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在JSP中，可以使用include指令来包含其他jsp文件。例如，一个网站中，多个网页具有很大部分是相同元素时，可以使用include指令来完成。include的指令的语法如下：
<%@ include file=”文件路径”%>
下面通过一个简单的实例来说明include指令实现的原理。使用Eclipse在web项目下新建一个jsp文件命名为header.jsp，具体代码如下
代码语言：javascript复制<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<!DOCTYPE HTML>
<html>
<head>
<title>My JSP 'header.jsp' starting page</title>
</head>
<body>
这是一个头文件
</body>
</html>然后新建一个jsp文件命名为include.jsp，具体代码如下：
代码语言：javascript复制<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<!DOCTYPE HTML>
<html>
<head>
<title>My JSP '03.include.jsp' starting page</title>
</head>
<body>
<%@ include file="header.jsp" %>
这是一个主体部分
</body>
</html>使用include指令时，在file属性中指定要包含的jsp文件的路径，推荐使用相对路径。在eclipse中，按住Ctrl键，并点击file属性指定的路径能够打开相对应的jsp文件，说明路径正确。
将web项目部署到Tomcat服务器并启动服务器，到work目录中找到jsp文件翻译成的java文件。代码如下：
代码语言：javascript复制/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/7.0.81
* Generated at: 2017-10-15 11:44:30 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.*;
import java.util.*;
public final class _include_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent {
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("/common/header.jsp", Long.valueOf(1508067769708L));
}
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 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 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("rn");
out.write("rn");
out.write("<!DOCTYPE HTML>rn");
out.write("<html>rn");
out.write(" <head>rn");
out.write(" rn");
out.write(" <title>My JSP '03.include.jsp' starting page</title>rn");
out.write(" rn");
out.write(" </head>rn");
out.write(" rn");
out.write(" <body>rn");
out.write(" t");
out.write("rn");
out.write("rn");
out.write("<!DOCTYPE HTML>rn");
out.write("<html>rn");
out.write(" <head>rn");
out.write(" rn");
out.write(" <title>My JSP 'header.jsp' starting page</title>rn");
out.write(" rn");
out.write(" </head>rn");
out.write(" rn");
out.write(" <body>rn");
out.write(" 这是一个头文件rn");
out.write(" </body>rn");
out.write("</html>rn");
out.write("rn");
out.write(" t这是一个主体部分rn");
out.write(" </body>rn");
out.write("</html>rn");
} 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);
}
}
}可以看出，只翻译成一个java文件，jsp引擎在处理include指令时，只是单纯的将被包含文件的内容copy到包含文件中。查看out.write()代码部分,发现两个其中有html标签有重复的。因此，我们可以在header.jsp中去掉部分重复的标签，这样include.jsp也能正常的翻译成servlet程序。
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