As a reminder, this book is not about Java EE role, so we can't go through all the specifications and detail them all but it is important to understand what Java EE is and what its role is to be able to start working on Java EE performances serenely.
Very often, a small annotation or line of code can hide a lot of logic. The entity manager is a good example: most of the methods are hiding some SQL generation and execution which is not a trivial operation.
With the standardization of CDI in applications, a simple call to a method with a simple complexity can imply to:
- Validate the call (BeanValidation) which can be impacting if the object graph is huge
- Validate the logged in user and its permissions (Security API) which can sometimes contact external systems depending on the configuration and implementations
- An integration of multiple external systems...