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發(fā)動(dòng)態(tài)web資源的技術(shù)。
JSP這門技術(shù)的最大的特點(diǎn)在于，寫jsp就像在寫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頁面的？
瀏覽器向服務(wù)器發(fā)請(qǐng)求，不管訪問的是什么資源，其實(shí)都是在訪問Servlet，所以當(dāng)訪問一個(gè)jsp頁面時(shí)，其實(shí)也是在訪問一個(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jsp時(shí)，其實(shí)不是在訪問jsp，而是在訪問jsp翻譯過后的那個(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)我們通過瀏覽器訪問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)瀏覽器訪問服務(wù)器上的index.jsp頁面時(shí)，其實(shí)就是在訪問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頁面中編寫的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頁面中的java代碼服務(wù)器是如何執(zhí)行的？
在jsp中編寫的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編寫的java代碼了，所以Jsp頁面中的java代碼服務(wù)器是通過調(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)沒有實(shí)例化的對(duì)象通過下面的方式實(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)在開發(fā)中該怎么去用。
不管是JSP還是Servlet，雖然都可以用于開發(fā)動(dòng)態(tài)web資源。但由于這2門技術(shù)各自的特點(diǎn)，在長期的軟件實(shí)踐中，人們逐漸把servlet作為web應(yīng)用中的控制器組件來使用，而把JSP技術(shù)作為數(shù)據(jù)顯示模板來使用。其原因?yàn)椋绦虻臄?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)通過轉(zhuǎn)發(fā)技術(shù)帶給jsp，數(shù)據(jù)的顯示jsp來做。
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容器來處理
- 在容器中找到需要執(zhí)行的*.jsp文件
- 之后*.jsp文件通過轉(zhuǎn)換變?yōu)?.java文件
- *.java文件經(jīng)過編譯后，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)換和編譯的過程
修改后執(zhí)行：
1.源文件已經(jīng)被修改過了，所以需要重新轉(zhuǎn)換，重新編譯。
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