What is differnece Between update n merg?
Both update() and merge() methods in hibernate are used to convert the object which is in detached state into persistence state. But there is little difference. Let us see which method will be used in what situation.
Let Us Take An Example
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| -----------SessionFactory factory = cfg.buildSessionFactory();Session session1 = factory.openSession();Student s1 = null;Object o = session1.get(Student.class, new Integer(101));s1 = (Student)o;session1.close();s1.setMarks(97);Session session2 = factory.openSession();Student s2 = null;Object o1 = session2.get(Student.class, new Integer(101));s2 = (Student)o1;Transaction tx=session2.beginTransaction();session2.merge(s1); |
Explanation
- See from line numbers 6 – 9, we just loaded one object s1 into session1 cache and closed session1 at line number 9, so now object s1 in the session1 cache will be destroyed as session1 cache will expires when ever we say session1.close()
- Now s1 object will be in some RAM location, not in the session1 cache
- Hear s1 is in detached state, and at line number 11 we modified that detached object s1, now if we call update() method then hibernate will throws an error, because we can update the object in the session only
- So we opened another session [session2] at line number 13, and again loaded the same student object from the database, but with name s2
- so in this session2, we called session2.merge(s1); now into s2 object s1 changes will be merged and saved into the database
What is Hibernate:
Hibernate is a powerful, high performance object/relational persistence and query service. This lets the users to develop persistent classes following object-oriented principles such as association, inheritance, polymorphism, composition, and collections
Hibernate Important Interview Question with Answers:
Q.1 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. 2 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.3 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.4 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.5 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.6 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().
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. 2 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.3 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.4 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.5 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.6 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.7 What is ORM ?
ORM stands for Object/Relational mapping. It is the programmed and translucent perseverance of objects in a Java application
in to the tables of a relational database using the metadata that
describes the mapping between the objects and the database. It works by
transforming the data from one representation to another.
Q.8 What are the different levels of ORM quality? There are four levels defined for ORM quality.
� Pure relational
� Light object mapping
� Medium object mapping
� Full object mapping
Q.9 How will you configure Hibernate?
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.
" 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.10 What are derived properties?
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.
Q.11 Define HibernateTemplate?
org.springframework.orm.hibernate.HibernateTemplate
is a helper class which provides different methods for
querying/retrieving data from the database. It also converts checked
HibernateExceptions into unchecked DataAccessExceptions.
Q.12 What are the benefits does HibernateTemplate provide?
The benefits of HibernateTemplate are :
- HibernateTemplate, a Spring Template class simplifies interactions with Hibernate Session.
- Common functions are simplified to single method calls.
- Sessions are automatically closed.
- Exceptions are automatically caught and converted to runtime exceptions.
Q.13 What is the difference between sorted and ordered collection in hibernate?
sorted collection vs. order collection :-
sorted collection
|
order collection
|
A
sorted collection is sorting a collection by utilizing the sorting
features provided by the Java collections framework. The sorting occurs
in the memory of JVM which running Hibernate, after the data being read
from database using java comparator.
|
Order collection is sorting a collection by specifying the order-by clause for sorting this collection when retrieval.
|
If your collection is not large, it will be more efficient way to sort it.
|
If your collection is very large, it will be more efficient way to sort it .
|
Q.14 What are the Collection types in Hibernate ?
- Bag
- Set
- List
- Array
- Map
Q.15 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 invokedQ.15 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.
Example :
name="com.test.User" table="user"> column="USER_NAME" length="255" name="userName" not-null="true" type="java.lang.String"/> column="USER_PASSWORD" length="255" name="userPassword" not-null="true" type="java.lang.String"/>
Q.16 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();
Q.17 What’s the difference between load() and get()?
load() vs. 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. |
Q.18 What is the difference between and merge and update ?
Use
update() if you are sure that the session does not contain an already persistent instance with the same identifier, and merge() if you want to merge your modifications at any time without consideration of the state of the session.Q19 How do you define sequence generated primary key in hibernate?
Using tag.
Example:-
Example:-
column="USER_ID" name="id" type="java.lang.Long">
class="sequence">
name="table">SEQUENCE_NAME Q21. What do you mean by Named – SQL query?
Named SQL queries are defined in the mapping xml document and called wherever required.
Example:
Example:
name = "empdetails">
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
Invoke Named Query :
List people = session.getNamedQuery("empdetails") .setString("TomBrady", name) .setMaxResults(50) .list();
Q.22 How do you invoke Stored Procedures?
name="selectAllEmployees_SP" callable="true">
alias="emp" class="employee">
name="empid" column="EMP_ID"/>
name="address" column="EMP_ADDRESS"/>
{ ? = call selectAllEmployees() }
Q.23 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.
Example :
Example :
List employees = session.createCriteria(Employee.class) .add(Restrictions.like("name", "a%") ) .add(Restrictions.like("address", "Boston")) .addOrder(Order.asc("name") ) .list();
Q.24 Define HibernateTemplate?
org.springframework.orm.hibernate.HibernateTemplate
is a helper class which provides different methods for
querying/retrieving data from the database. It also converts checked
HibernateExceptions into unchecked DataAccessExceptions.Q.25 What are the benefits does HibernateTemplate provide?
The benefits of HibernateTemplate are :
HibernateTemplate, a Spring Template class simplifies interactions with Hibernate Session.- Common functions are simplified to single method calls.
- Sessions are automatically closed.
- Exceptions are automatically caught and converted to runtime exceptions
Hibernate Vs. 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
the Core interfaces are of hibernate framework? - See more at:
http://www.fresherventure.net/frequently-asked-j2ee-interview-questions-and-answers/#sthash.oc5o7D4O.dpuf
What
the Core interfaces are of hibernate framework? - See more at:
http://www.fresherventure.net/frequently-asked-j2ee-interview-questions-and-answers/#sthash.oc5o7D4O.dpuf
What
the Core interfaces are of hibernate framework? - See more at:
http://www.fresherventure.net/frequently-asked-j2ee-interview-questions-and-answers/#sthash.oc5o7D4O.dpuf
What
the Core interfaces are of hibernate framework? - See more at:
http://www.fresherventure.net/frequently-asked-j2ee-interview-questions-and-answers/#sthash.oc5o7D4O.dpuf
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