example41 - StackOverflow caused by bidirectional association w/ @Data

cased by `Person.java` and `EazyClass.java`

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In a bidirectional association in JPA/Hibernate, having two entities reference each other can potentially lead to a stack overflow error if not handled correctly, especially when using Lombok's `@Data` annotation. The `@Data` annotation generates `toString()`, `equals()`, and `hashCode()` methods, which can cause issues with bidirectional relationships.

Why the Stack Overflow Occurs
The stack overflow occurs because the generated `toString()`, `equals()`, and `hashCode()` methods can end up in an infinite loop. For example:

Entity A references Entity B.
Entity B references Entity A.
When `toString()` is called on Entity A, it calls `toString()` on Entity B, which in turn calls `toString()` on Entity A, and so on, leading to a stack overflow.

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- Solutions

**1. Use @ToString.Exclude and @EqualsAndHashCode.Exclude**
By excluding the bidirectional field, you prevent the infinite loop in the toString(), equals(), and hashCode() methods.

`Person.java`
```
@ManyToOne(fetch = FetchType.LAZY, optional = true)
@JoinColumn(name = "class_id", referencedColumnName = "classId", nullable = true)
@JsonBackReference
@ToString.Exclude
@EqualsAndHashCode.Exclude
private EazyClass eazyClass;
```

`EazyClass.java`
```
@OneToMany(mappedBy = "eazyClass", cascade = CascadeType.PERSIST,targetEntity = Person.class)
@JsonManagedReference
@ToString.Exclude
@EqualsAndHashCode.Exclude
private Set persons;
```

**2. Manually Implement toString(), equals(), and hashCode()**

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- Conclusion

When using bidirectional associations with Lombok's `@Data` annotation, it's crucial to prevent infinite recursion in the generated methods. You can achieve this by excluding the bidirectional fields from `toString()`, `equals()`, and `hashCode()` using `@ToString.Exclude` and `@EqualsAndHashCode.Exclude`, or by manually implementing these methods to avoid the recursion.