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JSP動(dòng)態(tài)網(wǎng)頁(yè)開(kāi)發(fā)原理詳解

一、什么是jsp？

　    jsp全稱是java server pages，它和servle技術(shù)一樣，都是sun公司定義的一種用于開(kāi)發(fā)動(dòng)態(tài)web資源的技術(shù)。
　　jsp這門(mén)技術(shù)的最大的特點(diǎn)在于，寫(xiě)jsp就像在寫(xiě)html，但它相比html而言，html只能為用戶提供靜態(tài)數(shù)據(jù)，而jsp技術(shù)允許在頁(yè)面中嵌套java代碼，為用戶提供動(dòng)態(tài)數(shù)據(jù)。

二、jsp原理

2.1、web服務(wù)器是如何調(diào)用并執(zhí)行一個(gè)jsp頁(yè)面的？

　　瀏覽器向服務(wù)器發(fā)請(qǐng)求，不管訪問(wèn)的是什么資源，其實(shí)都是在訪問(wèn)servlet，所以當(dāng)訪問(wèn)一個(gè)jsp頁(yè)面時(shí)，其實(shí)也是在訪問(wèn)一個(gè)servlet，服務(wù)器在執(zhí)行jsp的時(shí)候，首先把jsp翻譯成一個(gè)servlet，所以我們?cè)L問(wèn)jsp時(shí)，其實(shí)不是在訪問(wèn)jsp，而是在訪問(wèn)jsp翻譯過(guò)后的那個(gè)servlet，例如下面的代碼：

index.jsp

<%@ page language="java" import="java.util.*" pageencoding="utf-8"%>
<%
string path = request.getcontextpath();
string basepath = request.getscheme()+"://"+request.getservername()+":"+request.getserverport()+path+"/";
%>

<!doctype html public "-//w3c//dtd html 4.01 transitional//en">
<html>
 <head>
  <base href="<%=basepath%>" rel="external nofollow" >

  <title>first jsp</title>

 </head>

 <body>
  <%
    out.print("hello jsp");
  %>
 </body>
</html>

　　當(dāng)我們通過(guò)瀏覽器訪問(wèn)index.jsp時(shí)，服務(wù)器首先將index.jsp翻譯成一個(gè)index_jsp.class，在tomcat服務(wù)器的work\catalina\localhost\項(xiàng)目名\org\apache\jsp目錄下可以看到index_jsp.class的源代碼文件index_jsp.java，index_jsp.java的代碼如下：

package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import java.util.*;

public final class index_jsp extends org.apache.jasper.runtime.httpjspbase
  implements org.apache.jasper.runtime.jspsourcedependent {

 private static final jspfactory _jspxfactory = jspfactory.getdefaultfactory();

 private static java.util.list _jspx_dependants;

 private javax.el.expressionfactory _el_expressionfactory;
 private org.apache.annotationprocessor _jsp_annotationprocessor;

 public object getdependants() {
  return _jspx_dependants;
 }

 public void _jspinit() {
  _el_expressionfactory = _jspxfactory.getjspapplicationcontext(getservletconfig().getservletcontext()).getexpressionfactory();
  _jsp_annotationprocessor = (org.apache.annotationprocessor) getservletconfig().getservletcontext().getattribute(org.apache.annotationprocessor.class.getname());
 }

 public void _jspdestroy() {
 }

 public void _jspservice(httpservletrequest request, httpservletresponse response)
    throws java.io.ioexception, servletexception {

  pagecontext pagecontext = null;
  httpsession session = null;
  servletcontext application = null;
  servletconfig config = null;
  jspwriter out = null;
  object page = this;
  jspwriter _jspx_out = null;
  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');
   out.write('\n');

string path = request.getcontextpath();
string basepath = request.getscheme()+"://"+request.getservername()+":"+request.getserverport()+path+"/";

   out.write("\r\n");
   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("  <base href=\"");
   out.print(basepath);
   out.write("\">\r\n");
   out.write("  \r\n");
   out.write("  <title>first jsp</title>\r\n");
   out.write("\t\r\n");
   out.write(" </head>\r\n");
   out.write(" \r\n");
   out.write(" <body>\r\n");
   out.write("  ");

    out.print("hello jsp");

   out.write("\r\n");
   out.write(" </body>\r\n");
   out.write("</html>\r\n");
  } catch (throwable t) {
   if (!(t instanceof 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);
  }
 }
}

　　我們可以看到，index_jsp這個(gè)類是繼承 org.apache.jasper.runtime.httpjspbase這個(gè)類的，通過(guò)查看tomcat服務(wù)器的源代碼，可以知道在apache-tomcat-6.0.20-src\java\org\apache\jasper\runtime目錄下存httpjspbase這個(gè)類的源代碼文件，如下圖所示：　

我們可以看看httpjsbase這個(gè)類的源代碼，如下所示：

/*
 * licensed to the apache software foundation (asf) under one or more
 * contributor license agreements. see the notice file distributed with
 * this work for additional information regarding copyright ownership.
 * the asf licenses this file to you under the apache license, version 2.0
 * (the "license"); you may not use this file except in compliance with
 * the license. you may obtain a copy of the license at
 *
 *   http://www.apache.org/licenses/license-2.0
 *
 * unless required by applicable law or agreed to in writing, software
 * distributed under the license is distributed on an "as is" basis,
 * without warranties or conditions of any kind, either express or implied.
 * see the license for the specific language governing permissions and
 * limitations under the license.
 */

package org.apache.jasper.runtime;

import java.io.ioexception;

import javax.servlet.servletconfig;
import javax.servlet.servletexception;
import javax.servlet.http.httpservlet;
import javax.servlet.http.httpservletrequest;
import javax.servlet.http.httpservletresponse;
import javax.servlet.jsp.httpjsppage;
import javax.servlet.jsp.jspfactory;

import org.apache.jasper.compiler.localizer;

/**
 * this is the super class of all jsp-generated servlets.
 *
 * @author anil k. vijendran
 */
public abstract class httpjspbase
  extends httpservlet
  implements httpjsppage


{

  protected httpjspbase() {
  }

  public final void init(servletconfig config)
  throws servletexception
  {
    super.init(config);
  jspinit();
    _jspinit();
  }

  public string getservletinfo() {
  return localizer.getmessage("jsp.engine.info");
  }

  public final void destroy() {
  jspdestroy();
  _jspdestroy();
  }

  /**
   * entry point into service.
   */
  public final void service(httpservletrequest request, httpservletresponse response)
  throws servletexception, ioexception
  {
    _jspservice(request, response);
  }

  public void jspinit() {
  }

  public void _jspinit() {
  }

  public void jspdestroy() {
  }

  protected void _jspdestroy() {
  }

  public abstract void _jspservice(httpservletrequest request,
           httpservletresponse response)
  throws servletexception, ioexception;
}

　　httpjspbase類是繼承httpservlet的，所以httpjspbase類是一個(gè)servlet，而index_jsp又是繼承httpjspbase類的，所以index_jsp類也是一個(gè)servlet，所以當(dāng)瀏覽器訪問(wèn)服務(wù)器上的index.jsp頁(yè)面時(shí)，其實(shí)就是在訪問(wèn)index_jsp這個(gè)servlet，index_jsp這個(gè)servlet使用_jspservice這個(gè)方法處理請(qǐng)求。

2.2、jsp頁(yè)面中的html排版標(biāo)簽是如何被發(fā)送到客戶端的？

瀏覽器接收到的這些數(shù)據(jù)

<!doctype html public "-//w3c//dtd html 4.01 transitional//en">
<html>
 <head>
  <base href="http://localhost:8080/javaweb_jsp_study_20140603/" rel="external nofollow" >

  <title>first jsp</title>

 </head>

 <body>
  hello jsp
 </body>
</html>

都是在_jspservice方法中使用如下的代碼輸出給瀏覽器的：

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

string path = request.getcontextpath();
string basepath = request.getscheme()+"://"+request.getservername()+":"+request.getserverport()+path+"/";

   out.write("\r\n");
   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("  <base href=\"");
   out.print(basepath);
   out.write("\">\r\n");
   out.write("  \r\n");
   out.write("  <title>first jsp</title>\r\n");
   out.write("\t\r\n");
   out.write(" </head>\r\n");
   out.write(" \r\n");
   out.write(" <body>\r\n");
   out.write("  ");

    out.print("hello jsp");

   out.write("\r\n");
   out.write(" </body>\r\n");
   out.write("</html>\r\n");

　　在jsp中編寫(xiě)的java代碼和html代碼都會(huì)被翻譯到_jspservice方法中去，在jsp中編寫(xiě)的java代碼會(huì)原封不動(dòng)地翻譯成java代碼，如<%out.print("hello jsp");%>直接翻譯成out.print("hello jsp");，而html代碼則會(huì)翻譯成使用out.write("<html標(biāo)簽>\r\n");的形式輸出到瀏覽器。在jsp頁(yè)面中編寫(xiě)的html排版標(biāo)簽都是以out.write("<html標(biāo)簽>\r\n");的形式輸出到瀏覽器，瀏覽器拿到html代碼后才能夠解析執(zhí)行html代碼。

2.3、jsp頁(yè)面中的java代碼服務(wù)器是如何執(zhí)行的？

　　在jsp中編寫(xiě)的java代碼會(huì)被翻譯到_jspservice方法中去，當(dāng)執(zhí)行_jspservice方法處理請(qǐng)求時(shí)，就會(huì)執(zhí)行在jsp編寫(xiě)的java代碼了，所以jsp頁(yè)面中的java代碼服務(wù)器是通過(guò)調(diào)用_jspservice方法處理請(qǐng)求時(shí)執(zhí)行的。

2.4、web服務(wù)器在調(diào)用jsp時(shí)，會(huì)給jsp提供一些什么java對(duì)象？

　　查看_jspservice方法可以看到，web服務(wù)器在調(diào)用jsp時(shí)，會(huì)給jsp提供如下的8個(gè)java對(duì)象

pagecontext pagecontext;
httpsession session;
servletcontext application;
servletconfig config;
jspwriter out;
object page = this;
httpservletrequest request,
httpservletresponse response

　　其中page對(duì)象，request和response已經(jīng)完成了實(shí)例化，而其它5個(gè)沒(méi)有實(shí)例化的對(duì)象通過(guò)下面的方式實(shí)例化

pagecontext = _jspxfactory.getpagecontext(this, request, response,null, true, 8192, true);
 application = pagecontext.getservletcontext();
 config = pagecontext.getservletconfig();
 session = pagecontext.getsession();
 out = pagecontext.getout();

 這8個(gè)java對(duì)象在jsp頁(yè)面中是可以直接使用的，如下所示：

<%
    session.setattribute("name", "session對(duì)象");//使用session對(duì)象,設(shè)置session對(duì)象的屬性
    out.print(session.getattribute("name")+"<br/>");//獲取session對(duì)象的屬性
    pagecontext.setattribute("name", "pagecontext對(duì)象");//使用pagecontext對(duì)象,設(shè)置pagecontext對(duì)象的屬性
    out.print(pagecontext.getattribute("name")+"<br/>");//獲取pagecontext對(duì)象的屬性
    application.setattribute("name", "application對(duì)象");//使用application對(duì)象,設(shè)置application對(duì)象的屬性
    out.print(application.getattribute("name")+"<br/>");//獲取application對(duì)象的屬性
    out.print("hello jsp"+"<br/>");//使用out對(duì)象
    out.print("服務(wù)器調(diào)用index.jsp頁(yè)面時(shí)翻譯成的類的名字是："+page.getclass()+"<br/>");//使用page對(duì)象
    out.print("處理請(qǐng)求的servlet的名字是："+config.getservletname()+"<br/>");//使用config對(duì)象
    out.print(response.getcontenttype()+"<br/>");//使用response對(duì)象
    out.print(request.getcontextpath()+"<br/>");//使用request對(duì)象
%>

運(yùn)行結(jié)果如下：

　

2.5、jsp最佳實(shí)踐

　　jsp最佳實(shí)踐就是jsp技術(shù)在開(kāi)發(fā)中該怎么去用。

　　不管是jsp還是servlet，雖然都可以用于開(kāi)發(fā)動(dòng)態(tài)web資源。但由于這2門(mén)技術(shù)各自的特點(diǎn)，在長(zhǎng)期的軟件實(shí)踐中，人們逐漸把servlet作為web應(yīng)用中的控制器組件來(lái)使用，而把jsp技術(shù)作為數(shù)據(jù)顯示模板來(lái)使用。其原因?yàn)?，程序的?shù)據(jù)通常要美化后再輸出：讓jsp既用java代碼產(chǎn)生動(dòng)態(tài)數(shù)據(jù)，又做美化會(huì)導(dǎo)致頁(yè)面難以維護(hù)。讓servlet既產(chǎn)生數(shù)據(jù)，又在里面嵌套html代碼美化數(shù)據(jù)，同樣也會(huì)導(dǎo)致程序可讀性差，難以維護(hù)。因此最好的辦法就是根據(jù)這兩門(mén)技術(shù)的特點(diǎn)，讓它們各自負(fù)責(zé)各的，servlet只負(fù)責(zé)響應(yīng)請(qǐng)求產(chǎn)生數(shù)據(jù)，并把數(shù)據(jù)通過(guò)轉(zhuǎn)發(fā)技術(shù)帶給jsp，數(shù)據(jù)的顯示jsp來(lái)做。

2.6、tomcat服務(wù)器的執(zhí)行流程

　　

第一次執(zhí)行：

  • 客戶端通過(guò)電腦連接服務(wù)器，因?yàn)槭钦?qǐng)求是動(dòng)態(tài)的，所以所有的請(qǐng)求交給web容器來(lái)處理
  • 在容器中找到需要執(zhí)行的*.jsp文件
  • 之后*.jsp文件通過(guò)轉(zhuǎn)換變?yōu)?.java文件
  • *.java文件經(jīng)過(guò)編譯后，形成*.class文件
  • 最終服務(wù)器要執(zhí)行形成的*.class文件

    • 第二次執(zhí)行：

    因?yàn)橐呀?jīng)存在了*.class文件，所以不在需要轉(zhuǎn)換和編譯的過(guò)程

    修改后執(zhí)行：

           1.源文件已經(jīng)被修改過(guò)了，所以需要重新轉(zhuǎn)換，重新編譯。

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