Research Singleton Pattern for the Mapper class DTOEntityMappings

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For now, a single ModelMapper instance is being returned from DTOEntitiesMappings class, which performs configuration loading by adding mappings from the defined PropertyMaps list in the class.

It does this every time a new instance of the DTOEntitiesMappings class is called, which seems futile, since each time the same mappings are gettings added to the ModelMapper instance. Also, no new mappings configurations for DTO <-> Entity mapping are expected within the application flow for now.

Try creating a single instance of ModelMapper inside the DTOEntitiesMappings class, instantiate this instance the first time the model mapper is required anywhere in the code, and returned the same instantiated ModelMapper instance thereafter.

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Good article explaining the Singleton pattern : https://www.callicoder.com/java-singleton-design-pattern-example/

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There are many articles online stating that the Singleton design pattern should be avoided wherever possible.

It causes performance issues (e.g. if using synchronized block), is hard to maintain and unit test.
Sources:
Youtube - Are Singletons Bad
Singletons are Pathological Liars
Avoid Singletons to Write Testable Code
SO Answer - explains why using Singletons may not results in repeatable unit tests. Even though we may have a Singleton whose state doesn't change for now, it is safe to avoid using it, since we may in a hurry to fix a bug or add a feature introduce changes that may cause the state of the singleton to change in the application flow, causing unrepeatable tests.

Although I did not understand all the reasons to ditch Singletons, NOT using the pattern for now.

Instead, the structure for DTOEntitiesMapping.java file is changed now, to make it more modular and testable.
Commit: db57ae5
It implements the DTOEntitiesMapping interface, which defines the API that developers can use to convert objects from DTO to Entities and vice versa, without concerning themselves with what does the underlying conversion.
The API methods only take the object to be converted (Source) as input and return the converted object as output (Destination).

But research the above new design too. Check if it can be improved.