Гибернейт – один из наиболее широко используемых инструментов ORM для приложений на Java. Он часто используется в корпоративных приложениях для операций с базами данных. Поэтому я решил написать пост о вопросах на собеседовании по Hibernate, чтобы освежить ваши знания перед интервью. Будь то новичок или опытный специалист, хорошее знание инструмента Hibernate ORM поможет вам успешно пройти интервью. Здесь я предоставляю важные вопросы на собеседование по Hibernate с ответами, чтобы помочь вам освежить знания и произвести впечатление на вашего интервьюера. Как и в других постах с вопросами на собеседование, есть вероятность, что я буду добавлять больше вопросов в этот список в будущем, поэтому вам может захотеться добавить эту страницу в закладки для будущего использования. 
Недавно я написал много постов о Hibernate, большинство из них содержат полные проекты для скачивания. Я буду предоставлять ссылки на них по мере необходимости, и вы сможете ознакомиться с ними, чтобы обновить свои знания.
Вопросы на собеседование по Hibernate
- Что такое фреймворк Hibernate?
- Что такое Java Persistence API (JPA)?
- Какие важные преимущества использования фреймворка Hibernate?
- Каковы преимущества Hibernate по сравнению с JDBC?
- Назовите некоторые важные интерфейсы фреймворка Hibernate?
- Что такое конфигурационный файл Hibernate?
- Что такое файл отображения Hibernate?
- Назовите некоторые важные аннотации, используемые для отображения в Hibernate?
- Что такое фабрика сессий Hibernate и как её настроить?
- Фабрика сессий Hibernate является потокобезопасной?
- Что такое сессия Hibernate и как её получить?
- Сессия Hibernate является потокобезопасной?
- Какова разница между openSession и getCurrentSession в Hibernate?
- В чем разница между методами get() и load() сессии Hibernate?
- Что такое кэширование в Hibernate? Объясните кэш первого уровня Hibernate?
- Как настроить вторичный уровень кэша Hibernate с использованием EHCache?
- Какие различные состояния сущности в бине?
- Для чего используется вызов merge() в сессии Hibernate?
- В чем разница между методами save(), saveOrUpdate() и persist() в Hibernate?
- Что произойдет, если у нас нет конструктора без аргументов в сущности бина?
- В чем разница между отсортированной коллекцией и упорядоченной коллекцией, какая из них лучше?
- Какие типы коллекций существуют в Hibernate?
- Как реализовать соединения в Hibernate?
- Почему мы не должны делать класс Entity финальным?
- Что такое HQL и в чем ее преимущества?
- Что такое кэш запросов в Hibernate?
- Можем ли мы выполнять запросы на языке SQL в Hibernate?
- Какова польза поддержки запросов на языке SQL в Hibernate?
- Что такое именованный SQL-запрос?
- Каковы преимущества именованного SQL-запроса?
- Какова польза API критериев Hibernate?
- Как вести журнал запросов на языке SQL, созданных Hibernate, в журнальных файлах?
- Что такое Hibernate Proxy и как он помогает в ленивой загрузке?
- Как реализовать отношения в Hibernate?
- Как работает управление транзакциями в Hibernate?
- Что такое каскадирование и какие существуют различные типы каскадирования?
- Как интегрировать логирование log4j в приложение на Hibernate?
- Как использовать источник данных JNDI сервера приложений с фреймворком Hibernate?
- Как интегрировать фреймворки Hibernate и Spring?
- Что такое класс HibernateTemplate?
- Как интегрировать Hibernate с веб-приложениями на Servlet или Struts2?
- Какие шаблоны проектирования используются в фреймворке Hibernate?
- Какие лучшие практики следует соблюдать при использовании фреймворка Hibernate?
- Что такое фреймворк Hibernate Validator?
- Какова польза от плагина Hibernate Tools для Eclipse?
Вопросы и ответы на интервью по Hibernate
-
Что такое фреймворк Hibernate?
Отображение объектов в реляционные структуры, или ORM, является программной техникой отображения объектов модели предметной области приложения на таблицы реляционных баз данных. Hibernate – это инструмент ORM на языке Java, который предоставляет фреймворк для отображения объектов модели предметной области приложения на таблицы реляционной базы данных и наоборот. Hibernate предоставляет эталонную реализацию Java Persistence API, что делает его отличным выбором в качестве инструмента ORM с преимуществами слабой связности. Мы можем использовать API постоянства Hibernate для операций CRUD. Фреймворк Hibernate предоставляет возможность отображать обычные объекты Java на традиционные таблицы баз данных с использованием аннотаций JPA, а также конфигурации на основе XML. Аналогично конфигурации Hibernate гибки и могут выполняться из XML-файла конфигурации, а также программно. Для быстрого обзора использования фреймворка Hibernate вы можете ознакомиться с Руководством для начинающих по Hibernate.
-
Что такое Java Persistence API (JPA)?
Java Persistence API (JPA) предоставляет спецификацию для управления реляционными данными в приложениях. Текущая версия JPA 2.1 была запущена в июле 2011 года как JSR 338. JPA 2.1 была утверждена в качестве окончательной 22 мая 2013 года. Спецификации JPA определены с помощью аннотаций в пакете javax.persistence. Использование аннотаций JPA помогает нам писать код, независимый от реализации.
-
Каковы важные преимущества использования фреймворка Hibernate?
Некоторые из важных преимуществ использования фреймворка Hibernate:
- Hibernate устраняет всю шаблонную кодировку, которая сопутствует JDBC, и заботится о управлении ресурсами, поэтому мы можем сосредоточиться на бизнес-логике.
- Фреймворк Hibernate поддерживает XML, а также аннотации JPA, что делает нашу реализацию кода независимой.
- Hibernate предоставляет мощный язык запросов (HQL), который похож на SQL. Однако HQL полностью объектно-ориентирован и понимает концепции наследования, полиморфизма и ассоциаций.
- Hibernate является проектом с открытым исходным кодом от сообщества Red Hat и используется по всему миру. Это делает его лучшим выбором, чем другие, потому что кривая обучения небольшая, и есть масса онлайн-документации, а помощь легко доступна на форумах.
- Hibernate легко интегрируется с другими фреймворками Java EE, он настолько популярен, что фреймворк Spring предоставляет встроенную поддержку для интеграции Hibernate с приложениями Spring.
- Hibernate поддерживает отложенную инициализацию с использованием прокси-объектов и выполняет фактические запросы к базе данных только при необходимости.
- Кеш Hibernate помогает нам достичь лучшей производительности.
- Для функций, специфичных для поставщика базы данных, Hibernate подходит, потому что мы также можем выполнять запросы к базе данных на языке SQL. В целом, Hibernate – лучший выбор на текущем рынке для инструмента ORM, он содержит все функции, которые вам когда-либо понадобятся в инструменте ORM.
-
Каковы преимущества Hibernate перед JDBC?
Некоторые из важных преимуществ фреймворка Hibernate перед JDBC:
- Hibernate убирает много шаблонного кода, который присутствует в JDBC API, код выглядит более чистым и читаемым.
- Hibernate поддерживает наследование, ассоциации и коллекции. Эти функции отсутствуют в JDBC API.
- Hibernate неявно обеспечивает управление транзакциями, фактически большинство запросов не могут быть выполнены вне транзакции. В JDBC API мы должны писать код для управления транзакциями, используя команды commit и rollback. Читать больше на JDBC Управление транзакциями.
- JDBC API генерирует исключение
SQLException, которое является проверяемым исключением, поэтому нам нужно писать много блоков try-catch. Большинство времени это избыточно в каждом вызове JDBC и используется для управления транзакциями. Hibernate оборачивает исключения JDBC и выбрасывает непроверяемое исключениеJDBCExceptionилиHibernateException, поэтому нам не нужно писать код для его обработки. Встроенное управление транзакциями Hibernate устраняет необходимость в блоках try-catch. - Язык запросов Hibernate (HQL) более объектно-ориентированный и близок к языку программирования Java. Для JDBC нам нужно писать запросы на языке SQL.
- Hibernate поддерживает кэширование, что лучше для производительности, запросы JDBC не кэшируются, следовательно, производительность низкая.
- Hibernate предоставляет возможность создавать таблицы базы данных, для JDBC таблицы должны существовать в базе данных.
- Конфигурация Hibernate помогает нам использовать JDBC-подобное подключение, а также JNDI DataSource для пула подключений. Это очень важная функция в корпоративных приложениях и полностью отсутствует в JDBC API.
- Hibernate поддерживает аннотации JPA, поэтому код не зависит от реализации и легко заменяется другими инструментами ORM. Код JDBC очень плотно связан с приложением.
-
Назовите некоторые важные интерфейсы фреймворка Hibernate?
Some of the important interfaces of Hibernate framework are:
1. **SessionFactory (org.hibernate.SessionFactory)**: SessionFactory is an immutable thread-safe cache of compiled mappings for a single database. We need to initialize SessionFactory once and then we can cache and reuse it. SessionFactory instance is used to get the Session objects for database operations.
2. **Session (org.hibernate.Session)**: Session is a single-threaded, short-lived object representing a conversation between the application and the persistent store. It wraps JDBC `java.sql.Connection` and works as a factory for `org.hibernate.Transaction`. We should open session only when it's required and close it as soon as we are done using it. Session object is the interface between java application code and hibernate framework and provide methods for CRUD operations.
3. **Transaction (org.hibernate.Transaction)**: Transaction is a single-threaded, short-lived object used by the application to specify atomic units of work. It abstracts the application from the underlying JDBC or JTA transaction. A org.hibernate.Session might span multiple org.hibernate.Transaction in some cases.
Hibernate configuration file contains database specific configurations and used to initialize SessionFactory. We provide database credentials or JNDI resource information in the hibernate configuration xml file. Some other important parts of hibernate configuration file is Dialect information, so that hibernate knows the database type and mapping file or class details.
Hibernate mapping file is used to define the entity bean fields and database table column mappings. We know that JPA annotations can be used for mapping but sometimes XML mapping file comes handy when we are using third party classes and we can't use annotations.
Hibernate supports JPA annotations and it has some other annotations in `org.hibernate.annotations` package. Some of the important JPA and hibernate annotations used are:
1. **javax.persistence.Entity**: Used with model classes to specify that they are entity beans.
2. **javax.persistence.Table**: Used with entity beans to define the corresponding table name in database.
3. **javax.persistence.Access**: Used to define the access type, either field or property. Default value is field and if you want hibernate to use getter/setter methods then you need to set it to property.
4. **javax.persistence.Id**: Used to define the primary key in the entity bean.
5. **javax.persistence.EmbeddedId**: Used to define composite primary key in the entity bean.
6. **javax.persistence.Column**: Used to define the column name in database table.
7. **javax.persistence.GeneratedValue**: Used to define the strategy to be used for generation of primary key. Used in conjunction with `javax.persistence.GenerationType` enum.
8. **javax.persistence.OneToOne**: Used to define the one-to-one mapping between two entity beans. We have other similar annotations as `OneToMany`, `ManyToOne` and `ManyToMany`
9. **org.hibernate.annotations.Cascade**: Used to define the cascading between two entity beans, used with mappings. It works in conjunction with `org.hibernate.annotations.CascadeType`
10. **javax.persistence.PrimaryKeyJoinColumn**: Used to define the property for foreign key. Used with `org.hibernate.annotations.GenericGenerator` and `org.hibernate.annotations.Parameter`
Here are two classes showing usage of these annotations.
```
package com.journaldev.hibernate.model;
import javax.persistence.Access;
import javax.persistence.AccessType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.Table;
import org.hibernate.annotations.Cascade;
@Entity
@Table(name = "EMPLOYEE")
@Access(value=AccessType.FIELD)
public class Employee {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "emp_id")
private long id;
@Column(name = "emp_name")
private String name;
@OneToOne(mappedBy = "employee")
@Cascade(value = org.hibernate.annotations.CascadeType.ALL)
private Address address;
//методы getter и setter
}
```
```
package com.journaldev.hibernate.model;
import javax.persistence.Access;
import javax.persistence.AccessType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.PrimaryKeyJoinColumn;
import javax.persistence.Table;
import org.hibernate.annotations.GenericGenerator;
import org.hibernate.annotations.Parameter;
@Entity
@Table(name = "ADDRESS")
@Access(value=AccessType.FIELD)
public class Address {
@Id
@Column(name = "emp_id", unique = true, nullable = false)
@GeneratedValue(generator = "gen")
@GenericGenerator(name = "gen", strategy = "foreign", parameters = { @Parameter(name = "property", value = "employee") })
private long id;
@Column(name = "address_line1")
private String addressLine1;
@OneToOne
@PrimaryKeyJoinColumn
private Employee employee;
//методы getter и setter
}
```
SessionFactory is the factory class used to get the Session objects. SessionFactory is responsible to read the hibernate configuration parameters and connect to the database and provide Session objects. Usually an application has a single SessionFactory instance and threads servicing client requests obtain Session instances from this factory. The internal state of a SessionFactory is immutable. Once it is created this internal state is set. This internal state includes all of the metadata about Object/Relational Mapping. SessionFactory also provide methods to get the Class metadata and Statistics instance to get the stats of query executions, second level cache details etc.
Internal state of SessionFactory is immutable, so it's thread safe. Multiple threads can access it simultaneously to get Session instances.
Hibernate Session is the interface between java application layer and hibernate. This is the core interface used to perform database operations. Lifecycle of a session is bound by the beginning and end of a transaction. Session provide methods to perform create, read, update and delete operations for a persistent object. We can execute HQL queries, SQL native queries and create criteria using Session object.
Hibernate Session object is not thread safe, every thread should get it's own session instance and close it after it's work is finished.
Hibernate SessionFactory getCurrentSession() method returns the session bound to the context. But for this to work, we need to configure it in hibernate configuration file. Since this session object belongs to the hibernate context, we don't need to close it. Once the session factory is closed, this session object gets closed.
```
<property name="hibernate.current_session_context_class">thread</property>
```
Hibernate SessionFactory openSession() method always opens a new session. We should close this session object once we are done with all the database operations. We should open a new session for each request in multi-threaded environment. There is another method openStatelessSession() that returns stateless session, for more details with examples please read [Hibernate openSession vs getCurrentSession](/community/tutorials/hibernate-sessionfactory).
Hibernate session comes with different methods to load data from database. get and load are most used methods, at first look they seems similar but there are some differences between them.
1. get() loads the data as soon as it's called whereas load() returns a proxy object and loads data only when it's actually required, so load() is better because it support lazy loading.
2. Since load() throws exception when data is not found, we should use it only when we know data exists.
3. We should use get() when we want to make sure data exists in the database.
For clarification regarding the differences, please read [Hibernate get vs load](/community/tutorials/hibernate-session-get-vs-load-difference-with-examples).
As the name suggests, hibernate caches query data to make our application faster. Hibernate Cache can be very useful in gaining fast application performance if used correctly. The idea behind cache is to reduce the number of database queries, hence reducing the throughput time of the application. Hibernate first level cache is associated with the Session object. Hibernate first level cache is enabled by default and there is no way to disable it. However hibernate provides methods through which we can delete selected objects from the cache or clear the cache completely. Any object cached in a session will not be visible to other sessions and when the session is closed, all the cached objects will also be lost. For better explanation, please read [Hibernate First Level Cache](/community/tutorials/hibernate-caching-first-level-cache).
EHCache is the best choice for utilizing hibernate second level cache. Following steps are required to enable EHCache in hibernate application.
- Add hibernate-ehcache dependency in your maven project, if it's not maven then add corresponding jars.
```
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-ehcache</artifactId>
<version>4.3.5.Final</version>
</dependency>
```
- Add below properties in hibernate configuration file.
```
<property name="hibernate.cache.region.factory_class">org.hibernate.cache.ehcache.EhCacheRegionFactory</property>
<!-- For singleton factory -->
<!-- <property name="hibernate.cache.region.factory_class">org.hibernate.cache.ehcache.SingletonEhCacheRegionFactory</property>
-->
<!-- enable second level cache and query cache -->
<property name="hibernate.cache.use_second_level_cache">true</property>
<property name="hibernate.cache.use_query_cache">true</property>
<property name="net.sf.ehcache.configurationResourceName">/myehcache.xml</property>
```
- Create EHCache configuration file, a sample file myehcache.xml would look like below.
```
<?xml version="1.0" encoding="UTF-8"?>
<ehcache xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="ehcache.xsd" updateCheck="true"
monitoring="autodetect" dynamicConfig="true">
<diskStore path="java.io.tmpdir/ehcache" />
<defaultCache maxEntriesLocalHeap="10000" eternal="false"
timeToIdleSeconds="120" timeToLiveSeconds="120" diskSpoolBufferSizeMB="30"
maxEntriesLocalDisk="10000000" diskExpiryThreadIntervalSeconds="120"
memoryStoreEvictionPolicy="LRU" statistics="true">
<persistence strategy="localTempSwap" />
</defaultCache>
<cache name="employee" maxEntriesLocalHeap="10000" eternal="false"
timeToIdleSeconds="5" timeToLiveSeconds="10">
<persistence strategy="localTempSwap" />
</cache>
<cache name="org.hibernate.cache.internal.StandardQueryCache"
maxEntriesLocalHeap="5" eternal="false" timeToLiveSeconds="120">
<persistence strategy="localTempSwap" />
</cache>
<cache name="org.hibernate.cache.spi.UpdateTimestampsCache"
maxEntriesLocalHeap="5000" eternal="true">
<persistence strategy="localTempSwap" />
</cache>
</ehcache>
```
- Annotate entity beans with @Cache annotation and caching strategy to use. For example,
```
import org.hibernate.annotations.Cache;
import org.hibernate.annotations.CacheConcurrencyStrategy;
@Entity
@Table(name = "ADDRESS")
@Cache(usage=CacheConcurrencyStrategy.READ_ONLY, region="employee")
public class Address {
}
```
That's it, we are done. Hibernate will use the EHCache for second level caching, read [Hibernate EHCache Example](/community/tutorials/hibernate-ehcache-hibernate-second-level-cache) for a complete example with explanation.
An entity bean instance can exist is one of the three states.
1. **Transient**: When an object is never persisted or associated with any session, it's in transient state. Transient instances may be made persistent by calling save(), persist() or saveOrUpdate(). Persistent instances may be made transient by calling delete().
2. **Persistent**: When an object is associated with a unique session, it's in persistent state. Any instance returned by a get() or load() method is persistent.
3. **Detached**: When an object is previously persistent but not associated with any session, it's in detached state. Detached instances may be made persistent by calling update(), saveOrUpdate(), lock() or replicate(). The state of a transient or detached instance may also be made persistent as a new persistent instance by calling merge().
Hibernate merge can be used to update existing values, however this method create a copy from the passed entity object and return it. The returned object is part of persistent context and tracked for any changes, passed object is not tracked. For example program, read [Hibernate merge](/community/tutorials/hibernate-session-merge-vs-update-save-saveorupdate-persist-example).
Hibernate save can be used to save entity to database. Problem with save() is that it can be invoked without a transaction and if we have mapping entities, then only the primary object gets saved causing data inconsistencies. Also save returns the generated id immediately. Hibernate persist is similar to save with transaction. I feel it's better than save because we can't use it outside the boundary of transaction, so all the object mappings are preserved. Also persist doesn't return the generated id immediately, so data persistence happens when needed. Hibernate saveOrUpdate results into insert or update queries based on the provided data. If the data is present in the database, update query is executed. We can use saveOrUpdate() without transaction also, but again you will face the issues with mapped objects not getting saved if session is not flushed. For example usage of these methods, read [Hibernate save vs persist](/community/tutorials/hibernate-session-merge-vs-update-save-saveorupdate-persist-example).
Hibernate uses [Reflection API](/community/tutorials/java-reflection-example-tutorial) to create instance of Entity beans, usually when you call get() or load() methods. The method `Class.newInstance()` is used for this and it requires no-args constructor. So if you won't have no-args constructor in entity beans, hibernate will fail to instantiate it and you will get `HibernateException`.
When we use Collection API sorting algorithms to sort a collection, it's called sorted list. For small collections, it's not much of an overhead but for larger collections it can lead to slow performance and OutOfMemory errors. Also the entity beans should implement `Comparable` or `Comparator` interface for it to work, read more at [java object list sorting](/community/tutorials/comparable-and-comparator-in-java-example). If we are using Hibernate framework to load collection data from database, we can use it's Criteria API to use "order by" clause to get ordered list. Below code snippet shows you how to get it.
```
List<Employee> empList = session.createCriteria(Employee.class)
.addOrder(Order.desc("id")).list();
```
Ordered list is better than sorted list because the actual sorting is done at database level, that is fast and doesn't cause memory issues.
There are five collection types in hibernate used for one-to-many relationship mappings.
1. Bag
2. Set
3. List
4. Array
5. Map
There are various ways to implement joins in hibernate.
- Using associations such as one-to-one, one-to-many etc.
- Using JOIN in the HQL query. There is another form "join fetch" to load associated data simultaneously, no lazy loading.
- We can fire native sql query and use join keyword.
Hibernate use proxy classes for lazy loading of data, only when it's needed. This is done by extending the entity bean, if the entity bean will be final then lazy loading will not be possible, hence low performance.
Hibernate Framework comes with a powerful object-oriented query language – Hibernate Query Language (HQL). It’s very similar to SQL except that we use Objects instead of table names, that makes it more close to object oriented programming. Hibernate query language is case-insensitive except for java class and variable names. So SeLeCT is the same as sELEct is the same as SELECT, but com.journaldev.model.Employee is not same as com.journaldev.model.EMPLOYEE. The HQL queries are cached but we should avoid it as much as possible, otherwise we will have to take care of associations. However it's a better choice than native sql query because of Object-Oriented approach. Read more at [HQL Example](/community/tutorials/hibernate-query-language-hql-example-tutorial).
Hibernate implements a cache region for queries resultset that integrates closely with the hibernate second-level cache. This is an optional feature and requires additional steps in code. This is only useful for queries that are run frequently with the same parameters. First of all we need to configure below property in hibernate configuration file.
```
<property name="hibernate.cache.use_query_cache">true</property>
```
And in code, we need to use setCacheable(true) method of Query, quick example looks like below.
```
Query query = session.createQuery("from Employee");
query.setCacheable(true);
query.setCacheRegion("ALL_EMP");
```
Hibernate provide option to execute native SQL queries through the use of `SQLQuery` object. For normal scenarios, it is however not the recommended approach because we loose benefits related to hibernate association and hibernate first level caching. Read more at [Hibernate Native SQL Query Example](/community/tutorials/hibernate-native-sql-query-example).
Native SQL Query comes handy when we want to execute database specific queries that are not supported by Hibernate API such as query hints or the CONNECT keyword in Oracle Database.
Hibernate provides Named Query that we can define at a central location and use them anywhere in the code. We can created named queries for both HQL and Native SQL. Hibernate Named Queries can be defined in Hibernate mapping files or through the use of JPA annotations @NamedQuery and @NamedNativeQuery.
Hibernate Named Query helps us in grouping queries at a central location rather than letting them scattered all over the code. Hibernate Named Query syntax is checked when the hibernate session factory is created, thus making the application fail fast in case of any error in the named queries. Hibernate Named Query is global, means once defined it can be used throughout the application. However one of the major disadvantage of Named query is that it’s hard to debug, because we need to find out the location where it’s defined.
Hibernate provides Criteria API that is more object oriented for querying the database and getting results. We can’t use Criteria to run update or delete queries or any DDL statements. It’s only used to fetch the results from the database using more object oriented approach. Some of the common usage of Criteria API are:
- Criteria API provides Projection that we can use for aggregate functions such as sum(), min(), max() etc.
- Criteria API can be used with ProjectionList to fetch selected columns only.
- Criteria API can be used for join queries by joining multiple tables, useful methods are createAlias(), setFetchMode() and setProjection()
- Criteria API can be used for fetching results with conditions, useful methods are add() where we can add Restrictions.
- Criteria API provides addOrder() method that we can use for ordering the results.
Learn some quick examples at [Hibernate Criteria Example](/community/tutorials/hibernate-criteria-example-tutorial).
We can set below property for hibernate configuration to log SQL queries.
```
<property name="hibernate.show_sql">true</property>
```
However we should use it only in Development or Testing environment and turn it off in production environment.
Hibernate uses proxy object to support lazy loading. Basically when you load data from tables, hibernate doesn't load all the mapped objects. As soon as you reference a child or lookup object via getter methods, if the linked entity is not in the session cache, then the proxy code will go to the database and load the linked object. It uses javassist to effectively and dynamically generate sub-classed implementations of your entity objects.
We can easily implement one-to-one, one-to-many and many-to-many relationships in hibernate. It can be done using JPA annotations as well as XML based configurations. For better understanding, you should go through following tutorials.
1. [Hibernate One to One Mapping](/community/tutorials/hibernate-one-to-one-mapping-example-annotation)
2. [Hibernate One to Many Mapping](/community/tutorials/hibernate-one-to-many-mapping-annotation)
3. [Hibernate Many to Many Mapping](/community/tutorials/hibernate-many-to-many-mapping-join-tables)
Transaction management is very easy in hibernate because most of the operations are not permitted outside of a transaction. So after getting the session from SessionFactory, we can call session `beginTransaction()` to start the transaction. This method returns the Transaction reference that we can use later on to either commit or rollback the transaction. Overall hibernate transaction management is better than JDBC transaction management because we don't need to rely on exceptions for rollback. Any exception thrown by session methods automatically rollback the transaction.
When we have relationship between entities, then we need to define how the different operations will affect the other entity. This is done by cascading and there are different types of it. Here is a simple example of applying cascading between primary and secondary entities.
```
import org.hibernate.annotations.Cascade;
@Entity
@Table(name = "EMPLOYEE")
public class Employee {
@OneToOne(mappedBy = "employee")
@Cascade(value = org.hibernate.annotations.CascadeType.ALL)
private Address address;
}
```
Note that Hibernate CascadeType enum constants are little bit different from JPA `javax.persistence.CascadeType`, so we need to use the Hibernate CascadeType and Cascade annotations for mappings, as shown in above example. Commonly used cascading types as defined in CascadeType enum are:
1. None: No Cascading, it's not a type but when we don't define any cascading then no operations in parent affects the child.
2. ALL: Cascades save, delete, update, evict, lock, replicate, merge, persist. Basically everything
3. SAVE\_UPDATE: Cascades save and update, available only in hibernate.
4. DELETE: Corresponds to the Hibernate native DELETE action, only in hibernate.
5. DETATCH, MERGE, PERSIST, REFRESH and REMOVE - for similar operations
6. LOCK: Corresponds to the Hibernate native LOCK action.
7. REPLICATE: Corresponds to the Hibernate native REPLICATE action.
Hibernate 4 uses JBoss logging rather than slf4j used in earlier versions. For log4j configuration, we need to follow below steps.
- Add log4j dependencies for maven project, if not maven then add corresponding jar files.
- Create log4j.xml configuration file or log4j.properties file and keep it in the classpath. You can keep file name whatever you want because we will load it in next step.
- For standalone projects, use static block to configure log4j using `DOMConfigurator` or `PropertyConfigurator`. For web applications, you can use ServletContextListener to configure it.
That's it, our setup is ready. Create `org.apache.log4j.Logger` instance in the java classes and start logging. For complete example code, you should go through [Hibernate log4j example](/community/tutorials/hibernate-log4j-logging) and [Servlet log4j example](/community/tutorials/servlet-jdbc-database-connection-example).
For web applications, it's always best to allow servlet container to manage the connection pool. That's why we define JNDI resource for DataSource and we can use it in the web application. It's very easy to use in Hibernate, all we need is to remove all the database specific properties and use below property to provide the JNDI DataSource name.
```
<property name="hibernate.connection.datasource">java:comp/env/jdbc/MyLocalDB</property>
```
For a complete example, go through [Hibernate JNDI DataSource Example](/community/tutorials/hibernate-tomcat-jndi-datasource-example-tutorial).
Spring is one of the most used Java EE Framework and Hibernate is the most popular ORM framework. That’s why Spring Hibernate combination is used a lot in enterprise applications. The best part with using Spring is that it provides out-of-box integration support for Hibernate with **Spring ORM** module. Following steps are required to integrate Spring and Hibernate frameworks together.
1. Add hibernate-entitymanager, hibernate-core and spring-orm dependencies.
2. Create Model classes and corresponding DAO implementations for database operations. Note that DAO classes will use SessionFactory that will be injected by Spring Bean configuration.
3. If you are using Hibernate 3, you need to configure `org.springframework.orm.hibernate3.LocalSessionFactoryBean` or `org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean` in Spring Bean configuration file. For Hibernate 4, there is single class `org.springframework.orm.hibernate4.LocalSessionFactoryBean` that should be configured.
4. Note that we don't need to use Hibernate Transaction Management, we can leave it to Spring declarative transaction management using `@Transactional` annotation.
For complete example go through [Spring Hibernate Integration](/community/tutorials/spring-hibernate-integration-example-tutorial) and [Spring MVC Hibernate Integration](/community/tutorials/spring-mvc-hibernate-mysql-integration-crud-example-tutorial).
When Spring and Hibernate integration started, Spring ORM provided two helper classes - `HibernateDaoSupport` and `HibernateTemplate`. The reason to use them was to get the Session from Hibernate and get the benefit of Spring transaction management. However from Hibernate 3.0.1, we can use `SessionFactory` _getCurrentSession()_ method to get the current session and use it to get the spring transaction management benefits. If you go through above examples, you will see how easy it is and that's why we should not use these classes anymore. One other benefit of `HibernateTemplate` was exception translation but that can be achieved easily by using `@Repository` annotation with service classes, shown in above spring mvc example. This is a trick question to judge your knowledge and whether you are aware of recent developments or not.
Hibernate integration with Servlet or Struts2 needs to be done using `ServletContextListener`, a complete example can be found at [Hibernate Struts2 Integration Example](/community/tutorials/struts2-hibernate-integration-example-tutorial).
Some of the design patterns used in Hibernate Framework are:
- Domain Model Pattern - An object model of the domain that incorporates both behavior and data.
- Data Mapper - A layer of Mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself.
- [Proxy Pattern](/community/tutorials/proxy-design-pattern) for lazy loading
- [Factory pattern](/community/tutorials/factory-design-pattern-in-java "Factory Design Pattern in Java") in SessionFactory
Some of the best practices to follow in Hibernate are:
- Always check the primary key field access, if it's generated at the database layer then you should not have a setter for this.
- By default hibernate set the field values directly, without using setters. So if you want hibernate to use setters, then make sure proper access is defined as `@Access(value=AccessType.PROPERTY)`.
- If access type is property, make sure annotations are used with getter methods and not setter methods. Avoid mixing of using annotations on both filed and getter methods.
- Use native sql query only when it can't be done using HQL, such as using database specific feature.
- If you have to sort the collection, use ordered list rather than sorting it using Collection API.
- Use named queries wisely, keep it at a single place for easy debugging. Use them for commonly used queries only. For entity specific query, you can keep them in the entity bean itself.
- For web applications, always try to use JNDI DataSource rather than configuring to create connection in hibernate.
- Avoid Many-to-Many relationships, it can be easily implemented using bidirectional One-to-Many and Many-to-One relationships.
- For collections, try to use Lists, maps and sets. Avoid array because you don't get benefit of lazy loading.
- Do not treat exceptions as recoverable, roll back the Transaction and close the Session. If you do not do this, Hibernate cannot guarantee that in-memory state accurately represents the persistent state.
- Prefer DAO pattern for exposing the different methods that can be used with entity bean
- Prefer lazy fetching for associations
Data validation is integral part of any application. You will find data validation at presentation layer with the use of Javascript, then at the server side code before processing it. Also data validation occurs before persisting it, to make sure it follows the correct format. Validation is a cross cutting task, so we should try to keep it apart from our business logic. That’s why JSR303 and JSR349 provides specification for validating a bean by using annotations. Hibernate Validator provides the reference implementation of both these bean validation specs. Read more at [Hibernate Validation Example](/community/tutorials/hibernate-validator-jsr303-example-tutorial).
Hibernate Tools plugin helps us in writing hibernate configuration and mapping files easily. The major benefit is the content assist to help us with properties or xml tags to use. It also validates them against the Hibernate DTD files, so we know any mistakes before hand. Learn how to install and use at [Hibernate Tools Eclipse Plugin](/community/tutorials/hibernate-tools-eclipse-plugin).
Вот и все по вопросам и ответам на собеседование по Hibernate, надеюсь, что это поможет вам на собеседовании, будь то вы новичок или опытный специалист. Пожалуйста, дайте мне знать, если я упустил какой-то важный вопрос, и я добавлю его в список.
Source:
https://www.digitalocean.com/community/tutorials/hibernate-interview-questions-and-answers