How can non-searchable fields in the User object be made searchable?

The option that involves creating a hook plugin that implements an indexer post processor to index non-searchable fields in the User object is correct because indexer post processors are designed specifically for modifying the indexing process of existing entities in Liferay. They allow developers to intercept and augment the data that gets indexed by the Liferay search engine, which is essential when additional fields need to be made searchable.

By using an indexer post processor, developers can listen for indexing events and manipulate the data just before it gets indexed. This mechanism is efficient and respects the framework’s extensibility, ensuring that updates to the search capabilities can be achieved without altering core functionalities or causing interference with updates.

This is the most effective and clean way to enhance search functionality without having to make deep modifications to the core indexing logic or re-implement large portions of the User service.

In contrast, options that involve overriding the User service or Struts action might lead to more complicated implementations that require maintaining additional code bases and could lead to compatibility issues during upgrades. An Ext plugin, while useful for certain modifications, tends to be more invasive and less flexible than a hook plugin, which is generally the preferred method for extending functionality without modifying the core installation.