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Listing 7.3 Specifying category identity using EmbeddedId
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@Embeddable public class CategoryPK { String name; Date createDate; public CategoryPK() {} public boolean equals(Object other) { if (other instanceof CategoryPK) { final CategoryPK otherCategoryPK = (CategoryPK)other; return (otherCategory.name.equals(name) && otherCategoryPK.createDate.equals(createDate)); } return false; } public int hashCode() { return super.hashCode(); } } @Entity public class Category { public Category() {} @EmbeddedId protected CategoryPK categoryPK; ... }
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In listing 7.3, notice that the identity fields, name and createDate, are absent altogether from the Category class. Instead, an Embeddable object instance, categoryPK b, is designated as the identity using the @EmbeddedId annotation D. The
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Implementing domain models
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CategoryPK object itself is almost identical to the IdClass used in listing 7.1 and contains the name and createDate fields. We still need to implement the equals and hashCode methods C. The only difference is that the @Embedded object need not be Serializable. In effect, the object designated as @EmbeddedId is expected to be a simple data holder encapsulating only the identity fields. Note that the @Id
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annotation is missing altogether since it is redundant. As a matter of fact, you are not allowed to use Id or IdClass in conjunction with EmbeddedId. As you can see, although this approach saves typing, it is a little awkward to justify in terms of object modeling (even the variable name, categoryPK, is more reminiscent of relational databases than OO). It is a little unwieldy too. Imagine having to write category.catetogyPK.name to use the name field for any other purpose than as a primary key, as opposed to using category.name. However, whatever method you choose is ultimately a matter of personal taste. Unless you really need a composite primary key because you are stuck with a legacy database, we don t recommend using it and instead recommend a simple generated key (also known as surrogate key) that you ll learn about in chapter 8. One concept is critical: identities can only be defined once in an entire entity hierarchy. Having had a sufficient introduction to the idea of entities and identities, you are now ready to explore the @Embeddable annotation in greater detail.
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7.2.4 The @Embeddable annotation
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Let s step back a second from the idea of identities into the world of pure OO domain modeling. Are all domain objects always identifiable on their own How about objects that are simply used as convenient data holders/groupings inside other objects An easy example would be an Address object used inside a User object as an elegant OO alternative to listing street address, city, zip, and so forth directly as fields of the User object. It would be overkill for the Address object to have an identity since it is not likely to be used outside a User object. This is exactly the kind of scenario for which the @Embeddable annotation was designed. The @Embeddable annotation is used to designate persistent objects that need not have an identity of their own. This is because Embeddable objects are identified by the entity objects they are nested inside and never persisted or accessed on their own. Put another way, Embeddable objects share their identities with the enclosing entity. An extreme case of this is the @EmbeddedId situation where the Embeddable object is the identity. Listing 7.4 contains a user/address example to help you gain a better understanding of the most commonly used Embeddable object semantic patterns.
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Listing 7.4 Using embedded objects
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@Embeddable public class Address protected String protected String protected String protected String protected String protected String ... }
Embeddable address
{ streetLine1; streetLine2; city; state; zipCode; country;
@Entity public class User { @Id protected Long id; protected String username; protected String firstName; protected String lastName; @Embedded protected Address address; protected String email; protected String phone; ... }
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