{"data":{"post":{"fields":{"slug":"/jpa-and-uuid/","date":"2018-10-24"},"html":"

Long story short

\n

Using a manually generated id in your entities, you should initialize it no later than when\ninstantiating the the object to avoid null value comparison in equals() if storing instances\nin a Set. @PrePersist is too late or you’ll end up with only one persisted object out of the\nwhole collection.

\n

You can still achieve this if using Lombok’s Builder and the @Builder.Default feature.

\n

Best practices in place - what can possibly go wrong?

\n

Manually generated UUID

\n

In our JPA-mapped project, we had Venues and Prices in a One-to-many relationship -\neach venue can have several prices, keeping them in a Set. Being all good boys and girls here,\nwe had them identified by UUIDs. We generated the UUIDs using the javax.persistence.PrePersist\nevent listener:

\n
public class Price {\n  @Id\n  @Type(type = \"pg-uuid\")\n  @Column(unique = true, nullable = false, columnDefinition = \"uuid\")\n  private UUID id;\n\n  // other fields...\n  \n  @PrePersist\n  public void prePersist() {\n    id = UUID.randomUUID();\n  }\n  \n}
\n

Implemented equals() and hashCode()

\n

As good girls and boys, we also know we should take care of a proper equals() and hashCode\nimplementation, which in case of JPA entities comes down to comparing the unique id:

\n
@Builder\n@Entity\npublic class Price {\n// ...\n  @Override\n  public boolean equals(Object o) {\n    if (this == o) {\n      return true;\n    }\n    if (o == null || getClass() != o.getClass()) {\n      return false;\n    }\n    Price price = (Price) o;\n    return Objects.equals(id, price.id);\n  }\n\n  @Override\n  public int hashCode() {\n    return Objects.hash(id);\n  }\n // ...
\n

(Here we’d like to say “hi” to one of our favorite writers\nJakub Kubryński )

\n

Let’s get all the credit

\n

So far so good. In the above setting, Venue is the managing entity, and PERSIST is one of\nthe cascaded operations, so adding a few prices to a venue is as simple as:

\n
  gasStation.getPrices().addAll(Set.of(\n          Price.builder()\n              .priceType(PriceType.REGULAR)\n              .price(new BigDecimal(\"12.345\"))\n              .venue(gasStation)\n              .build(),\n          Price.builder()\n              .priceType(PriceType.DIESEL)\n              .price(new BigDecimal(\"34.345\"))\n              .venue(gasStation)\n              .build()\n      ));
\n

(since we also use the pretty @Builder syntax, generated for us by Lombok).

\n

…but no. A single price will be created correctly, but since Objects.equals() implementation\nis such that a null equals a null, two freshly created prices with a uuid = null will be\ntreated as equal by the Set implementation and in result… only the one added last will be\nstored in the database!

\n

This is definitely not what we had in mind.

\n

Side note: We were lucky enough to have discovered this thanks to having some test coverage\nas well as using Java9’s java.util.ImmutableCollections which gave us the following error,\nclearly indicating our problem:

\n
java.lang.IllegalArgumentException: duplicate element: Price(id=null, price=12.345, priceType=REGULAR, createdAt=null)\n\n\tat java.base/java.util.ImmutableCollections$Set2.<init>(ImmutableCollections.java:380)\n\tat java.base/java.util.Set.of(Set.java:484)
\n

Solving the problem

\n

If the @PrePersist phase is too late for generating the UUID for our needs, why not use\nfield initialization, like that?

\n
@Builder\n@Entity\npublic class Price {\n  @Id\n  @Type(type = \"pg-uuid\")\n  @Column(unique = true, nullable = false, columnDefinition = \"uuid\")\n  private UUID id = UUID.randomUUID();\n  }
\n

Well, almost. This would have worked if we hadn’t used the goodness of Lombok's @Builder\nwhose own fields override the Price object’s fields the moment we call build().

\n

The new hope

\n

Fortunately, Lombok didn’t leave us alone in our misery and provided a tool to preserve\nthe initial value of a @Builder - decorated class. It’s the @Builder.Default annotation\nand we use it like this:

\n
@Builder\n@Entity\npublic class Price {\n\n  @Id\n  @Type(type = \"pg-uuid\")\n  @Column(unique = true, nullable = false, columnDefinition = \"uuid\")\n  @Builder.Default\n  private UUID id = UUID.randomUUID();\n}
\n

Ta-da, persistence functionality is saved, all Price instances get stored, and we can still use\nthe pretty @Builder generated by Lombok.

\n

Conclusions

\n
    \n
  1. @PrePersist is too late for programmatically generating an id,
    2. \n
  2. Many thanks to the Lombok team for including the Builder.Default annotation, otherwise\nwe’d have to give up using Builder on this class,
    3. \n
  3. All hail to Java9 and its ImmutableCollections for their validation and nice error messages.
    4. \n
\n

Hope you save some time and trouble with this couple of tips!

\n

Don’t hesitate to reach out.

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