Spring Data, JPA @OneToMany Lazy fetch not working in Spring Boot

It seems to be a debugging artifact.

At debugging time, because the transaction is still open, the watched lazy loaded entity properties will be loaded at the breakpoint evaluation time.

To check the "production" behavior you should insert a em.detach statement just before the breakpoint or use logging (as suggested by Manza) and check em.getEntityManagerFactory().getPersistenceUnitUtil().isLoaded(fabricRoll1.fabricDefects()) returns false on the detached entity.

(remember to inject EntityManager for example by declaring @PersistenceContext private EntityManager em;)

You don't need to use @JoinColumn, and you don't need to instantiate fabricDefects

@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY) private Set<FabricDefect> fabricDefects ; 

See more in this question.

FabricDefect class:

@ManyToOne @JoinColumn(name = "fabric_roll_id") private FabricRoll roll; 

FabricRoll class:

@OneToMany(mappedBy = "roll") private Set<FabricDefect> fabricDefects; 

Collections are by default loaded lazily, JPA will query the db only when the method getFabricDefects will be called. You can see it by yourself enabling logging.

I found solution in this tutorial.

You have to modify FabricRoll OneToMany map as below:

@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "fabricRoll") private Set<FabricDefect> fabricDefects; 

FabricDefect ManyToOne as below (remember to remove fabric_roll_id field if you included it in your entity):

@ManyToOne(fetch = FetchType.LAZY) @JoinColumn(name = "fabric_roll_id") private FabricRoll fabricRoll; 

And you don't need to add @Transactional(propagation = Propagation.REQUIRED) before getAllFabricRoll() function.

The issue occurs because, despite `fetch = FetchType.LAZY`, something in your application is accessing `getFabricDefects()`, triggering Hibernate to load the `FabricDefect` collection. Common causes include JSON serialization (e.g., Jackson in a REST API), Lombok annotations like `@Data` or `@ToString`, or logging/debugging that inadvertently calls the getter. Here's how to fix it and ensure `FabricDefect` is only loaded when explicitly requested.

### Why Lazy Loading Isn't Working

1. Serialization: - When returning `List<FabricRoll>` from a REST controller, Jackson calls `getFabricDefects()` to serialize the collection, fetching the data if the session is open or throwing a `LazyInitializationException` if closed. 2. Lombok: - If `FabricRoll` uses `@Data` or `@ToString`, the generated `toString()` or getters may be called during logging (e.g., `System.out.println(fabricRoll)`) or debugging, loading `fabricDefects`. 3. Transactional Context: - In a `@Transactional` method, accessing `fabricDefects` (even unintentionally) fetches the data because the Hibernate session is open. 

### Solutions

#### 1. Prevent Serialization Issues:

Use `@JsonIgnore` to exclude `fabricDefects` from JSON serialization or configure Jackson to handle lazy-loaded collections.

 @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY) @JoinColumn(name = "fabric_roll_id", referencedColumnName = "fabricRollId") @JsonIgnore private Set<FabricDefect> fabricDefects = new HashSet<>() 

#### 2. Avoid Lombok Pitfalls

If using Lombok, avoid `@Data` as it generates `toString()`, `equals()`, and `hashCode()`, which may access `fabricDefects`. Use `@Getter`, `@Setter`, and exclude `fabricDefects` from `toString()`:

@ToString(exclude = "fabricDefects")