Velocity determines null and null

There are several ways to determine if it is null:

1. #if (! $foo) determine that $foo is empty and that non-null is #if ($foo)

2. Use #ifnull () or #ifnotnull ()

#ifnull ($foo)

To use this feature, you must include the following in the Velocity.properties file:

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

3. Use the null tool to determine

#if ($null. IsNull ($foo))

Note that this is especially useful, especially if you need this judgment as a judgment sentence, such as when I want you to judge whether a set is null or empty, you can only use this method:

$if ($null. IsNull ($mycoll) | | $mycoll. Size () ==0)

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1. Determine if velocity is null
Method one, #ifnull () or #ifnotnull ()
Eg: #ifnull ($XXX)

Warn: To use this feature, you must include the following in the Velocity.properties file:
Userdirective = Org.apache.velocity.tools.generic.directive.Ifnull
Userdirective = Org.apache.velocity.tools.generic.directive.Ifnotnull

Method Two, $null. IsNull ()
Eg: #if ($null. IsNull ())

It is recommended to use method two, especially when judging that the set object is null.

2. Determine if velocity is empty

Judge null: #if (! $xxx)
Judgment non-null: #if ($XXX)

3, for more detailed information, please refer to:
      http://wiki.apache.org/velocity/velocitynullsupport    
      http://wiki.apache.org/velocity/checkingfornull