与Hibernate交易代码有关的问题

Question

While I am working on spring boot with hibernate I got some confusion on transaction management code. Can any one clarify my doubt?

public class HibernateConfig {

    @Value("${db.driver-class-name}")
    private String DB_DRIVER;
    @Value("${db.password}")
    private String DB_PASSWORD;
    @Value("${db.url}")
    private String DB_URL;
    @Value("${db.username}")
    private String DB_USERNAME;
    @Value("${hibernate.dialect}")
    private String DB_DIALECT;
    @Value("${hibernate.show_sql}")
    private String DB_SHOW_SQL;
    @Value("${entitymanager.packagesToScan}")
    private String DB_PACAKAGESTOSCAN;

    public DataSource dataSource() {
        DriverManagerDataSource datasources = new DriverManagerDataSource();
        datasources.setDriverClassName(DB_DRIVER);
        datasources.setUrl(DB_URL);
        datasources.setUsername(DB_USERNAME);
        datasources.setPassword(DB_PASSWORD);
        return datasources;
    }

    public LocalSessionFactoryBean sessionFactory() {
        LocalSessionFactoryBean sessionfactory = new LocalSessionFactoryBean();
        sessionfactory.setDataSource(dataSource());
        sessionfactory.setPackagesToScan(DB_PACAKAGESTOSCAN);
        Properties hb_properties = new Properties();
        hb_properties.put("hibernate.dialect", DB_DIALECT);
        hb_properties.put("hibernate.show_sql", DB_SHOW_SQL);
        return sessionfactory;

    }

@Bean
public HibernateTransactionManager transactionManager() {
    HibernateTransactionManager transactionManager = new HibernateTransactionManager();
    transactionManager.setSessionFactory(sessionFactory().getObject());
    return transactionManager;

    }

}

Well in the above code the last method transaction manager why we need to write? In this method what is happening internally can anyone explain me?

Answer 1

Because hibernate needs transactions in order to perform its operations. Example would be @OneToMany relationship. Once an entity is loaded you actualy need an active transaction in order to fetch its Lazy relationships. Only within an active transaction the entity is in managed state.

Make the following experiment. Try to load an entity with Lazy relationships without active transaction, the load will succeed but then try to access one of the relationships.

Without an active transaction you will not be able to perform a simple save operation.

Answer 2

Just based on the docs:

Binds a Hibernate Session from the specified factory to the thread, potentially allowing for one thread-bound Session per factory.

Basically thanks to a transaction manager you are able to perform various CRUD operations and conform to the ACID database rules.

On top of that it:

Supports custom isolation levels, and timeouts

Then if certain operations are only fetching data, then you can set the transaction as readOnly increasing the performance.