JAVA_WEB之JSP

JSP

一、概念

ldea下面的tomcat的目录：C:\Users\adiminstor\AppData\Local\JetBrains

1.jsp的运行原理：

1.jsp实际上编译后会变成一个class文件，继承了一个类org.apache.jasper.runtime.HttpJspBase;会发现这实际上就是一个servlet类

1 2	public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase----> ===>public abstract class HttpJspBase extends HttpServlet implements HttpJspPage

2.内置对象

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;

3.运行过程

用户客户端发出请求，服务器在自己项目内寻找jsp，找到后，先把jsp转换成Java文件，在把Java文件编译成class文件，再交给服务器，由服务器交给用户，那么用户拿到的就是真正的class对象，实际上就是一个servlet；

4.对比jsp和jsp.java

jsp:

<%@ page contentType="text/html;charset=UTF-8" language="java"  pageEncoding="utf-8" %>
<html>
<body>
<h2>Hello World!</h2>
<%--pageContext.request.contextPath 当前目录/url pattern--%>
<% String name="saxon";%>
name=<%=name%>
<form action="${pageContext.request.contextPath}/re" method="get" >
    <p><label>username：<input type="text" name="username"></label></p>
    <p><label>password：<input type="password" name="password"></label></p>
    <p>爱好：
        <input type="checkbox" name="hobbies" value="篮球" required>篮球
        <input type="checkbox" name="hobbies" value="游戏">游戏
        <input type="checkbox" name="hobbies" value="休息">休息
        <input type="checkbox" name="hobbies" value="唱歌">唱歌
    </p>
    <input type="submit">
</form>
</body>
</html>

java

response.setContentType("text/html;charset=UTF-8");//识别
      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("<html>\n");
      out.write("<body>\n");
      out.write("<h2>Hello World!</h2>\n");
      out.write('\n');
 String name="saxon";//我们写的Java代码
      out.write("\n");
      out.write("name=");
      out.print(name);//原样输出
      out.write("\n");
      out.write("<form action=\"");
      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("/re\" method=\"get\" >\n");
      out.write("    <p><label>username：<input type=\"text\" name=\"username\"></label></p>\n");
      out.write("    <p><label>password：<input type=\"password\" name=\"password\"></label></p>\n");
      out.write("    <p>爱好：\n");
      out.write("        <input type=\"checkbox\" name=\"hobbies\" value=\"篮球\" required>篮球\n");
      out.write("        <input type=\"checkbox\" name=\"hobbies\" value=\"游戏\">游戏\n");
      out.write("        <input type=\"checkbox\" name=\"hobbies\" value=\"休息\">休息\n");
      out.write("        <input type=\"checkbox\" name=\"hobbies\" value=\"唱歌\">唱歌\n");
      out.write("    </p>\n");
      out.write("    <input type=\"submit\">\n");
      out.write("</form>\n");
      out.write("</body>\n");
      out.write("</html>\n");

==通过对比我们明白，如果我们写在jsp中的代码是Java，那么就会原模原样的输出Java代码，如果是html代码就会改写成out.write方法输出到页面上==

二、基本语法

一、导入依赖的包

<dependencies>
<!--    servlet的依赖包-->
    <dependency>
      <groupId>javax.servlet</groupId>
      <artifactId>servlet-api</artifactId>
      <version>2.5</version>
    </dependency>
<!--    jsp的包-->
    <dependency>
      <groupId>javax.servlet.jsp</groupId>
      <artifactId>javax.servlet.jsp-api</artifactId>
      <version>2.3.3</version>
    </dependency>
    <!-- https://mvnrepository.com/artifact/javax.servlet.jsp.jstl/jstl-api -->
<!--    JSTL的包-->
    <dependency>
      <groupId>javax.servlet.jsp.jstl</groupId>
      <artifactId>jstl-api</artifactId>
      <version>1.2</version>
    </dependency>
    <!-- https://mvnrepository.com/artifact/taglibs/standard -->
<!--   standard标签库-->
    <dependency>
      <groupId>taglibs</groupId>
      <artifactId>standard</artifactId>
      <version>1.1.2</version>
    </dependency>

  </dependencies>

二、基本语法

1.表达式(<%= %>)

1	<h1><%=3%></h1>

2.jsp脚本片段(<% %>)

<%
int sum=0;
    for (int i = 0; i <1000 ; i++) {
        sum+=i;
    }
    out.println (sum);
%>
<hr>
<%
    for (int i = 0; i <5 ; i++) {
%>
<h1>hello world!</h1>
<%}    %>

3.jsp声明(<%! %>)

<%!
static {
    int i=0;
}    
%>

<%--
  Created by IntelliJ IDEA.
  User: saxon
  Date: 2020/7/28
  Time: 21:08
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1><%=3%></h1>
<hr>
<%
int sum=0;
    for (int i = 0; i <1000 ; i++) {
        sum+=i;
    }
    out.println (sum);
%>
<hr>
<%
    for (int i = 0; i <5 ; i++) {
%>
<h1>hello world!</h1>
<%}    %>
    
<%!
static {
    int i=0;
}    
%>
</body>
</html>

除了声明，编译后会在类的里面，其他的编译了会在service方法里面；

三、jsp指令

1、配置错误页面

<!-- code要写在location前面-->
<error-page>
        <error-code>404</error-code>
        <location>/error/404.jsp</location>
    </error-page>
    <error-page>
        <error-code>500</error-code>
        <location>/error/500.jsp</location>
    </error-page>

404错误界面，没有页面时跳转

<%--
  Created by IntelliJ IDEA.
  User: Saxon
  Date: 2020/7/28
  Time: 22:18
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>404</title>
</head>
<body>
<img src="${pageContext.request.contextPath}/images/404.jpg" alt="404 error">
</body>
</html>

主页面显示：

<%--
  Created by IntelliJ IDEA.
  User: Saxon
  Date: 2020/7/28
  Time: 22:16
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<% int x=1/0;%> <%--500错误--%>
</body>
</html>

2.include标签的区别

1 2	<%@include file="index.jsp"%> <jsp:include page="index.jsp"/>

<%@include file=”index.jsp”%> ,就是一个页面，把全部的代码用Java写出来，就是out.write();可能出现变量重名的情况

out.write("<html>\r\n");
out.write("<head>\r\n");
out.write("    <title>Title</title>\r\n");
out.write("</head>\r\n");
out.write("<body>\r\n");
out.write("<h1>this is my include</h1>\r\n");
out.write("</body>\r\n");
out.write("</html>\r\n");
out.write("\r\n");
out.write("<h1>hello world</h1>\r\n");

<jsp:include page=”index.jsp”/>是页面的拼接，是对象拼在一起，区别就在于如果写Java代码会不会出现变量重名的情况

  static {
    _jspx_imports_packages = new java.util.HashSet<>();
    _jspx_imports_packages.add("javax.servlet");
    _jspx_imports_packages.add("java.util");
    _jspx_imports_packages.add("javax.servlet.http");
    _jspx_imports_packages.add("javax.servlet.jsp");
    _jspx_imports_classes = null;
  }
org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "index.jsp", out, false);

四、jsp的四大作用域和9大内置对象

1.9大内置对象

1.PageContext: javax.servlet.jsp.PageContext 存储信息
2**.request :javax.servlet.http.HttpServletrequest **存储信息
3.response: javax.servlet.http.HttpServletResponse 服务器向客户端的回应信息
4**.session**: javax.servlet.http.HttpSession 存储信息
5.application: javax.servlet.ServletContext 存储信息
6.config :javax.servlet.ServletConfig 配置文件
7.out :javax.servlet.jsp.jspWriter
8.page ：不常用
9.exception java.lang.Throwable 异常

2、四大作用域

pageContext.setAttribute ("NO1", "saxon1");
    request.setAttribute ("NO2", "saxon2");
    session.setAttribute ("NO3", "saxon3");
    application.setAttribute ("NO4", "saxon4");

pageContext：只有当前页面有用，其他页面数据就失效；
request：只有这个请求页有效；但是转发数据也会被带到新的页面；
session：在这个浏览器关闭前有效；
application：只有服务器炸裂时，才会失效；

测试：

<%--
  Created by IntelliJ IDEA.
  User: Saxon
  Date: 2020/7/28
  Time: 21:08
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Index</title>
</head>
<body>
<%
    pageContext.setAttribute ("NO1", "saxon1");
    request.setAttribute ("NO2", "saxon2");
    session.setAttribute ("NO3", "saxon3");
    application.setAttribute ("NO4", "saxon4");
%>
<%
    String no1 = (String) pageContext.findAttribute ("No1");
    String no2 = (String) pageContext.findAttribute ("No2");
    String no3 = (String) pageContext.findAttribute ("No3");
    String no4 = (String) pageContext.findAttribute ("No4");
    String no5 = (String) pageContext.findAttribute ("No2");
%>
<%pageContext.forward("/Test.jsp");%>
${NO1}
${NO2}
${NO3}
${NO4}
${NO5}
</body>
</html>

转发页：

<%--
  Created by IntelliJ IDEA.
  User:Saxon
  Date: 2020/7/28
  Time: 22:16
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>

<html>
<head>
    <title>test</title>
</head>
<body>
<%
    String no1 = (String) pageContext.findAttribute ("No1");
    String no2 = (String) pageContext.findAttribute ("No2");
    String no3 = (String) pageContext.findAttribute ("No3");
    String no4 = (String) pageContext.findAttribute ("No4");
    String no5 = (String) pageContext.findAttribute ("No5");
%>
${NO1}
${NO2}
${NO3}
${NO4}
${NO5}
</body>
</html>

常见场景：

1.看了一次就不会在看的，例如新闻，可以放在request中

2.用户会一直使用的信息，例如购物车，放在session中

3.这个网页，网站一直要用的，例如，浏览人数，放在application中

4.一些只在当前页面要用到的，就放在pagecontext

五、JSP标签，JSTL标签，EL表达式

一、JSTL标签

种类：

核心标签
sql标签
xml标签
格式化标签

测试一：

<form action="#" method="get">
    用户：<input type="text" name="username" value="${param.username}">
    分数：<input type="text" name="score" value="${param.score}">
    <br>
    <input type="submit" value="提交">
</form>

JSTL标签

<c:if test="${param.username=='saxon'}" var="saxon"><!-- 这里的值-->
    <c:out value="登录成功"/>
    <c:out value="${saxon}"/>
</c:if>
<c:set value="${param.score}" var="score"/>
    <c:choose>
        <c:when test="${score>90}">
            你的成绩优秀
        </c:when>
        <c:when test="${score>80}">
            你的成绩一般
        </c:when>
        <c:when test="${score>70}">
            你的成绩可以
        </c:when>
        <c:when test="${score>60}">
            你的成绩罪恶
        </c:when>
    </c:choose>

java实现：

<%
    if (request.getParameter ("username") != null && request.getParameter ("username").equals ("saxon")) {
        out.print (request.getParameter ("username") + ",欢迎你");
    }

%>
<%
    if (request.getParameter ("username") != null && request.getParameter ("username").equals ("saxon")) {
        if (request.getParameter ("score") != null) {
            String score = request.getParameter ("score");
            int i = Integer.parseInt (score);
            out.print ("\n");
            out.print ("你的成绩是：");
            switch (i / 10) {
                case 9:
                    out.println ("good");
                    break;
                case 8:
                    out.println ("better");
                    break;
                case 7:
                    out.println ("Just-so-so");
                    break;
                case 6:
                    out.println ("Usual");
                    break;
                default:
                    out.println ("worse or in put again");
            }
        }
    }
%>

测试二：

<%
    ArrayList<String> str = new ArrayList<> ();
    str.add ("张三");
    str.add ("李四");
    str.add ("王五");
    str.add ("田六");
    str.add ("李七");
    request.setAttribute ("people",str);
%>

JSTL标签：

<!--
var:名字；
items：来源
begin：开始节点
step:下标递增的次序
end:最后一个的下标
-->
<c:forEach var="list" items="${people}" begin="0" step="2" end="5">
    <c:out value="${list}"/>
</c:forEach>

Java代码：

<%
    for (int i = 0; i <5 ; i+=2) {
        out.println (str.get (i));
    }
%>

通过对比我们就可以明白，标签可以做的，我们的Java语言也可以做。那么用标签的原因就是简化代码，看起来美观一些；

二、EL表达式

格式: ${}

作用：

获取数据
执行运算
获得web执行的对象；

1
2
3

${param.score} //获得数据
${score>90}    //执行运算
${param.score}//获得web对象