Hibernate Interview Questions

1. Does Hibernate implement its functionality using a minimal number of database queries to ensure optimal output?

Hibernate can make certain optimizations all the time:
• Caching objects - The session is a transaction-level cache of persistent objects. You may also enable a JVM-level/cluster cache to memory and/or local disk.
• Executing SQL statements later, when needed - The session never issues an INSERT or UPDATE until it is actually needed. So if an exception occurs and you need to abort the transaction, some statements will never actually be issued. Furthermore, this keeps lock times in the database as short as possible (from the late UPDATE to the transaction end).
• Never updating unmodified objects - It is very common in hand-coded JDBC to see the persistent state of an object updated, just in case it changed.....for example, the user pressed the save button but may not have edited any fields. Hibernate always knows if an object's state actually changed, as long as you are inside the same (possibly very long) unit of work.
• Efficient Collection Handling - Likewise, Hibernate only ever inserts/updates/deletes collection rows that actually changed.
• Rolling two updates into one - As a corollary to (1) and (3), Hibernate can roll two seemingly unrelated updates of the same object into one UPDATE statement.
• Updating only the modified columns - Hibernate knows exactly which columns need updating and, if you choose, will update only those columns.
• Outer join fetching - Hibernate implements a very efficient outer-join fetching algorithm! In addition, you can use subselect and batch pre-fetch optimizations.
• Lazy collection initialization
• Lazy object initialization - Hibernate can use runtime-generated proxies (CGLIB) or interception injected through byte code instrumentation at build-time.

2. Why not implement instance-pooling in Hibernate?

Firstly, it would be pointless. There is a lower bound to the amount of garbage Hibernate creates every time it loads or updates and object - the garbage created by getting or setting the object's properties using reflection.
More importantly, the disadvantage of instance-pooling is developers who forget to reinitialize fields each time an instance is reused. We have seen very subtle bugs in EJBs that don't reinitialize all fields in ejbCreate.
On the other hand, if there is a particular application object that is extremely expensive to create, you can easily implement your own instance pool for that class and use the version of Session.load() that takes a class instance. Just remember to return the objects to the pool every time you close the session.

3. Does Hibernate use runtime reflection?

Many former C or C++ programmers prefer generated-code solutions to runtime reflection. This is usually justified by reference to the performance red-herring. However, modern JVMs implement reflection extremely efficiently and the overhead is minimal compared to the cost of disk access or IPC. Developers from other traditions (e.g. Smalltalk) have always relied upon reflection to do things that C/C++ needs code-generation for.
In the very latest versions of Hibernate, "reflection" is optimised via the CGLIB runtime byte code generation library. This means that "reflected" property get / set calls no longer carry the overhead of the Java reflection API and are actually just normal method calls. This results in a (very) small performance gain.

4. How do I use Hibernate in an EJB 2.1 session bean?

1. Look up the SessionFactory in JNDI.
2. Call getCurrentSession() to get a Session for the current transaction.
3. Do your work.
4. Don't commit or close anything, let the container manage the transaction.

5. What’s the easiest way to configure Hibernate in a plain Java application (without using JNDI)?

Build a SessionFactory from a Configuration object.

6. What is Middlegen?

Middlegen is an open source code generation framework that provides a general-purpose database-driven engine using various tools such as JDBC, Velocity, Ant and XDoclet.

7. How can I count the number of query results without actually returning them?

Integer count = (Integer) session.createQuery("select count(*) from ....").uniqueResult();
8. How can I find the size of a collection without initializing it?

Integer size = (Integer) s.createFilter( collection, "select count(*)" ).uniqueResult();

9. How can I order by the size of a collection?

Use a left join, together with group by
select user
from User user
left join user.messages msg
group by user
order by count(msg)

10. How can I place a condition upon a collection size?

If your database supports subselects:
from User user where size(user.messages) >= 1
or:
from User user where exists elements(user.messages)
If not, and in the case of a one-to-many or many-to-many association:
select user
from User user
join user.messages msg
group by user
having count(msg) >= 1
Because of the inner join, this form can't be used to return a User with zero messages, so the following form is also useful
select user
from User as user
left join user.messages as msg
group by user
having count(msg) = 0

11. How can I query for entities with empty collections?

from Box box
where box.balls is empty
Or, try this:
select box
from Box box
left join box.balls ball
where ball is null

12. How can I sort / order collection elements?

There are three different approaches:
1. Use a SortedSet or SortedMap, specifying a comparator class in the sort attribute or < set > or < map >. This solution does a sort in memory.
2. Specify an order-by attribute of < set >, < map > or < bag >, naming a list of table columns to sort by. This solution works only in JDK 1.4+.
3. Use a filter session.createFilter( collection, "order by ...." ).list()

13. Are collections pageable?

Query q = s.createFilter( collection, "" );
q.setMaxResults(PAGE_SIZE);
q.setFirstResult(PAGE_SIZE * pageNumber);
List page = q.list();
I have a one-to-one association between two classes. Ensuring that associated objects have matching identifiers is bug-prone. Is there a better way?
< generator class="foreign" >
< param name="property" > parent < / param >
< / generator >
I have a many-to-many association between two tables, but the association table has some extra columns (apart from the foreign keys). What kind of mapping should I use?
Use a composite-element to model the association table. For example, given the following association table:
create table relationship (
fk_of_foo bigint not null,
fk_of_bar bigint not null,
multiplicity smallint,
created date )
you could use this collection mapping (inside the mapping for class Foo):
< set name="relationship" >
< key column="fk_of_foo" / >
< composite-element class="Relationship" >
< property name="multiplicity" type="short" not-null="true" / >
< property name="created" type="date" not-null="true" / >
< many-to-one name="bar" class="Bar" not-null="true" / >
< / composite-element >
< / set >
You may also use an with a surrogate key column for the collection table. This would allow you to have nullable columns.
An alternative approach is to simply map the association table as a normal entity class with two bidirectional one-to-many associations.
In an MVC application, how can we ensure that all proxies and lazy collections will be initialized when the view tries to access them?
One possible approach is to leave the session open (and transaction uncommitted) when forwarding to the view. The session/transaction would be closed/committed after the view is rendered in, for example, a Servlet filter (another example would by to use the ModelLifetime.discard() callback in Maverick). One difficulty with this approach is making sure the session/transaction is closed/rolled back if an exception occurs rendering the view.
Another approach is to simply force initialization of all needed objects using Hibernate.initialize(). This is often more straightforward than it sounds.

14. How can I bind a dynamic list of values into an in query expression?

Query q = s.createQuery("from foo in class Foo where foo.id in (:id_list)");
q.setParameterList("id_list", fooIdList);
List foos = q.list();

15. How can I bind properties of a JavaBean to named query parameters?

Query q = s.createQuery("from foo in class Foo where foo.name=:name and foo.size=:size");
q.setProperties(fooBean); // fooBean has getName() and getSize()
List foos = q.list();

16. Can I map an inner class?

You may persist any static inner class. You should specify the class name using the standard form i.e. eg.Foo$Bar

17. How can I assign a default value to a property when the database column is null?

Use a UserType.

18. How can I truncate String data?

Use a UserType.

19. How can I trim spaces from String data persisted to a CHAR column?

Use a UserType.

20. How can I convert the type of a property to/from the database column type?

Use a UserType.

21. How can I get access to O/R mapping information such as table and column names at runtime?

This information is available via the Configuration object. For example, entity mappings may be obtained using Configuration.getClassMapping(). It is even possible to manipulate this metamodel at runtime and then build a new SessionFactory.

22. How can I create an association to an entity without fetching that entity from the database (if I know the identifier)?

If the entity is proxyable (lazy="true"), simply use load(). The following code does not result in any SELECT statement:
Item itemProxy = (Item) session.load(Item.class, itemId);
Bid bid = new Bid(user, amount, itemProxy);
session.save(bid);

23. How can I retrieve the identifier of an associated object, without fetching the association?

Just do it. The following code does not result in any SELECT statement, even if the item association is lazy.
Long itemId = bid.getItem().getId();
This works if getItem() returns a proxy and if you mapped the identifier property with regular accessor methods. If you enabled direct field access for the id of an Item, the Item proxy will be initialized if you call getId(). This method is then treated like any other business method of the proxy, initialization is required if it is called.

24. How can I manipulate mappings at runtime?

You can access (and modify) the Hibernate metamodel via the Configuration object, using getClassMapping(), getCollectionMapping(), etc.
Note that the SessionFactory is immutable and does not retain any reference to the Configuration instance, so you must re-build it if you wish to activate the modified mappings.

25. How can I avoid n+1 SQL SELECT queries when running a Hibernate query?

Follow the best practices guide! Ensure that all and mappings specify lazy="true" in Hibernate2 (this is the new default in Hibernate3). Use HQL LEFT JOIN FETCH to specify which associations you need to be retrieved in the initial SQL SELECT.
A second way to avoid the n+1 selects problem is to use fetch="subselect" in Hibernate3.
If you are still unsure, refer to the Hibernate documentation and Hibernate in Action.
I have a collection with second-level cache enabled, and Hibernate retrieves the collection elements one at a time with a SQL query per element!
Enable second-level cache for the associated entity class. Don't cache collections of uncached entity types.

26. How can I insert XML data into Oracle using the xmltype() function?

Specify custom SQL INSERT (and UPDATE) statements using and in Hibernate3, or using a custom persister in Hibernate 2.1.
You will also need to write a UserType to perform binding to/from the PreparedStatement.

27. How can I execute arbitrary SQL using Hibernate?

PreparedStatement ps = session.connection().prepareStatement(sqlString);
Or, if you wish to retrieve managed entity objects, use session.createSQLQuery().
Or, in Hibernate3, override generated SQL using , , and in the mapping document.
I want to call an SQL function from HQL, but the HQL parser does not recognize it!
Subclass your Dialect, and call registerFunction() from the constructor.

28. Why to use HQL?
• Full support for relational operations: HQL allows representing SQL queries in the form of objects. Hibernate Query Language uses Classes and properties instead of tables and columns.
• Return result as Object: The HQL queries return the query result(s) in the form of object(s), which is easy to use. This eliminates the need of creating the object and populate the data from result set.
• Polymorphic Queries: HQL fully supports polymorphic queries. Polymorphic queries results the query results along with all the child objects if any.
• Easy to Learn: Hibernate Queries are easy to learn and it can be easily implemented in the applications.
• Support for Advance features: HQL contains many advance features such as pagination, fetch join with dynamic profiling, Inner/outer/full joins, Cartesian products. It also supports Projection, Aggregation (max, avg) and grouping, Ordering, Sub queries and SQL function calls.
• Database independent: Queries written in HQL are database independent (If database supports the underlying feature).

Java Spring and Hibernate Interview Questions

1. Explain DI or IOC pattern.

Dependency injection (DI) is a programming design pattern and architectural model, sometimes also referred to as inversion of control or IOC, although technically speaking, dependency injection specifically refers to an implementation of a particular form of IOC. Dependency Injection describes the situation where one object uses a second object to provide a particular capacity. For example, being passed a database connection as an argument to the constructor method instead of creating one inside the constructor. The term "Dependency injection" is a misnomer, since it is not a dependency that is injected; rather it is a provider of some capability or resource that is injected. There are three common forms of dependency injection: setter, constructor and interface-based injection.
Dependency injection is a way to achieve loose coupling. Inversion of control (IOC) relates to the way in which an object obtains references to its dependencies. This is often done by a lookup method. The advantage of inversion of control is that it decouples objects from specific lookup mechanisms and implementations of the objects it depends on. As a result, more flexibility is obtained for production applications as well as for testing.

2. What are the different IOC containers available?


Spring is an IOC container. Other IOC containers are HiveMind, Avalon, PicoContainer.

3. What are the different types of dependency injection? Explain with examples.


There are two types of dependency injection: setter injection and constructor injection.
Setter Injection: Normally in all the java beans, we will use setter and getter method to set and get the value of property as follows:

public class namebean {
String name;
public void setName(String a) {
name = a; }
public String getName() {
return name; }
}

We will create an instance of the bean 'namebean' (say bean1) and set property as bean1.setName("tom"); Here in setter injection, we will set the property 'name' in spring configuration file as shown below:

< bean id="bean1" class="namebean" >
< property name="name" >
< value > tom < / value >
< / property >
< / bean >

The subelement < value > sets the 'name' property by calling the set method as setName("tom"); This process is called setter injection.

To set properties that reference other beans , subelement of is used as shown below,
< bean id="bean1" class="bean1impl" >
< property name="game" >
< ref bean="bean2" / >
< / property >
< / bean >
< bean id="bean2" class="bean2impl" / >

Constructor injection: For constructor injection, we use constructor with parameters as shown below,

public class namebean {
String name;
public namebean(String a) {
name = a;
}
}

We will set the property 'name' while creating an instance of the bean 'namebean' as namebean bean1 = new namebean("tom");

Here we use the < constructor-arg > element to set the property by constructor injection as
< bean id="bean1" class="namebean" >
< constructor-arg >
< value > My Bean Value < / value >
< / constructor-arg >
< / bean >


4. What is spring? What are the various parts of spring framework? What are the different persistence frameworks which could be used with spring?


Spring is an open source framework created to address the complexity of enterprise application development. One of the chief advantages of the Spring framework is its layered architecture, which allows you to be selective about which of its components you use while also providing a cohesive framework for J2EE application development. The Spring modules are built on top of the core container, which defines how beans are created, configured, and managed. Each of the modules (or components) that comprise the Spring framework can stand on its own or be implemented jointly with one or more of the others. The functionality of each component is as follows:

The core container: The core container provides the essential functionality of the Spring framework. A primary component of the core container is the BeanFactory, an implementation of the Factory pattern. The BeanFactory applies the Inversion of Control (IOC) pattern to separate an application’s configuration and dependency specification from the actual application code.

Spring context: The Spring context is a configuration file that provides context information to the Spring framework. The Spring context includes enterprise services such as JNDI, EJB, e-mail, internalization, validation, and scheduling functionality.

Spring AOP: The Spring AOP module integrates aspect-oriented programming functionality directly into the Spring framework, through its configuration management feature. As a result you can easily AOP-enable any object managed by the Spring framework. The Spring AOP module provides transaction management services for objects in any Spring-based application. With Spring AOP you can incorporate declarative transaction management into your applications without relying on EJB components.

Spring DAO: The Spring JDBC DAO abstraction layer offers a meaningful exception hierarchy for managing the exception handling and error messages thrown by different database vendors. The exception hierarchy simplifies error handling and greatly reduces the amount of exception code you need to write, such as opening and closing connections. Spring DAO’s JDBC-oriented exceptions comply to its generic DAO exception hierarchy.

Spring ORM: The Spring framework plugs into several ORM frameworks to provide its Object Relational tool, including JDO, Hibernate, and iBatis SQL Maps. All of these comply to Spring’s generic transaction and DAO exception hierarchies.

Spring Web module: The Web context module builds on top of the application context module, providing contexts for Web-based applications. As a result, the Spring framework supports integration with Jakarta Struts. The Web module also eases the tasks of handling multi-part requests and binding request parameters to domain objects.

Spring MVC framework: The Model-View-Controller (MVC) framework is a full-featured MVC implementation for building Web applications. The MVC framework is highly configurable via strategy interfaces and accommodates numerous view technologies including JSP, Velocity, Tiles, iText, and POI.

5. What is AOP? How does it relate with IOC? What are different tools to utilize AOP?


Aspect-oriented programming, or AOP, is a programming technique that allows programmers to modularize crosscutting concerns, or behaviour that cuts across the typical divisions of responsibility, such as logging and transaction management. The core construct of AOP is the aspect, which encapsulates behaviours affecting multiple classes into reusable modules. AOP and IOC are complementary technologies in that both apply a modular approach to complex problems in enterprise application development. In a typical object-oriented development approach you might implement logging functionality by putting logger statements in all your methods and Java classes. In an AOP approach you would instead modularize the logging services and apply them declaratively to the components that required logging. The advantage, of course, is that the Java class doesn't need to know about the existence of the logging service or concern itself with any related code. As a result, application code written using Spring AOP is loosely coupled. The best tool to utilize AOP to its capability is AspectJ. However AspectJ works at the byte code level and you need to use AspectJ compiler to get the aop features built into your compiled code. Nevertheless AOP functionality is fully integrated into the Spring context for transaction management, logging, and various other features. In general any AOP framework control aspects in three possible ways:

Joinpoints: Points in a program's execution. For example, joinpoints could define calls to specific methods in a class
Pointcuts: Program constructs to designate joinpoints and collect specific context at those points
Advices: Code that runs upon meeting certain conditions. For example, an advice could log a message before executing a joinpoint

6. What are the advantages of spring framework?


Spring has layered architecture. Use what you need and leave you don't need.
Spring Enables POJO Programming. There is no behind the scene magic here. POJO programming enables continuous integration and testability.
Dependency Injection and Inversion of Control Simplifies JDBC
Open source and no vendor lock-in.
7. Can you name a tool which could provide the initial ant files and directory structure for a new spring project?


Appfuse or equinox.

8. Explain BeanFactory in spring.


Bean factory is an implementation of the factory design pattern and its function is to create and dispense beans. As the bean factory knows about many objects within an application, it is able to create association between collaborating objects as they are instantiated. This removes the burden of configuration from the bean and the client. There are several implementation of BeanFactory. The most useful one is "org.springframework.beans.factory.xml.XmlBeanFactory" It loads its beans based on the definition contained in an XML file. To create an XmlBeanFactory, pass a InputStream to the constructor. The resource will provide the XML to the factory.
BeanFactory factory = new XmlBeanFactory(new FileInputStream("myBean.xml"));

This line tells the bean factory to read the bean definition from the XML file. The bean definition includes the description of beans and their properties. But the bean factory doesn't instantiate the bean yet. To retrieve a bean from a 'BeanFactory', the getBean() method is called. When getBean() method is called, factory will instantiate the bean and begin setting the bean's properties using dependency injection.

myBean bean1 = (myBean)factory.getBean("myBean");

9. Explain the role of ApplicationContext in spring.


While Bean Factory is used for simple applications; the Application Context is spring's more advanced container. Like 'BeanFactory' it can be used to load bean definitions, wire beans together and dispense beans upon request. It also provide

1) A means for resolving text messages, including support for internationalization.
2) A generic way to load file resources.
3) Events to beans that are registered as listeners.

Because of additional functionality, 'Application Context' is preferred over a BeanFactory. Only when the resource is scarce like mobile devices, 'BeanFactory' is used. The three commonly used implementation of 'Application Context' are

1. ClassPathXmlApplicationContext : It Loads context definition from an XML file located in the classpath, treating context definitions as classpath resources. The application context is loaded from the application's classpath by using the code

ApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");

2. FileSystemXmlApplicationContext : It loads context definition from an XML file in the filesystem. The application context is loaded from the file system by using the code

ApplicationContext context = new FileSystemXmlApplicationContext("bean.xml");

3. XmlWebApplicationContext : It loads context definition from an XML file contained within a web application.

10. How does Spring supports DAO in hibernate?


Spring’s HibernateDaoSupport class is a convenient super class for Hibernate DAOs. It has handy methods you can call to get a Hibernate Session, or a SessionFactory. The most convenient method is getHibernateTemplate(), which returns a HibernateTemplate. This template wraps Hibernate checked exceptions with runtime exceptions, allowing your DAO interfaces to be Hibernate exception-free.
Example:

public class UserDAOHibernate extends HibernateDaoSupport {

public User getUser(Long id) {
return (User) getHibernateTemplate().get(User.class, id);
}
public void saveUser(User user) {
getHibernateTemplate().saveOrUpdate(user);
if (log.isDebugEnabled()) {
log.debug(“userId set to: “ + user.getID());
}
}
public void removeUser(Long id) {
Object user = getHibernateTemplate().load(User.class, id);
getHibernateTemplate().delete(user);
}

}


11. What are the id generator classes in hibernate?


increment: It generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. It should not the used in the clustered environment.
identity: It supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type long, short or int.
sequence: The sequence generator uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is of type long, short or int
hilo: The hilo generator uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and column (by default hibernate_unique_key and next_hi respectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database. Do not use this generator with connections enlisted with JTA or with a user-supplied connection.
seqhilo: The seqhilo generator uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence.
uuid: The uuid generator uses a 128-bit UUID algorithm to generate identifiers of type string, unique within a network (the IP address is used). The UUID is encoded as a string of hexadecimal digits of length 32.
guid: It uses a database-generated GUID string on MS SQL Server and MySQL.
native: It picks identity, sequence or hilo depending upon the capabilities of the underlying database.
assigned: lets the application to assign an identifier to the object before save() is called. This is the default strategy if no element is specified.
select: retrieves a primary key assigned by a database trigger by selecting the row by some unique key and retrieving the primary key value.
foreign: uses the identifier of another associated object. Usually used in conjunction with a primary key association.

12. How is a typical spring implementation look like?

For a typical Spring Application we need the following files

1. An interface that defines the functions.
2. An Implementation that contains properties, its setter and getter methods, functions etc.,
3. A XML file called Spring configuration file.
4. Client program that uses the function.


13. How do you define hibernate mapping file in spring?

Add the hibernate mapping file entry in mapping resource inside Spring’s applicationContext.xml file in the web/WEB-INF directory.

< property name="mappingResources" >
< list >
< value > org/appfuse/model/User.hbm.xml < / value >
< / list >
< / property >


14. How do you configure spring in a web application?

It is very easy to configure any J2EE-based web application to use Spring. At the very least, you can simply add Spring’s ContextLoaderListener to your web.xml file:

< listener >
< listener-class > org.springframework.web.context.ContextLoaderListener < / listener-class >
< / listener >


15. Can you have xyz.xml file instead of applicationcontext.xml?

ContextLoaderListener is a ServletContextListener that initializes when your webapp starts up. By default, it looks for Spring’s configuration file at WEB-INF/applicationContext.xml. You can change this default value by specifying a element named “contextConfigLocation.” Example:

< listener >
< listener-class > org.springframework.web.context.ContextLoaderListener

< context-param >
< param-name > contextConfigLocation < / param-name >
< param-value > /WEB-INF/xyz.xml< / param-value >
< / context-param >

< / listener-class >
< / listener >



16. How do you configure your database driver in spring?

Using datasource "org.springframework.jdbc.datasource.DriverManagerDataSource". Example:

< bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource" >
< property name="driverClassName" >
< value > org.hsqldb.jdbcDriver < / value >
< / property >
< property name="url" >
< value > jdbc:hsqldb:db/appfuse < / value >
< / property >
< property name="username" > < value > sa < / value > < / property >
< property name="password" > < value > < / value > < / property >
< / bean >


17. How can you configure JNDI instead of datasource in spring applicationcontext.xml?

Using "org.springframework.jndi.JndiObjectFactoryBean". Example:

< bean id="dataSource" class="org.springframework.jndi.JndiObjectFactoryBean" >
< property name="jndiName" >
< value > java:comp/env/jdbc/appfuse < / value >
< / property >
< / bean >

18. What are the key benefits of Hibernate?

These are the key benifits of Hibernate:
Transparent persistence based on POJOs without byte code processing
Powerful object-oriented hibernate query language
Descriptive O/R Mapping through mapping file.
Automatic primary key generation
Hibernate cache: Session Level, Query and Second level cache.
Performance: Lazy initialization, Outer join fetching, Batch fetching

19. What is hibernate session and session factory? How do you configure sessionfactory in spring configuration file?

Hibernate Session is the main runtime interface between a Java application and Hibernate. SessionFactory allows applications to create hibernate session by reading hibernate configurations file hibernate.cfg.xml.

// Initialize the Hibernate environment
Configuration cfg = new Configuration().configure();
// Create the session factory
SessionFactory factory = cfg.buildSessionFactory();
// Obtain the new session object
Session session = factory.openSession();

The call to Configuration().configure() loads the hibernate.cfg.xml configuration file and initializes the Hibernate environment. Once the configuration is initialized, you can make any additional modifications you desire programmatically. However, you must make these modifications prior to creating the SessionFactory instance. An instance of SessionFactory is typically created once and used to create all sessions related to a given context.
The main function of the Session is to offer create, read and delete operations for instances of mapped entity classes. Instances may exist in one of three states:

transient: never persistent, not associated with any Session
persistent: associated with a unique Session
detached: previously persistent, not associated with any Session

A Hibernate Session object represents a single unit-of-work for a given data store and is opened by a SessionFactory instance. You must close Sessions when all work for a transaction is completed. The following illustrates a typical Hibernate session:
Session session = null;
UserInfo user = null;
Transaction tx = null;
try {
session = factory.openSession();
tx = session.beginTransaction();
user = (UserInfo)session.load(UserInfo.class, id);
tx.commit();
} catch(Exception e) {
if (tx != null) {
try {
tx.rollback();
} catch (HibernateException e1) {
throw new DAOException(e1.toString()); }
} throw new DAOException(e.toString());
} finally {
if (session != null) {
try {
session.close();
} catch (HibernateException e) { }
}
}

20. What is the difference between hibernate get and load methods?

The load() method is older; get() was added to Hibernate’s API due to user request. The difference is trivial:
The following Hibernate code snippet retrieves a User object from the database:
User user = (User) session.get(User.class, userID);

The get() method is special because the identifier uniquely identifies a single instance of a class. Hence it’s common for applications to use the identifier as a convenient handle to a persistent object. Retrieval by identifier can use the cache when retrieving an object, avoiding a database hit if the object is already cached.
Hibernate also provides a load() method:
User user = (User) session.load(User.class, userID);

If load() can’t find the object in the cache or database, an exception is thrown. The load() method never returns null. The get() method returns
null if the object can’t be found. The load() method may return a proxy instead of a real persistent instance. A proxy is a placeholder instance of a runtime-generated subclass (through cglib or Javassist) of a mapped persistent class, it can initialize itself if any method is called that is not the mapped database identifier getter-method. On the other hand, get() never returns a proxy. Choosing between get() and load() is easy: If you’re certain the persistent object exists, and nonexistence would be considered exceptional, load() is a good option. If you aren’t certain there is a persistent instance with the given identifier, use get() and test the return value to see if it’s null. Using load() has a further implication: The application may retrieve a valid reference (a proxy) to a persistent instance without hitting the database to retrieve its persistent state. So load() might not throw an exception when it doesn’t find the persistent object in the cache or database; the exception would be thrown later, when the proxy is accessed.

21. What type of transaction management is supported in hibernate?

Hibernate communicates with the database via a JDBC Connection; hence it must support both managed and non-managed transactions.

Non-managed in web containers:

< bean id="transactionManager" class="org.springframework.orm.hibernate.HibernateTransactionManager" >
< property name="sessionFactory" >
< ref local="sessionFactory" / >
< / property >
< / bean >

Managed in application server using JTA:

< bean id="transactionManager" class="org.springframework.transaction.jta.JtaTransactionManager." >
< property name="sessionFactory" >
< ref local="sessionFactory" / >
< / property >
< / bean >


22. What is lazy loading and how do you achieve that in hibernate?


Lazy setting decides whether to load child objects while loading the Parent Object. You need to specify parent class.Lazy = true in hibernate mapping file. By default the lazy loading of the child objects is true. This make sure that the child objects are not loaded unless they are explicitly invoked in the application by calling getChild() method on parent. In this case hibernate issues a fresh database call to load the child when getChild() is actually called on the Parent object. But in some cases you do need to load the child objects when parent is loaded. Just make the lazy=false and hibernate will load the child when parent is loaded from the database. Examples: Address child of User class can be made lazy if it is not required frequently. But you may need to load the Author object for Book parent whenever you deal with the book for online bookshop.

Hibernate does not support lazy initialization for detached objects. Access to a lazy association outside of the context of an open Hibernate session will result in an exception.

23. What are the different fetching strategies in Hibernate?

Hibernate3 defines the following fetching strategies:

Join fetching - Hibernate retrieves the associated instance or collection in the same SELECT, using an OUTER JOIN.

Select fetching - a second SELECT is used to retrieve the associated entity or collection. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only be executed when you actually access the association.

Subselect fetching - a second SELECT is used to retrieve the associated collections for all entities retrieved in a previous query or fetch. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only be executed when you actually access the association.

Batch fetching - an optimization strategy for select fetching - Hibernate retrieves a batch of entity instances or collections in a single SELECT, by specifying a list of primary keys or foreign keys.

24. What are different types of cache hibernate supports?

Caching is widely used for optimizing database applications. Hibernate uses two different caches for objects: first-level cache and second-level cache. First-level cache is associated with the Session object, while second-level cache is associated with the Session Factory object. By default, Hibernate uses first-level cache on a per-transaction basis. Hibernate uses this cache mainly to reduce the number of SQL queries it needs to generate within a given transaction. For example, if an object is modified several times within the same transaction, Hibernate will generate only one SQL UPDATE statement at the end of the transaction, containing all the modifications. To reduce database traffic, second-level cache keeps loaded objects at the Session Factory level between transactions. These objects are available to the whole application, not just to the user running the query. This way, each time a query returns an object that is already loaded in the cache, one or more database transactions potentially are avoided. In addition, you can use a query-level cache if you need to cache actual query results, rather than just persistent objects. The query cache should always be used in conjunction with the second-level cache. Hibernate supports the following open-source cache implementations out-of-the-box:

EHCache is a fast, lightweight, and easy-to-use in-process cache. It supports read-only and read/write caching, and memory and disk-based caching. However, it does not support clustering.
OSCache is another open-source caching solution. It is part of a larger package, which also provides caching functionalities for JSP pages or arbitrary objects. It is a powerful and flexible package, which, like EHCache, supports read-only and read/write caching, and memory- and disk-based caching. It also provides basic support for clustering via either JavaGroups or JMS.
SwarmCache is a simple cluster-based caching solution based on JavaGroups. It supports read-only or nonstrict read/write caching (the next section explains this term). This type of cache is appropriate for applications that typically have many more read operations than write operations.
JBoss TreeCache is a powerful replicated (synchronous or asynchronous) and transactional cache. Use this solution if you really need a true transaction-capable caching architecture.
Commercial Tangosol Coherence cache.
25. What are the different caching strategies?

The following four caching strategies are available:
Read-only: This strategy is useful for data that is read frequently but never updated. This is by far the simplest and best-performing cache strategy.
Read/write: Read/write caches may be appropriate if your data needs to be updated. They carry more overhead than read-only caches. In non-JTA environments, each transaction should be completed when Session.close() or Session.disconnect() is called.
Nonstrict read/write: This strategy does not guarantee that two transactions won't simultaneously modify the same data. Therefore, it may be most appropriate for data that is read often but only occasionally modified.
Transactional: This is a fully transactional cache that may be used only in a JTA environment.
26. How do you configure 2nd level cache in hibernate?

To activate second-level caching, you need to define the hibernate.cache.provider_class property in the hibernate.cfg.xml file as follows:
< hibernate-configuration >
< session-factory >
< property name="hibernate.cache.provider_class" >org.hibernate.cache.EHCacheProvider< / property >
< / session-factory >
< / hibernate-configuration >

By default, the second-level cache is activated and uses the EHCache provider.
To use the query cache you must first enable it by setting the property hibernate.cache.use_query_cache to true in hibernate.properties.

27. What is the difference between sorted and ordered collection in hibernate?

A sorted collection is sorted in-memory using java comparator, while order collection is ordered at the database level using order by clause.

28. What are the types of inheritance models and describe how they work like vertical inheritance and horizontal?

There are three types of inheritance mapping in hibernate:

Example: Let us take the simple example of 3 java classes. Class Manager and Worker are inherited from Employee Abstract class.
1. Table per concrete class with unions: In this case there will be 2 tables. Tables: Manager, Worker [all common attributes will be duplicated]
2. Table per class hierarchy: Single Table can be mapped to a class hierarchy. There will be only one table in database called 'Employee' that will represent all the attributes required for all 3 classes. But it needs some discriminating column to differentiate between Manager and worker;
3. Table per subclass: In this case there will be 3 tables represent Employee, Manager and Worker