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javaweb學習總結(十四)——JSP原理一、什麼是JSP？ JSP全稱是Java Server Pages，它和servle技術一樣，都是SUN公司定義的一種用於開發動態web資源的技術。
JSP這門技術的最大的特點在於，寫jsp就像在寫html，但它相比html而言，html只能為使用者提供待用資料，而Jsp技術允許在頁面中嵌套java代碼，為使用者提供動態資料。
瀏覽器向伺服器發請求，不管訪問的是什麼資源，其實都是在訪問Servlet，所以當訪問一個jsp頁面時，其實也是在訪問一個Servlet，伺服器在執行jsp的時候，首先把jsp翻譯成一個Servlet，所以我們訪問jsp時，其實不是在訪問jsp，而是在訪問jsp翻譯過後的那個Servlet，例如下面的代碼：
index.jsp
1 <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%> 2 <% 3 String path = request.getContextPath(); 4 String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; 5 %> 6 7 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 8 <html> 9 <head>10 <base href="<%=basePath%>">11 12 <title>First Jsp</title>13 14 </head>15 16 <body>17 <%18 out.print("Hello Jsp");19 %>20 </body>21 </html>
當我們通過瀏覽器訪問index.jsp時，伺服器首先將index.jsp翻譯成一個index_jsp.class，在Tomcat伺服器的work\Catalina\localhost\項目名\org\apache\jsp目錄下可以看到index_jsp.class的原始碼檔案index_jsp.java，index_jsp.java的代碼如下：
1 package org.apache.jsp; 2 3 import javax.servlet.*; 4 import javax.servlet.http.*; 5 import javax.servlet.jsp.*; 6 import java.util.*; 7 8 public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase 9 implements org.apache.jasper.runtime.JspSourceDependent {10 11 private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory();12 13 private static java.util.List _jspx_dependants;14 15 private javax.el.ExpressionFactory _el_expressionfactory;16 private org.apache.AnnotationProcessor _jsp_annotationprocessor;17 18 public Object getDependants() {19 return _jspx_dependants;20 }21 22 public void _jspInit() {23 _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();24 _jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName());25 }26 27 public void _jspDestroy() {28 }29 30 public void _jspService(HttpServletRequest request, HttpServletResponse response)31 throws java.io.IOException, ServletException {32 33 PageContext pageContext = null;34 HttpSession session = null;35 ServletContext application = null;36 ServletConfig config = null;37 JspWriter out = null;38 Object page = this;39 JspWriter _jspx_out = null;40 PageContext _jspx_page_context = null;41 42 43 try {44 response.setContentType("text/html;charset=UTF-8");45 pageContext = _jspxFactory.getPageContext(this, request, response,46 null, true, 8192, true);47 _jspx_page_context = pageContext;48 application = pageContext.getServletContext();49 config = pageContext.getServletConfig();50 session = pageContext.getSession();51 out = pageContext.getOut();52 _jspx_out = out;53 54 out.write(‘\r‘);55 out.write(‘\n‘);56 57 String path = request.getContextPath();58 String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";59 60 out.write("\r\n");61 out.write("\r\n");62 out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n");63 out.write("<html>\r\n");64 out.write(" <head>\r\n");65 out.write(" <base href=\"");66 out.print(basePath);67 out.write("\">\r\n");68 out.write(" \r\n");69 out.write(" <title>First Jsp</title>\r\n");70 out.write("\t\r\n");71 out.write(" </head>\r\n");72 out.write(" \r\n");73 out.write(" <body>\r\n");74 out.write(" ");75 76 out.print("Hello Jsp");77 78 out.write("\r\n");79 out.write(" </body>\r\n");80 out.write("</html>\r\n");81 } catch (Throwable t) {82 if (!(t instanceof SkipPageException)){83 out = _jspx_out;84 if (out != null && out.getBufferSize() != 0)85 try { out.clearBuffer(); } catch (java.io.IOException e) {}86 if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);87 }88 } finally {89 _jspxFactory.releasePageContext(_jspx_page_context);90 }91 }92 }
我們可以看到，index_jsp這個類是繼承 org.apache.jasper.runtime.HttpJspBase這個類的，通過查看Tomcat伺服器的原始碼，可以知道在apache-tomcat-6.0.20-src\java\org\apache\jasper\runtime目錄下存HttpJspBase這個類的原始碼檔案，如所示：
我們可以看看HttpJsBase這個類的原始碼，如下所示：
1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.010 * 11 * Unless required by applicable law or agreed to in writing, software12 * distributed under the License is distributed on an "AS IS" BASIS,13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.14 * See the License for the specific language governing permissions and15 * limitations under the License.16 */17 18 package org.apache.jasper.runtime;19 20 import java.io.IOException;21 22 import javax.servlet.ServletConfig;23 import javax.servlet.ServletException;24 import javax.servlet.http.HttpServlet;25 import javax.servlet.http.HttpServletRequest;26 import javax.servlet.http.HttpServletResponse;27 import javax.servlet.jsp.HttpJspPage;28 import javax.servlet.jsp.JspFactory;29 30 import org.apache.jasper.compiler.Localizer;31 32 /**33 * This is the super class of all JSP-generated servlets.34 *35 * @author Anil K. Vijendran36 */37 public abstract class HttpJspBase 38 extends HttpServlet 39 implements HttpJspPage 40 41 42 {43 44 protected HttpJspBase() {45 }46 47 public final void init(ServletConfig config) 48 throws ServletException 49 {50 super.init(config);51 jspInit();52 _jspInit();53 }54 55 public String getServletInfo() {56 return Localizer.getMessage("jsp.engine.info");57 }58 59 public final void destroy() {60 jspDestroy();61 _jspDestroy();62 }63 64 /**65 * Entry point into service.66 */67 public final void service(HttpServletRequest request, HttpServletResponse response) 68 throws ServletException, IOException 69 {70 _jspService(request, response);71 }72 73 public void jspInit() {74 }75 76 public void _jspInit() {77 }78 79 public void jspDestroy() {80 }81 82 protected void _jspDestroy() {83 }84 85 public abstract void _jspService(HttpServletRequest request, 86 HttpServletResponse response) 87 throws ServletException, IOException;88 }
HttpJspBase類是繼承HttpServlet的，所以HttpJspBase類是一個Servlet，而index_jsp又是繼承HttpJspBase類的，所以index_jsp類也是一個Servlet，所以當瀏覽器訪問伺服器上的index.jsp頁面時，其實就是在訪問index_jsp這個Servlet，index_jsp這個Servlet使用_jspService這個方法處理請求。
2.2、Jsp頁面中的html排版標籤是如何被發送到用戶端的？瀏覽器接收到的這些資料
1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2 <html> 3 <head> 4 <base href="http://localhost:8080/JavaWeb_Jsp_Study_20140603/"> 5 6 <title>First Jsp</title> 7 8 </head> 9 10 <body>11 Hello Jsp12 </body>13 </html>
都是在_jspService方法中使用如下的代碼輸出給瀏覽器的：
1 out.write(‘\r‘); 2 out.write(‘\n‘); 3 4 String path = request.getContextPath(); 5 String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; 6 7 out.write("\r\n"); 8 out.write("\r\n"); 9 out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n");10 out.write("<html>\r\n");11 out.write(" <head>\r\n");12 out.write(" <base href=\"");13 out.print(basePath);14 out.write("\">\r\n");15 out.write(" \r\n");16 out.write(" <title>First Jsp</title>\r\n");17 out.write("\t\r\n");18 out.write(" </head>\r\n");19 out.write(" \r\n");20 out.write(" <body>\r\n");21 out.write(" ");22 23 out.print("Hello Jsp");24 25 out.write("\r\n");26 out.write(" </body>\r\n");27 out.write("</html>\r\n");
在jsp中編寫的java代碼和html代碼都會被翻譯到_jspService方法中去，在jsp中編寫的java代碼會原封不動地翻譯成java代碼，如<%out.print("Hello Jsp");%>直接翻譯成out.print("Hello Jsp");，而HTML代碼則會翻譯成使用out.write("<html標籤>\r\n");的形式輸出到瀏覽器。在jsp頁面中編寫的html排版標籤都是以out.write("<html標籤>\r\n");的形式輸出到瀏覽器，瀏覽器拿到html代碼後才能夠解析執行html代碼。
2.3、Jsp頁面中的java代碼伺服器是如何執行的？在jsp中編寫的java代碼會被翻譯到_jspService方法中去，當執行_jspService方法處理請求時，就會執行在jsp編寫的java代碼了，所以Jsp頁面中的java代碼伺服器是通過調用_jspService方法處理請求時執行的。
2.4、Web伺服器在調用jsp時，會給jsp提供一些什麼java對象？查看_jspService方法可以看到，Web伺服器在調用jsp時，會給Jsp提供如下的8個java對象
1 PageContext pageContext;2 HttpSession session;3 ServletContext application;4 ServletConfig config;5 JspWriter out;6 Object page = this;7 HttpServletRequest request, 8 HttpServletResponse response
其中page對象，request和response已經完成了執行個體化，而其它5個沒有執行個體化的對象通過下面的方式執行個體化
1 pageContext = _jspxFactory.getPageContext(this, request, response,null, true, 8192, true);2 application = pageContext.getServletContext();3 config = pageContext.getServletConfig();4 session = pageContext.getSession();5 out = pageContext.getOut();
這8個java對象在Jsp頁面中是可以直接使用的，如下所示：
1 <% 2 session.setAttribute("name", "session對象");//使用session對象,設定session對象的屬性 3 out.print(session.getAttribute("name")+"<br/>");//擷取session對象的屬性 4 pageContext.setAttribute("name", "pageContext對象");//使用pageContext對象,設定pageContext對象的屬性 5 out.print(pageContext.getAttribute("name")+"<br/>");//擷取pageContext對象的屬性 6 application.setAttribute("name", "application對象");//使用application對象,設定application對象的屬性 7 out.print(application.getAttribute("name")+"<br/>");//擷取application對象的屬性 8 out.print("Hello Jsp"+"<br/>");//使用out對象 9 out.print("伺服器調用index.jsp頁面時翻譯成的類的名字是："+page.getClass()+"<br/>");//使用page對象10 out.print("處理請求的Servlet的名字是："+config.getServletName()+"<br/>");//使用config對象11 out.print(response.getContentType()+"<br/>");//使用response對象12 out.print(request.getContextPath()+"<br/>");//使用request對象13 %>
運行結果如下：
2.5、Jsp最佳實務
Jsp最佳實務就是jsp技術在開發中該怎麼去用。
不管是JSP還是Servlet，雖然都可以用於開發動態web資源。但由於這2門技術各自的特點，在長期的軟體實踐中，人們逐漸把servlet作為web應用中的控制器組件來使用，而把JSP技術作為資料顯示模板來使用。其原因為，程式的資料通常要美化後再輸出：讓jsp既用java代碼產生動態資料，又做美化會導致頁面難以維護。讓servlet既產生資料，又在裡面嵌套html代碼美化資料，同樣也會導致程式可讀性差，難以維護。因此最好的辦法就是根據這兩門技術的特點，讓它們各自負責各的，servlet只負責響應請求產生資料，並把資料通過轉寄技術帶給jsp，資料的顯示jsp來做。
2.6、 Tomcat伺服器的執行流程
第一次執行：
第二次執行：
修改後執行：
1.源檔案已經被修改過了，所以需要重新轉換，重新編譯。
javaweb學習總結(十四)——JSP原理
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