JSP运行原理及优势

JSP本质上就是把Java代码嵌套到HTML中，然后经过JSP容器（Tomcat、Resin、Weblogic等）的编译执行，再根据这些动态代码的运行结果生成对应的HTML代码，从而可以在客户端的浏览器中正常显示。

运行原理

如果JSP页面是第一次被请求运行，服务器的JSP编译器会生成JSP页面对应的JAVA代码，并且编译成类文件。当服务器再次收到对这个JSP页面请求的时候，会判断这个JSP页面是否被修改过，如果被修改过就会重新生成Java代码并且重新编译，而且服务器中的垃圾回收方易做图把没用的类文件删除。如果没有修改过，服务器就会直接调用以前已经编译过的类文件。

举个例子，代码如下：

[html]
<%@ page language="java" import="java.util.*" pageEncoding="gb2312"%>
<html>
    <head>
        <title>简单JSP页面示例</title>
    </head>
<body>
    这是一个简单的 JSP页面示例 <br>
    </body>
</html>
<%@ page language="java" import="java.util.*" pageEncoding="gb2312"%>
<html>
<head>
    <title>简单JSP页面示例</title>
</head>
<body>
这是一个简单的 JSP页面示例 <br>
</body>
</html>

上面这个JSP页面在被请求的时候，Web服务器中JSP编译器会生成对应的Java文件，上面这个JSP程序在服务器中生成的对应Java代码如下：

[java]
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/7.0.22
* Generated at: 2012-03-22 08:40:10 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 Simple_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 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() {
    _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
    _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
}

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=gb2312");
      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("<html>\r\n");
      out.write(" <head>\r\n");
      out.write("    <title>简单JSP页面示例</title>\r\n");
      out.write(" </head>\r\n");
      out.write("<body>\r\n");
      out.write("    这是一个简单的 JSP页面示例 <br>\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 { out.clearBuffer(); } catch (java.io.IOException e) {}
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
      }
    } finally {
      _jspxFactory.releasePageContext(_jspx_page_context);
    }
}
}
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/7.0.22
* Generated at: 2012-03-22 08:40:10 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 Simple_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent {

private static final javax.servlet.jsp.JspFactory _jspxFactory =
&n

补充：Web开发 , Jsp ,