Jsp的原理

在学习javaweb中jsp这块的时候，会详细了解到jsp的内容，一般，从网上的解释来看，说的比较难懂，JSP，JSP教程；

可以看到这样的解释：

JSP全名为Java Server Pages，中文名叫java服务器页面，其根本是一个简化的Servlet设计，它是由Sun Microsystems公司倡导、许多公司参与一起建立的一种动态网页技术标准。JSP技术有点类似ASP技术，它是在传统的网页HTML（标准通用标记语言的子集）文件(*.htm,*.html)中插入Java程序段(Scriptlet)和JSP标记(tag)，从而形成JSP文件，后缀名为(*.jsp)。 用JSP开发的Web应用是跨平台的，既能在Linux下运行，也能在其他操作系统上运行。

其实啊，JSP是一种特殊的Servlet，当JSP页面首次被访问时，容器（Tomcat）会先把JSP编译成Servlet，然后再去执行Servlet。所以JSP其实就是一个Servlet！

当你去查看tomcat下的work目录，你随便找一个带有jsp的项目，打开来看，比如我的miniblog项目，

当客户端访问find.jsp时，服务器通过find.jsp产生find_jsp.java，即servlet，再把find_jsp.java编译成find_jsp.class，最后通过find_jsp.class完成响应，

来看一看jsp（find_jsp.java）里面的内容就明白了：

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.42
 * Generated at: 2016-10-25 05:29:30 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.*;
import java.util.*;

public final class find_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 org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fc_005furl_0026_005fvalue_005fnobody;
  private org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fc_005fforEach_0026_005fvar_005fitems;
  private org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fc_005fchoose;
  private org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fc_005fwhen_0026_005ftest;
  private org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fc_005fotherwise;
  private org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fc_005fset_0026_005fvar_005fvalue_005fnobody;
  private org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fc_005fif_0026_005ftest;
  private org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fc_005fforEach_0026_005fvar_005fend_005fbegin;

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

  public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
    return _jspx_dependants;
  }

  public void _jspInit() {
    _005fjspx_005ftagPool_005fc_005furl_0026_005fvalue_005fnobody = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
    _005fjspx_005ftagPool_005fc_005fforEach_0026_005fvar_005fitems = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
    _005fjspx_005ftagPool_005fc_005fchoose = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
    _005fjspx_005ftagPool_005fc_005fwhen_0026_005ftest = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
    _005fjspx_005ftagPool_005fc_005fotherwise = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
    _005fjspx_005ftagPool_005fc_005fset_0026_005fvar_005fvalue_005fnobody = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
    _005fjspx_005ftagPool_005fc_005fif_0026_005ftest = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
    _005fjspx_005ftagPool_005fc_005fforEach_0026_005fvar_005fend_005fbegin = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
    _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
    _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
  }

  public void _jspDestroy() {
    _005fjspx_005ftagPool_005fc_005furl_0026_005fvalue_005fnobody.release();
    _005fjspx_005ftagPool_005fc_005fforEach_0026_005fvar_005fitems.release();
    _005fjspx_005ftagPool_005fc_005fchoose.release();
    _005fjspx_005ftagPool_005fc_005fwhen_0026_005ftest.release();
    _005fjspx_005ftagPool_005fc_005fotherwise.release();
    _005fjspx_005ftagPool_005fc_005fset_0026_005fvar_005fvalue_005fnobody.release();
    _005fjspx_005ftagPool_005fc_005fif_0026_005ftest.release();
    _005fjspx_005ftagPool_005fc_005fforEach_0026_005fvar_005fend_005fbegin.release();
  }

  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('\n');
      out.write('\n');

	String path = request.getContextPath();
	String basePath = request.getScheme() + "://"
			+ request.getServerName() + ":" + request.getServerPort()
			+ path + "/";

      out.write("\n");
      out.write("\n");
      out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\n");
      out.write("<html>\n");
      out.write("<head>\n");
      out.write("\n");
      out.write("<base href=\"");
      out.print(basePath);
      out.write("\">\n");
      out.write("\n");
      out.write("<meta charset=\"UTF-8\">\n");
      out.write("<link rel=\"icon\" href=\"assets/img/favicon.ico\">\n");
      out.write("<title>miniblog-find</title>\n");
      out.write("<meta http-equiv=\"pragma\" content=\"no-cache\">\n");
      out.write("<meta http-equiv=\"cache-control\" content=\"no-cache\">\n");
      out.write("<meta http-equiv=\"expires\" content=\"0\">\n");
      out.write("<meta http-equiv=\"keywords\" content=\"keyword1,keyword2,keyword3\">\n");
      out.write("<meta http-equiv=\"description\" content=\"This is my page\">\n");

当然，这只是片段，但我们可以看出，你会发现，在JSP中的静态信息（例如<html>等）在“真身”中都是使用out.write()完成打印！这些静态信息都是作为字符串输出给了客户端。JSP的整篇内容都会放到名为_jspService的方法中！

所以，从上面我们可以看出，jsp实际上通过servlet来完成的，这也是我们为什么将之称其为运行在服务端的语言（Java Server Pages）。