1.

Explain how exceptions are handled in Hibernate?

Answer»

Hibernate supports the three basic inheritance mapping strategies: 

  • table per class hierarchy 
  • table per subclass 
  • table per concrete class 
1. Table per class Hierarchy 

Let us assume we have an interface Payment which is IMPLEMENTED by classesCreditCardPayment, CashPayment, andChequePayment. When we map using the table per hierarchy mapping strategy, the xml would be written as below:: 

<class NAME="Payment" table="PAYMENT">      <id name="id" type="long" column="PAYMENT_ID">          <generator class="native"/>      </id>      <discriminator column="PAYMENT_TYPE" type="string"/>      <PROPERTY name="amount" column="AMOUNT"/>      ...      <subclass name="CreditCardPayment" discriminator-value="CREDIT">          <property name="creditCardType" column="CCTYPE"/>          ...      </subclass>      <subclass name="CashPayment" discriminator-value="CASH">          ...      </subclass>      <subclass name="ChequePayment" discriminator-value="CHEQUE">          ...      </subclass>  </class>

In this strategy, exactly on table is required. The restrictions or limitations based on this strategy is that the subclass properties cannot have NULL values. 

2. Table per subclass 

A table per subclass mapping looks like this: 

<class name="Payment" table="PAYMENT">      <id name="id" type="long" column="PAYMENT_ID">          <generator class="native"/>      </id>      <property name="amount" column="AMOUNT"/>      ...      <joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">          <key column="PAYMENT_ID"/>          <property name="creditCardType" column="CCTYPE"/>          ...      </joined-subclass>  <joined-subclass name="CashPayment" table="CASH_PAYMENT">          <key column="PAYMENT_ID"/>          ...      </joined-subclass>      <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">          <key column="PAYMENT_ID"/>          ...      </joined-subclass>  </class>

In this strategy four TABLES are required i.e. 1 superclass and 3 subclasses. The three subclass tables are associated with the superclass table through the Primary Key. So this RELATIONAL model is actually a one-to-one association between the superclass and subclasses. 

3. Table per concrete class 

We can map the table per concrete class strategy by using <union-subclass>. 

<class name="Payment">      <id name="id" type="long" column="PAYMENT_ID">          <generator class="sequence"/>      </id>      <property name="amount" column="AMOUNT"/>      ...      <union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">          <property name="creditCardType" column="CCTYPE"/>          ...      </union-subclass>      <union-subclass name="CashPayment" table="CASH_PAYMENT">          ...      </union-subclass>      <union-subclass name="ChequePayment" table="CHEQUE_PAYMENT">          ...      </union-subclass>  </class>

3 tables are mapped to the subclasses. Each table defines columns for all properties of the class, including inherited properties. The limitation of this strategy is that if a property is mapped on the superclass, the column name must be the same on all subclass tables. Another limitation is that the identity generator strategy is not allowed in union subclass inheritance. The primary key has to be shared across all unioned subclasses of a hierarchy. If the superclass is abstract, then the mapping must show it as abstract with abstract="true". If it is not abstract, an additional table is required to hold the instances of the superclass. 


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