jsp简介

一、概念部分
　　JSP，即Java Server Pages首字母的缩写，Java服务器页面，即运行在服务器端的页面。它是由Sun公司倡导，许多国际性大公司一起参与，建立的一种动态网页技术。JSP技术是在传统的静态网页HTML页面中插入Java代码片段和JSP标签后形成的一种文件，其后缀名为.jsp。使用JSP开发的Web应用和Java一样是跨平台的，也就是既能在linux上运行，也能在其它操作系统上运行。

　　JSP文件的本质是Servlet。只不过JSP与Servlet不同的是:JSP是专门用于数据展示的Servlet，其有特殊的的写法。而普通的Servlet是用于完成业务逻辑处理的。由于Servlet是运行在单例多线程环境下的，所以JSP同样也是运行在单例多线程环境下的。
二、JSP规范
　　将JSP页面翻译为Servlet的过程是由Tomcat完成的。在Tomcat中内置了一个JSP翻译引擎，当第一次访问该JSP页面时，该翻译引擎会将JSP页面翻译为Servlet的.java源文件，再将该.java源文件编译为.class字节码文件进行运行。
　　SUN公司制定的Java EE规范中包含两个很重要的子规范：Servlet规范和JSP规范。其中JSP规范中就包含了如何将JSP页面翻译为Servlet。例如，JSP页面中的HTML、CSS、JavaScript及普通文本部分军徽被翻译到out.write()中。Tomcat中的JSP翻译引擎，就遵循了这个JSP规范。

三、jsp页面组成部分图解

四、示例

　　4.1我的第一个jsp页面

　　发布到tomcat后该页面存放在：..workspace.metadata.pluginsorg.eclipse.wst.server.core mp0wtpwebappsjsp　　

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>first_jsp</title>
</head>
<body>
My First jsp!
</body>
</html>

　　4.2经Tomcat采用JSP规范将上面jsp页面翻译成的.java源码

　　存放地址：..workspace.metadata.pluginsorg.eclipse.wst.server.core mp0workCatalinalocalhostjsporgapachejsp

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.85
 * Generated at: 2019-01-22 16:01:50 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 {

  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("
");
      out.write("<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
");
      out.write("<html>
");
      out.write("<head>
");
      out.write("<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
");
      out.write("<title>first_jsp</title>
");
      out.write("</head>
");
      out.write("<body>
");
      out.write("My First jsp!
");
      out.write("</body>
");
      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);
    }
  }
}

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原文地址：https://www.cnblogs.com/aeon/p/10306822.html