SessionFactory is built once and shared by all threads. A Session is one unit of work. It isn't thread-safe, so each request gets its own.LazyInitializationException both start here.hibernate.cfg.xml hibernate.properties new Configuration()persistence.xmlTipHibernate detects the dialect from JDBC metadata, so that setting is usually optional.
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AskedWhat are the ways to configure Hibernate?EntityManager and JPQL.@BatchSize or custom typesUse Hibernate's own features. They tie you to it.TipHibernate adds caching, and HQL that runs on any supported database. Session extends JPA's EntityManager.
ConfigurationSessionFactorySessionTransactionQueryCriteriaBuilderTrapBuild the SessionFactory once at startup, because it's expensive. Open a Session per request or thread, and close it.
close()remove(user)TipA new object is transient. A persistent object is also called managed, and the session saves its changes.
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AskedWhat are the different states of a Hibernate entity?save(user)persist(user)saveOrUpdate(user) TipHibernate 7 removed save() and saveOrUpdate(). Use persist(), or merge() for a detached object.
TrapWith an identity column, save() inserts at once, even outside a transaction. persist() never does.
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AskedWhat is the difference between save(), persist(), and saveOrUpdate()?update(user)managed = merge(user)Tipupdate() throws if the session already holds that id. Hibernate 7 removed it.
Trapmerge() leaves your object detached. Keep using the one it returns.
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AskedWhat is the difference between merge() and update()?find(User.class, id)get(User.class, id)getReference(User.class, id)load(User.class, id)TipA proxy sets a foreign key without loading the row, as in order..
TrapA proxy fails later on a missing row. Its first use throws Object.
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AskedWhat is the difference between get() and load() in Hibernate?openSession()getCurrentSession() It needs an active transaction, and Hibernate closes it at the end.Tiphibernate. sets the context to thread or jta.
TrapNeither is thread-safe. A bound session is safe only because each thread or transaction gets its own.
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AskedWhat is the difference between getCurrentSession() and openSession()?user.setName("New") No save call. Flush compares it with the snapshot.@DynamicUpdateHibernate then builds the SQL at runtime.TipBy default, the UPDATE sets every mapped column, so one cached statement serves all updates.
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AskedExplain the concept of dirty checking in Hibernate.AUTOKeep queries correct, the defaultCOMMITRun faster, accept stale resultsMANUALFlush only when you call flush()ALWAYSFlush before every queryTipAUTO flushes only before a query the changes could affect. Set a mode with set.
TrapBy default, Hibernate doesn't flush before a native SQL query.
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AskedExplain Hibernate's flush modes and when to use each.@Version.spring.jpa.open-in-view=false TipAfter an exception, roll back and discard the session. It's no longer usable.
TrapNever share one session across the app. It isn't thread-safe, and its cache keeps growing.
@Entity@Table(name = "users")@Id@GeneratedValue@Column(name = "x")@TransientTipWithout @Table, the table takes the entity's name.
TrapAn entity needs an @Id and a no-argument constructor that isn't private. The class can't be final.
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AskedWhat is the purpose of @Entity and @Table annotations?@Embeddable@Embedded@AttributeOverride @EmbeddedId@Id field@IdClass(RoleId.class) TipA value type has no table and no id. Its fields become columns in the entity's table. @Embedded is optional.
TrapA key class must be Serializable, have a no-argument constructor, and override equals() and hashCode().
SINGLE_TABLEThe default, usually fastest, with a discriminator column.JOINEDLoading joins the tables, so queries are slower.TABLE_PER_CLASSA query on the parent needs a UNION.TipChoose it with @Inheritance. One table can waste space on subclass columns.
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AskedExplain the different inheritance mapping strategies in Hibernate.@ConvertWrite a JPA AttributeConverter .@Type(MyType.class) Implement UserType, or CompositeUserType for several columns.TrapA UserType from Hibernate 5 doesn't compile on 6, and 7 changed nullSafeGet() and nullSafeSet() again.
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AskedHow do you implement custom Hibernate types?PhysicalNamingStrategy ImplicitNamingStrategy PhysicalNamingStrategySnakeCaseImpl TipRegister yours in hibernate..
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AskedHow do you implement custom Hibernate naming strategies?@NaturalIdAdd a unique constraint too.o instanceof Book bA proxy is a subclass, so getClass() fails.TrapA generated id is null until Hibernate persists the entity. A hash code built on it changes, and a HashSet loses the entity.
@OneToOne@ManyToManyTrapIn a bidirectional link, Hibernate writes only changes made on the owning side. A change to the mappedBy side alone writes nothing.
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AskedWhat are the different association mappings in Hibernate?PERSISTMERGEREMOVEREFRESHDETACHALLorphanRemoval = trueTipSet them with CascadeType. Cascade fits a parent that owns its children.
TrapAvoid REMOVE and ALL on @ManyToMany. Removing one entity deletes rows that other entities still use.
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AskedWhat are the different cascade types in Hibernate?@OneToMany@ManyToMany@ManyToOne@OneToOnefetch = FetchType.LAZYTipMake almost every association lazy. Fetch what a use case needs with a join fetch or an entity graph.
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AskedExplain the difference between lazy and eager loading in Hibernate.Getting the orders of 5 users. Each bar is one query, and the dark one loads the users. A lazy loop sends 6, a batch size of 2 sends 4, a fetch join 1.
LEFT JOIN FETCH u.orders@BatchSize(size = 10)@Fetch(FetchMode.SUBSELECT) TipA lazy association read in a loop causes it. A JPA entity graph also fixes it, by naming what to fetch.
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AskedWhat is the N+1 problem and how can it be solved?join fetch o.itemsHibernate.initialize(o.getItems()) TipThe cause is code reading a lazy field after its session closed, often in a controller or a JSON serializer.
TrapEAGER and Open Session in View only hide it. EAGER loads on every query, and the view runs extra lazy queries.
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AskedWhat causes a LazyInitializationException and how do you fix it?FROM User u WHERE u.name = :ncreateNativeQuery(sql) setFirstResult(20) setMaxResults(10) TipHQL is a superset of JPQL. A query on a parent class also returns its subclasses.
TrapClass and property names are case-sensitive in HQL. Keywords such as select aren't.
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AskedWhat is HQL and how is it different from SQL?getCriteriaBuilder() It also does joins and grouping. The JPA metamodel makes it type-safe.Example.of(user) It can't nest or group conditions.org.hibernate.criterion Hibernate 6.0 removed it, with its Example class.TrapExample.of() is from Spring Data JPA, not Hibernate.
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AskedWhat is the difference between Criteria API and Query by Example?@FilterDef@FilterenableFilter("active") TipA new session starts with every filter off.
TrapBy default, filters don't apply to find() or to @ManyToOne associations. Set apply (6.6+) to include them.
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AskedWhat are Hibernate filters and how are they used?@Cache@Cacheablehibernate.cache.use_second_level_cache=true hibernate.cache.use_query_cache=true TipThe first level ends with its session. The second level suits data read often and changed rarely.
TrapThe query cache is rarely worth it. Any change to a queried table clears its results.
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AskedExplain the Hibernate caching mechanism.READ_ONLYNONSTRICT_READ_WRITE READ_WRITETRANSACTIONALTipHibernate has no second-level cache of its own. Most providers plug in through JCache, with hibernate-jcache.
@VersionOptimisticLockException Reload and retry, or show the conflict.SELECT ... FOR UPDATEThis is pessimistic locking.TipHibernate adds the old version to the WHERE clause. If another transaction changed the row, no row matches and Hibernate throws.
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AskedHow do you handle optimistic locking in Hibernate?READ_UNCOMMITTED Allow all three read problemsREAD_COMMITTEDStop dirty readsREPEATABLE_READAlso stop non-repeatable readsSERIALIZABLEAlso stop phantom readsTipA dirty read sees uncommitted data, a non-repeatable read a new value, and a phantom read new rows. Set the level with @Transactional(isolation = ...).
TrapHigher levels hold more locks or abort more transactions, so throughput drops.
hibernate.jdbc.batch_size=50 flush()clear()TrapHibernate can't batch inserts with IDENTITY ids. It must run each INSERT to learn the id, so use a sequence.
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AskedHow would you optimize Hibernate performance for large datasets?UPDATE User SET status = :sexecuteUpdate()DELETE FROM User WHERE lastLogin < :dscroll(ScrollMode.FORWARD_ONLY) TrapA bulk statement bypasses the persistence context and optimistic locking. Loaded entities keep their old values.
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AskedHow do you implement bulk operations efficiently in Hibernate?openStatelessSession() insert(user)update(user)No dirty checking or cascades, and lazy data needs fetch().setJdbcBatchSize(50) TipIt uses little memory on any number of rows. Two reads of one row can return two objects.
TrapSince Hibernate 7, it uses the second-level cache by default, and hibernate. doesn't apply to it.
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AskedWhat is the difference between stateless and stateful sessions?hibernate.hikari.maximumPoolSize=20 TipWhen every connection is busy, a request waits up to connection, then fails.
TrapHibernate's built-in pool is for testing, not production.
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AskedExplain Hibernate's connection pooling and configuration.hibernate.show_sql=true logging.level.org.hibernate.SQL=DEBUG logging.level.org.hibernate.orm.jdbc.bind=TRACE hibernate.generate_statistics=true TipMost problems are too many queries, not one slow one. Count the statements per request first.
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AskedHow do you troubleshoot and debug Hibernate performance issues?validateupdatecreatecreate-dropvalidate.TipSet hibernate. to one of these values.
TrapNever use create or create-drop against production data. They drop the existing tables.
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AskedHow do you handle database schema evolution with Hibernate?@Audited@NotAuditedAuditReaderFactory.get(session) @org.hibernate.annotations.Audited TrapEnvers records passwords unless they're @NotAudited.
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AskedExplain the concept of Hibernate Envers for auditing.InterceptorPreInsertEventListener TipAn interceptor can fill audit columns. Its onPersist() returns true only when it changed the state array.
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AskedWhat are Hibernate Interceptors and Event Listeners?MultiTenantConnectionProvider @TenantIdCurrentTenantIdentifierResolver TrapNative SQL queries aren't filtered by tenant. Add the condition yourself.
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AskedHow do you implement multi-tenancy in Hibernate?TrapMany instances multiply the connections a database must hold. Size each pool for its service's load.
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