Explain about the dirty checking feature of Hibernate?

Dirty checking feature of the Hibernate allows users or developers to avoid time consuming data base write actions. This feature makes necessary updations and changes to the fields which require a change, remaining fields are left unchanged or untouched.

What is query cache in Hibernate ?

This question, Some times asked as a follow-up of last Hibernate Interview question, QueryCache actually stores result of sql query for future calls. Query cache can be used along with second level cache for improved performance. Hibernate support various open source caching solution to implement Query cache e.g. EhCache.

What is Second level Cache in Hibernate?

This is one of the first interview question related to caching in Hibernate, you can expect few more. Second level Cache is maintained at SessionFactory level and can improve performance by saving few database round trip. Another worth noting point is that second level cache is available to whole application rather than any particular session.

What does Session lock() method do in Hibernate?

This one is one of the tricky Hibernate Interview question, because Session's lock() method reattach object without synchronizing or updating with database. So you need to be very careful while using lock() method. By the way you can always use Session's update() method to sync with database during reattachment. Some time this Hibernate question is also asked as what is difference between Session's lock() and update() method. You can use this key point to answer that question as well.

What is difference between transient, persistent and detached object in Hibernate?

In Hibernate, Object can remain in three state transient, persistent or detached.  An object which is associated with Hibernate session is called persistent object. Any change in this object will reflect in database based upon your flush strategy i.e. automatic flush whenever any property of object change or explicit flushing by calling Session.flush() method. On the other hand if an object which is earlier associated with Session, but currently not associated with it are called detached object. You can reattach detached object to any other session by calling either update() or saveOrUpdate() method on that session. Transient objects are newly created instance of persistence class, which is never associated with any Hibernate Session. Similarly you can call persist() or save() methods to make transient object persistent. Just remember, here transient doesn’t represent transient keyword in Java, which is altogether different thing.

What is difference between sorted and ordered collection in hibernate?

This is one of the easy Hibernate interview question you ever face. A sorted collection is sorted in memory by using Java Comparator, while a ordered collection uses database's order by clause for ordering. For large data set it's better to use ordered collection to avoid any OutOfMemoryError in Java, by trying to sort them in memory.

Difference between save, persist and saveOrUpdate methods in Hibernate?

After get vs load, this is another Hibernate Interview question which appears quite often. All three methods i.e. save(), saveOrUpdate() and persist() is used to save objects into database, but has subtle differences e.g. save() can only INSERT records but saveOrUpdate() can either INSERT or UPDATE records. Also, return type of save() is a Serializable object, while return type of persist() method is void. You can also check save vs persist vs saveOrUpdate for complete differences between them in hibernate.

Hibernate interview Questions and answers ?

One Link

What are the types of Hibernate instance states ?

Three types of instance states:

• Transient -The instance is not associated with any persistence context
• Persistent -The instance is associated with a persistence context
• Detached -The instance was associated with a persistence context which has been closed – currently not associated

What are Callback interfaces?

Callback interfaces allow the application to receive a notification when something interesting happens to an object—for example, when an object is loaded, saved, or deleted. Hibernate applications don't need to implement these callbacks, but they're useful for implementing certain kinds of generic functionality.

What is automatic dirty checking?

Automatic dirty checking is a feature that saves us the effort of explicitly asking Hibernate to update the database when we modify the state of an object inside a transaction.

What is the use of dynamic-insert and dynamic-update attributes in a class mapping?

Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.

• dynamic-update (defaults to false): Specifies that UPDATE SQL should be generated at runtime and contain only those columns whose values have changed
• dynamic-insert (defaults to false): Specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null.

How can a whole class be mapped as immutable?

Mark the class as mutable="false" (Default is true),. This specifies that instances of the class are (not) mutable. Immutable classes, may not be updated or deleted by the application.

How can Hibernate be configured to access an instance variable directly and not through a setter method ?

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.

What is Hibernate proxy?

The proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will initially return CGLIB proxies which implement the named interface. The actual persistent object will be loaded when a method of the proxy is invoked.

What are the ways to express joins in HQL?

HQL provides four ways of expressing (inner and outer) joins:-

• An implicit association join
• An ordinary join in the FROM clause
• A fetch join in the FROM clause.
• A theta-style join in the WHERE clause.

What is the advantage of Hibernate over jdbc?

JDBC	Hibernate
With JDBC, developer has to write code to map an object model's data representation to a relational data model and its corresponding database schema.	Hibernate is flexible and powerful ORM solution to map Java classes to database tables. Hibernate itself takes care of this mapping using XML files so developer does not need to write code for this.
With JDBC, the automatic mapping of Java objects with database tables and vice versa conversion is to be taken care of by the developer manually with lines of code.	Hibernate provides transparent persistence and developer does not need to write code explicitly to map database tables tuples to application objects during interaction with RDBMS.
JDBC supports only native Structured Query Language (SQL). Developer has to find out the efficient way to access database, i.e. to select effective query from a number of queries to perform same task.	Hibernate provides a powerful query language Hibernate Query Language (independent from type of database) that is expressed in a familiar SQL like syntax and includes full support for polymorphic queries. Hibernate also supports native SQL statements. It also selects an effective way to perform a database manipulation task for an application.
Application using JDBC to handle persistent data (database tables) having database specific code in large amount. The code written to map table data to application objects and vice versa is actually to map table fields to object properties. As table changed or database changed then it’s essential to change object structure as well as to change code written to map table-to-object/object-to-table.	Hibernate provides this mapping itself. The actual mapping between tables and application objects is done in XML files. If there is change in Database or in any table then the only need to change XML file properties.
With JDBC, it is developer’s responsibility to handle JDBC result set and convert it to Java objects through code to use this persistent data in application. So with JDBC, mapping between Java objects and database tables is done manually.	Hibernate reduces lines of code by maintaining object-table mapping itself and returns result to application in form of Java objects. It relieves programmer from manual handling of persistent data, hence reducing the development time and maintenance cost.
With JDBC, caching is maintained by hand-coding.	Hibernate, with Transparent Persistence, cache is set to application work space. Relational tuples are moved to this cache as a result of query. It improves performance if client application reads same data many times for same write. Automatic Transparent Persistence allows the developer to concentrate more on business logic rather than this application code.
In JDBC there is no check that always every user has updated data. This check has to be added by the developer.	Hibernate enables developer to define version type field to application, due to this defined field Hibernate updates version field of database table every time relational tuple is updated in form of Java class object to that table. So if two users retrieve same tuple and then modify it and one user save this modified tuple to database, version is automatically updated for this tuple by Hibernate. When other user tries to save updated tuple to database then it does not allow saving it because this user does not have updated data.

What is component mapping in Hibernate?

• A component is an object saved as a value, not as a reference
• A component can be saved directly without needing to declare interfaces or identifier properties
• Required to define an empty constructor
• Shared references not supported

If you want to see the Hibernate generated SQL statements on console, what should we do?

In Hibernate configuration file set as follows: 
<property name="show_sql">true</property>

How do you switch between relational databases without code changes?

Using Hibernate SQL Dialects , we can switch databases. Hibernate will generate appropriate hql queries based on the dialect defined.

Explain Criteria API ?

Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.

List employees = session.createCriteria(Employee.class)
                          .add(Restrictions.like("name", "a%") )
                          .add(Restrictions.like("address", "Boston"))
                          .addOrder(Order.asc("name") )
                          .list();

How do you invoke Stored Procedures?

<sql-query name="selectAllEmployees_SP" callable="true">
   <return alias="emp" class="employee">
     <return-property name="empid" column="EMP_ID"/>       

     <return-property name="name" column="EMP_NAME"/>       
     <return-property name="address" column="EMP_ADDRESS"/>
      { ? = call selectAllEmployees() }
   </return>
  </sql-query> 

What do you mean by Named – SQL query?

Named SQL queries are defined in the mapping xml document and called wherever required.

<sql-query name = "empdetails">
     <return alias="emp" class="com.test.Employee"/>
        SELECT emp.EMP_ID AS {emp.empid},
                   emp.EMP_ADDRESS AS {emp.address},
                   emp.EMP_NAME AS {emp.name} 
        FROM Employee EMP WHERE emp.NAME LIKE :name
  </sql-query> 

Invoke Named Query :

List people = session.getNamedQuery("empdetails")
                      .setString("TomBrady", name)
                      .setMaxResults(50)
                      .list();

How do you define sequence generated primary key in hibernate?

Using <generator> tag.

<id column="USER_ID" name="id" type="java.lang.Long"> 
     <generator class="sequence"> 
       <param name="table">SEQUENCE_NAME</param>
     <generator>
  </id> 

What’s the difference between load() and get()?

load()	get()
Only use the load() method if you are sure that the object exists.	If you are not sure that the object exists, then use one of the get()methods.
load() method will throw an exception if the unique id is not found in the database.	get() method will return null if the unique id is not found in the database.
load() just returns a proxy by default and database won’t be hit until the proxy is first invoked.	get() will hit the database immediately.

How do you map Java Objects with Database tables?

• First we need to write Java domain objects (beans with setter and getter).
• Write hbm.xml, where we map java class to table and database columns to Java class variables.

• <hibernate-mapping>
    <class name="com.test.User"  table="user">
     <property  column="USER_NAME" length="255" 
        name="userName" not-null="true"  type="java.lang.String"/>
     <property  column="USER_PASSWORD" length="255"
  name="userPassword" not-null="true"  type="java.lang.String"/>
   </class>
  </hibernate-mapping>

What is Hibernate Query Language (HQL)?

Hibernate offers a query language that embodies a very powerful and flexible mechanism to query, store, update, and retrieve objects from a database. This language, the Hibernate query Language (HQL), is an object-oriented extension to SQL.

What is the general flow of Hibernate communication with RDBMS?

The general flow of Hibernate communication with RDBMS is :

• Load the Hibernate configuration file and create configuration object. It will automatically load all hbm mapping files
• Create session factory from configuration object
• Get one session from this session factory
• Create HQL Query
• Execute query to get list containing Java objects

What role does the SessionFactory interface play in Hibernate?

• The application obtains Session instances from a SessionFactory.
• There is typically a single SessionFactory for the whole applicationå¹¼reated during application initialization. 
•  The SessionFactory caches generate SQL statements and other mapping metadata that Hibernate uses at runtime. 
• It also holds cached data that has been read in one unit of work and may be reused in a future unit of work

SessionFactory sessionFactory = configuration.buildSessionFactory();

What role does the Session interface play in Hibernate?

The Session interface is the primary interface used by Hibernate applications. It is a single-threaded, short-lived object representing a conversation between the application and the persistent store. 
Session session = sessionFactory.openSession(); 
Session interface role:

• Wraps a JDBC connection
• Factory for Transaction
• Holds a mandatory (first-level) cache of persistent objects, used when navigating the object graph or looking up objects by identifier

What are the Core interfaces are of Hibernate framework?

The five core interfaces are used in just about every Hibernate application. Using these interfaces, you can store and retrieve persistent objects and control transactions.

• Session interface
• SessionFactory interface
• Configuration interface
• Transaction interface
• Query and Criteria interfaces

What are the important tags of hibernate.cfg.xml?

What are the important tags of hibernate.cfg.xml?

What are the most common methods of Hibernate configuration?

The most common methods of Hibernate configuration are:

• Programmatic configuration
• XML configuration (hibernate.cfg.xml)

What does ORM consists of ?

An ORM solution consists of the followig four pieces:

• API for performing basic CRUD operations
• API to express queries refering to classes
• Facilities to specify metadata
• Optimization facilities : dirty checking,lazy associations fetching

What is ORM ?

ORM stands for object/relational mapping. ORM is the automated persistence of objects in a Java application to the tables in a relational database.

What is Hibernate?

• Hibernate is a pure Java object-relational mapping (ORM) and persistence framework that allows you to map plain old Java objects to relational database tables using (XML) configuration files.
• Its purpose is to relieve the developer from a significant amount of relational data persistence-related programming tasks.

What is the need for Hibernate xml mapping file?

Hibernate mapping file tells Hibernate which tables and columns to use to load and store objects.