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EntityManager is a core interface in object-relational mapping frameworks that manage the persistence of domain objects. It serves as the primary API for interacting with the persistence context, mediating between in-memory entity instances and database records. The EntityManager tracks the lifecycle of entities, maintains a first-level cache (the persistence context) to ensure identity and consistency within a session, and implements the Unit of Work pattern to coordinate changes to the database.

Key responsibilities include creating, reading, updating, and deleting entities. Common operations are persist to insert, remove to delete, merge to update, and find or getReference to retrieve entities by their identifiers. It also supports querying through createQuery, createNamedQuery, and createNativeQuery, and can synchronize changes with the database using flush. Additional methods such as refresh, detach, clear, and contains help manage an entity’s state and its association with the persistence context.

Transaction management is closely tied to the EntityManager. In many frameworks, transactions can be container-managed or resource-local. The EntityManager may provide access to an underlying transaction object or participate in a larger transactional context. In resource-local setups, one can begin, commit, or rollback a transaction via the EntityTransaction interface; in container-managed environments, the container handles transaction boundaries.

EntityManager instances are typically not thread-safe and are designed to be used within a defined scope, such as a single request or a single business transaction. In practice, it is obtained from a factory or injected by the framework and used to perform data access operations, manage cascading persistence, and coordinate object lifecycles across related entities.