Friday, June 24, 2011

More Hibernate Questions

Question: What are common mechanisms of configuring Hibernate?
Answer: 1. By placing file in the classpath.
2. Including elements in hibernate.cfg.xml in the classpath.

Question:How can you create a primary key using Hibernate?
Answer: The 'id' tag in .hbm file corresponds to primary key of the table:
Here Id ="empid", that will act as primary key of the table "EMPLOYEE".

Question: In how many ways one can map files to be configured in Hibernate?
Answer: 1. Either mapping files are added to configuration in the application code or,
2.hibernate.cfg.xml can be used for configuring in .

Question: How to set Hibernate to log all generated SQL to the console?
Answer: By setting the hibernate.show_sql property to true.

Question: What happens when both and hibernate.cfg.xml are in the classpath?
Answer: The settings of the XML configuration file will override the settings used in the properties.

Question: What methods must the persistent classes implement in Hibernate?
Answer: Since Hibernate instantiates persistent classes using Constructor.newInstance(), it requires a constructor with no arguments for every persistent class. And getter and setter methods for all the instance variables.

Question: How can Hibernate be configured to access an instance variable directly and not through a setter method?
Answer: By mapping the property with access="field" in Hibernate metadata. This forces hibernate to bypass the setter method and access the instance variable directly while initializing a newly loaded object.

Question: How to declare mappings for multiple classes in one mapping file?
Answer:Use multiple elements. But, the recommended practice is to use one mapping file per persistent class.

Question: How are the individual properties mapped to different table columns?
Answer: By using multiple elements inside the element.

Question: What are derived properties?
Answer: The properties that are not mapped to a column, but calculated at runtime by evaluation of an expression are called derived properties. The expression can be defined using the formula attribute of the element.

Question: How can you make a property be read from the database but not modified in anyway (make it immutable)?
Answer: Use insert="false" and update="false" attributes.

Question: How can a whole class be mapped as immutable?
Answer: By using the mutable="false" attribute in the class mapping.

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