测试JSP EL表达式中BigDecimal值是否为零

Question

The following does not always behave as you would expect:

<c:if test="${someBigDecimal == 0}">

If someBigDecimal has a value of 0, but has a scale other than 0, the == operation returns false. That is, it returns true when someBigDecimal is new BigDecimal("0"), but false when someBigDecimal is new BigDecimal("0.00").

This results from the JSP 2.0, 2.1, and 2.2 specifications, which state:

For <, >, <=, >=:

If A or B is BigDecimal, coerce both A and B to BigDecimal and use the return value of A.compareTo(B).

For ==, !=:

If A or B is BigDecimal, coerce both A and B to BigDecimal and then:

• If operator is ==, return A.equals(B)
• If operator is !=, return !A.equals(B)

This means the == and != operators result in a call to the .equals() method, which compares not only the values, but also the scale of the BigDecimals. The other comparison operators result in a call to the .compareTo() method, which compares only the values.

Of course, the following would work:

<c:if test="${not ((someBigDecimal < 0) or (someBigDecimal > 0))}">

But this is rather ugly, is there a better way to do this?

Answer 1

In JSP 2.2 EL and above this expression will evaluate to true :

${someBigDecimal.unscaledValue() == 0}

This will avoid any loss of precision but assumes that someBigDecimal is always of type BigDecimal .

A custom EL function is probably the best approach for older versions of EL:

${fn:isZero(someBigDecimal)}

---

The core of the problem is that this Java code evaluates to false because ZERO has a scale of 0 and the new BigDecimal has a non-zero scale:

BigDecimal.ZERO.setScale(3).equals(BigDecimal.ZERO)

Answer 2

<c:if test="${someBigDecimal.compareTo(BigDecimal.ZERO) == 0}">

Answer 3

<c:if test="${someBigDecimal eq 0}">

Answer 4

使用最新版本的EL（例如Tomcat 7支持），您可以尝试：

<c:if test="${someBigDecimal.doubleValue() == 0}">

Answer 5

您可以尝试signum功能：

<c:if test="#{someBigDecimal.signum() == 0}">