Spring: Is using EntitiyManager in a @Component+@Transactional class thread safe?

Question

I am using JPA (Hibernate) in a Spring project and asking myself if my class 'BooksHandler' (which is a DAO) is thread safe ?

Code for my App Config:

@Configuration
@EnableTransactionManagement
public class AppConfig {

@Bean
public LocalContainerEntityManagerFactoryBean getEntityManagerFactory() {

    LocalContainerEntityManagerFactoryBean emf = new LocalContainerEntityManagerFactoryBean();
    emf.setDataSource(getDataSource());
    emf.setPackagesToScan("jpa.models");
    JpaVendorAdapter adapter = new HibernateJpaVendorAdapter();
    emf.setJpaVendorAdapter(adapter);
    emf.setJpaProperties(getProperties());
    return emf;
}

@Bean
public DataSource getDataSource() {
    DriverManagerDataSource dtSrc = new DriverManagerDataSource();
    dtSrc.setDriverClassName("com.mysql.jdbc.Driver");
    dtSrc.setUrl("jdbc:mysql://localhost:3306/jpa_example");
    dtSrc.setUsername("dbuser1");
    dtSrc.setPassword("dbuser1");
    return dtSrc;
}
private Properties getProperties() {
    Properties p = new Properties();
    p.setProperty("hibernate.dialect", "org.hibernate.dialect.MySQL5Dialect");
    p.setProperty("hibernate.hbm2ddl.auto", "create");
    p.setProperty("hibernate.show_sql", "true");
    return p;
}

//auto transaction management
@Bean
public PlatformTransactionManager getTransactionManager(EntityManagerFactory emf) {
    JpaTransactionManager manager = new JpaTransactionManager();
    manager.setEntityManagerFactory(emf);

    return manager;
}

//auto exception management
@Bean
public PersistenceExceptionTranslationPostProcessor getPostProcessor() {
    return new PersistenceExceptionTranslationPostProcessor();
}
}

DAO class:

@Component 
@Transactional 
public class BooksHandler {

@PersistenceContext
private EntityManager em;

public Book createBook(String title, String isbn, int year) {
    Book b = new Book();
    b.setIsbn(isbn);b.setTitle(title);b.setYear(year);
    em.persist(b);
    System.out.println("book created: "+b.getId());
    return b;
}

public Book getBook(int id) {
    return em.find(Book.class, id);
}
//other CRUD methods
}

The method of BooksHandler will be used by multiple threads and I know that EntityManager itself is NOT thread-safe. But an online course I am attending does it this way. Maybe there is some behind the scene magic by spring to make it thread safe ?

Answer 1

Yes, it is thread-safe. Spring injects a proxy that delegates to the EM associated to the current transaction/thread. See the documentation, which says:

The injected JPA EntityManager behaves like an EntityManager fetched from an application server’s JNDI environment, as defined by the JPA specification. It delegates all calls to the current transactional EntityManager, if any. Otherwise, it falls back to a newly created EntityManager per operation, in effect making its usage thread-safe.

(emphasis mine).