J2EE：自定义标签属性的默认值
因此，根据 Sun 的 J2EE 文档 (http ://docs.sun.com/app/docs/doc/819-3669/bnani?l=en&a=view)，“如果不需要标签属性，则标签处理程序应提供默认值价值。”
我的问题是如何根据文档的描述定义默认值。代码如下：
<%@ attribute name="visible" required="false" type="java.lang.Boolean" %>
<c:if test="${visible}">
My Tag Contents Here
</c:if>
显然，这个标签无法编译，因为它缺少标签指令和核心库导入。我的观点是，我希望“visible”属性默认为 TRUE。 “标签属性不是必需的”，因此“标签处理程序应该提供默认值”。我想提供一个默认值，那么我缺少什么？
非常感谢任何帮助。
So according to Sun's J2EE documentation (http://docs.sun.com/app/docs/doc/819-3669/bnani?l=en&a=view), "If a tag attribute is not required, a tag handler should provide a default value."
My question is how do I define a default value as per the documentation's description. Here's the code:
<%@ attribute name="visible" required="false" type="java.lang.Boolean" %>
<c:if test="${visible}">
My Tag Contents Here
</c:if>
Obviously, this tag won't compile because it's lacking the tag directive and the core library import. My point is that I want the "visible" property to default to TRUE. The "tag attribute is not required," so the "tag handler should provide a default value." I want to provide a default value, so what am I missing?
Any help is greatly appreciated.
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我会回答我自己的问题。我突然顿悟，意识到
java.lang.Boolean是一个类，而不是一个原语。这意味着该值可以为null，经过测试，这个值肯定是null。当未定义值时，则传入 null。否则，该值就是传入的值。因此，声明属性后我要做的第一件事就是检查它是否为 null。如果它是 null，那么我知道没有传入值或有人向我传递了 null，并且应该将其转换为某个默认值：
I'll answer my own question. I had an epiphany and realized that
java.lang.Booleanis a class and not a primitive. This means that the value can be null, and after testing, this value most certainly is null.When a value is not defined, then null is passed in. Otherwise, the value is whatever was passed in. So the first thing I do after declaring the attribute is to check if it's null. If it is null, then I know a value wasn't passed in or someone passed me null, and it should be converted to some default value:
使用 JSP EL 和条件运算符，它会更简洁，甚至更短：
With JSP EL and conditional operator it's a little bit cleaner and even shorter: