What is the recommended way to create new methods in a core service?

The recommended way to create new methods in a core service is to use a service wrapper hook. This approach allows developers to extend existing core services without altering the original source code directly, preserving upgradability and maintainability. By utilizing a service wrapper, you create a new class that can call the original service methods while adding new functionalities or overriding existing behaviors as needed.

Modifying the core service source code directly is not advisable because it leads to challenges during upgrades, as your changes may be overwritten or become incompatible with future versions of Liferay. Additionally, it could result in issues with dependencies and potentially introduce bugs into the core framework.

Creating a separate plugin is a viable approach for modular functionality but does not specifically address the need to add methods directly to an existing service. Instead, if there is a strong need to extend the service's capabilities directly, a service wrapper is more suitable.

Using a portal property is also unrelated to adding methods to a core service. Portal properties primarily configure settings and parameters, not code-level modifications.

Overall, a service wrapper hook provides the flexibility required to safely enhance core services while maintaining a clean upgrade path.