BASIC MAPPING AND OBJECT IDENTITY in Font

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CHAPTER 2 BASIC MAPPING AND OBJECT IDENTITY
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<key-property name="idCardNo" type="string" column="ID_CARD_NO"/> </composite-id> <property name="firstName" type="string" column="FIRST_NAME" /> <property name="lastName" type="string" column="LAST_NAME" /> <property name="address" type="string" column="ADDRESS" /> <property name="email" type="string" column="EMAIL" /> </class> </hibernate-mapping> When a new persistent object is added to your application, you need to define it in the hibernate.cfg.xml configuration file: <mapping resource="com/metaarchit/bookshop/Customer.hbm.xml" /> When you use load() or get() to retrieve a specified object from the database, you need to provide that object s ID. Which type of object should be passed as the ID At the moment, you can pass a newly created Customer object with countryCode and idCardNo set. Note that Hibernate requires that any ID class must implement the java.io.Serializable interface: public class Customer implements Serializable { ... } Customer customerId = new Customer(); customerId.setCountryCode("mo"); customerId.setIdCardNo("1234567(8)"); Customer customer = (Customer) session.get(Customer.class, customerId); It doesn t make sense to pass a whole persistent object as the ID. A better way is to extract the fields that form the ID as a separate class: public class CustomerId implements Serializable { private String countryCode; private String idCardNo; public CustomerId(String countryCode, String idCardNo) { this.countryCode = countryCode; this.idCardNo = idCardNo; } } Then, modify the Customer persistent class to use this new ID class: public class Customer implements Serializable { private CustomerId id; private String firstName; private String lastName; private String address; private String email; // Getters and Setters } The mapping definition should also be modified to use the ID class:
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CHAPTER 2 BASIC MAPPING AND OBJECT IDENTITY
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<hibernate-mapping> <class name="Customer" table="CUSTOMER"> <composite-id name="id" class="CustomerId"> <key-property name="countryCode" type="string" column="COUNTRY_CODE" /> <key-property name="idCardNo" type="string" column="ID_CARD_NO"/> </composite-id> <property name="firstName" type="string" column="FIRST_NAME" /> <property name="lastName" type="string" column="LAST_NAME" /> <property name="address" type="string" column="ADDRESS" /> <property name="email" type="string" column="EMAIL" /> </class> </hibernate-mapping> To retrieve a Customer object, you need to specify the ID. This time, you pass in an instance of CustomerId type: CustomerId customerId = new CustomerId("mo", "1234567(8)"); Customer customer = (Customer) session.get(Customer.class, customerId); To persist a Customer object, you use an instance of CustomerId type as its ID: Customer customer = new Customer(); customer.setId(new CustomerId("mo", "9876543(2)")); customer.setFirstName("Peter"); customer.setLastName("Lou"); customer.setAddress("Address for Peter"); customer.setEmail("peter@lou.com"); session.save(customer); For Hibernate caching to work correctly, you need to override the equals() and hashCode() methods of the custom ID class. The equals() method is used to compare two objects for equality, and the hashCode() method provides an object s hash code. You use EqualsBuilder and HashCodeBuilder to simplify the equals() and hashCode() implementations. These classes are the provided by Jakarta Commons Lang library; you can download it from http://jakarta.apache.org/site/downloads/downloads_commons-lang.cgi. After you download the library, include the commons-lang-2.1.jar in your project s Java build path: public class CustomerId implements Serializable { ... public boolean equals(Object obj) { if (!(obj instanceof CustomerId)) return false; CustomerId other = (CustomerId) obj; return new EqualsBuilder().append(countryCode, other.countryCode) .append(idCardNo, other.idCardNo) .isEquals(); } public int hashCode() { return new HashCodeBuilder().append(countryCode) .append(idCardNo)
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CHAPTER 2 BASIC MAPPING AND OBJECT IDENTITY
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.toHashCode(); } } However, if this Customer persistent class is designed from scratch, you should provide it with an auto-generated primary ID. Define the business keys countryCode and idCardNo as not-null, and add a multicolumn unique constraint. The <properties> tag can be used to group several properties: public class Customer { private Long id; private String countryCode; private String idCardNo; private String firstName; private String lastName; private String address; private String email; // Getters and Setters } <hibernate-mapping> <class name="Customer" table="CUSTOMER"> <id name="id" type="long" column="ID"> <generator class="native"/> </id> <properties name="customerKey" unique="true"> <property name="countryCode" type="string" column="COUNTRY_CODE" not-null="true" /> <property name="idCardNo" type="string" column="ID_CARD_NO" not-null="true" /> </properties> ... </class> </hibernate-mapping>
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2.3 SaveOrUpdate in Hibernate Problem
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How does save and update work for the saveOrUpdate() method in Hibernate
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Hibernate provides a method saveOrUpdate() for persisting objects. It determines whether an object should be saved or updated. This method is very useful for transitive object persistence. session.saveOrUpdate(book);
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CHAPTER 2 BASIC MAPPING AND OBJECT IDENTITY
How It Works
If a persistent object using an autogenerated ID type is passed to the saveOrUpdate() method with an empty ID value, it s treated as a new object that should be inserted into the database. Hibernate first generates an ID for this object and then issues an INSERT statement. Otherwise, if the ID value isn t empty, Hibernate treats it as an existing object and issues an UPDATE statement for it. How does Hibernate treat an ID as empty For the Book class, the isbn type is a primitive long. You should assign a number as the unsaved value. Typically, you choose 0 as unsaved, because it s the default value for the long data type. But it s a problem that you can t have an object whose ID value is really 0 : <id name="isbn" type="long" column="ISBN" unsaved-value="0"> <generator class="native"/> </id> The solution to this problem is to use a primitive wrapper class as your ID type (java.lang.Long in this case). Then, null is treated as the unsaved value. You can use any number within the range of the long data type as the ID value: public class Book { private Long isbn; private String name; private Publisher publisher; private Date publishDate; private int price; private List chapters; // Getters and Setters } This is also the case for other persistent properties, such as the price property in the Book class. When the price of a book is unknown, which value should be assigned to this field Should it be 0 or a negative number Neither seems suitable. Instead, you can change the type to a primitive wrapper class (java.lang.Integer in this case) and use null to represent an unknown value: public class Book { private Long isbn; private String name; private Publisher publisher; private Date publishDate; private Integer price; private List chapters; // Getters and Setters }
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