1.JSP指令简介
JSP指令是为JSP引擎设计的，它们并不直接产生任何可见输出，而只是告诉引擎如何处理JSP页面中的其余部分。
在JSP 2.0规范中共定义了三个指令：page指令，Include指令，taglib指令。
JSP指令的基本语法格式：<%@ 指令 属性名="值" %>
例如：
<%@ page contentType="text/html;charset=gb2312"%>如果一个指令有多个属性，这多个属性可以写在一个指令中，也可以分开写。
例如：
<%@ page contentType="text/html;charset=gb2312"%>
<%@ page import="java.util.Date"%>也可以写作：
<%@ page contentType="text/html;charset=gb2312" import="java.util.Date"%>2.page指令
page指令用于定义JSP页面的各种属性，无论page指令出现在JSP页面中的什么地方，它作用的都是整个JSP页面，为了保持程序的可读性和遵循良好的编程习惯，page指令最好是放在整个JSP页面的起始位置。
JSP 2.0规范中定义的page指令的完整语法：
<%@ page
[ language="java" ]
[ extends="package.class" ]
[ import="{package.class | package.*}, ..." ]
[ session="true | false" ]
[ buffer="none | 8kb | sizekb" ]
[ autoFlush="true | false" ]
[ isThreadSafe="true | false" ]
[ info="text" ]
[ errorPage="relative_url" ]
[ isErrorPage="true | false" ]
[ contentType="mimeType [ ;charset=characterSet ]" | "text/html ; charset=ISO-8859-1" ]
[ pageEncoding="characterSet | ISO-8859-1" ]
[ isELIgnored="true | false" ]
%>2.1 import属性
在jsp页面中，jsp引擎会自动导入下面的包和类：
java.lang.*
javax.servlet.*
javax.servlet.jsp.*
javax.servlet.http.*
可以在一条page指令引入多个类和包，其中的每个包和类之间使用逗号分隔开，例如，
<%@ page import="java.util.Date,java.sql.*,java.io.*"%>2.2 errorPage属性
- errorPage属性的设置值必须使用相对路径，如果以“/”开头，表示相对于当前Web应用程序的根目录(注意不是站点根目录)，否则，表示相对于当前页面。
- 可以在web.xml文件中使用<error-page>元素为整个Web应用程序设置错误处理页面。
- <error-page>元素有3个子元素，<error-code>、<exception-type>、<location>
- <error-code>子元素指定错误的状态码，例如：<error-code>404</error-code>
- <exception-type>子元素指定异常类的完全限定名，例如：<exception-type>java.lang.ArithmeticException</exception-type>
- <location>子元素指定以“/”开头的错误处理页面的路径，例如：<location>/ErrorPage/404Error.jsp</location>
- 如果设置了某个JSP页面的errorPage属性，那么在web.xml文件中设置的错误处理将不对该页面起作用。
jsperrorPage的相对路径，“/”表示当前web应用程序的根目录（WebRoot），“./”代表当前目录（即当前文件所在的目录），“../”代表当前文件所在目录的上一级目录。
例如有以下的工程目录结构：
testA.jsp中page指令的errorPage路径为：
<%@ page import="java.util.Date" errorPage="/jspTest/error.jsp"%>即路径"/jspTest/error.jsp"为“WebRoot/jspTest/error.jsp”。
使用errorPage属性可以指明出错后跳转的错误页面，比如如下的testA.jsp代码：
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@ page import="java.util.Date" errorPage="/jspTest/error.jsp"%>
<%
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 'testA.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>
<%
int i=2/0;
%>
</body>
</html>int i=2/0，显然出错，第二行page指令的errorPage属性<%@ page import="java.util.Date" errorPage="/jspTest/error.jsp"%>指明出错后跳转到error.jsp文件，error.jsp的内容为：
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@ page import="java.io.PrintWriter" %>
<%
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 'error.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>
<%
PrintWriter outs=response.getWriter();
outs.write("出错啦!");
%>
</body>
</html>运行结果如下：
2.3 在web.xml中使用<error-page>标签为整个web应用设置错误处理页面
例如，使用<error-page>标签配置针对404错误的处理页面，在web.xml中的配置如下：
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
<display-name></display-name>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
<!-- 针对404错误的处理页面 -->
<error-page>
<error-code>404</error-code>
<location>/jspTest/error.jsp</location>
</error-page>
</web-app>要跳转的error.jsp的代码如下：
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<html>
<head>
<title>404错误友好提示页面</title>
<!-- 3秒钟后自动跳转回首页 -->
<meta http-equiv="refresh" content="3;url=${pageContext.request.contextPath}/WEB-INF/index.jsp">
</head>
<body>
<p>404错误</p>
<br/>
3秒钟后自动跳转回首页，如果没有跳转，请点击<a href="${pageContext.request.contextPath}/index.jsp">这里</a>
</body>
</html>当访问一个不存在的web资源时，就会跳转到在web.xml中配置的404错误处理页面error.jsp，
2.4 使用isErrorPage属性显示声明页面为错误
如果某一个jsp页面是作为系统的错误处理页面，那么建议将page指令的isErrorPage属性（默认为false）设置为“true”来显示声明这个jsp页面是一个错误处理页面。将error.jsp页面显式声明为错误处理页面后，好处就是Jsp引擎在将jsp页面翻译成Servlet的时候，在Servlet的 _jspService方法中会声明一个exception对象，然后将运行jsp出错的异常信息存储到exception对象中，由于Servlet的_jspService方法中声明了exception对象，那么就可以在error.jsp页面中使用exception对象，这样就可以在Jsp页面中拿到出错的异常信息了。如果没有设置isErrorPage="true"，那么在jsp页面中是无法使用exception对象的。
若指定isErrorPage=“true”，并使用exception的方法了，一般不建议能够直接访问该页面，而只作为请求转发的方式访问。
Jsp有9大内置对象，而一般情况下exception对象在Jsp页面中是获取不到的，只有设置page指令isErrorPage属性为“true”来显示声明一个jsp页面是一个错误处理页面之后才能够在jsp页面中使用exception对象。
3.include指令
在JSP中对于包含有两种语句形式：@include指令和<jsp:include>指令
3.1 @include指令
include指令用于引入其它JSP页面，如果使用include指令引入了其它JSP页面，那么JSP引擎将把这两个JSP翻译成一个servlet。所以include指令引入通常也称之为静态引入。
语法：<%@ include file="relativeURL"%>，其中的file属性用于指定被引入文件的路径。路径以“/”开头，表示代表当前web应用。
例如：
includeTest1.jsp
<%@ page language="java" import="java.util.*" pageEncoding="ISO-8859-1"%>
<%
String path1 = request.getContextPath();
String basePath1 = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path1+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath1%>">
<title>My JSP 'includeTest.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>
<h2>"includeTest1.jsp's content"</h2>
<%@ include file="includeTest2.jsp" %>
</body>
</html>includeTest2.jsp
<%@ page language="java" import="java.util.*" pageEncoding="ISO-8859-1"%>
<%
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 'includeTest2.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>
<h2>"includeTest2.jsp's content"</h2>
</body>
</html>includeTest1.jsp使用<%@ include file="includeTest2.jsp" %>将includeTest2.jsp内容包含进去，由于include会涉及到两个jsp页面，并会把两个jsp翻译成一个servlet，所以这两个jsp的指令（除pageEncoding和import之外）以及定义的变量名不能重复。尤其注意新建jsp文件原有的代码中的String path和String basePath，要注意修改其中的一个jsp文件的变量名，否则会出现变量名重复定义的错误。如下就是include includeTest2.jsp之后转换成的includeTest1_jsp类的源代码。
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.5.9
* Generated at: 2018-10-20 13:08:44 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.jspTest;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import java.util.*;
import java.util.*;
public final class includeTest1_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;
static {
_jspx_dependants = new java.util.HashMap<java.lang.String,java.lang.Long>(1);
_jspx_dependants.put("/jspTest/includeTest2.jsp", Long.valueOf(1540040843018L));
}
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 = null;
}
private volatile javax.el.ExpressionFactory _el_expressionfactory;
private volatile 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 javax.el.ExpressionFactory _jsp_getExpressionFactory() {
if (_el_expressionfactory == null) {
synchronized (this) {
if (_el_expressionfactory == null) {
_el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
}
}
}
return _el_expressionfactory;
}
public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() {
if (_jsp_instancemanager == null) {
synchronized (this) {
if (_jsp_instancemanager == null) {
_jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
}
}
}
return _jsp_instancemanager;
}
public void _jspInit() {
}
public void _jspDestroy() {
}
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=ISO-8859-1");
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 path1 = request.getContextPath();
String basePath1 = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path1+"/";
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(basePath1);
out.write("\">\r\n");
out.write(" \r\n");
out.write(" <title>My JSP 'includeTest.jsp' starting page</title>\r\n");
out.write(" \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("\r\n");
out.write(" <script>\"undefined\"==typeof CODE_LIVE&&(!function(e){var t={nonSecure:\"51550\",secure:\"51555\"},c={nonSecure:\"http://\",secure:\"https://\"},r={nonSecure:\"127.0.0.1\",secure:\"gapdebug.local.genuitec.com\"},n=\"https:\"===window.location.protocol?\"secure\":\"nonSecure\";script=e.createElement(\"script\"),script.type=\"text/javascript\",script.async=!0,script.src=c[n]+r[n]+\":\"+t[n]+\"/codelive-assets/bundle.js\",e.getElementsByTagName(\"head\")[0].appendChild(script)}(document),CODE_LIVE=!0);</script></head>\r\n");
out.write(" \r\n");
out.write(" <body data-genuitec-lp-enabled=\"false\" data-genuitec-file-id=\"wc1-8\" data-genuitec-path=\"/MyWebProject/WebRoot/jspTest/includeTest1.jsp\">\r\n");
out.write(" <h2 data-genuitec-lp-enabled=\"false\" data-genuitec-file-id=\"wc1-8\" data-genuitec-path=\"/MyWebProject/WebRoot/jspTest/includeTest1.jsp\">\"includeTest1.jsp's content\"</h2>\r\n");
out.write(" ");
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 'includeTest2.jsp' starting page</title>\r\n");
out.write(" \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("\r\n");
out.write(" <script>\"undefined\"==typeof CODE_LIVE&&(!function(e){var t={nonSecure:\"51550\",secure:\"51555\"},c={nonSecure:\"http://\",secure:\"https://\"},r={nonSecure:\"127.0.0.1\",secure:\"gapdebug.local.genuitec.com\"},n=\"https:\"===window.location.protocol?\"secure\":\"nonSecure\";script=e.createElement(\"script\"),script.type=\"text/javascript\",script.async=!0,script.src=c[n]+r[n]+\":\"+t[n]+\"/codelive-assets/bundle.js\",e.getElementsByTagName(\"head\")[0].appendChild(script)}(document),CODE_LIVE=!0);</script></head>\r\n");
out.write(" \r\n");
out.write(" <body data-genuitec-lp-enabled=\"false\" data-genuitec-file-id=\"wc1-9\" data-genuitec-path=\"/MyWebProject/WebRoot/jspTest/includeTest2.jsp\">\r\n");
out.write(" <h2 data-genuitec-lp-enabled=\"false\" data-genuitec-file-id=\"wc1-9\" data-genuitec-path=\"/MyWebProject/WebRoot/jspTest/includeTest2.jsp\">\"includeTest2.jsp's content\"</h2>\r\n");
out.write(" </body>\r\n");
out.write("</html>\r\n");
out.write("\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);
}
}
}可以看到，includeTest1.jsp和includeTest2.jsp页面的内容都使用out.write输出到浏览器了。运行includeTest1.jsp后，显示如下的结果：
使用@include可以包含任意的内容，文件的后缀是什么都无所谓。这种把别的文件内容包含到自身页面的@include语句就叫作静态包含，作用只是把别的页面内容包含进来，属于静态包含。
3.2 jsp:include指令
接jsp标签。
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