在JSP中用兩種聲明變量的方法，一種是在<%! %>內，一種是在<% %>內。他們之間有什么區別呢？我們直接看一個JSP文件來理解。
count.jsp
代碼如下：
<%@ page contentType="text/html; charset=gb2312" language="java" errorPage="" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<HTML>
<HEAD>
<TITLE>JSP Declarations</TITLE>
<BODY>
<H1>JSP Declarations</H1>
<%!
int count = 0; //聲明在<%！>內的變量
void method(){}
%>
<%
int i= 0; //聲明在<% >內的變量
%>
<H1>
count:<%= ++count %></H1>
<br/>
<H1>i:<%= ++i %></H1>
</BODY>
</HTML>
啟動tomcat運行這個jsp會發現當我們不斷刷新頁面時count的值會不斷增加 而i的值始終不變，也就是說定義在<%!%>內的變量會累加，而定義在<%%>內的變量不會累加。
這是為什么呢？
我們再看下count.jsp文件自動轉換成的類文件的代碼
文件所在地
TOMCAT安裝目錄/work/Catalina/localhost/你的工程名稱/org/apache/jsp下的count_jsp.java
代碼如下：
package org.apache.jsp;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
public final class count_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent {
int count = 0; //聲明 在<%！內的 變量
void method(){} //方法
private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory();
private static java.util.List _jspx_dependants;
private javax.el.ExpressionFactory _el_expressionfactory;
private org.apache.AnnotationProcessor _jsp_annotationprocessor;
public Object getDependants() {
return _jspx_dependants;
}
public void _jspInit() {
_el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
_jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName());
}
public void _jspDestroy() {
}
public void _jspService(HttpServletRequest request, HttpServletResponse response)
throws java.io.IOException, ServletException { //tomcat生成的方法
PageContext pageContext = null;
HttpSession session = null;
ServletContext application = null;
ServletConfig config = null;
JspWriter out = null;
Object page = this;
JspWriter _jspx_out = null;
PageContext _jspx_page_context = null;
try {
response.setContentType("text/html; charset=gb2312");
pageContext = _jspxFactory.getPageContext(this, request, response,
"", 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("<!DOCTYPE html PUBLIC /"-//W3C//DTD XHTML 1.0 Transitional//EN/" /"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd/">/r/n");
out.write("<html xmlns=/"http://www.w3.org/1999/xhtml/">/r/n");
out.write("<head>/r/n");
out.write("<meta http-equiv=/"Content-Type/" content=/"text/html; charset=gb2312/" />/r/n");
out.write("<HTML>/r/n");
out.write("<HEAD>/r/n");
out.write("<TITLE>JSP Declarations</TITLE>/r/n");
out.write("<BODY>/r/n");
out.write("<H1>JSP Declarations</H1>/r/n");
out.write("/r/n");
out.write("/r/n");
out.write("/r/n");
int i= 0; //聲明在<%內的變量
out.write("/r/n");
out.write("/r/n");
out.write("<H1>/r/n");
out.write("/r/n");
out.write("count:");
out.print( ++count );
out.write("</H1>/r/n");
out.write("<br/>/r/n");
out.write("/r/n");
out.write("<H1>i:");
out.print( ++i );
out.write("</H1> /r/n");
out.write("/r/n");
out.write("</BODY>/r/n");
out.write("</HTML>");
} catch (Throwable t) {
if (!(t instanceof SkipPageException)){
out = _jspx_out;
if (out != null && out.getBufferSize() != 0)
try { out.clearBuffer(); } catch (java.io.IOException e) {}
if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
}
} finally {
_jspxFactory.releasePageContext(_jspx_page_context);
}
}
}
通過觀察代碼后我們發現，原先聲明在<%! %>內的變量和方法是一個類內的變量和方法也就是成員變量和成員方法。聲明在<%%>內的變量是一個方法的變量也就是局部變量。
我們學習JAVA的時候就知道成員變量會被多個對象共享，而局部變量單獨對象訪問。所以count會自動增加，i不對遞增。
在這里還有個問題要告訴讀者，如果在<% %>內添加一個method()方法那么該程序就會報錯，其實原因不難想象，這樣聲明一個方法等於就是一個類里的方法里又聲明了個方法
結果肯定是錯誤的。