java jstl 的 c:url 标签的目的是什么？
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What is the purpose of jstl's c:url tag?
提问by Ole
TutorialPointhas a simple example of the c:urltag that looks like this:
TutorialPoint有一个简单的c:url标签示例，如下所示：
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<html>
<head>
<title><c:url> Tag Example</title>
</head>
<body>
<a href="<c:url value="/jsp/index.htm"/>">TEST</a>
</body>
</html>
When looking at the corresponding rendering with Chrome's developer tooling it renders like this:
当使用 Chrome 的开发人员工具查看相应的渲染时，它会呈现如下：
<a href="/jsp/index.htm">TEST</a>
So the c:urltag seems redundant, but I'm sure I'm missing something?
所以c:url标签似乎是多余的，但我确定我遗漏了什么？
回答by msagala25
As Tutorials Points says, It is for formattingpurposes of the URLyou put in and it can be stored in a variable.
作为教程点说，这是格式化的目的URL你把它可以存储在一个变量。
Example you have this:
例如你有这个：
<a href="<c:url value="/test.html"/>">TEST</a>
if you click TEST, it will go to page test.html. simple as that. but the question is, what is the value of <c:url value="/test.html"/>?
如果您单击TEST，它将转到页面test.html。就那么简单。但问题是， 的价值是<c:url value="/test.html"/>什么？
are you thinking the value is only /test.html?
你以为只有价值/test.html吗？
try to test it, like this:
尝试测试它，像这样：
<a href="<c:url value="/test.html" var="testvar" />">TEST</a> // testvar is where you put the url formatted by c:url
<c:out value="${testvar}"/> // you print what is the formatted url
the answer will be the Context Folderof your project plus the URLyou put in.
答案将是您项目的上下文文件夹加上您输入的 URL。
context/test.html will be the output.
上下文/test.html 将是输出。
I think that its purpose is to have the context(Current Application) already given to the URL, and you only need to add the remaining URLpart.
我认为它的目的是将context（当前应用程序）已经提供给URL，您只需要添加其余URL部分。
回答by LexSav
Here is a short snippet from my training app where I use <c:url>tag:
这是我使用<c:url>标签的训练应用程序中的一个简短片段：
<table>
<tr>
<th>First Name</th>
<th>Last Name</th>
<th>Email</th>
<th>Action</th>
</tr>
<c:forEach var="student" items="${student_list}">
<c:url var="loadStudentLink" value="StudentControllerServlet">
<c:param name="command" value="load"/>
<c:param name="id" value="${student.id}"/>
</c:url>
<tr>
<td> ${student.firstName} </td>
<td> ${student.lastName} </td>
<td> ${student.email} </td>
<td>
<a href="${loadStudentLink}">Update</a>
</td>
</tr>
</c:forEach>
</table>
Of course in this case I could just use the link below and it would be the same:
当然，在这种情况下，我可以只使用下面的链接，它会是一样的：
<a href="StudentControllerServlet?command=load&id=${student.id}">Update</a>
In a nutshell, <c:url>creates an ordinary link which you can store in a variable and define its scope.
With <c:param>tags you can set parameters for the link.
In addition, some could say that it looks more neatly with JSTL.
简而言之，<c:url>创建一个普通链接，您可以将其存储在变量中并定义其范围。使用<c:param>标签，您可以为链接设置参数。另外，有人会说用JSTL看起来更整洁。
Also, as it was said before, <c:url>already has context path of the app.
So, for instance, you can do this<link rel="stylesheet" href="<c:url value="/resources/css/test.css"/>" />
instead of that<link rel="stylesheet" href="${pageContext.request.contextPath}/resources/css/test.css" />
此外，正如之前所说，<c:url>已经有了应用程序的上下文路径。因此，例如，您可以这样做<link rel="stylesheet" href="<c:url value="/resources/css/test.css"/>" />
而不是那样<link rel="stylesheet" href="${pageContext.request.contextPath}/resources/css/test.css" />
I hope you catch my meaning :)
我希望你明白我的意思:)
回答by LazerBass
In addition to the purposes already mentioned in the other answers an additional purpose of the c:urltag is to rewrite URLs returned from a JSP page. For example for session tacking. In case the user agent does not support (or has disabled) cookies the jsessionid will be appended to the URL.
除了其他答案中已经提到的目的之外，该c:url标记的另一个目的是重写从 JSP 页面返回的 URL。例如用于会话跟踪。如果用户代理不支持（或已禁用）cookie，则 jsessionid 将附加到 URL。
The tag includes the session ID in the URL only if cookies are disabled; otherwise, it returns the URL unchanged.
仅当 cookie 被禁用时，该标签才会在 URL 中包含会话 ID；否则，它将返回未更改的 URL。
See: https://docs.oracle.com/javaee/5/tutorial/doc/bnakh.html#bnakn
请参阅：https: //docs.oracle.com/javaee/5/tutorial/doc/bnakh.html#bnakn
