ServletContext对象

1、ServletContext（应用程序）

得到ServletContext的两种方式：

/**
 * Servlet implementation class servletDemo
 */
@WebServlet("/servletDemo")
public class servletDemo extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public servletDemo() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        //得到servletcontext的第一种方式
        ServletContext context = this.getServletConfig().getServletContext();
        //第二种方式
        context = this.getServletContext();
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

 2、ServletContext域（这是一个容器，说明了应用程序范围）

就比如一个人往一个应用程序里面放东西，如果这个应用程序没有退出，那么其他人也可以访问这个东西，示例如下：

ServletContextDemo1.java

package test;

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 ServletContextDemo1
 */
@WebServlet("/ServletContextDemo1")  //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo1 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        this.getServletContext().setAttribute("name", "bbb");
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

ServletContextDemo2.java

package test;

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 ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String name = (String) this.getServletContext().getAttribute("name");
        System.out.println(name);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

先访问ServletContextDemo1后访问ServletContextDemo2控制台就会输出bbb。

中间报了一个错：[X]和 [Y] 的servlet不能映射为一个url模式(url-pattern)

这是因为使用“创建Servlet”向导创建了一个Servlet，url-pattern，然后添加了一个 @WebServlet 注释，它们可以代替到web.xml中的任何内容，但是你又在web.xml文件里面加了文件映射路径。

你应该使用一个或另一个，而不是两者。从 web.xml 中删除​​映射，然后继续使用 @WebServlet 注释。

 

2、通过getInitParameter获得给所有应用程序配置的参数

在web.xml文件里面通过context-param标签就可以给所有应用程序（也就是所有项目）添加参数。

<context-param>
      <param-name>data</param-name>
      <param-value>xxxx</param-value>
  </context-param>
  <context-param>
      <param-name>data1</param-name>
      <param-value>xxxxxxx</param-value>
  </context-param>

Java文件：

package test;

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 ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String name = this.getServletContext().getInitParameter("data1");
        System.out.println(name);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

3、转发

由于在Java文件里面写html代码太麻烦，例如要向浏览器写入<div color='red'></div>，那么我们还需要转成二进制流，之后再输出

write("<div color='red'></div>".getbytes())，太麻烦了，所以我们转发给jsp处理。

注意：不可以通过上面讲的ServletContext域设置参数的方式把所需数据带给jsp处理，因为应用程序只有一个servlet，其他用户使用servlet的时候可能会修改上一个用户还没来得及邮递给JSP的数据。

4、资源文件的访问

资源文件一般有两种，一种是properties文件，主要存放没有关系的数据。另一种是xml文件，用来存放有关系的数据

这一点看一下：在eclipse完成对Java_web项目里面资源文件的读取  博客，我分开写了，要不然篇幅太长。