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CHAPTER 10 WORKING WITH OBJECTS
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10.3 Persisting Detached Objects
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Problem
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How do you persist a detached object
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Solution
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You can persist a detached object two ways: by reattaching it or by merging it.
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How It Works
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You first reattach a detached object and then merge one.
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Reattaching a Detached Object
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In Hibernate, you use the session s update() method to reattach a detached object to the session and persistence context: Session session = getSession(); Transaction tx = session.beginTransaction(); Book book = (Book) session.get(Book.class, new Long(294912)); tx.commit(); session.close(); book.setName("Detached Hibernate"); Session session2 = getSession(); Transaction tx2 = session2.beginTransaction(); session2.update(book); tx2.commit(); session2.close(); Hibernate keeps track of modifications made to the detached object. Hence, when the update() method is called, it always schedules an update SQL command. If you don t want to make an update call, you can configure the class mapping to have select-before-update='true'. Then, a get() call is made to check whether the object has been updated , before the call to the update() method. JPA doesn t have an update() method. It only supports merging detached objects.
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Merging a Detached Object
In Hibernate, you use the session s merge() method to persist a detached object. The merge() method call results in a complex set of actions. Hibernate first looks to see if the instance of the entity is in the current persistence context. If it finds an instance of the entity equal to the detached object, then it copies the state from the detached object to the instance in the persistence context: Session session = getSession();
CHAPTER 10 WORKING WITH OBJECTS
Transaction tx = session.beginTransaction(); Book book = (Book) session.get(Book.class, new Long(294912)); tx.commit(); session.close(); book.setName("Detached Hibernate"); Session session2 = getSession(); Transaction tx2 = session2.beginTransaction(); Book book2 = (Book) session2.merge(book); tx2.commit(); session2.close(); If the persistence context doesn t contain an equal instance of the entity, Hibernate makes a get() call internally to load the persistent entity object into the persistence context. Then, the detached object state is copied onto the newly created persistent entity object. Note that if a transient object is passed to the merge() method, a new persistent instance is created and the state is copied to the new instance. The same applies to JPA. Here s the implementation: EntityManager manager = SessionManager.getEntityManager(); EntityTransaction tran = manager.getTransaction(); tran.begin(); Book book = manager.find(Book.class, new Long(262144)); tran.commit(); manager.close(); book.setBookName("Detached Hibernate merged"); EntityManager manager2 = SessionManager.getEntityManager(); EntityTransaction tran2 = manager2.getTransaction(); tran2.begin(); Book book2 = (Book) manager2.merge(book); tran2.commit(); manager2.close();
10.4 Using Data Filters
Problem
Suppose your bookshop has a lot of children and teenagers as users. You want to make the bookshop kid friendly by classifying users according to their age range. (You don t want a child to be able to buy a book meant for an adult reader.) How can you achieve this by using the data-filtering mechanism provided by Hibernate
Solution
Every Book class has a UserRank class that specifies the lowest rank that can access a book. Books for adults have a user rank of 3, and books for children have a user rank of 1. You define a filter definition (<filter-def>) in the metadata, which can be declared in any XML mapping file as long as it s inside a <hibernate-mapping> element. You need to provide a globally unique
CHAPTER 10 WORKING WITH OBJECTS
name to the filter definition. You apply and implement the filter in Book s mapping XML file. And finally, you enable the filter on the session that is used to query.
How It Works
First, you create the UserRank class and the Book class. The UserRank class includes the user rank and the user type. The Book class is associated with the UserRank class: public class UserRank { private long rank; private String userType; // getters and setters } public class Book { private Long book_id; private String isbn; private String name; private String publisher; private Date publishDate; private Integer price; private UserRank userRank; // getters and setters } The mapping files for UserRank and Book are shown next. You define the filter in the UserRank mapping file by providing a globally unique name and the filter parameters it requires. You use the <filter-def> element and <filter-param> elements to define a filter: <hibernate-mapping package="com.hibernaterecipes.chapter10" auto-import="false" > <import class="UserRank" rename="userRank"/> <class name="UserRank" table="UserRank" dynamic-insert="true" dynamic-update="true" schema="BOOK"> <id name="rank" column="RANK" type="long"> <generator class="assigned"> </generator> </id> <property name="userType" type="string" column="USERTYPE" /> </class> <filter-def name="filterByRank"> <filter-param name="currentRank" type="long"/> <filter-param name="requestedRank" type="long"/> </filter-def> </hibernate-mapping>
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