问题：
- 父 JSP 页面中声明了一个变量，子 JSPF 文件中不显示的声明能直接使用这个变量吗
- 上面的情况如果是子 JSP 又如何
实验
新建一个 servlet 在 request 中传入 name 属性，然后 forward 到 parent.jsp 页面中。parent 页面包含三个子页面，分别是 sub.jspf, sub2.jsp 和 sub3.jsp. 前两个通过 <%@ include file="xxx" %> 引入，sub3.jsp 通过 <jsp:include page="xxx"/> 引入。目录结构如下
1
2
3
4
5
6
7
8
9
10
11
12
13
| .
├── java
│ └── com
│ └── jzheng
│ └── servlet
│ └── ParentServlet.java
└── webapp
├── WEB-INF
│ └── web.xml
├── parent.jsp
├── sub.jspf
├── sub2.jsp
└── sub3.jsp
|
代码实现如下
1
2
3
4
5
6
7
| public class ParentServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
req.setAttribute("name", "jack");
getServletContext().getRequestDispatcher("/parent.jsp").forward(req, resp);
}
}
|
parent.jsp 页面取得 request 中的属性，并重命名为 myname
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
| <%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>Title</title>
</head>
<body>
<%
String myname = (String)request.getAttribute("name");
%>
<hr>
<h1> include jspf file </h1>
<%@ include file="sub.jspf" %>
<hr>
<h1> include jsp file </h1>
<%@ include file="sub2.jsp" %>
<hr>
<h1> jsp:include jsp file </h1>
<jsp:include page="sub3.jsp"/>
</body>
</html>
|
sub 页面内容如下
1
2
3
4
5
6
7
8
9
10
11
| <!-- sub.jsp -->
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<h2> Name: <%= myname%> <h2/>
<!-- sub2.jsp -->
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<h2> Name: <%=myname%> </h2>
<!-- sub3.jsp -->
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<h2>Name: ${name}</h2>
|
PS: 这里插入一个语法点，<%= myname%> 这种语法是结合 String myname = (String)request.getAttribute("name"); 只有 jsp 中声明的变量可以这么用，其实这种写法有点累赘，可以通过 EL 表示式 ${name} 直接从内置对象中取值。
配置 web.xml
1
2
3
4
5
6
7
8
| <servlet>
<servlet-name>ParentServlet</servlet-name>
<servlet-class>com.jzheng.servlet.ParentServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>ParentServlet</servlet-name>
<url-pattern>/parent</url-pattern>
</servlet-mapping>
|
启动服务器访问 /parent 可以看到如下结果, 几种方式都能 work.
深入理解
查看编译生成的 JSP 文件，可以看到 parent.jsp 和 sub3.jsp 被编译成了 java/class, sub.jsp 和 sub2.jsp 没有。原因是 <%@ include file="xxx" %> 会将页面直接整合到父页面中，而 <jsp:include page="xxx"/> 则是通过 request 转发达到这个效果的。查看 parent_jsp.java 文件可以更清晰一点. sub 页面处理部分已表明。所以 <%@ include file="xxx" %> 中直接使用父页面定义的变量是可以，这种做法更像是定义了一写通用脚本做包含。但是这些变量都会爆红，非常的不爽
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
| 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("\n");
out.write("\n");
out.write("<html>\n");
out.write("<head>\n");
out.write(" <title>Title</title>\n");
out.write("</head>\n");
out.write("<body>\n");
out.write("\n");
String myname = (String)request.getAttribute("name");
out.write("\n");
out.write("\n");
out.write("<hr>\n");
out.write("<h1> include jspf file </h1>\n");
out.write('\n');
out.write('\n');
out.write("\n");
out.write("<h2> Name: ");
out.print( myname);
out.write(" <h2/>");
out.write("\n");
out.write("\n");
out.write("<hr>\n");
out.write("<h1> include jsp file </h1>\n");
out.write("\n");
out.write("<h2> Name: ");
out.print(myname);
out.write(" </h2>\n");
out.write("\n");
out.write("\n");
out.write("<hr>\n");
out.write("<h1> jsp:include jsp file </h1>\n");
org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "sub3.jsp", out, false);
out.write("\n");
out.write("\n");
out.write("</body>\n");
out.write("</html>\n");
|