velocity set值为null

前几天发现了一个问题,用set指令给一个变量赋值,如果一开始那个变量有值,然后赋值为null,则赋值不起作用.
比如如下代码

public class Bar {
 public String getBar() {
  return "hello";
 }
 public String getFoo() {
  return null;
 }
}

#set($v=$bar.bar)
$v
#set($v=$bar.foo)
$v

产生输出为

hello
hello
文档上说

引用

If the RHS is a property or method reference that evaluates to null, it will not be assigned to the LHS. Depending on how Velocity is configured, it is usually not possible to remove an existing reference from the context via this mechanism. (Note that this can be permitted by changing one of the Velocity configuration properties).

需要在配置文件中设置directive.set.null.allowed = true才行
或者
#set($v="")
#set($v=$bar.bar)
$v
#set($v="")
#set($v=$bar.foo)
$v
顺便吐槽下velocity的文档,
Note that this can be permitted by changing one of the Velocity configuration properties
你就不能顺便说下改哪个property?