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.