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/** * A DocumentHandler implementation to delegate responsibility to * based on a files extension. Currently only .html and .txt * files are handled, other extensions ignored. * * @author Erik Hatcher * @created October 28, 2001 * @todo Implement dynamic document type lookup */ public class FileExtensionDocumentHandler implements DocumentHandler { // implementation omitted }

Can there be more than one Bid for a particular Item Can there be more than one Item for a particular Bid

17-5

After glancing at the domain model (see figure 6.8), you can conclude that the association from Bid to Item is a many-to-one association. Recalling that associations are directional, you classify the inverse association from Item to Bid as a oneto-many association. There are only two more possibilities: many-to-many and one-to-one. We ll get back to these in the next chapter. In the context of object persistence, we aren t interested in whether many means two or a maximum of five or unrestricted. And we re only barely interested in optionality of most associations; we don t especially care whether an associated instance is required or if the other end in an association can be NULL (meaning zero-to-many and to-zero association) However, these are important aspects in your relational data schema that influence your choice of integrity rules and the constraints you define in SQL DDL (see chapter 8, section 8.3, Improving schema DDL ).

When running <javadoc> from JDK 1.4 over this source, it complains that you are using a tag that they plan to support in future:

The association from Bid to Item (and vice versa) is an example of the simplest possible kind of entity association. You have two properties in two classes. One is a collection of references, and the other a single reference. First, here s the Java class implementation of Bid:

Custom tags that could override future standard tags: @todo. To avoid potential overrides, use at least one period character (.) in custom tag names.

public class Bid { ... private Item item; public void setItem(Item item) { this.item = item; } public Item getItem() { return item; } ... }

Note that if multiple files have the same name in the source fileset, regardless of directory, only one of them will make it to the destination directory; it is unspecified which one it will be. 3.10.3 Merge mapper The target file name remains fixed to the to attribute specified. All source file names map to the single target.

Next, this is the Hibernate mapping for this association:

<class name="Bid" table="BID"> ... <many-to-one name="item" column="ITEM_ID" class="Item" not-null="true"/> </class>

The merge mapper is used with <uptodate> in cases where many files map to a single destination. For example, many files are bundled together into a single Zip file. A property can be set if the Zip contains all the latest sources:

This mapping is called a unidirectional many-to-one association. (Actually, because it s unidirectional, you don t know what is on the other side, and you could just as well call this mapping a unidirectional to-one association mapping.) The column ITEM_ID in the BID table is a foreign key to the primary key of the ITEM table. You name the class Item, which is the target of this association, explicitly. This is usually optional, because Hibernate can determine the target type with reflection on the Java property. You added the not-null attribute because you can t have a bid without an item a constraint is generated in the SQL DDL to reflect this. The foreign key column ITEM_ID in the BID can never be NULL, the association is not to-zero-orone. The table structure for this association mapping is shown in figure 6.9.

<uptodate property="zip.notRequired"> <srcfiles dir="src" includes="**/*.java"/> <mapper type="merge" to="${dist.dir}/src.zip"/> </uptodate>

In JPA, you map this association with the @ManyToOne annotation, either on the field or getter method, depending on the access strategy for the entity (determined by the position of the @Id annotation):

The <uptodate> task is covered in section 3.12.4. The merge mapper in <copy> is not extremely useful since all files get copied to the same file, with the last unpredictable file becoming the sole new file. There is one interesting case, however, that is worthy of mention. If, for example, you have a directory containing a single file whose name is not precisely known (perhaps with a timestamp suffix), you can copy this file to a known file name using the merge mapper:

public class Bid { ... @ManyToOne( targetEntity = auction.model.Item.class ) @JoinColumn(name = "ITEM_ID", nullable = false) private Item item; ... }