12 - Annotations - Spring Boot, JPA vs Lombok

how to avoid common pitfalls

Introduction

Annotations make coding more readable, but introduce an abstraction layer that can hide incompatibilities with other libraries. That’s specially true when combining JPA and Lombok. This article by Thorben Janssen explains in detail many of the common pitfalls a developer can fall into.

Lombok vs. JPA

Lombok annotations you need to avoid

Dont use @EqualsAndHashCode

Thorben explains it very well in his article, but to sum up the @EqualsAndHashCode Lombok annotations don’t comply with what Hibernate expects:

_… your hashCode() method should always return a fixed value […]. In the equals() method, you should only compare the type of the objects and their primary key values. If at least one of the primary keys is null, the equals method has to return false.

Take the example models of the article:

Order.java

@Entity
public class Order {
 
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private Long id;
 
    private String customer;
 
    @OneToMany(mappedBy = "order")
    private Set<OrderPosition> positions = new HashSet<>();
 
    public Long getId() {
        return id;
    }
    public String getCustomer() {
        return customer;
    }
 
    public void setCustomer(String customer) {
        this.customer = customer;
    }
 
    public Set<OrderPosition> getPositions() {
        return positions;
    }
 
    public void setPositions(Set<OrderPosition> positions) {
        this.positions = positions;
    }
 
    @Override
    public int hashCode() {
        return 42;
    }
 
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Order other = (Order) obj;
        if (id == null) {
            return false;
        } else if (!id.equals(other.id))
            return false;
        return true;
    }
 
    @Override
    public String toString() {
        return "Order [customer=" + customer + ", id=" + id + "]";
    }
     
}

OrderPosition.java

@Entity
public class OrderPosition {
     
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private Long id;
 
    private String product;
 
    private int quantity;
 
    @ManyToOne(fetch = FetchType.LAZY)
    private Order order;
 
    public Long getId() {
        return id;
    }
 
    public String getProduct() {
        return product;
    }
 
    public void setProduct(String product) {
        this.product = product;
    }
 
    public int getQuantity() {
        return quantity;
    }
 
    public void setQuantity(int quantity) {
        this.quantity = quantity;
    }
 
    public Order getOrder() {
        return order;
    }
 
    public void setOrder(Order order) {
        this.order = order;
    }
 
    @Override
    public int hashCode() {
        return 42;
    }
 
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        OrderPosition other = (OrderPosition) obj;
        if (id == null) {
            return false;
        } else if (!id.equals(other.id))
            return false;
        return true;
    }
}

Lombok equals() and hashCode implementations include all non-final attributes of the class. This can be changed by setting the onlyExplicitlyIncluded attribute of the @EqualsAndHashCode annotation to true and annotating the primary key attribute with @EqualsAndHashCode.Include:

Example of configured @EqualsAndHashCode annotated class

@Entity
@EqualsAndHashCode(onlyExplicitlyIncluded = true)
public class Order {
 
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    @EqualsAndHashCode.Include
    private Long id;
 
    private String customer;
 
    @OneToMany(mappedBy = "order")
    private Set<OrderPosition> positions = new HashSet<>();
     
    ...
}

…but this won’t fix all issues. As Thorben says:

Your equals() method should return false if the primary key value of both entity objects is null. But Lombok’s equals() method returns true. Because of that, you can’t add two new entity objects to a Set.

In the example shown avobe, that means that you can’t add two new OrderPosition objects to an Order (TODO: Explain why). You should, therefore, avoid Lombok’s @EqualsAndHashCode annotation.