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

 更新時(shí)間:2020年07月22日 14:38:25   作者:孤傲蒼狼  
這篇文章主要介紹了JSP動(dòng)態(tài)網(wǎng)頁(yè)開(kāi)發(fā)原理詳解,文中通過(guò)示例代碼介紹的非常詳細(xì),對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值,需要的朋友們下面隨著小編來(lái)一起學(xué)習(xí)學(xué)習(xí)吧

一、什么是JSP?

     JSP全稱(chēng)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(hù)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(hù)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(lèi)是繼承 org.apache.jasper.runtime.HttpJspBase這個(gè)類(lèi)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(lèi)的源代碼文件,如下圖所示: 

我們可以看看HttpJsBase這個(gè)類(lèi)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(lèi)是繼承HttpServlet的,所以HttpJspBase類(lèi)是一個(gè)Servlet,而index_jsp又是繼承HttpJspBase類(lèi)的,所以index_jsp類(lèi)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(hù)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(lèi)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:

  1. 客戶(hù)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
  2. 在容器中找到需要執(zhí)行的*.jsp文件
  3. 之后*.jsp文件通過(guò)轉(zhuǎn)換變?yōu)?.java文件
  4. *.java文件經(jīng)過(guò)編譯后,形成*.class文件
  5. 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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