ServletContext上下文对象

ServletContext的总结：
①什么是ServletContext呢？
首先我们来其API给的解释：Defines a set of methods that a servlet uses to communicate with its servlet container, for example, to get the MIME type of a file, dispatch requests, or write to a log file.
这是一个用于和servlet容器交流的的servlet类，它定义一个方法的集合。例如获得文件的类型，分派调遣request对象 或者写一个日志文件。
 There is one context per "web application" per Java Virtual Machine. (A "web application" is a collection of servlets and content installed under a specific subset of the server's URL namespace such as /catalog and possibly installed via a .war file.)
java虚拟机的每一个web应用都有一个servletcontext上下文对象。（一个web应用是servlet的集合和一个被安装在服务器的url命名空间下具体的子集的上下文，例如/catalog和可能被安装取道一个.war文件）
In the case of a web application marked "distributed" in its deployment descriptor, there will be one context instance for each virtual machine. In this situation, the context cannot be used as a location to share global information  (because the information won't be truly global). Use an external resource like a database instead.
至于一个web应用被标记为分布式在他的部署描述器中，每一个虚拟机器将会有一个上下文实例。在这种情况下，下文不能被用来当作一个分享全面信息的场所。（因为信息不将是真正的全的）使用一个额外的资源例如数据库代替。
The ServletContext object is contained within the ServletConfig object, which the Web server provides the servlet when the servlet is initialized.
上下文对象被包含在servletconfig对象中的，web服务器提供一个servletcontext当这个servlet被初始化
可能看官方网站的API有点模糊，其实总结下来也就是一下几点：
（1）servletContext就是代表web应用的一个servlet类。也就是说一个web应用只有一个servletContext对象。
（2）servletContext是用来管理web应用下的资源的。
（3）servletContext是可以被所有访问其代表web应用的用户共享的。
（4）servletContext是在服务器端被创建和销毁的。
②servletContext的生命周期
在web应用被创建时创建与其对用的servletContext,在其管理的web应用被关闭时servletContext也被销毁。所以servletcontext中的数据会长时间的保存在服务器内存中，因此最后不要往servletcontext中存储过大的数据，以免降低服务器的运行效率。
③那么我们如何获得servletContext呢？
在servlet类中获得servletContext主要有两种方式：
（1）通过this.getServletContext()方法获得上下文的引用。
（2）通过servletConfig.getServletContext()方法获得上下文的引用。
这两种方式都能获得servletContext引用。并且没有任何区别。
④ServletContext的应用场景有哪些呢？
（1）多个Servlet类通过ServletContext实现数据的共享。
（2）获得web应用的初始化参数。之前学习Servlet时就会给一个Servlet配置初始化参数。
servlet初始化参数配置如下：
<servlet>
    <servlet-name>注册名</servlet-name>
    <servlet-class>类路径</servlet-class>
    <init-param>
        <param-name>参数名</param-name>
        <param-value>参数值</param-value>
    </init-param>
<servlet>
<servlet-mapping>
    <servlet-name>注册名</servlet-name>
    <url-pattern>访问地址</url-pattern>
<servlet-mapping>
如果给web应用配置初始化参数或者说是让web应用下的所有servlet都可以获得该初始化参数则应该这样配置初始化参数
<context-param>
    <param-name>参数名</param-name>
    <param-value>参数值</param-value>
</context-param>
那么我们应该如何来获得给web应用配置的初始化参数呢?
其实很简单就是通过我们的ServletContext对象来获得初始化参数的。

给我们的web应用配置初始化参数如下：

获得参数的java代码如下：

package com.yd.serlvlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * Servlet implementation class ReadAttribute */@WebServlet("/ReadAttribute")public class ReadAttribute extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public ReadAttribute() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {response.setContentType("text/html;charset=utf-8");//获得上下文对象获得设置的属性ServletContext servletContext=this.getServletConfig().getServletContext();String name=(String) servletContext.getAttribute("name");//显示到网页上PrintWriter out=response.getWriter();out.println("没有移除属性之前name属性的值是："+name);//移除name属性servletContext.removeAttribute("name");String name2=(String) servletContext.getAttribute("name");out.println("移除属性之后name属性的值是："+name2);}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

（3）实现servlet转发，通过ServletContext可以实现页面的转发，和我们的request.getRequestDispatcher("资源名").forward(request,response);的用法一样。可以共享request对象，也是在服务器中发生页面跳转，只能访问该web应用下的资源。

java代码如下：

package com.yd.serlvlet;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * Servlet implementation class Forward */@WebServlet("/Forward")public class Forward extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public Forward() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {//直接转发request.setCharacterEncoding("utf-8");//给request对象添加属性request.setAttribute("age", 18);//通过servletcontext实现转发this.getServletConfig().getServletContext().getRequestDispatcher("/GetInitParam").forward(request, response);}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

package com.yd.serlvlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * Servlet implementation class GetInitParam */@WebServlet("/GetInitParam")public class GetInitParam extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public GetInitParam() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {response.setContentType("text/html;charset=utf-8");//获得参数ServletContext servletContext=this.getServletContext();String name=servletContext.getInitParameter("name");PrintWriter out=response.getWriter();//获得请求对象的属性Integer age=(Integer) request.getAttribute("age");out.println("初始化参数name的值："+name+"request的属性age:"+age);}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

运行结果如下：

（4）因为之前我们就说过servletContext是用来管理整个web应用的，所以可以通过servletContext来读取web应用下的资源文件。但是这里需要注意的是读取的web应用资源只能是webcontent下的资源不能是src目录下的资源。要想读取src下的资源需要通过类加载器。并且获得src资源时一定要注意资源地址包含在哪个包下，资源之间用/隔开。否则就不能正确的访问到src下的文件。
java代码如下：

package com.yd.servlet;import java.io.IOException;import java.io.InputStream;import java.io.PrintWriter;import java.util.Properties;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import com.sun.corba.se.spi.orbutil.fsm.Input;/** * Servlet implementation class GetResource */@WebServlet("/GetResource")public class GetResource extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public GetResource() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {//设置响应对象response.setContentType("text/html;charset=utf-8");//首先获得上下文对象ServletContext servletContext=this.getServletContext();//以流的形式获得资源InputStream is=servletContext.getResourceAsStream("/properties/count.properties");//将流作为参数放大属性对象中Properties properties=new Properties();properties.load(is);//获得属性对象中的键值PrintWriter out=response.getWriter();out.println("获得属性对象中的值username:"+properties.getProperty("count")+"<br/>");//如果要获得src下资源需要使用类加载器InputStream inputStream=this.getClass().getClassLoader().getResourceAsStream("/com/yd/serlvlet/dbinfo.properties");Properties properties2=new Properties();properties2.load(inputStream);System.out.println(properties2);out.println("src获得属性对象中的值username:"+properties2.getProperty("username"));}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

文件内容截图如下：

运行结果如图：

（5）获得资源在服务器的绝对路径。这个对于项目部署的服务器中要找到一个资源是很有用的。
java代码如下：

package com.yd.serlvlet;import java.io.IOException;import java.io.PrintWriter;import java.net.URL;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * Servlet implementation class GetRealPath */@WebServlet("/GetRealPath")public class GetRealPath extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public GetRealPath() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {//设置响应对象的响应文件类型response.setContentType("text/html;charset=utf-8");//获取上下文对象ServletContext servletContext=this.getServletContext();//获得资源在服务器的位置String path=servletContext.getRealPath("/image/context初始化参数.png");//获得资源的urlURL url=servletContext.getResource("/image/context初始化参数.png");PrintWriter out=response.getWriter();out.println("服务器位置path="+path+"<br/>");out.println("url="+url.getPath());}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

运行结果如下：