Hibernate Interview Questions

Q. How will you configure Hibernate?

Answer:

The configuration files hibernate.cfg.xml (or hibernate.properties) and mapping files *.hbm.xml are used by the Configuration class to create (i.e. configure and bootstrap hibernate) the SessionFactory, which in turn creates the Session instances. Session instances are the primary interface for the persistence service.

" hibernate.cfg.xml (alternatively can use hibernate.properties): These two files are used to configure the hibernate sevice (connection driver class, connection URL, connection username, connection password, dialect etc). If both files are present in the classpath then hibernate.cfg.xml file overrides the settings found in the hibernate.properties file.

" Mapping files (*.hbm.xml): These files are used to map persistent objects to a relational database. It is the best practice to store each object in an individual mapping file (i.e mapping file per class) because storing large number of persistent classes into one mapping file can be difficult to manage and maintain. The naming convention is to use the same name as the persistent (POJO) class name. For example Account.class will have a mapping file named Account.hbm.xml. Alternatively hibernate annotations can be used as part of your persistent class code instead of the *.hbm.xml files.


Q. What is a SessionFactory? Is it a thread-safe object?

Answer:

SessionFactory is Hibernate?s concept of a single datastore and is threadsafe so that many threads can access it concurrently and request for sessions and immutable cache of compiled mappings for a single database. A SessionFactory is usually only built once at startup. SessionFactory should be wrapped in some kind of singleton so that it can be easily accessed in an application code.

SessionFactory sessionFactory = new Configuration().configure().buildSessionfactory();


Q. What is a Session? Can you share a session object between different theads?

Answer:

Session is a light weight and a non-threadsafe object (No, you cannot share it between threads) that represents a single unit-of-work with the database. Sessions are opened by a SessionFactory and then are closed when all work is complete. Session is the primary interface for the persistence service. A session obtains a database connection lazily (i.e. only when required). To avoid creating too many sessions ThreadLocal class can be used as shown below to get the current session no matter how many times you make call to the currentSession() method.

&
public class HibernateUtil {
&
public static final ThreadLocal local = new ThreadLocal();

public static Session currentSession() throws HibernateException {
Session session = (Session) local.get();
//open a new session if this thread has no session
if(session == null) {
session = sessionFactory.openSession();
local.set(session);
}
return session;
}
}

It is also vital that you close your session after your unit of work completes. Note: Keep your Hibernate Session API handy.


Q. What are the benefits of detached objects?

Answer:


Detached objects can be passed across layers all the way up to the presentation layer without having to use any DTOs (Data Transfer Objects). You can later on re-attach the detached objects to another session.

Q. What are the pros and cons of detached objects?

Answer:

Pros:

" When long transactions are required due to user think-time, it is the best practice to break the long transaction up into two or more transactions. You can use detached objects from the first transaction to carry data all the way up to the presentation layer. These detached objects get modified outside a transaction and later on re-attached to a new transaction via another session.


Cons

" In general, working with detached objects is quite cumbersome, and better to not clutter up the session with them if possible. It is better to discard them and re-fetch them on subsequent requests. This approach is not only more portable but also more efficient because - the objects hang around in Hibernate's cache anyway.

" Also from pure rich domain driven design perspective it is recommended to use DTOs (DataTransferObjects) and DOs (DomainObjects) to maintain the separation between Service and UI tiers.


Q. How does Hibernate distinguish between transient (i.e. newly instantiated) and detached objects?

Answer

" Hibernate uses the ?version? property, if there is one.
" If not uses the identifier value. No identifier value means a new object. This does work only for Hibernate managed surrogate keys. Does not work for natural keys and assigned (i.e. not managed by Hibernate) surrogate keys.
" Write your own strategy with Interceptor.isUnsaved().

Q. What is the difference between the session.get() method and the session.load() method?

Both the session.get(..) and session.load() methods create a persistent object by loading the required object from the database. But if there was not such object in the database then the method session.load(..) throws an exception whereas session.get(&) returns null.


Q. What is the difference between the session.update() method and the session.lock() method?

Both of these methods and saveOrUpdate() method are intended for reattaching a detached object. The session.lock() method simply reattaches the object to the session without checking or updating the database on the assumption that the database in sync with the detached object. It is the best practice to use either session.update(..) or session.saveOrUpdate(). Use session.lock() only if you are absolutely sure that the detached object is in sync with your detached object or if it does not matter because you will be overwriting all the columns that would have changed later on within the same transaction.

Note: When you reattach detached objects you need to make sure that the dependent objects are reatched as well.

Q. How would you reatach detached objects to a session when the same object has already been loaded into the session?

You can use the session.merge() method call.


Q. What are the general considerations or best practices for defining your Hibernate persistent classes?


1.You must have a default no-argument constructor for your persistent classes and there should be getXXX() (i.e accessor/getter) and setXXX( i.e. mutator/setter) methods for all your persistable instance variables.

2.You should implement the equals() and hashCode() methods based on your business key and it is important not to use the id field in your equals() and hashCode() definition if the id field is a surrogate key (i.e. Hibernate managed identifier). This is because the Hibernate only generates and sets the field when saving the object.


3. It is recommended to implement the Serializable interface. This is potentially useful if you want to migrate around a multi-processor cluster.

4.The persistent class should not be final because if it is final then lazy loading cannot be used by creating proxy objects.

5.Use XDoclet tags for generating your *.hbm.xml files or Annotations (JDK 1.5 onwards), which are less verbose than *.hbm.xml files.

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

Integer count = (Integer) session.createQuery("select count(*) from ....").uniqueResult();

How can I find the size of a collection without initializing it?

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

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)

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

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

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()

Are collections pageable?

Query q = s.createFilter( collection, "" ); // the trivial filter

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 bugprone. 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 <idbag> 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.

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();

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();

Can I map an inner class?

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

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

Use a UserType.

How can I trucate String data?

Use a UserType.

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

Use a UserType.

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

Use a UserType.

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.

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);

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.

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.

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

Follow the best practices guide! Ensure that all <class> and <collection> 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.

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

Specify custom SQL INSERT (and UPDATE) statements using <sql-insert> and <sql-update> 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.

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 <sql-insert>, <sql-update>, <sql-delete> and <loader> 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.