一道练习题引发的思考

小灰灰 2021-03-16 15:23 625阅读 0赞

在一个JSP页面中，有如下代码，请问最终的输出结果为：

<%! int i = 1;%>
     <%! int getNumber(){
         return i;
     }%>
     <% int i = 10;
         out.println("first i = "+i);%>
     <% out.println("second i = "+i);%>
     <% out.println("third i = "+getNumber());%>

其输出结果为：first i = 10  second i = 10 third i = 1

这道题容易出错的原因是在于分不清在<%! %>声明语句中声明的变量和在<% %>小脚本中声明的变量的区别，它们两个区别具体如下：JSP声明语句中声明的变量在生成Servlet后位于类的成员变量部分，属于全局变量；而JSP脚本段声明的变量在生成Servlet后位于service方法中，属于局部变量，当方法运行完后就被销毁了。Servlet是一个单实例对象，成员变量只有一个，每个用户都可访问到它，所以会污染全局变量，在多用户访问时，也不利于维护数据的正确性和安全性。

其实 我们可以从jsp生成的.java文件看出区别，从源代码可以看出这个jsp页面程序到底是怎么执行的：

````
 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 {
  //JSP声明语句声明的变量和方法，在全局部分
  int i = 1;
  int getNumber(){
         return i;
     }
   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 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() {
   }
   //JSP页面生成的Service方法，小脚本语句位于这个方法中
   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("<html>\r\n");
       out.write("<head>\r\n");
       out.write("    <title>test</title>\r\n");
       out.write("</head>\r\n");
       out.write("<body>\r\n");
       out.write("    ");
       out.write("\r\n");
       out.write("    ");
       out.write("\r\n");
       out.write("    ");
       //JSP小脚本中声明的变量
       int i = 10;
       //JSP小脚本中的语句
         out.println("first i = "+i);
       out.write("\r\n");
       out.write("    ");
       //JSP小脚本中的语句
  out.println("second i = "+i);
       out.write("\r\n");
       out.write("    ");
       //JSP小脚本中的语句
  out.println("third i = "+getNumber());
       out.write("\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);
     }
   }
 }
````