2.5.5. Provide identifier attribute(s)

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Historically this was considered optional.
However, not defining identifier attribute(s) on the entity should be considered a deprecated feature that will be removed in an upcoming release.

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The identifier attribute does not necessarily need to be mapped to the column(s) that physically define the primary key.
However, it should map to column(s) that can uniquely identify each row.

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We recommend that you declare consistently-named identifier attributes on persistent classes and that you use a nullable (i.e., non-primitive) type.

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The placement of the `@Id` annotation marks the [persistence state access strategy](#access).

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<pre class="prettyprint highlight">`@Id
private Integer id;`</pre>
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Hibernate offers multiple identifier generation strategies, see the [Identifier Generators](#identifiers) chapter for more about this topic.

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