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jsp编译过程---二

2017-06-07 22:17 197 查看

jsp编译过程---二

<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>

<%! int a=3;%>  <%--声明 ,全局变量--%>
<% int b=3; %>   <%--脚本段 ，局部变量，定义与方法中，调用完释放--%>

<%=a-- %><br>
<%=b-- %>
</body>
</html>

输出结果：a=3 2 1 0 -1 -2 ...

b=3 3 3 3 3 3 ....

结果分析：每次刷新，a的值每次减1，b的值始终保持不变。在该jsp中变量a被声明为全局变量，调用完毕不会被释放，每次刷新，该值都会被减少1.b为方法中的局部变量，每次刷新，执行该方法时，取到的值都是一个3，因为减1之后的2被释放掉了。每一个jsp都会转化成一个与之对应的servlet文件（具体转化过程可看我的上一篇文章），如下所示：

/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/7.0.30
* Generated at: 2017-06-07 14:08:56 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.*;

public final class test_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent {

int a=3;
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;

private javax.el.ExpressionFactory _el_expressionfactory;
private org.apache.tomcat.InstanceManager _jsp_instancemanager;

public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
return _jspx_dependants;
}

public void _jspInit() {
_el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
_jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
}

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("<!DOCTYPE html PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">\r\n");
out.write("<html>\r\n");
out.write("<head>\r\n");
out.write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=UTF-8\">\r\n");
out.write("<title>Insert title here</title>\r\n");
out.write("</head>\r\n");
out.write("<body>\r\n");
out.write("\r\n");
out.write("  ");
out.write(' ');
out.write(' ');
out.write("\r\n");
out.write("  ");
int b=3;
out.write(' ');
out.write(' ');
out.write(' ');
out.write("\r\n");
out.write("  \r\n");
out.write("  ");
out.print(a-- );
out.write("<br>\r\n");
out.write("  ");
out.print(b-- );
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 { 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);
}
}
}

详解二：

JSP声明的格式：<%! int a=3;%>，脚本端的形式：<% int b=3;%>。当服务器将JSP转换为Servlet时，会将JSP中的声明转换为类的成员变量，而将脚本段转换为方法中的局部变量，Servlet是单实例的，这样成员变量的值就只有一个,每个用户都会访问到它，而脚本段中的值则是局部变量，每个用户访问的时候各有一份，互不影响。

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