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How to Get the Result As List<Class> In Hibernate?

5 minutes read

To get the result as a list of a specific class in Hibernate, you can use the Criteria API or HQL (Hibernate Query Language). By using the Criteria API, you can create criteria queries to retrieve objects of a specific class from the database. Alternatively, you can use HQL to write custom queries that retrieve objects of a specific class. To execute the query and obtain the result as a list, you can use the setResultTransformer method with the Criteria query, or simply execute the HQL query and cast the result to a list of the desired class. This way, you can retrieve the data from the database and store it in a list of the specified class in Hibernate.


How to configure Hibernate for a Spring application?

To configure Hibernate for a Spring application, follow these steps:

  1. Add necessary dependencies in your pom.xml file:
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<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-rest</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
</dependency>


  1. Create a configuration class for Hibernate:
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@Configuration
@EnableTransactionManagement
@EnableJpaRepositories(basePackages = { "com.example.repository" })
public class HibernateConfig {

    @Autowired
    private Environment env;

    @Bean
    public LocalContainerEntityManagerFactoryBean entityManagerFactory() {
        LocalContainerEntityManagerFactoryBean em = new LocalContainerEntityManagerFactoryBean();
        em.setDataSource(dataSource());
        em.setPackagesToScan("com.example.model");

        JpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
        em.setJpaVendorAdapter(vendorAdapter);
        em.setJpaProperties(additionalProperties());

        return em;
    }

    @Bean
    public DataSource dataSource() {
        DriverManagerDataSource dataSource = new DriverManagerDataSource();
        dataSource.setDriverClassName(env.getProperty("spring.datasource.driver-class-name"));
        dataSource.setUrl(env.getProperty("spring.datasource.url"));
        dataSource.setUsername(env.getProperty("spring.datasource.username"));
        dataSource.setPassword(env.getProperty("spring.datasource.password"));
        return dataSource;
    }

    @Bean
    public PlatformTransactionManager transactionManager(EntityManagerFactory emf) {
        JpaTransactionManager transactionManager = new JpaTransactionManager();
        transactionManager.setEntityManagerFactory(emf);
        return transactionManager;
    }

    Properties additionalProperties() {
        Properties properties = new Properties();
        properties.setProperty("hibernate.hbm2ddl.auto", "create-drop");
        properties.setProperty("hibernate.dialect", "org.hibernate.dialect.H2Dialect");
        return properties;
    }
}


  1. Configure the database connection properties in your application.properties file:
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spring.datasource.driver-class-name=org.h2.Driver
spring.datasource.url=jdbc:h2:mem:myDb;DB_CLOSE_DELAY=-1;DB_CLOSE_ON_EXIT=FALSE
spring.datasource.username=sa
spring.datasource.password=sa


  1. Create Entity classes and Repository interfaces for your database tables:
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@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    //Getters and Setters
}

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
}


  1. Use these repositories in your service classes to perform CRUD operations on your database.


With these steps, Hibernate will be configured for a Spring application and you can use it to interact with your database.


What is the purpose of Criteria API in Hibernate?

The Criteria API in Hibernate is used to create and execute dynamic queries to retrieve data from a database. It provides a programmatic and type-safe way to build queries using a set of predefined classes and methods instead of writing SQL queries directly.


Some of the benefits of using Criteria API in Hibernate are:

  1. Type-safety: Criteria API ensures that queries are syntactically correct at compile time, reducing the chances of runtime errors.
  2. Dynamic query building: Criteria API allows for the creation of queries dynamically based on runtime conditions, such as user input or business logic.
  3. No SQL injection: Since queries are built using Criteria API methods, there is no risk of SQL injection attacks.
  4. Improved code readability: The Criteria API provides a more readable and maintainable way to build queries compared to writing SQL queries directly.


Overall, the Criteria API in Hibernate helps in creating flexible and efficient queries for retrieving data from a database.


How to use projections in Hibernate?

Projections in Hibernate are used to retrieve specific data from the database by selecting only the required columns instead of fetching entire rows. Projections can be used with criteria queries or HQL queries.


Here is an example of how to use projections with criteria queries in Hibernate:

  1. Create a Criteria instance:


Criteria criteria = session.createCriteria(Employee.class);

  1. Add projections to the criteria:


criteria.setProjection(Projections.projectionList() .add(Projections.property("firstName"), "firstName") .add(Projections.property("lastName"), "lastName") .add(Projections.property("salary"), "salary"));


In this example, we are specifying the columns ("firstName", "lastName", "salary") to be selected from the Employee table.

  1. Execute the query and retrieve the results:


List<Object[]> results = criteria.list();


for(Object[] result : results) { String firstName = (String) result[0]; String lastName = (String) result[1]; double salary = (Double) result[2];

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System.out.println("First Name: " + firstName + ", Last Name: " + lastName + ", Salary: " + salary);


}


This will fetch only the specified columns from the Employee table and print the results.


Projections can also be used with HQL queries by using the select clause with the desired columns. Here is an example:


String hql = "select firstName, lastName, salary from Employee"; Query query = session.createQuery(hql); List<Object[]> results = query.list();


for(Object[] result : results) { String firstName = (String) result[0]; String lastName = (String) result[1]; double salary = (Double) result[2];

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System.out.println("First Name: " + firstName + ", Last Name: " + lastName + ", Salary: " + salary);


}


This will select only the specified columns from the Employee table using an HQL query.


Overall, using projections in Hibernate helps to optimize performance by fetching only the required data from the database.


How to query a database using Hibernate?

To query a database using Hibernate, you can use Hibernate's query API. Here is an example:

  1. Create a Hibernate Session:
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Session session = sessionFactory.openSession();


  1. Create a query using HQL (Hibernate Query Language) or Criteria API:
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Query query = session.createQuery("from EntityName where columnName = :value");
query.setParameter("value", value);


  1. Execute the query and get the results:
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List<EntityName> results = query.list();


  1. Iterate over the results (if needed):
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for (EntityName entity : results) {
    System.out.println(entity);
}


  1. Close the session:
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session.close();


Make sure to replace "EntityName" with the name of your entity class and "columnName" with the name of the column you want to query on. Also, replace ":value" with the parameter value you want to pass to the query.


Alternatively, you can use the Criteria API to build dynamic queries in a type-safe way. Here is an example:

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CriteriaBuilder builder = session.getCriteriaBuilder();
CriteriaQuery<EntityName> query = builder.createQuery(EntityName.class);
Root<EntityName> root = query.from(EntityName.class);
query.select(root).where(builder.equal(root.get("columnName"), value));

List<EntityName> results = session.createQuery(query).getResultList();


This example demonstrates how to create a query using the Criteria API to select entities where a specific column equals a certain value.


Remember to handle exceptions, manage transactions, and close the session properly to avoid memory leaks in your application.

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