What is a JPA persistence context, and what does EntityManager do?

Explains the persistence context as an identity map and unit of work that tracks managed entities through EntityManager.

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TL;DR

A persistence context tracks entity instances and their database identity. Within one context, loading the same entity identity normally returns the same managed object. EntityManager is the JPA API used to find, persist, remove, query, flush, detach, and otherwise interact with that context. In a typical Spring transaction, one transaction-bound persistence context acts as an identity map and unit of work until the transaction ends.

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Managed identity, entity tracking, transaction scope, first-level caching, flushing, and thread confinement.

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Overview
The persistence context is the working set in which one database identity maps to one managed Java object.
A persistence context tracks entity instances and their database identity. Within one context, loading the same entity identity normally returns the same managed object. EntityManager is the JPA API used to find, persist, remove, query, flush, detach, and otherwise interact with that context. In a typical Spring transaction, one transaction-bound persistence context acts as an identity map and unit of work until the transaction ends.
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Changing a managed entity does not immediately require an explicit update call; Hibernate can detect the change and write it during flush. The context can also return an already managed entity without querying again, but that is not a general query-result cache. A long-lived context retains managed objects and snapshots, so large imports can consume memory and make dirty checking expensive. and Hibernate instances are not designed to be shared concurrently between threads.

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Calling the persistence context a shared application cache.
Sharing one EntityManager between threads.
Keeping a context open for an unbounded batch.

What is a JPA persistence context, and what does EntityManager do?

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