velocity如何判空

问题: I want to check for null, something like this:

#if ($car.fuel == null)

Approach 1: null在条件表达式中表示false. (null is evaluated as a false conditional.)

#if( ! $car.fuel )

• 备注: 如果$car.fuel为'false',依然可以通过判断.What this approach is actually checking is whether the reference is null or false.

Approach 2: null是一个空string (null is evaluated as an empty string in quiet references.)

#if( "$!car.fuel" == "" )

• 备注: 当$car.fuel为空String ''依然可以通过pass. What this approach is actually checking is whether the reference is null or empty.

BTW, just checking for empty can be achieved by:

#if( "$car.fuel" == "" )

Approach 3: 结合Approach 1 and 2，就可以判断null并且只判断null.

#if ((! $car.fuel) && ("$!car.fuel" == ""))

• 
  

Approach 4: Use a Tool that can check for null (NullTool,ViewNullTool).

#if( $null.isNull($car.fuel) )

• Note: Of course, NullTool must be in the Context as $null in this case.

Approach 5: Don't check for null directly, use a self-explaining method.

#if( $car.fuelEmpty )

• Note: 建议的方法. You have to implement the method, but it makes the template so easy-to-read.

public boolean isFuelEmpty()
{
  // return true if fuel is empty.
}

Approach 6: Use a custom directive. cf. IfNullDirective, IfNotNullDirective

#ifnull( $car.fuel )
#ifnotnull( $car.fuel )

• Note: You will have to register the directive in your velocity.properties.

userdirective = org.apache.velocity.tools.generic.directive.Ifnull
userdirective = org.apache.velocity.tools.generic.directive.Ifnotnull