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You tell Hibernate that the user property of the Address class is the inverse of a property on the other side of the association. You can now call anAddress.getUser() to access the user who s shipping address you ve given. There is no additional column or foreign key constraint; Hibernate manages this pointer for you. Should you make this association bidirectional As always, the decision is up to you and depends on whether you need to navigate through your objects in that direction in your application code. In this case, we d probably conclude that the bidirectional association doesn t make much sense. If you call anAddress.getUser(), you are saying give me the user who has this address has its shipping address, not a very reasonable request. We recommend that a foreign key-based one-to-one association, with a unique constraint on the foreign key column is almost always best represented without a mapping on the other side. Let s repeat the same mapping with annotations. Mapping a foreign key with annotations The JPA mapping annotations also support a one-to-one relationship between entities based on a foreign key column. The main difference compared to the mappings earlier in this chapter is the use of @JoinColumn instead of @PrimaryKeyJoinColumn. First, here s the to-one mapping from User to Address with the unique constraint on the SHIPPING_ADDRESS_ID foreign key column. However, instead of a @ManyToOne annotation, this requires a @OneToOne annotation:

The fact that this program does nothing but print the argument list is unimportant; it is still Java code that we need to build, package, and execute work we will delegate to Ant.

public class User { ... @OneToOne @JoinColumn(name="SHIPPING_ADDRESS_ID") private Address shippingAddress; ... }

First, ensure that Ant is installed and ready to run. You will also need a properly installed Java development kit appropriate for your platform. Appendix A describes how to set up an Ant development system on both Unix and Windows. After having installed everything, at a command prompt type

Hibernate will now enforce the multiplicity with the unique constraint. If you want to make this association bidirectional, you need another @OneToOne mapping in the Address class:

public class Address { ... @OneToOne(mappedBy = "shippingAddress") private User user; ... }

A good response would be something listing a recent version of Ant, ideally 1.5 or later:

The effect of the mappedBy attribute is the same as the property-ref in XML mapping: a simple inverse declaration of an association, naming a property on the target entity side. The equivalent mapping in JPA XML descriptors is as follows:

A bad response would be any error message saying Ant was a not a recognized command, such as this one on a Unix system:

<entity-mappings> <entity class="auction.model.User" access="FIELD"> ... <one-to-one name="shippingAddress"> <join-column name="SHIPPING_ADDRESS_ID"/> </one-to-one> </entity> <entity class="auction.model.Address" access="FIELD"> ... <one-to-one name="user" mapped-by="shippingAddress"/> </entity> </entity-mappings>

On Windows, the response will be less terse, but contains the same underlying message:

You ve now completed two basic single-ended association mappings: the first with a shared primary key, the second with a foreign key reference. The last option we want to discuss is a bit more exotic: mapping a one-to-one association with the help of an additional table.

Let s take a break from the complex CaveatEmptor model and consider a different scenario. Imagine you have to model a data schema that represents an office allocation plan in a company. Common entities include people working at desks. It seems reasonable that a desk may be vacant and have no person assigned to it. On the other hand, an employee may work at home, with the same result. You re dealing with an optional one-to-one association between Person and Desk.

Any such response indicates you have not installed or configured Ant yet, so turn to appendix A: Installation and follow the instructions there on setting up and testing the system. The rest of this chapter, and indeed the entire book, assumes that Ant is installed and working.

If you apply the mapping techniques we discussed in the previous sections, you may come to the following conclusions: Person and Desk are mapped to two tables, with one of them (let s say the PERSON table) having a foreign key column that references the other table (such as ASSIGNED_DESK_ID) with an additional unique constraint (so two people can t be assigned the same desk) The relationship is optional if the foreign key column is nullable On second thought, you realize that the assignment between persons and desks calls for another table that represents ASSIGNMENT In the current design, this table has only two columns: PERSON_ID and DESK_ID The multiplicity of these foreign key columns is enforced with a unique constraint on both a particular person and desk can only be assigned once, and only one such an assignment can exist.

With Ant installed, and the source file created, it is time for Ant to compile our project. Ant is controlled by providing a text file that tells how to perform all the stages of building, testing, and deploying a project. These files are build files, and every project that uses Ant must have at least one. The most minimal build file useful in Java development is one that builds all Java source in and below the current directory: