JavaWeb入门:ServletContext详解和应用

<!--1、获取web应用程序初始化参数-->
<context-param>
    <param-name>name</param-name>
    <param-value>crush</param-value>
</context-param>
<context-param>
    <param-name>school</param-name>
    <param-value>hngy</param-value>
</context-param>

/**
 * @Author: crush
 * @Date: 2021-05-09 16:32
 * version 1.0
 */
@WebServlet("/servlet")
public class ServletContextTest extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //设置响应头
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter writer = resp.getWriter();
        ServletContext servletContext = this.getServletContext();
        // 根据名称 获取单个初始化参数
        String parameter = servletContext.getInitParameter("name");
        writer.print(parameter);
        writer.print("<br><hr>");
        // 获取全部初始化参数
        Enumeration<String> initParameterNames = servletContext.getInitParameterNames();
        while (initParameterNames.hasMoreElements()){
            String name = initParameterNames.nextElement();
            String value = servletContext.getInitParameter(name);
            writer.print(name+":"+value);
            writer.print("<hr>");
        }
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

<servlet>
    <servlet-name>/servlet1</servlet-name>
    <servlet-class>com.crush.servlet.ServletContextTest</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>/servlet1</servlet-name>
    <url-pattern>/servlet1</url-pattern>
</servlet-mapping>

/**
 * @Author: crush
 * @Date: 2021-05-09 16:59
 * version 1.0
 */
@WebServlet("/servlet1")
public class ServletContextTest1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 获取ServletContext对象
        ServletContext servletContext = this.getServletContext();
        //设置值  ServletContext 域属性 name 域属性名 obj是值
        // 往ServletContext 中放进 username=crush 这个键值对
        servletContext.setAttribute("username","crush");
        // 在控制台给出提示
        System.out.println("值已经设置完成");
        // 重定向
//        resp.sendRedirect("/servlet2");
        // 转发
        req.getRequestDispatcher("servlet2").forward(req, resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

/**
 * @Author: crush
 * @Date: 2021-05-09 16:59
 * version 1.0
 */
@WebServlet("/servlet2")
public class ServletContextTest2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 获取ServletContext对象
        ServletContext servletContext = this.getServletContext();

        // 通过之前的设置的名字 取出username 的值
        String username =(String) servletContext.getAttribute("username");
        PrintWriter writer = resp.getWriter();
        writer.print(username);
        //返回一个Enumeration其中包含该ServletContext中可用的属性名称
        // Enumeration<String> attributeNames = servletContext.getAttributeNames();
        //从此ServletContext中删除具有给定名称的属性。
//        servletContext.removeAttribute("");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

user=root
password=123456
url=jdbc:mysql://localhost:3306/mysql
drive=com

/**
 * 读取资源内容
 * @Author: crush
 * @Date: 2021-05-09 16:59
 * version 1.0
 */
@WebServlet("/servlet3")
public class ServletContextTest3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        ServletContext servletContext = this.getServletContext();
        // 获取相对路径中的输入流对象  是要获取web应用程序的路径
        InputStream inputStream = servletContext.getResourceAsStream("\\WEB-INF\\classes\\mysql.properties");
        // 资源类对象
        Properties properties = new Properties();
        // load 从输入字节流中读取属性列表（键和元素对）
        properties.load(inputStream);
        PrintWriter writer = resp.getWriter();
        writer.print("user"+properties.getProperty("user")+"<br>");
        writer.print("password"+properties.getProperty("password")+"<br>");
        writer.print("url"+properties.getProperty("url")+"<br>");
        writer.print("drive"+properties.getProperty("drive")+"<br>");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

/**
 * 请求转发
 * @Author: crush
 * @Date: 2021-05-09 16:59
 * version 1.0
 */
@WebServlet("/servlet5")
public class ServletContextTest5 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        ServletContext servletContext = this.getServletContext();
        // RequestDispatcher对象可用于将请求转发到资源或将资源包括在响应中。 资源可以是动态的也可以是静态的。
        //路径名必须以/开头，并被解释为相对于当前上下文根。 使用getContext获取外部上下文中资源的RequestDispatcher
        servletContext.getRequestDispatcher("/servlet").forward(req,resp);
        //forward:将请求从servlet转发到服务器上的另一个资源（servlet，JSP文件或HTML文件）
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

/**
 * 乱码问题
 * @Author: crush
 * @Date: 2021-05-09 16:59
 * version 1.0
 */
@WebServlet("/servlet6")
public class ServletContextTest6 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 解决乱码 只是解决单独的响应乱码 如果有请求 还要设置请求的编码
        req.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter writer = resp.getWriter();
        writer.print("你好啊，JavaWeb");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}