JavaWeb - El&JSTL-MVC-三层架构-小demo - 《Java Web》

JSP:
1. 指令
2. 注释
3. 内置对象
MVC开发模式
EL表达式
JSTL标签
三层架构

JSP

指令：用于配置JSP页面
- 导包导库等等
注释：
内置对象, 9个
- 变量名真实类型作用
  pageContext PageContext 当前页面共享数据，还可以获取其他八个内置对象
  request HttpServletRequest 一次请求访问的多个资源(转发)
  session HttpSession 一次会话的多个请求间
  application ServletContext 所有用户间共享数据
  response HttpServletResponse 响应对象
  page Object 当前页面(Servlet)的对象 this
  out JspWriter 输出对象，数据输出到页面上
  config ServletConfig Servlet的配置对象
  * exception Throwable 异常对象
为什么jsp中能直接使用这些对象，因为jsp 本质就是一个servlet
找到项目在Tomcat中的-work目录：(out是项目部署目录)
- Using CATALINA_BASE: “C:\Users\quan\AppData\Local\JetBrains\IntelliJIdea2020
- 访问这个jsp页面会编译出来.class 和 .java 文件，里面有这9个内置对象
- ```java /*
- Generated by the Jasper component of Apache Tomcat
- Version: Apache Tomcat/8.5.70
- Generated at: 2021-09-25 09:19:38 UTC
- Note: The last modified time of this file was set to
- the last modified time of the source file after
- generation to assist with modification tracking. */ package org.apache.jsp;

import javax.servlet.; import javax.servlet.http.; import javax.servlet.jsp.*;

public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase implements org.apache.jasper.runtime.JspSourceDependent, org.apache.jasper.runtime.JspSourceImports {

private static final javax.servlet.jsp.JspFactory _jspxFactory = javax.servlet.jsp.JspFactory.getDefaultFactory();

private static java.util.Map _jspx_dependants;

private static final java.util.Set _jspx_imports_packages;

private static final java.util.Set _jspx_imports_classes;

static { _jspx_imports_packages = new java.util.HashSet<>(); _jspx_imports_packages.add(“javax.servlet”); _jspx_imports_packages.add(“javax.servlet.http”); _jspx_imports_packages.add(“javax.servlet.jsp”); _jspx_imports_classes = null; }

private volatile javax.el.ExpressionFactory _el_expressionfactory; private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;

public java.util.Map getDependants() { return _jspx_dependants; }

public java.util.Set getPackageImports() { return _jspx_imports_packages; }

public java.util.Set getClassImports() { return _jspx_imports_classes; }

public javax.el.ExpressionFactory _jsp_getExpressionFactory() { if (_el_expressionfactory == null) { synchronized (this) { if (_el_expressionfactory == null) { _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory(); } } } return _el_expressionfactory; }

public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() { if (_jsp_instancemanager == null) { synchronized (this) { if (_jsp_instancemanager == null) { _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig()); } } } return _jsp_instancemanager; }

public void _jspInit() { }

public void _jspDestroy() { }

public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response) throws java.io.IOException, javax.servlet.ServletException {

final java.lang.String _jspx_method = request.getMethod();
if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
  response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSP 只允许 GET、POST 或 HEAD。Jasper 还允许 OPTIONS");
  return;
}
final javax.servlet.jsp.PageContext pageContext;
javax.servlet.http.HttpSession session = null;
final javax.servlet.ServletContext application;
final javax.servlet.ServletConfig config;
javax.servlet.jsp.JspWriter out = null;
final java.lang.Object page = this;
javax.servlet.jsp.JspWriter _jspx_out = null;
javax.servlet.jsp.PageContext _jspx_page_context = null;
try {
  response.setContentType("text/html;charset=UTF-8");
  pageContext = _jspxFactory.getPageContext(this, request, response,
              null, true, 8192, true);
  _jspx_page_context = pageContext;
  application = pageContext.getServletContext();
  config = pageContext.getServletConfig();
  session = pageContext.getSession();
  out = pageContext.getOut();
  _jspx_out = out;
  out.write("\n");
  out.write("\n");
  out.write("<!DOCTYPE html>\n");
  out.write("<html lang=\"zh-CN\">\n");
  out.write("<head>\n");
  out.write("  <meta charset=\"utf-8\"/>\n");
  out.write("  <meta http-equiv=\"X-UA-Compatible\" content=\"IE=edge\"/>\n");
  out.write("  <meta name=\"viewport\" content=\"width=device-width, initial-scale=1\"/>\n");
  out.write("  <title>首页</title>\n");
  out.write("\n");
  out.write("  <!-- 1. 导入CSS的全局样式 -->\n");
  out.write("  <link href=\"css/bootstrap.min.css\" rel=\"stylesheet\">\n");
  out.write("  <!-- 2. jQuery导入，建议使用1.9以上的版本 -->\n");
  out.write("  <script src=\"js/jquery-2.1.0.min.js\"></script>\n");
  out.write("  <!-- 3. 导入bootstrap的js文件 -->\n");
  out.write("  <script src=\"js/bootstrap.min.js\"></script>\n");
  out.write("  <script type=\"text/javascript\">\n");
  out.write("  </script>\n");
  out.write("</head>\n");
  out.write("<body>\n");
  out.write("\n");
  out.write("<div align=\"center\">\n");
  out.write("  <a\n");
  out.write("          href=\"");
  out.write((java.lang.String) org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate("${pageContext.request.contextPath}", java.lang.String.class, (javax.servlet.jsp.PageContext)_jspx_page_context, null));
  out.write("/userListServlet\" style=\"text-decoration:none;font-size:33px\">查询所有用户信息\n");
  out.write("  </a>\n");
  out.write("</div>\n");
  out.write("\n");
  out.write("</body>\n");
  out.write("</html>\n");
} catch (java.lang.Throwable t) {
  if (!(t instanceof javax.servlet.jsp.SkipPageException)){
    out = _jspx_out;
    if (out != null && out.getBufferSize() != 0)
      try {
        if (response.isCommitted()) {
          out.flush();
        } else {
          out.clearBuffer();
        }
      } catch (java.io.IOException e) {}
    if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
    else throw new ServletException(t);
  }
} finally {
  _jspxFactory.releasePageContext(_jspx_page_context);
}

} }

``` 关于IDEA中Tomcat项目的部署多说几句：

在idea中配置的tomcat，在运行时idea不会把项目放到自己的webapps路径下，而是复制三份文件到 ${user.home}/.IntelliJIdea/system/tomcat 目录(Mac)下的各自项目。

MCV 开发模式

1. M：Model，模型。JavaBean - Dao
  - 完成具体的业务操作，如：查询数据库，封装对象
    2. V：View，视图。JSP - Web层，界面层
    展示数据
    3. C：Controller，控制器。Servlet - Service
    获取用户的输入
    调用模型
    将数据交给视图进行展示

三层架构

界面层(表示层)：用户看的得界面。用户可以通过界面上的组件和服务器进行交互
业务逻辑层：处理业务逻辑的。
数据访问层：操作数据存储文件。data access object

and Demo-用户信息查询

EL表达式

概念：Expression Language 表达式语言
作用：替换和简化jsp页面中java代码的编写
语法：${表达式}
使用：
1. 运算
2. 获取值
  1. el表达式只能从域对象中获取值
  2. 语法
    1. ${域名称.键名}：从指定域中获取指定键的值
    2. ${键名}：表示依次从最小的域中查找是否有该键对应的值，直到找到为止。
    3. 获取对象、List集合、Map集合的值
      1. 对象：${域名称.键名.属性名} #属性名即：getter setter方法去掉set get ,剩余部分首字母变小写
      2. List集合：${域名称.键名[索引]}
      3. Map集合：${域名称.键名.key名称} ， * ${域名称.键名[“key名称”]}
    4. 隐式对象：
      1. el表达式中有11个隐式对象 pageContex
      2. 可获取jsp其他八个内置对象

JSTL

概念：JavaServer Pages Tag Library JSP标准标签库，是由Apache组织提供的开源的免费的jsp标签 <标签>
作用：用于简化和替换jsp页面上的java代码
使用步骤：
1. 导入jstl相关jar包
2. 引入标签库：taglib指令： <%@ taglib %> #比如要JSTL标签；就像java中导包
3. 使用标签
常用JSTL 标签
- if:相当于java代码的if语句
  - 属性：test
- choose:相当于java代码的switch语句
- foreach:相当于java代码的for语句