JSP指令

悠悠 2022-06-08 11:25 196阅读 0赞

JSP指令简介：JSP指令(directive)是为JSP引擎而设计的，他们并不直接产生任何可见的输出，而只是告诉

引擎如何处理JSP页面中的剩余部分。

JSP指令的基本语法：

<%@ 指令 属性名="值"%>

例如：

<%@ page language="java" import="java.util.\*" pageEncoding="ISO-8859-1"%> 
    注意：属性名部分是大小写敏感的

在目前的JSP2.0当中，定于了page,include和taglib这三个指令，每种指令中又定义了一些各自的属性。

如果要遭一个JSP页面中设置同一条指令的多个属性，可以使用多条指令语句单独设置每个属性，也可以使用同一条指令

语句设置该指令的多个属性。

第一种方式：

<%@ page language="java" import="java.util.\*" pageEncoding="ISO-8859-1"%>

<%@  import="java.util.\*" %>

第二种方式：

<%@ page language="java" import="java.util.\*" pageEncoding="ISO-8859-1"%>

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变成习惯，page指令最好是放在JSP页面的起始位置。

page指令的完整语法:

1）.language="java"

2）.extends="package.class"  当前JSP页面翻译成Servlet之后可以继承的类

3）.import="java.util.\*"    导入类之后，才可以使用这些类中的方法。

![Center][]

查看jsp的java文件，可以看到jsp页面会自动导入一些包：

import javax.servlet.\*;  
             import javax.servlet.http.\*;  
             import javax.servlet.jsp.\*;

4）.session="true|false"  表示当前页面上是否允许使用session.

允许使用session：

<%@ 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 'Jsp1.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">
 	
 
 </head>
 
 <body>
 <%= session.getId() %>
 </body>
 </html>

查看页面输出:

![Center 1][]

如果不允许使用session:

在jsp页面加上 <%@ page session="false" %> 之后，再使用session则会报错!!!

5.1）.<%@ page errorPage="/page/error.jsp" %>  出现异常时跳转的页面.

Jsp1.jsp:

<%@ page language="java" import="java.util.*" pageEncoding="ISO-8859-1"%>
 <%@ page errorPage="/page/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 'Jsp1.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">
 	
 
 </head>
 
 <body>
 <%int i=10/0; %>
 </body>
 </html>

error.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 '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">
 	
 
 </head>
 
 <body>
 ERROR PAGE! 
 
 </body>
 </html>

浏览器进入该页面时报异常，这时转发进入errorPage.

![Center 2][]

5.2）.<%@ page isErrorPage="true" %>  . 指定当前页面是否为错误处理页面，可以说明当前页面是否可以使用

使用exception这个隐藏变量：

<%@ page language="java" import="java.util.*" pageEncoding="ISO-8859-1"%>
 <%@page isErrorPage="true"%>
 <%
 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">
 	
 
 </head>
 
 <body>
 ERROR PAGE! 
 ERROR:<%= exception.getMessage()%>
 </body>
 </html>

这样再次浏览器测试：

![Center 3][]

5.3）需要注意的是若指定isErrorPage="true" ,并使用exception的方法，一般不建议直接访问该错误页面，对于TOMCAT服务器而言，

浏览器不能通过直接输入文件地址的形式访问WEB-INF下的文件，因为WEB-INF下报错的是隐私文件，如果想访问的话，可以通过请求的转发

可以访问：

将error.jsp放入WEB-INF目录下：

![Center 4][]

修改Jsp1.jsp里的<%@ page errorPage="/WEB-INF/error.jsp" %>

浏览器测试：

![Center 5][]

OK!!!!

5.4）.  还可以在web.xml文件中配置错误页面:

不在某一jsp页面指定errorPage，而改为web.xml文件中配置：

5.4.1）.根据错误代码500,404等等:

<?xml version="1.0" encoding="UTF-8"?>
 <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
 <display-name>MyLab2</display-name>
 <welcome-file-list>
 <welcome-file>index.html</welcome-file>
 <welcome-file>index.htm</welcome-file>
 <welcome-file>index.jsp</welcome-file>
 <welcome-file>default.html</welcome-file>
 <welcome-file>default.htm</welcome-file>
 <welcome-file>default.jsp</welcome-file>
 </welcome-file-list>
 <error-page> <error-code>500</error-code> <location>/WEB-INF/error.jsp</location> </error-page>
 </web-app>

测试：

![Center 6][]

5.4.2)  根据错误类型，例如java.lang.ArithmeticException：

<?xml version="1.0" encoding="UTF-8"?>
 <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
 <display-name>MyLab2</display-name>
 <welcome-file-list>
 <welcome-file>index.html</welcome-file>
 <welcome-file>index.htm</welcome-file>
 <welcome-file>index.jsp</welcome-file>
 <welcome-file>default.html</welcome-file>
 <welcome-file>default.htm</welcome-file>
 <welcome-file>default.jsp</welcome-file>
 </welcome-file-list>
 <error-page> <exception-type>java.lang.ArithmeticException</exception-type> <location>/WEB-INF/error.jsp</location> </error-page>
 </web-app>

测试：

![Center 7][]

6）. <%@ page contentType="text/html; charset=ISO-8859-1" %>      指定当前JSP页面的响应类型，实际调用的是

response.setContentType("text/html;charset=ISO-8859-1");

通常情况下，对于JSP页面而言，其取值不需要改变。响应内容为html,表明JSP页面被servlet引擎编译之后，响应的是html。

charset指定返回的页面的字符编码是多少，通常取值为UTF-8。

7）.  <%@ page  pageEncoding="ISO-8859-1"%>   指定当前JSP页面的字符编码，通常和charset一致。

8）.  <%@page isElIgnored="true" %>  指定当前JSP页面是否可以使用EL表达式。通常取值为true.

include指令：   用于通知JSP引擎翻译当前JSP页面 时将其他文件中的内容合并进当前JSP页面转换成的servlet源文件中，

这种在源文件级别进行引入的方式称之为静态引入。当前JSP页面与静态引入的页面紧密结合为一个servlet.

语法：<%@include file="relativeUrl"%>

其中的file属性用于指定被引入文件的相对路径。如果以"/"开头，表示相对于当前WEB应用的根目录。

被引入的文件需要遵守的规则：

1.被引入的文件必须遵循JSP语法，其中的内容可以包含静态HTML,JSP脚本元素,JSP指令和JSP行为元素等普通JSP页面所具有的一切内容。

2.被引入的文件可以使用任意的扩展名，即使其扩展名为HTML,JSP引擎也会按照处理JSP页面的方式处理他里面的内容，为了见名知意，

JSP规范建议使用.jspf(jsp fragments) 组我诶静态引入文件的扩展名。

3.在将JSP文件翻译成servlet源文件时，JSP引擎将合并被引入的文件与当前JSP页面中的指令元素（设置pageEncoding属性的page指令除外），

所以，除了import和pageEncoding属性之外,page指令的其他属性不能再这个页面中有不同的设置值。

测试：

jsp1.jsp中包含jsp2.jsp（静态包含，源码级包含

![Center 8][]

发现只有jsp1的文件，但是没有jsp2文件，打开jsp1的java文件，发现jsp2的代码都在该文件中：

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/8.5.6
 * Generated at: 2017-09-24 07:45:15 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.page;
 
 import javax.servlet.*;
 import javax.servlet.http.*;
 import javax.servlet.jsp.*;
 import java.util.*;
 import java.util.*;
 
 public final class Jsp1_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("/page/Jsp2.jsp", Long.valueOf(1506239106121L));
 }
 
 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\n");
 out.write("\r\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 'Jsp1.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("\r\n");
 out.write(" </head>\r\n");
 out.write(" \r\n");
 out.write(" <body>\r\n");
 out.write(" This is my JSP1 page. \r\n");
 out.write(" \r\n");
 out.write(" ");
 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 'Jsp2.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("\r\n"); out.write(" </head>\r\n"); out.write(" \r\n"); out.write(" <body>\r\n"); out.write(" This is my JSP2 page. \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);
 }
 }
 }

）：

jsp1.jsp:

<%@ page language="java" import="java.util.*" pageEncoding="ISO-8859-1"%>
 <%@ page contentType="text/html; charset=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 'Jsp1.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">
 	
 
 </head>
 
 <body>
 This is my JSP1 page. 
  <%@ include file="Jsp2.jsp" %>
 </body>
 </html>

jsp2.jsp：

<%@ page language="java" import="java.util.*" pageEncoding="ISO-8859-1"%>
 
 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
 <html>
 <head>
 <base href="<%=basePath%>">
 
 <title>My JSP 'Jsp2.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">
 	
 
 </head>
 
 <body>
 This is my JSP2 page. 
 </body>
 </html>

浏览器测试：

![Center 9][]  
jsp1.jsp和jsp2.jsp有一点不同的是jsp2,jsp没有:

<%
 String path = request.getContextPath();
 String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
 %>

是因为若两个页面都有这段JAVA代码的话，相当于jsp1的servlet中的service方法中有两个path变量，两个basePath变量，造成冲突。

[Center]: /images/20220608/e0e0a39cf5664743b76a33cab96d09c9.png
[Center 1]: /images/20220608/87f99afe9364453ab56c6dba72373e39.png
[Center 2]: /images/20220608/3f198ac2bf2a402a9e4c645c4fa1f41f.png
[Center 3]: /images/20220608/e8ee7adfece64a7ba4f1c07aee520c74.png
[Center 4]: /images/20220608/e87c7324324445dd9037874a40a45cc9.png
[Center 5]: /images/20220608/cd4d796ce6b94492895d3fdff11930ad.png
[Center 6]: /images/20220608/f239e375b5d34118aa568e6920e57f64.png
[Center 7]: /images/20220608/ac6108e893f64793a2c4dd754ed628dc.png
[Center 8]: /images/20220608/7fca1bc6bcb043d2b44f419928b30ff5.png
[Center 9]: /images/20220608/a1b82228e9ee429cbb091823c90027d4.png