javaee学习日记之Servlet，过滤器，拦截器，jstl,EL,jsp

最新推荐文章于 2021-09-23 21:57:24 发布

原创最新推荐文章于 2021-09-23 21:57:24 发布 · 358 阅读

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文章标签：

#servlet #java ee #jstl #jsp #servlets

servlet 同时被 3 个专栏收录

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本文详细介绍了Servlet的实现方式，包括实现Servlet接口、继承GenericServlet和HttpServlet的方法，并解释了Servlet在web.xml中的配置方式及如何被容器加载。此外，还介绍了过滤器和监听器的工作原理及其配置。

Servlet
在web.xml容器中的加载

    <servlet>
        <servlet-name>servlets</servlet-name><!--一般用类名首字母小写表示-->
        <servlet-class>cn.ii.user.web.UserServlet</servlet-class><!--全限定类名(包+类名)-->
    </servlet>
    <!--Servlet映射-->
    <servlet-mapping>
        <servlet-name>servlets</servlet-name><!--与上名同-->
        <url-pattern>/servlets</url-pattern><!--访问时所用名-->
    </servlet-mapping>

我的理解是：让web容器加载这个类到tomcat上，通过映射，定义的名字，进行访问！
Servlet:java代码
一,实现Servlet接口

import javax.servlet.*;
import java.io.IOException;
public class BServlet implements Servlet {
    //初始化即产生
    public void init(ServletConfig servletConfig) throws ServletException {
    System.out.println("Servlet出生");
    }
    //ServletConfig:上下文对象的获取
    public ServletConfig getServletConfig() {
        return null;
    }
    //请求方法
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
    System.out.println("Servlet正在处理请求");
    }
    //servlet的一些信息
    public String getServletInfo() {
        return null;
    }
    //销毁即死去
    public void destroy() {
    System.out.println("可怜的Servlet去世了");
    }

二.继承GenericServlet

import javax.servlet.*;
import java.io.IOException;

public class BServlet extends GenericServlet {
    //初始化即产生
    public void init(ServletConfig servletConfig) throws ServletException {

    }
    //ServletConfig:上下文对象的获取
    public ServletConfig getServletConfig() {
        return null;
    }
    //请求方法
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {

    }
    //Servlet的一些信息
    public String getServletInfo() {
        return null;
    }
    //销毁即死去
    public void destroy() {

    }
}

三.继承HttpServlet(用的最多)

import javax.servlet.*;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

/**
 * 将service请求细化成get与post请求,
 */
public class BServlet extends HttpServlet {
    //get请求(明文)
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         req.setAttribute("x","");//Request域，有效范围：单次请求内有效
        req.getSession().setAttribute("x","");//Session域，有效范围：浏览器与服务器的一次会话中有效，不同浏览器Session保留时间不同！
        req.getRequestDispatcher("index.jsp").forward(req,resp);//转发，配合Request域使用！地址栏不变，还是一次请求。
        resp.sendRedirect("index.jsp");//重定向,请求改变，地址栏改变！
        resp.setContentType("text/html;charset=UTF-8");//设置响应类型
        resp.getWriter().print("IO");//响应流
        //applicationcontext域：服务器运行中有效，重启或关闭后失效
        //page域：在本页面内有效（jsp中）
        req.getHeaderNames();//请求头信息(全部)
        //jsp九大内置对象：四个域 + response响应对象 + out输出对象 +　config配置对象 +　exception异常对象 +　page页面对象
    }
    //post请求(隐藏的)
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

jsp = Servlet

jstl:标签库(为了更好的让前后端分离)

<%--设置本页编码,后端语言，响应编码--%>
<%@ page language="java" contentType="text/html; charset=UTF-8"
         pageEncoding="UTF-8"%>
<%--导入jar包--%>
<%@ page import="java.util.List"%>
<%--导入jstl标签库,前缀为c,由':'分离--%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<head>
    <title>bug</title>
</head>
<body>
<!--forEach遍历List集合中的User对象-->
    <c:forEach items="${list}" var="l">
        ${l.username}<br/>
    </c:forEach>
<% //往四大域中添加值！
   //用El表达式取(优先级,由以下顺序，由大到小)
    pageContext.setAttribute("message","0");
    request.setAttribute("message","1");
    session.setAttribute("message","2");
    application.setAttribute("message","3");
%>
<!--EL表达式-->
${message}
<!--if语句，test中是判断条件-->
<c:if test="${w} == 0">
    </c:if>
  <!--switch语句-->  
<c:choose>
    <c:when test="${x} == 1">1</c:when>
    <c:when test="${x} == 2">2</c:when>
    <c:otherwise>3</c:otherwise>
</c:choose>
<!--jsp内置标签转发-->
<jsp:forward page="login.html"></jsp:forward>
</body>
</html>

过滤器在web容器中的加载

    <filter>
        <filter-name>userFilter</filter-name>
        <filter-class>cn.ii.user.UserFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>userFilter</filter-name>
        <url-pattern>/*</url-pattern><!--过滤范围（这里是全部资源）-->
    </filter-mapping>

Filter：过滤器

import javax.servlet.*;
import java.io.IOException;

public class BBFilter implements Filter {
    public void destroy() {
        System.out.println("Filter销毁");
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        System.out.println("过滤内容");
        //可以在这里实现，解决中文乱码问题！
    }

    public void init(FilterConfig config) throws ServletException {
        System.out.println("Filter出生");
    }
}

拦截器在web容器中的加载

    <listener>
        <listener-class>cn.ii.user.ListenerS</listener-class>
    </listener>

拦截器实现
(可以实现一些资源监控,用户权限监控)

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import javax.servlet.http.HttpSessionBindingEvent;

public class ListenerS implements ServletContextListener,
        HttpSessionListener, HttpSessionAttributeListener {

    // Public constructor is required by servlet spec
    public ListenerS() {
    }

    // -------------------------------------------------------
    // ServletContextListener implementation
    // -------------------------------------------------------
    //监控context：上下文对象的创建
    public void contextInitialized(ServletContextEvent sce) {
      /* This method is called when the servlet context is
         initialized(when the Web application is deployed). 
         You can initialize servlet context related data here.
      */
    }
    //监控context：上下文对象的销毁
    public void contextDestroyed(ServletContextEvent sce) {
      /* This method is invoked when the Servlet Context 
         (the Web application) is undeployed or 
         Application Server shuts down.
      */
    }

    // -------------------------------------------------------
    // HttpSessionListener implementation
    // -------------------------------------------------------
    //监控session：会话对象的创建
    public void sessionCreated(HttpSessionEvent se) {
      /* Session is created. */
    }
    //监控session：会话对象的销毁
    public void sessionDestroyed(HttpSessionEvent se) {
      /* Session is destroyed. */
    }

    // -------------------------------------------------------
    // HttpSessionAttributeListener implementation
    // -------------------------------------------------------
    //监控session：会话中的属性添加
    public void attributeAdded(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute 
         is added to a session.
      */
    }
    //监控session：会话中的属性移除
    public void attributeRemoved(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute
         is removed from a session.
      */
    }
    //监控session：会话中的属性取代
    public void attributeReplaced(HttpSessionBindingEvent sbe) {
      /* This method is invoked when an attibute
         is replaced in a session.
      */
    }
}