JSP指令是指：用于设置JSP页面相关属性的一个语法命令，例如：设置页面编码字符集、导入其他包等等。JSP中提供了三个指令，分别是：page指令、include指令、taglib指令。其中page指令用于设置JSP页面属性，include指令用于引入其他的JSP文件，taglib指令用于引入标签库。这一小节内容介绍include指令的使用。
1.1、include指令作用
include指令作用：将指定的文件引入到当前JSP页面里面。include指令会将引入的文件内容嵌入到当前JSP页面中的对应位置。
<%@ include file="文件的相对路径" %>案例代码：
- 创建一个header.jsp文件。
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>这是HTML头部</title>
</head>- 创建一个include.jsp文件。
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%-- 引入头部文件 --%>
<%@ include file="header.jsp" %>
<body>
<div style="background-color: cadetblue">
这是正文内容区域
</div>
<%-- 引入底部文件 --%>
<%@ include file="footer.html" %>
</body>
</html>- 创建一个footer.jsp文件。
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<div style="background-color: antiquewhite;">
这是footer底部内容区域
</div>启动Tomcat容器，浏览器访问http://localhost:8080/servlet/include.jsp，结果如下：
1.2、include指令的本质
include指令的本质是什么呢？？？我们来查看下编译之后的Java源代码，找到上面include.jsp文件编译之后的源文件，如下所示：
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.5.98
* Generated at: 2024-02-25 05:06:41 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 com.gitcode.servlet;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
public final class include_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>(2);
_jspx_dependants.put("/footer.jsp", Long.valueOf(1708837593266L));
_jspx_dependants.put("/header.jsp", Long.valueOf(1708837593271L));
}
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("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, "JSP 只允许 GET、POST 或 HEAD。Jasper 还允许 OPTIONS");
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');
out.write('\r');
out.write('\n');
out.write("\r\n");
out.write("<html>\r\n");
out.write("<head>\r\n");
out.write(" <title>这是HTML头部</title>\r\n");
out.write("</head>");
out.write("\r\n");
out.write("<body>\r\n");
out.write("\r\n");
out.write("<div style=\"background-color: cadetblue\">\r\n");
out.write(" 这是正文内容区域\r\n");
out.write("</div>\r\n");
out.write("\r\n");
out.write('\r');
out.write('\n');
out.write("\r\n");
out.write("<div style=\"background-color: antiquewhite;\">\r\n");
out.write(" 这是footer底部内容区域\r\n");
out.write("</div>");
out.write("\r\n");
out.write("</body>\r\n");
out.write("</html>");
} 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);
}
}
}通过上面源代码，可以看到，使用include指令引入的两个文件，最终都会将两个文件中的内容直接嵌入到当前include.jsp文件里面，如下所示：
所以include指令的本质就是将引入文件中的内容，直接拼接到当前JSP页面的对应位置。这里也就会存在一个问题，引入的JSP文件中，不能存在和当前JSP页面相同的变量名称，因为变量名称相同会导致编译失败。另外，使用include指令引入其他的JSP文件时候，只会生成访问的那个JSP文件的源代码，被引入的JSP文件不会生成对应的源代码。
以上，就是include指令的使用及其本质。
今天就到这里，未完待续~~
本文暂时没有评论，来添加一个吧(●'◡'●)