加信息

servletContext.getServerInfo()

返回Servlet容器的名稱和版本號(hào)

servletContext.getMajorVersion()

返回Servlet容器所支持Servlet的主版本號(hào)。

servletContext.getMinorVersion()

返回Servlet容器所支持Servlet的副版本號(hào)。

public class GetBaseInfoServlet extends HttpServlet {
   @Override
   protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       this.doPost(req, resp);
   }

   @Override
   protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       //獲取ServletContext對(duì)象
       ServletContext servletContext = this.getServletContext();
       //獲取容器的基本信息
       String serverInfo = servletContext.getServerInfo();
       //獲取容器所支持的主版本號(hào)
       int majorVersion = servletContext.getMajorVersion();
       //獲取容器所支持的副版本號(hào)
       int minorVersion = servletContext.getMinorVersion();
       //設(shè)置響應(yīng)編碼
       resp.setContentType("text/plain;charset=utf-8");
       PrintWriter pw = resp.getWriter();
       pw.println("容器的基本信息："+serverInfo);
       pw.println("容器所支持Servlet的版本號(hào)為："+majorVersion+"."+minorVersion);
   }
}

    <servlet>
        <servlet-name>getBaseInfoServlet</servlet-name>
        <servlet-class>com.java.GetBaseInfoServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getBaseInfoServlet</servlet-name>
        <url-pattern>/getBaseInfo.do</url-pattern>
    </servlet-mapping>

獲取web.xml文件中的信息

<context-param>
  <param-name>key</param-name>
  <param-value>value</param-value>
</context-param>

servletContext.getInitParameter("key")

該方法可以讀取web.xml文件中標(biāo)簽中的配置信息。

servletContext.getInitParameterNames()

該方法可以讀取web.xml文件中所有param-name標(biāo)簽中的值。

    <context-param>
        <param-name>key1</param-name>
        <param-value>java1</param-value>
    </context-param>

    <context-param>
        <param-name>key2</param-name>
        <param-value>java2</param-value>
    </context-param>

public class ContextReadInfoServlet extends HttpServlet {
   @Override
   protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       this.doPost(req, resp);
   }

   @Override
   protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       //獲取ServletContext對(duì)象
       ServletContext servletContext = this.getServletContext();
       //獲取所有的<param-name>的值
       Enumeration<String> initParameterNames = servletContext.getInitParameterNames();
       PrintWriter pw = resp.getWriter();
       while (initParameterNames.hasMoreElements()){
           String name = initParameterNames.nextElement();
           String value = servletContext.getInitParameter(name);
           pw.println("name:"+name+" = value:"+value);
       }
       pw.flush();
       pw.close();
   }
}

	<servlet>
        <servlet-name>contextReadInfoServlet</servlet-name>
        <servlet-class>com.java.ContextReadInfoServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>contextReadInfoServlet</servlet-name>
        <url-pattern>/readInfo.do</url-pattern>
    </servlet-mapping>
    

全局容器

servletContext.setAttribute("key",ObjectValue)

向全局容器中存放數(shù)據(jù)。

servletContext.getAttribute("key")

從全局容器中獲取數(shù)據(jù)。

servletContext.removeAttribute("key")

根據(jù)key刪除全局容器中的value。

public class GlobalContainerServlet extends HttpServlet {
   @Override
   protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       this.doPost(req,resp);
   }

   @Override
   protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       //獲取ServletContext對(duì)象
       ServletContext servletContext = this.getServletContext();
       String s1 = (String) servletContext.getAttribute("key1");
       String s2 = (String)servletContext.getAttribute("key2");
       PrintWriter pw=resp.getWriter();
       pw.println(s1);
       pw.println(s2);
       pw.flush();
       pw.close();
   }

   @Override
   public void init() throws ServletException {
       //獲取ServletContext對(duì)象
       ServletContext servletContext = this.getServletContext();
       servletContext.setAttribute("key1","java1");
       servletContext.setAttribute("key2","java2");
   }
}

ServletContext對(duì)象生命周期

? 當(dāng)容器啟動(dòng)時(shí)會(huì)創(chuàng)建ServletContext對(duì)象并一直緩存該對(duì)象，直到容器關(guān)閉后該對(duì)象生命周期結(jié)束。ServletContext對(duì)象的生命周期非常長，所以在使用全局容器時(shí)不建議存放業(yè)務(wù)數(shù)據(jù)。