Hibernate and java persistence by example pdf
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- Uma introdução prática ao JPA com Hibernate
- Java Persistence and Hibernate Guide for developer
- Java Persistence with Hibernate, Second Edition
Esse buraco entre esses dois paradigmas gera bastante trabalho: a todo momento devemos "transformar" objetos em registros e registros em objetos.
Uma introdução prática ao JPA com Hibernate
We also show you how you can override the SQL Hibernate generates auto- matically. You can consider a large part of this chapter to be reference material and come back when you face a particular issue. If your development process is top-down, however, you may want to skip this section. Furthermore, we recommend that you first read all chap- ters about class, collection, and association mappings before you attempt to reverse-engineer a complex legacy schema.
We have to warn you: When your application inherits an existing legacy data- base schema, you should usually make as few changes to the existing schema as possible. Every change that you make to the schema could break other existing applications that access the database. Possibly expensive migration of existing data is also something you need to evaluate. It should be clear that the first kind of problem is usually visible by looking at just the logical data model.
The problems that can be solved without schema changes usually involve inconvenient table or column definitions for a particular entity. Alternatively, you can try to write the mapping completely by hand without the reverse-engineering tools. Natural keys often make it difficult to refactor the data model when business requirements change.
They may even, in extreme cases, impact performance. Unfortunately, many legacy schemas use natural composite keys heavily and, for the reason we discourage the use of composite keys, it may be difficult to change the legacy schema to use noncomposite natural or surrogate keys.
Therefore, Hibernate supports the use of natural keys. In this case, you have no surrogate identifier that is auto- matically generated. This is certainly not the best solution, because it triggers an additional hit on the database.
Hibernate manages both values internally for optimistic concurrency con- trol discussed later in the book. As a side effect, an empty timestamp or a 0 or NULL version indicates that an instance is new and has to be inserted, not updated. This extension interface allows you to implement the method isTransient with any custom procedure you may need to distinguish old and new objects. This issue is, in practice, the only reason to not use saveOrUpdate all the time, by the way.
Mapping natural primary keys with JPA annotations is straightforward: Id private String username; If no identifier generator is declared, Hibernate assumes that it has to apply the regular select-to-determine-state-unless-versioned strategy and expects the appli- cation to take care of the primary key value assignment.
You can again avoid the SELECT by extending your application with an interceptor or by adding a version- control property version number or timestamp. Composite natural keys extend on the same ideas. Identifier classes are also expected to implement Serializable. You now remove the username and departmentNr properties from User and add a userId property. In practice you can move all these details into the constructor of your com- posite identifier class. Foreign keys to composite primary keys Because USERS has a composite primary key, any referencing foreign key is also composite.
For example, the association from Item to User the seller is now mapped with a composite foreign key. This completes our discussion of the basic composite key mapping technique in Hibernate. Mapping composite keys with annotations is almost the same, but as always, small differences are important.
Composite keys with annotations The JPA specification covers strategies for handling composite keys. Include a property of this compo- nent type in your entity class, and map it with Id for an application- assigned strategy. Include a property of this type in your entity class, and map it with EmbeddedId. Now—and this is different that what you usually do in native Hibernate—duplicate all the identifier properties in the entity class.
Then, annotate the entity class with IdClass and specify the name of your encapsulated identifier class. The first option is straightforward.
Hence, no Embeddable and no other annotation on that class is needed. First, you encapsulate all identifier attributes in a separate class—as in the previous strategy, no extra annotations on that class are needed. All primary key properties are annotated with Id, and depending on the position of these elements field or getter method , the entity defaults to field or property access. Composite foreign keys are also possible with annotations. Foreign key referencing nonprimary keys Usually, a foreign key constraint references a primary key.
A foreign key constraint is an integrity rule that guarantees that the referenced table has one row with a key value that matches the key value in the referencing table and given row. Note that a foreign key constraint can be self-referencing; in other words, a column with a foreign key constraint can reference the primary key column of the same table.
Sometimes a foreign key references a nonprimary key: a simple unique column, a natural nonprimary key. If you try to map this association with JPA annotations, you may look for an equivalent to the property-ref attribute. To complete this example, you make this association mapping between the two classes bidirectional, with a mapping of an itemsForAuction collection on the User class. A foreign key may reference this natural key, as shown in figure 8.
This completes our exploration of natural, composite, and foreign key-related problems you may have to deal with when you try to map a legacy schema. Hibernate can map classes, properties, and even parts of asso- ciations to a simple SQL statement or expression.
We call these kinds of mappings formula mappings. The second formula calculated the discriminator in an inher- itance mapping; see chapter 5, section 5. Understanding the use case You now map a literal join condition between two entities. This sounds more com- plex than it is in practice. Look at the two classes shown in figure 8. A particular Item may have several Bids—this is a one-to-many association.
The second association allows you to call anItem. Logically, one of the elements in the collec- tion is also the successful bid object referenced by getSuccessfulBid.
Mapping a formula join condition Imagine that each row in the BID table has a flag column to mark the winning bid, as shown in figure 8. One BID row has the flag set to true, and all other rows for this auction item are naturally false.
Your goal is to map this flag column to a successfulBid property of the Item class. To map this as an object reference, you need a literal join condition, because there is no foreign key Hiber- nate can use for a join. If there is no such row, the anItem. However, by using a formula with a property-ref, you can apply it to a foreign key relationship. Working with the association The full SQL query for retrieval of an auction item and its successful bid looks like this: select i.
When you load an Item, Hibernate now joins a row from the BID table by applying a join condition that involves the columns of the successfulReference property. Because this is a grouped property, you can declare individual expressions for each of the columns involved, in the right order.
The first one, 'T', is a literal, as you can see from the quotes. You can expand this and add more join conditions if you need additional restrictions. Note that an outer join is generated because the item in question may not have a successful bid, so NULL is returned for each b. You can now call anItem.
Doing so may trigger the loading of the collection—a price you have to pay with this strat- egy. Then, the new successful bid is marked and set as an instance variable. To complete this and to fix the legacy schema , your database-level con- straints need to do the same as this method.
One of the things to remember about this literal join condition mapping is that it can be applied in many other situations, not only for successful or default relationships. Whenever you need some arbitrary join condition appended to your queries, a formula is the right choice. Our usual recommendation to have more classes than tables may not work, and you may have to do the opposite and join arbitrary tables into one class.
Any properly designed system should have more classes than tables. Splitting a single class into separate tables is something you should do only when you need to merge several tables in a legacy schema into a single class. This is shown in figure 8.
Hibernate also executes an outer join instead of an inner join to retrieve the row from the secondary table. The notion of a secondary table is also included in the Java Persistence specifi- cation. This is the default join condition, so you can only declare the secondary table name, and nothing else. You can probably see that the syntax of annotations is starting to become an issue and code is more difficult to read. The actual component property, billingAddress, is mapped as a regular Embedded class, just like a regular component.
This table contains all the open payments, executed in a nightly batch, for any auctions. The table has a foreign key column to ITEM, as you can see in figure 8. In CaveatEmptor, it would be convenient if you could access the price of a particular auction by calling anItem. The derived property results in an embedded subselect in the select clause of the original query.
Java Persistence and Hibernate Guide for developer
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Java Persistence with Hibernate, Second Edition
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