In our hook that created a custom login action, we modified the
login.events.pre portal property. This property accepts multiple values, so
our value was appended to the existing login.events.pre values. We can
repeatedly modify the property from additional hooks because it accepts
multiple values. Some portal properties only accept a single value, such as
the terms.of.use.required property, which is either true or false. Only
modify single value properties from one hook; otherwise Liferay won’t know
which value to use.
To find out which properties accept multiple values, look in the Configuring Liferay’s Properties document.
Now let’s look at overriding and adding Struts actions from a hook plugin.