servlet 监听器和过滤器入门

监听器

三种域对象的分类和性质

全局作用域对象共享数据发生变化时触发该监听器

首先定义一个servlet用来响应请求，更新全局作用域的共享数据来触发监听器

public class OneServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        ServletContext app = req.getServletContext();
        app.setAttribute("key",100);
        app.setAttribute("key",200);
        app.removeAttribute("key");

    }
}

监听器使用

@WebListener()
public class OneListener implements ServletContextListener, ServletContextAttributeListener {
    /**
     * ServletContextListener负责监听
     * application被初始化时调用
     * @param sce
     */
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("全局作用域对象被初始化");
    }

    /**
     * ServletContextListener负责监听
     * application被销毁时调用
     * @param sce
     */
    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("全局作用域对象被销毁");
    }

    /**
     * ServletContextAttributeListener负责监听
     * application添加新的共享数据时触发调用
     * @param scae
     */
    @Override
    public void attributeAdded(ServletContextAttributeEvent scae) {
        System.out.println("新增共享数据");
    }

    /**
     * ServletContextAttributeListener负责监听
     * application更新共享数据时触发
     * @param scae
     */
    @Override
    public void attributeReplaced(ServletContextAttributeEvent scae) {
        System.out.println("更新共享数据");
    }

    /**
     * ServletContextAttributeListener负责监听
     * application共享数据被删除时触发
     * @param scae
     */
    @Override
    public void attributeRemoved(ServletContextAttributeEvent scae) {
        System.out.println("删除共享数据");
    }
}

web.xml注册监听器

<listener>
        <listener-class>com.bjpowernode.listener.OneListener</listener-class>
    </listener>

    <servlet>
        <servlet-name>OneServlet</servlet-name>
        <servlet-class>com.bjpowernode.controller.OneServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>OneServlet</servlet-name>
        <url-pattern>/one.do</url-pattern>
    </servlet-mapping>

过滤器Filter

应用：过滤器拦截请求

public class OneFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
            throws IOException, ServletException {

        //从拦截的request上获得密码
        String passwd = servletRequest.getParameter("password");
        //根据密码判断当前请求的合法性
        if("123456".equals(passwd)){
            // 合法请求，交给下一个过滤器处理
            filterChain.doFilter(servletRequest,servletResponse);
        }else { // 终止请求
            servletResponse.setContentType("text/html;charset=UTF-8");
            servletResponse.getWriter().print("密码错误，拒绝进入");
        }
    }
}

在web.xml注册

<filter>
        <filter-name>OneFilter</filter-name>
        <filter-class>com.bjpowernode.filter.OneFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>OneFilter</filter-name>
        <url-pattern>*.do</url-pattern>
    </filter-mapping>

FilterChain 理解：

通过两个过滤器对请求进行增强

写一个初始页面index.jsp用来发送请求

<body>
    <form action="/myWeb/one.do" method="post">
      参数：<input type="text" name="uname">
      <input type="submit" value="OneServlet">
    </form>

    <form action="/myWeb/two.do" method="post">
      参数：<input type="text" name="uname">
      <input type="submit" value="TwoServlet">
    </form>
  </body>

两个servlet处理请求

public class OneServlet extends HttpServlet {

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String value = req.getParameter("uname");
        System.out.println("One 得到的参数：" + value);
    }
}

public class TwoServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String value = req.getParameter("uname");
        System.out.println("Two 得到的参数：" + value);
    }
}

两个过滤器链式调用，增强请求后再发送给servlet进行处理

OneFilter

@WebFilter(filterName = "OneFilter")
public class OneFilter implements Filter {
    /**
     * 对拦截的请求进行处理
     * @param req
     * @param resp
     * @param chain
     * @throws ServletException
     * @throws IOException
     */
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        System.out.println("OneFilter 进行过滤");
        chain.doFilter(req, resp);
    }
}

TwoFilter

@WebFilter(filterName = "TwoFilter")
public class TwoFilter implements Filter {
    /**
     * 对拦截的请求进行处理
     * @param req
     * @param resp
     * @param chain
     * @throws ServletException
     * @throws IOException
     */
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        System.out.println("TwoFilter 进行过滤");
        chain.doFilter(req, resp);
    }
}

过滤器注册先后顺序会影响到其在调用链的位置

<filter>
        <filter-name>OneFilter</filter-name>
        <filter-class>com.bjpowernode.filter.OneFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>OneFilter</filter-name>
        <url-pattern>*.do</url-pattern>
    </filter-mapping>

    <filter>
        <filter-name>TwoFilter</filter-name>
        <filter-class>com.bjpowernode.filter.TwoFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>TwoFilter</filter-name>
        <url-pattern>*.do</url-pattern>
    </filter-mapping>