简介：
JSP就是一种特俗的servlet，是java对前端页面功能的一种补充，jsp文件在被第一次请求时会变被服务器编译成java文件，然后再把java文件编译成.class文件，然后再调用它的service()方法；
在以后的请求时便不会再进行上面的步骤，这就是第一次惩罚（第一次请求的时间会长一些）
JSP使用java代码的三个标签：
NUM |
标签 |
作用 |
| 1 | <% %> | 代码块，可以在这个标签内编写任意的java代码 |
| 2 | <%! %> | 声明代码块，可以在这个标签内编写当前JSP的成员属性和成员方法 |
| 3 | <%= %> | 输出代码块，等号后必须是输出的内容 |
从作用上应该就能感觉出，2是不常用的。
<!-- jsp指令，一种特俗的标签 -->
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<%
String path = request.getContextPath(); //获取项目名
// 基础路径 // http://localhost:8080/项目名
String basePath = request.getScheme() + "://" + request.getServerName() + ":" + request.getServerPort() + path + "/";
%>
<html>
<head>
<!-- 向页面输出basePath -->
<base href="<%=basePath%>"> <!-- <base>标签如果有值，那么下面<body>标签中的所有标签的href="basePath/xxx",相当于吧basePath的值加在所有href连接前面 -->
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>JSP</title>
</head>
<body>
<h1>This is a GG</h1>
<% // 局部变量
int a = 10;
%>
<%=a%>
<%! // 声明成员变量和成员方法
int a = 100;
public void fun1(){
System.out.println(a);
}
%>
<%
out.println(a++); // 此时也是输出的局部变量的a, 重复请求并不会加一
out.println(this.a++); // 此时输出的就是成员变量的a,并且在重复请求时会加一,但是在修改了jsp页面后会重置
fun1(); //掉用方法
%>
</body>
</html>
在JSP中使用java代码块遍历表格标签内容：
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>a.jsp</title>
</head>
<body>
<table border="1" align="center" width="69%">
<tr>
<th>姓名</th>
<th>年级</th>
</tr>
<tr align="center">
<td>张三</td>
<td>19</td>
</tr>
<tr align="center">
<td>李四</td>
<td>15</td>
</tr>
<tr align="center">
<td>王五</td>
<td>21</td>
</tr>
<!-- 向上面那样加太麻烦了 -->
<% for(int i = 0; i < 10; i++) { %>
<tr align="center">
<td>王五<%=i %></td>
<td><% out.print(i); %></td>
</tr>
<% } %>
</table>
</body>
</html>
JSP与Servlet的联动：
通过JSP提交表单信息交给Servlet处理然后保存在request中再转发到另一个JSP页面并显示结果：
form.jsp:
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>title</title>
</head>
<body>
<div align="center">
<form action="/day0420_JSP01/AServlet" method="post">
int1: <input type="text" name="int1"><br/>
int2: <input type="text" name="int2"><br/>
<input type="submit" value="提交">
</form>
</div>
</body>
</html>
res.jsp:
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>title</title>
</head>
<body>
<%
Integer res = (Integer)request.getAttribute("res");
%>
<h1 align="center">res:<%=res %></h1>
</body>
</html>
AServlet:
package demoServlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/AServlet")
public class AServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// 获取参数
String s1 = request.getParameter("int1");
String s2 = request.getParameter("int2");
int int1 = Integer.parseInt(s1);
int int2 = Integer.parseInt(s2);
int res = int1 + int2;
// 设置参数
request.setAttribute("res", res);
// 转发
request.getRequestDispatcher("/jsp/res.jsp").forward(request, response);
}
}
输出信息提交：
结果显示：
这样就成将JSP页面提交的数据在Servlet中处理然后转发到另外的JSP中了。
可以在文件夹中发现编译后的JSP文件，我的路劲是下面的这个：
D:\SoftWare\SoftWareFiles\EclipsProject\ServletDemo\.metadata\.plugins\org.eclipse.wst.server.core\tmp1\work\Catalina\localhost\day0420_JSP01\org\apache\jsp\jsp
当我们写JSP页面时，在编译为java文件的时候都会被转换为字符串进行输出，而使用java代码块标签的在java文件中则是使用的原生java代码，例如：
上面的form.jsp在编译为form_jsp.java文件后的内容是：
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/9.0.30
* Generated at: 2022-04-20 06:04:55 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.jsp;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
public final class form_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<java.lang.String,java.lang.Long> _jspx_dependants;
static {
_jspx_dependants = new java.util.HashMap<java.lang.String,java.lang.Long>(2);
_jspx_dependants.put("/WEB-INF/lib/taglibs-standard-impl-1.2.5.jar", Long.valueOf(1649844437910L));
_jspx_dependants.put("jar:file:/D:/SoftWare/SoftWareFiles/EclipsProject/ServletDemo/.metadata/.plugins/org.eclipse.wst.server.core/tmp1/wtpwebapps/day0420_JSP01/WEB-INF/lib/taglibs-standard-impl-1.2.5.jar!/META-INF/c.tld", Long.valueOf(1425949870000L));
}
private static final java.util.Set<java.lang.String> _jspx_imports_packages;
private static final java.util.Set<java.lang.String> _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<java.lang.String,java.lang.Long> getDependants() {
return _jspx_dependants;
}
public java.util.Set<java.lang.String> getPackageImports() {
return _jspx_imports_packages;
}
public java.util.Set<java.lang.String> 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 {
if (!javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
final java.lang.String _jspx_method = request.getMethod();
if ("OPTIONS".equals(_jspx_method)) {
response.setHeader("Allow","GET, HEAD, POST, OPTIONS");
return;
}
if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method)) {
response.setHeader("Allow","GET, HEAD, POST, OPTIONS");
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("\r\n");
out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n");
out.write("<html>\r\n");
out.write("<head>\r\n");
out.write("\r\n");
out.write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=UTF-8\">\r\n");
out.write("<title>title</title>\r\n");
out.write("</head>\r\n");
out.write("<body>\r\n");
out.write("<div align=\"center\">\r\n");
out.write("<form action=\"/day0420_JSP01/AServlet\" method=\"post\">\r\n");
out.write("\tint1: <input type=\"text\" name=\"int1\"><br/>\r\n");
out.write("\tint2: <input type=\"text\" name=\"int2\"><br/>\r\n");
out.write("\t<input type=\"submit\" value=\"提交\">\t\r\n");
out.write("</form>\r\n");
out.write("</div>\r\n");
out.write("</body>\r\n");
out.write("</html>");
} 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);
}
}
}
可以发现，JSP里面除java代码块标签以外的代码在java文件中都是通过字符串输出，显示在JSP的编写页面（不是浏览器的请求页面）
这里有个小注意事项：jsp被编译为java文件时，会忽略jsp中的<%-- --%>注释部分，所以，java文件中没有jsp代码中的注释部分。
