一、为什么需要Jsp
前面我们学习Servlet时，servlet既要写业务逻辑也要写页面展现，耦合性太大，那么有没有一种技术把业务逻辑和界面展现分开呢？所以后来引入了Jsp。
- Servlet:
优点：逻辑处理方便
缺点：页面表现麻烦 - JSP：
优点：页面表现方便
缺点：逻辑处理麻烦
所以，一般在Web项目中，采用Jsp+Servlet+JavaBean的形式。后面还有更高级的框架SSM。
二、Jsp的本质是Servlet
JSP是一种动态网页技术（动态生成网页数据，而不是有动态效果的网页！），JSP就是Servlet，JSP也是Java类，通过JSP引擎将JSP转译成Servlet。
JSP≈java+html
我们探究一下Jsp的本质：
首先创建一个项目，命名JspProject，然后创建一个1.jsp文件
然后网页访问1.jsp，正常显示：
然后我们找到编译后的jsp文件，如果用Idea编辑器的话就是在这个目录：C:\Users\Administrator.IntelliJIdea2019.2\system\tomcat\Unnamed_JspProject\work\Catalina\localhost\JspProject\org\apache\jsp
然后我们打开_1_jsp.java文件看一下：
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/7.0.100
* Generated at: 2020-03-17 02:13:24 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 _1_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent {
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;
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 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 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("\r\n");
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("这是jsp页面\r\n");
out.write("</body>\r\n");
out.write("</html>\r\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);
}
}
}
从上述文件我们可以得到如下信息：
- 继承于org.apache.jasper.runtime.HttpJspBase
- 我们在jsp中写的html界面都是通过response写出来了。
接下来我们去Tomcat中找到jasper.jar包，然后反编译找到org.apache.jasper.runtime.HttpJspBase类看一下：
看到了吧，最终还是继承HttpServlet，所以以上验证了我们刚才说的：
jsp文件是java了，本质就是一个servlet
三、Jsp执行过程
- 浏览器输入：localhost:8080/jsp/1.jsp
- tomcat收到*.jsp请求，则会到org.apache.jasper.servlet.JspServlet处理
- JspServlet调用相应的java类，jsp引擎根据_1_.jsp生成的java类（位于tomcat子目录work下面：_1_jsp.java,_1_jsp.class）
- 执行_1_.jsp.class,将HTML数据输出给tomcat，tomcat再将这些数据输出给客户端。
四、搭建一个jsp页面
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>Title</title>
</head>
<body>
这是jsp页面
</body>
</html>
