JSP(Java Server Page)頁面是指擴展名爲 .jsp 的文件。
JSP是服務器端運行的頁面,不像html文件直接就可以在瀏覽器中運行。JSP頁面必須部署到web容器中編譯成Servlet才能執行,可以說,JSP就是Servlet的一種形式。
事實上,JSP是Servlet發展的產物,關於JSP和Servlet的關係以後再在博客中介紹。
JSP基本頁面由指令標籤,HTML標記語言,註釋,Java代碼,JSP動作標籤5個部分組成。
1.JSP指令標籤不會產生任何內容輸出到網頁中,主要用於定義整個JSP頁面的相關信息,如使用的語言,導入的類包,指定錯誤處理頁面等。
JSP指令有3種分別爲:
page----用於定義整個JSP頁面的相關屬性,這些屬性在JSP被服務器解析成Servlet時會轉換成相應的java代碼
主要屬性有language extends import pageEncoding contentType session 等
include--文件包含指令,它可以在JSP頁面中包含另一個文件內容(可以是一段java代碼,html代碼或JSP頁面)
taglib---引用標籤庫指令
2.HTML標記語言
3 Java代碼
語法格式 <% 編寫java代碼%>
JSP表達式<%=表達式%>可以直接把Java表達式結果輸出到JSP頁面中。表達式最終結果將被轉換爲字符串類型。
4 註釋 包括html註釋JSP註釋java代碼註釋等
5 JSP動作標籤,主要用來實現特殊的功能如轉發用戶請求,包含其他文件 操作JavaBean等。
<jsp:include>包含外部文件的動作標籤,可以將另外一個文件的內容包含到當前JSP頁面中,可以是靜態文本,也可以是動態代碼
<jsp:include>動作標籤與include指令區別:include指令包含的資源爲靜態,如果將jsp動態內容用include指令包含,也會被當做靜態資源包含到當前頁面中
即是靜態包含,先包含再編譯成class文件(靜態包含不能傳遞參數)。而<jsp:include>動作標籤是動態包含,對於一個動態頁面先編譯,後包含,即當前頁面與被包含的資源是兩個獨立的實體。被包含JSP頁面會對包含它的JSP頁面中的請求對象進行處理,將處理結果作爲當前JSP頁面中的處理內容,與當前頁面內容一起發送到客戶端。
<jsp:forward>請求轉發標籤,將當前頁面請求轉發給其他Web資源。
<jsp:param>設置參數標籤,可作爲其他標籤子標籤,爲其他標籤傳遞參數。如
<jsp:forward page="login.jsp">
<jsp:param name="userName" value="admin">
</jsp:forward>
在JSP頁面第一次被請求時,JSP引擎會將JSP原始文件轉換爲Servlet源代碼,然後調用Java編譯器,編譯成Servlet,並在Servlet引擎中執行,容器將生成的頁面返回給客戶端顯示。當再次有請求時,JSP引擎會根據JSP文件是否存在更新,如果沒有更新,運行Servlet;如果存在更新,就會重新執行轉換和編譯的過程。JSP的執行性能和Servlet相比,差別只在第一次的執行,當重複調用時,就直接執行第一次產生的Servlet。
如下爲index.jsp的一個最簡單代碼
<%@ 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%>">
<title>My JSP 'index.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
This is my JSP page. <br>
</body>
</html>
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.0.17
* Generated at: 2015-05-18 12:09:56 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 index_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent,
org.apache.jasper.runtime.JspSourceImports {
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 static final java.util.Set<java.lang.String> _jspx_imports_packages;
private static final java.util.Set<java.lang.String> _jspx_imports_classes;
static {
_jspx_imports_packages = new java.util.HashSet<>();
_jspx_imports_packages.add("javax.servlet");
_jspx_imports_packages.add("java.util");
_jspx_imports_packages.add("javax.servlet.http");
_jspx_imports_packages.add("javax.servlet.jsp");
_jspx_imports_classes = new java.util.HashSet<>();
}
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 java.util.Set<java.lang.String> getPackageImports() {
return _jspx_imports_packages;
}
public java.util.Set<java.lang.String> getClassImports() {
return _jspx_imports_classes;
}
public void _jspInit() { //初始化
_el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
_jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
}
public void _jspDestroy() { //銷燬
}
public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
throws java.io.IOException, javax.servlet.ServletException {
final java.lang.String _jspx_method = request.getMethod();
if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD");
return;
}
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=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>My JSP 'index.jsp' starting page</title>\r\n");
out.write("\t<meta http-equiv=\"pragma\" content=\"no-cache\">\r\n");
out.write("\t<meta http-equiv=\"cache-control\" content=\"no-cache\">\r\n");
out.write("\t<meta http-equiv=\"expires\" content=\"0\"> \r\n");
out.write("\t<meta http-equiv=\"keywords\" content=\"keyword1,keyword2,keyword3\">\r\n");
out.write("\t<meta http-equiv=\"description\" content=\"This is my page\">\r\n");
out.write("\t<!--\r\n");
out.write("\t<link rel=\"stylesheet\" type=\"text/css\" href=\"styles.css\">\r\n");
out.write("\t-->\r\n");
out.write(" </head>\r\n");
out.write(" \r\n");
out.write(" <body>\r\n");
out.write(" This is my JSP page. <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 {
if (response.isCommitted()) {
out.flush();
} else {
out.clearBuffer();
}
} catch (java.io.IOException e) {}
if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
else throw new ServletException(t);
}
} finally {
_jspxFactory.releasePageContext(_jspx_page_context);
}
}
}
Web容器處理JSP文件請求的執行過程主要包括如下4個部分
1客戶端發出Request請求
2JSP容器將JSP轉譯成Servlet代碼
3將轉譯的Servlet代碼經過編譯後,加載到內存執行
4把結果Request至客戶端