Java interview questions and answers

Component scanning finds @Service classes and registers bean definitions.

Singleton is one instance per container, which is what almost every stateless service should be.

They are all stereotypes on top of @Component.

Constructor injection makes dependencies required and final, which is easier to reason about and to unit test without Spring.

Aspect-Oriented Programming intercepts join points so you can run extra logic without cluttering business methods.

Spring puts a proxy around the bean

Auto-configuration classes are @Configuration beans gated by @ConditionalOnClass and similar, loaded via spring.factories or AutoConfiguration.imports.

If two beans implement the same type, injection is ambiguous and startup fails.

Entities are tied to the persistence context, lazy proxies, and often more fields than the API should expose.

The many side usually owns the foreign key with @ManyToOne, which is the side you need for inserts.

Lazy is the safe default for associations so you do not pull an entire object graph on every find.

It is the first-level cache: a map of managed entities for the current unit of work, usually the transaction.

persist inserts a new entity and fails if the id already exists in some setups.

You pass PageRequest.of(page, size, sort) into a repository method that returns Page or Slice.

EntityManager is the JPA API for the persistence context.

It means persist, merge, remove, and refresh cascade to children.

INNER keeps only customers who have orders.

The engine walks a balanced tree of page pointers instead of reading every row.

Functions on the column, implicit type conversion, leading wildcards like LIKE '%foo', or low selectivity can force a sequential scan.

1NF: no repeating mark columns like mark1, mark2