# 01 — get() vs load() / getReference()

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Backs [ankurm.com: Hibernate 7 — get() vs load()](https://ankurm.com/hibernate-7-get-vs-load-which-one-should-you-actually-use/).

Source: [`GetVsLoadRunner`](../src/main/java/com/ankurm/hibernatedemo/scenario/GetVsLoadRunner.java).
Run it yourself with `./scripts/run.sh getvsload`; the full transcript below is committed at
[`docs/output/get-vs-load.txt`](output/get-vs-load.txt).

## What the six steps show

1. **`session.get()` on an existing id** — one `SELECT` immediately, returns the real entity.
2. **`session.get()` on a missing id** — one `SELECT`, returns `null`. No exception, ever.
3. **`session.getReference()` on an existing id** — no `SELECT` at the call site. The `SELECT`
   only happens the moment a non-identifier accessor (`getTitle()`) is called on the returned
   proxy. The log ordering in the transcript is the actual evidence for this, not a claim about
   it — the `about to call` / `now calling` markers in the code bracket exactly where each
   `SELECT` does or doesn't appear.
4. **`session.getReference()` on a missing id** — same deferral: the proxy is handed back with
   no exception, and only accessing it throws
   `jakarta.persistence.EntityNotFoundException: No row with the given identifier exists`.
5. **A proxy outlives its session** — `getReference()`, then commit and close, then access:
   `org.hibernate.LazyInitializationException: Could not initialize proxy [...] - no session`.
6. **Proxy identity** — `Book.class.isInstance(proxy)` is `true`, but `real.getClass() ==
   proxy.getClass()` is `false` (the proxy's runtime class is `Book$HibernateProxy`), and
   `real.equals(proxy)` is `false` even though both represent the same row.

## What surprised me building this

Step 6 is the one worth sitting with. `Book` here does not override `equals()`, so
`real.equals(proxy)` falling through to reference equality is not a Hibernate quirk — it's plain
Java doing exactly what an un-overridden `equals()` always does. But it means the moment `get()`
and `getReference()` are mixed for the *same row* in code that ever compares entities by
`equals()` (a `Set`, a `List.contains()`, a manual reconciliation), the proxy boundary silently
breaks that comparison. `instanceof` survives it. `equals()`, `hashCode()`-based collections, and
naive `==` do not. The fix is the one every Effective-Java-style guide already recommends —
implement `equals()`/`hashCode()` off the identifier, never off the object's identity — but this
is what makes skipping that advice actually bite: not a compile error, a `false` you don't
notice until two supposedly-identical entities land in the same `HashSet` and both show up.

Also unexpected going in: `getReference()` on a *missing* row throws `EntityNotFoundException`
on first non-id access, not `ObjectNotFoundException`. The two names get used interchangeably in
older Hibernate discussion; running it pins down which one this exact version actually throws
(see the transcript for the full class name and message).

## Full captured transcript

```console
Hibernate: create global temporary table HTE_book(rn_ integer not null, id bigint, version bigint, author varchar(255), title varchar(255), primary key (rn_)) transactional
Hibernate: create global temporary table HTE_widget_sequence(rn_ integer not null, id bigint, name varchar(255), primary key (rn_)) transactional
Hibernate: create table book (id bigint not null, author varchar(255), title varchar(255), version bigint not null, primary key (id))
Hibernate: create table widget_identity (id bigint generated by default as identity, name varchar(255), primary key (id))
Hibernate: create table widget_sequence (id bigint not null, name varchar(255), primary key (id))
Hibernate: create sequence book_seq start with 1 increment by 50
Hibernate: create sequence widget_seq start with 1 increment by 25
Hibernate: select next value for book_seq
Hibernate: /* insert for com.ankurm.hibernatedemo.model.Book */insert into book (author,title,version,id) values (?,?,?,?)
binding parameter (1:VARCHAR) <- [Joshua Bloch]
binding parameter (2:VARCHAR) <- [Effective Java]
binding parameter (3:BIGINT) <- [0]
binding parameter (4:BIGINT) <- [1]
SEED: inserted Book id=1
--- Step 1: session.get() on an existing id ---
about to call session.get(Book.class, 1)
Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
binding parameter (1:BIGINT) <- [1]
get() returned: Book{id=1, title=Effective Java, author=Joshua Bloch, version=0}
--- Step 2: session.get() on a missing id ---
about to call session.get(Book.class, 999001)
Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
binding parameter (1:BIGINT) <- [999001]
get() returned: null (no exception thrown)
--- Step 3: session.getReference() on an existing id ---
getReference() returned proxy of class com.ankurm.hibernatedemo.model.Book$HibernateProxy -- no SELECT above this line
now calling proxy.getTitle() ...
Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
binding parameter (1:BIGINT) <- [1]
getTitle() returned 'Effective Java' -- the SELECT for this ran just above this line
--- Step 4: session.getReference() on a missing id ---
getReference() returned a proxy for a row that does not exist -- no exception yet: com.ankurm.hibernatedemo.model.Book$HibernateProxy
Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
binding parameter (1:BIGINT) <- [999001]
accessing the proxy threw jakarta.persistence.EntityNotFoundException: No row with the given identifier exists for entity [com.ankurm.hibernatedemo.model.Book with id '999001']
--- Step 5: proxy accessed after its session is closed ---
session closed. proxy in hand: com.ankurm.hibernatedemo.model.Book$HibernateProxy
accessing the proxy after close threw org.hibernate.LazyInitializationException: Could not initialize proxy [com.ankurm.hibernatedemo.model.Book#1] - no session
--- Step 6: proxy identity vs a real loaded instance ---
Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
binding parameter (1:BIGINT) <- [1]
real.getClass()  = com.ankurm.hibernatedemo.model.Book
proxy.getClass() = com.ankurm.hibernatedemo.model.Book$HibernateProxy
proxy instanceof Book.class:            true
real.getClass() == proxy.getClass():    false
real.equals(proxy) before proxy access: false
```

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