过滤器filter，监听器Listener

过滤器（filter）

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将浏览器请求的数据进行过滤，对于服务器向浏览器的相应进行过滤

//TODO:当访问/servlet1时，会先经过MyFilter1过滤器，再经过MyServlet1
@WebFilter("/servlet1")
public class MyFilter1 implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("MyFilter1过滤器执行了");
        // 放行，继续执行下一个过滤器或目标资源
        filterChain.doFilter(servletRequest,servletResponse);
    }
}

Filter的生命周期：

初始化方法（init）：

Filter的创建和销毁由Web服务器负责（Tomcat）

拦截请求方法（doFilter）：

当浏览器客户端访问与过滤器有关联的URL的时候，Servlet过滤器会先执行doFilter方法，在通过FilterChain参数去访问后续的过滤器或者直接访问对应的资源

销毁方法（destroy):

Filter对象在创建了之后会驻留在内存中，当Web应用移除的时候或者服务器停止的时候才会销毁，在Web容器卸载Filter对象之前被调用，用于释放过滤器使用的资源。

@WebFilter("/servlet1")
public class MyFilter2 implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("MyFilter2过滤器初始化");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("MyFilter2过滤器执行了");
        // 放行，继续执行下一个过滤器或目标资源
        filterChain.doFilter(servletRequest,servletResponse);
    }

    //关闭Tomcat的时候会执行该方法
    @Override
    public void destroy() {
        System.out.println("MyFilter2过滤器销毁");
    }
}

过滤器实现设置字符编码

//过滤器实现设置字符编码
@WebFilter("/servlet2")
public class MyFilter3 implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        // TODO:设置为utf-8编码
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        request.setCharacterEncoding("UTF-8");
        filterChain.doFilter(servletRequest,servletResponse);
    }
}

通过Filter实现页面总的统计量PV：通过Ctrl+shift+delete去进行清除缓存

//TODO:通过Filter实现统计页面总的访问量PV
@WebFilter("/*")
public class MyFilter4 implements Filter {
    private final static String PV_COUNT = "pvCount";
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        ServletContext application = request.getServletContext();

        Integer count = (Integer)application.getAttribute(PV_COUNT);
        if (count == null){
            //第一次访问
            application.setAttribute(PV_COUNT,1);
        }else {
            //不是第一次访问
            int count1 = count;
            application.setAttribute(PV_COUNT,count1+1);
        }
        System.out.println("当前的页面访问量为："+ application.getAttribute(PV_COUNT));
        filterChain.doFilter(servletRequest,servletResponse);
    }
}

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监听器（Listener）

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在Javaweb中用于监听Servlet的三个作用域的程序就是监听器，Javaweb的Servlet作用域对象：ServletContext/HttpSession/HttpServletRequest三个对象

（应用程序/会话/请求）—》三个对象

监听器的基础：

监听生命周期的变化：

ServletContextListener：监听ServletContext生命周期

HttpSessionListener：监听HttpSession生命周期

HttpServletRequestListener：监听HttpServletRequest生命周期

监听属性变化：（setAttribute，getAttribute，removeAttribute）

ServletContextAttributeListener：监听ServletContext属性变化

HttpSessionAttributeListener：监听HttpSessionListener属性变化

HttpServletRequestAttributeListener：监听HttpServletRequest属性变化

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Request域的监听器：–一次请求的时间是很短的

两个监听器的接口

ServletRequestListener：生命周期监听

ServletRequestAttributeListener：属性监听

@WebListener
public class RequestListener implements ServletRequestListener, ServletRequestAttributeListener {
    // 请求初始化监听
    public void requestInitialized(javax.servlet.ServletRequestEvent servletRequestEvent) {
        System.out.println("请求初始化");
    }

    // 请求销毁监听
    public void requestDestroyed(javax.servlet.ServletRequestEvent servletRequestEvent) {
        System.out.println("请求销毁");
    }
    // 请求属性添加监听
    public void attributeAdded(javax.servlet.ServletRequestAttributeEvent servletRequestAttributeEvent) {
        System.out.println("添加属性");
    }
    // 请求属性删除监听
    public void attributeRemoved(javax.servlet.ServletRequestAttributeEvent servletRequestAttributeEvent) {
        System.out.println("删除属性");
    }
    // 请求属性替换监听
    public void attributeReplaced(javax.servlet.ServletRequestAttributeEvent servletRequestAttributeEvent) {
        System.out.println("替换属性");
    }
}

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Session域的监听器：

两个监听器的接口

HttpSessionListener：生命周期监听

HttpSessionAttributeListener：属性监听

@WebListener
public class SessionListener implements HttpSessionListener, HttpSessionAttributeListener {
    @Override
    public void attributeAdded(HttpSessionBindingEvent event) {
        System.out.println("SessionListener监听到属性添加了");
    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent event) {
        System.out.println("SessionListener监听到属性删除了");
    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent event) {
        System.out.println("SessionListener监听到属性替换了");
    }

    @Override
    public void sessionCreated(HttpSessionEvent se) {
        System.out.println("SessionListener监听到session创建了");
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        System.out.println("SessionListener监听到session销毁了");
    }
}

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ServletContext域的监听器：

两个监听器的接口

ServletContextListener：生命周期监听—Tomcat一启动就监听

ServletContextAttributeListener：属性监听

public class ContextListener implements ServletContextListener, ServletContextAttributeListener {
    @Override
    public void attributeAdded(ServletContextAttributeEvent event) {
        System.out.println("ServletContextAttributeListener监听器监听到ServletContext的属性被添加了");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("ServletContextListener监听器监听到ServletContext被销毁了");
    }

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("ServletContextListener监听器监听到ServletContext被创建了");
    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent event) {
        System.out.println("ServletContextAttributeListener监听器监听到ServletContext的属性被替换了");
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent event) {
        System.out.println("ServletContextAttributeListener监听器监听到ServletContext的属性被移除了");
    }
}