ServletContext对象

Java知识点总结：想看的可以从这里进入

3.5、ServletContext

ServletContex是 servlet上下文，是一个代表了整个项目的对象，全局唯一，项目内部所有servlet都能共享这个对象中保存的数据所以 ServletContext 又叫全局应用程序共享对象。

ServletContext对象在服务器启动的时候创建，服务器关闭的时候销毁。它共用四种获取方式

//第一种。 通过 GenericServlet 提供的 getServletContext() 方法
ServletContext servletContext = this.getServletContext();
//第二种。通过 ServletConfig 提供的 getServletContext() 方法
ServletContext servletContext = this.getServletConfig().getServletContext();
//第三种。通过 HttpSession 提供的 getServletContext() 方法
ServletContext servletContext = req.getSession().getServletContext();
//第四种。通过 HttpServletRequest的 getServletContext() 方法
ServletContext servletContext = req.getServletContext();
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ServletContext的作用：

1. 在各个 servlet 间实现全局数据共享

2. 获取web.xml配置的上下文参数context-param

3. 获取项目路径，读取 Web 应用下的资源文件

3.5.1 数据共享

因为ServletContext的全局性，所以可以在Servlet之间共享数据

public class Test1Servlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         doPost(request, response);
    }
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Userinfo userinfo = new Userinfo("张三","123456");
        //获取context将对象userinfo保存进去
        ServletContext context = request.getServletContext();
        context.setAttribute("userinfo",userinfo);
    }
}

public class Test2Servlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         doPost(request, response);
    }
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = request.getServletContext();
        //从context对象中获取userinfo
        Userinfo userinfo = (Userinfo) context.getAttribute("userinfo");
        System.out.println("username："+userinfo.getUsername());
        //获取所有的attributeName
        Enumeration<String> attributeNames = context.getAttributeNames();
        while (attributeNames.hasMoreElements()){
            System.out.println(attributeNames.nextElement());
        }
        //移除userinfo对象
        context.removeAttribute("userinfo");
        Userinfo userinfo1 = (Userinfo) context.getAttribute("userinfo");
        if(userinfo1 == null){
            System.out.println("移除成功");
        }
    }
}
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3.5.2 上下文参数

获取在XML文件中上下文初始化的参数

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                             http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
  
  <context-param>
    <param-name>nameparam-name>
    <param-value>yuparam-value>
  context-param>
  <context-param>
    <param-name>genderparam-name>
    <param-value>男param-value>
  context-param>
web-app>
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@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=utf-8");
    PrintWriter out = response.getWriter();

    //获取ServletContext对象
    ServletContext servletContext = this.getServletContext();
    //获取全局param-name
    Enumeration<String> initParameterNames = servletContext.getInitParameterNames();
    while (initParameterNames.hasMoreElements()){
        String paramName = initParameterNames.nextElement();
        String paramValue = servletContext.getInitParameter(paramName);
        out.write("param-name:"+paramName+".paramValue:"+paramValue+"
");
    }
    out.close();
}
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3.5.3 页面转发

访问Test1Servlet时会将页面转发到指定的转发路径，访问路径不会改变，但是会显示转发后的页面

public class Test1Servlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
       response.setContentType("text/html;charset=utf-8");
       request.setCharacterEncoding("utf-8");
		//获取ServletContext对象
        ServletContext context = this.getServletContext();
        //设置需要转发的路径
        RequestDispatcher requestDispatcher = context.getRequestDispatcher("/Test2Servlet");
        //开始转发
        requestDispatcher.forward(request,response);
    }
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}
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3.5.4、项目路径

因为是使用的Tomcat部署项目，我们的项目随然显示在idea 的路径中，但实际上还是部署到了Tomcat的webapps中了，如果直接在代码中写相对、绝对路径，很可能会导致找不到文件，而直接写磁盘路径的话，在更换服务器后一样会出现找不到资源的情况，所以ServletContext提供了getRealPath()获取项目路径，这样不管再怎么更换服务器都能获取到正确的路径。

• 根据项目内文件的相对路径获取绝对路径：context.getRealPath(“文件相对路径”);
• 返回webapp目录下的类目录列表，指示子目录路径的路径以/结尾。：getResourcePaths(String path)
• 返回映射到资源文件的 URL 对象（该路径必须以/开头）：getResource(String path)
• 根据相对路径获取服务器上资源的输入字节流：getResourceAsStream(path)

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=utf-8");

    PrintWriter out = response.getWriter();
    //获取ServletContext对象
    ServletContext servletContext = this.getServletContext();
    String realPath = servletContext.getRealPath("index.xml");
    out.write("获取的路径:"+realPath+"
");
    Set<String> servlet = servletContext.getResourcePaths("/");
    out.write("获取的web资源：");
    servlet.forEach(s -> out.write(s+","));
    out.write("
");
}
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