<span role="heading" aria-level="2">JSP的include指令

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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

<%@ 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：

<%@ 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：

/*
 * 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("\r\n");
      out.write("\r\n");
      out.write("<!DOCTYPE HTML>\r\n");
      out.write("<html>\r\n");
      out.write("  <head>\r\n");
      out.write("    \r\n");
      out.write("    <title>My JSP '03.include.jsp' starting page</title>\r\n");
      out.write("   \r\n");
      out.write("  </head>\r\n");
      out.write("  \r\n");
      out.write("  <body>\r\n");
      out.write("   \t");
      out.write("\r\n");
      out.write("\r\n");
      out.write("<!DOCTYPE HTML>\r\n");
      out.write("<html>\r\n");
      out.write("  <head>\r\n");
      out.write("    \r\n");
      out.write("    <title>My JSP 'header.jsp' starting page</title>\r\n");
      out.write("   \r\n");
      out.write("  </head>\r\n");
      out.write("  \r\n");
      out.write("  <body>\r\n");
      out.write("   这是一个头文件\r\n");
      out.write("  </body>\r\n");
      out.write("</html>\r\n");
      out.write("\r\n");
      out.write("   \t这是一个主体部分\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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JSP的include指令

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JSP的include指令

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