Today I found something very interesting. I wanted to pass a java.math.BigDecimal
to a custom JSP Tag Library and format it as a percentage, however if the BigDecimal
was null
I didn’t want to show it.
Easy, I thought. So I created a class similar to the following…
package name.kayley.blog.taglib; import javax.servlet.jsp.JspException; import javax.servlet.jsp.tagext.TagSupport; import java.io.IOException; import java.math.BigDecimal; public class BigDecimalTagSupport extends TagSupport { private BigDecimal value; public void setValue(BigDecimal value) { this.value = value; } public int doEndTag() throws JspException { try { if (value != null) { pageContext .getOut() .print(formatAsPercentage(value)); } } catch (IOException e) { throw new JspException(e); } return EVAL_PAGE; } // ... }
However, when I called this taglib with a null
value I still got a value of 0.
<mytag:bigDecimal value="${aBigDecimal}" />
The output:
0
After a bit of trial and error I discovered that the JSP tab library framework does a little bit of magic on known classes, such as String
s and Numeric
objects. Classes of Numeric
types that are null
get converted to 0
and String
s get converted into the empty string ""
I discovered that if you change your internal value to an java.lang.Object
instead of a java.math.BigDecimal
the magic doesn’t know what to do and just passes null
to your class.
So the new class looks like the following…
package name.kayley.blog.taglib; import javax.servlet.jsp.JspException; import javax.servlet.jsp.tagext.TagSupport; import java.io.IOException; public class BigDecimalTagSupport extends TagSupport { private Object value; public void setValue(Object value) { this.value = value; } public int doEndTag() throws JspException { try { if (value != null) { // Cast value to a BigDecimal // if you need to. pageContext .getOut() .print(formatAsPercentage(value)); } } catch (IOException e) { throw new JspException(e); } return EVAL_PAGE; } //... }
So now when I use the tag and pass a null BigDecimal
to it I get the required result of no output.
WIN
I have an example web app demonstrating this, if anyone wants a copy just let me know.
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