Java JSP 自定义标签属性的默认值
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Default value on JSP custom-tag attribute
提问by Vivin Paliath
When defining an attribute for a custom JSP tag, is it possible to specify a default value? The attributedirective doesn't have a default value attribute. Currently I'm making do with:
为自定义 JSP 标记定义属性时，是否可以指定默认值？该attribute指令没有默认值属性。目前我正在做：
<%@ attribute name="myAttr" required="false" type="java.lang.String" %>
<c:if test="${empty myAttr}" >
<c:set var="myAttr" value="defaultValue" />
</c:if>
Is there a better way?
有没有更好的办法？
采纳答案by G. Demecki
There is a better way:
有一个更好的方法：
<c:set var="title" value="${(empty title) ? 'Default title' : title}" />
No need for custom tag in Java nor tld. Just plain JSP EL and conditional operator.
Java 和 tld 中不需要自定义标签。只是普通的 JSP EL 和条件运算符。
In my opinion it is shorter and cleaner than old:
在我看来，它比旧的更短更干净：
<c:if test="${empty title}" >
<c:set var="title" value="Default title" />
</c:if>
回答by Vivin Paliath
So I wasn't able to figure out a way to add this to the attributedirective itself; it appears that the directive does not support this functionality. I was, however, able to create a tag that encapsulates the <c:if>...</c:if>logic. I had to write the tag in Java since there is no way (that I know of) to use an attribute value as a variable name.
所以我无法想出一种方法将它添加到attribute指令本身；该指令似乎不支持此功能。但是，我能够创建一个封装<c:if>...</c:if>逻辑的标签。我不得不用 Java 编写标签，因为没有办法（据我所知）使用属性值作为变量名。
First I wrote the tag file as a Java class:
首先，我将标记文件编写为 Java 类：
DefaultTag.java
默认标签.java
public class DefaultTag extends BodyTagSupport {
private String var;
private Object value;
//for tag attribute
public void setVar(String var) {
this.var = var;
}
//for tag attribute
public void setValue(Object value) {
this.value = value;
}
public int doEndTag() throws JspException {
Object oldValue = pageContext.getAttribute(var);
Object newValue;
if(value != null) {
newValue = value;
}
else {
if(bodyContent == null || bodyContent.getString() == null) {
newValue = "";
}
else {
newValue = bodyContent.getString().trim();
}
}
if(oldValue == null) {
pageContext.setAttribute(var, newValue);
}
else if(oldValue.toString().trim().length() == 0) {
pageContext.setAttribute(var, newValue);
}
return EVAL_PAGE;
}
}
Then I made a tldfile:
然后我做了一个tld文件：
utils.tld:
utils.tld：
<?xml version="1.0" encoding="ISO-8859-1"?>
<taglib xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
version="2.1">
<tlib-version>2.0</tlib-version>
<short-name>utils</short-name>
<uri>http://utils</uri>
<tag>
<name>default</name>
<tag-class>com.mystuff.mvc.tag.DefaultTag</tag-class>
<body-content>JSP</body-content>
<attribute>
<name>var</name>
<required>true</required>
<rtexprvalue>true</rtexprvalue>
</attribute>
<attribute>
<name>value</name>
<required>true</required>
<rtexprvalue>true</rtexprvalue>
</attribute>
</tag>
</taglib>
Then I made a custom tag that uses this tag:
然后我制作了一个使用这个标签的自定义标签：
defaultTest.tag
defaultTest.tag
<%@ taglib prefix="utils" uri="/WEB-INF/tlds/utils.tld" %>
<%@ attribute name="value" required="true"%>
<%@ attribute name="optValue" required="false"%>
<utils:default var="optValue" value="optional monkeys"/>
${value} ${optValue}
After that I made a page to test the tag I just created:
之后我做了一个页面来测试我刚刚创建的标签：
tagTest.jsp
标签测试.jsp
<mystuff:defaultTest value="helloThar" /><br/><br/>
<mystuff:defaultTest value="helloThere" optValue="monkeys" /><br/><br/>
<mystuff:defaultTest value="helloYou" optValue="${1 + 2 + 4 + 10}" /><br/><br/>
And that gave me:
这给了我：
helloThar optional monkeys
helloThere monkeys
helloYou 17
你好塔尔可选猴子
你好有猴子
你好你 17
