Reduce Repetitive Code in Spring MVC Controllers

After spending some time doing sustained engineering (a.k.a. maintaining legacy code), I ventured to reduce repetitive code in Spring MVC @Controllers. I started with an abstract base controller class. But I soon found out that it was a dead-end because @RequestMapping is not inherited from (or combined with) parent classes and/or interfaces (see Spring MVC @RequestMapping Inheritance).

With some free time to further think about this, I took different approach.

@Controller
public class RepositoryCrudController {

	private final Repositories repositories;
	private final RepositoryInvokerFactory repositoryInvokerFactory;

	// ...

	@RequestMapping("/{repository}")
	String index(@PathVariable("repository") String repositoryKey,
			Pageable pageable, Model model) {
		// ... (only if repository has findAll(Pageable) method)
		return repositoryKey + "/index";
	}

	@GetMapping("/{repository}/{id}")
	String show(@PathVariable("repository") String repositoryKey,
			@PathVariable("id") Object id, Model model) {
		// ... (only if repository has findById method)
		return repositoryKey + "/show";
	}

	@GetMapping(path = "/{repository}", param = "create")
	String create(...) {
		// ... (only if repository has save method)
		return repositoryKey + "/create";
	}

	@PostMapping("/{repository}")
	String save(...) {
		// ... (only if repository has save method)
		return "redirect:/" + repositoryKey + "/{id}";
	}

	@GetMapping(path = "/{repository}/{id}", param = "edit")
	// ... edit (only if repository has findById and save methods)
	@PutMapping("/{repository}/{id}")
	// ... update (only if repository has save method)
	// @DeleteMapping("/{repository}/{id}")
	// ... delete (only if repository has deleteById method)
}

This approach is largely inspired by RepositoryEntityController of Spring Data REST.

Instead of defining an abstract base controller class, I created a concrete controller class with the default (or repetitive) behavior. The default behavior relies on invoking methods on Spring Data repositories.

For custom controllers, instead of creating subclasses (of the abstract base class), the controller can simply define handler methods that behave differently. For example:

@Entity
public class Article {...}

// Spring Data JPA
public interface ArticleRepository extends CrudRepository<Article, ...> {...}

@Controller
@RequestMapping("/articles")
public class ArticlesController {

	// no need for index() handler method
	// just set-up "articles/index" view

	// no need for show() handler method
	// just set-up "articles/show" view

	@GetMapping(param = "create")
	String create(Model model) {
		// Do something that is _not_ *default* behavior
		// e.g. provide options for dropdowns, or use form-backing object/JavaBean
		// ...
		return "articles/create";
	}

	// no need for save() handler method
	// just set-up "articles/show" view

	@GetMapping(path = "/{id}", param = "edit")
	String edit(@PathVariable("id") ... id, Model model) {
		// Do something that is _not_ *default* behavior
		// e.g. provide options for dropdowns, or use form-backing object/JavaBean
		// ...
		return "articles/edit";
	}

	// no need for update() handler method
	// just set-up "articles/show" view

}

The above will work because Spring MVC chooses the more specific mapping first. When a GET /articles?create request is received, the ArticlesController will be chosen to handle it (and not RepositoryCrudController). But if ArticlesController handler methods were not defined, the GET /articles?create request would have been handled by the RepositoryCrudController.

With this simple fall-back controller that has the default behavior, developers (team mates) can then focus on the domain, creating views, or creating controllers with custom behavior (e.g. Ajax, Server-generated JavaScript Responses).

That's all for now.

DataSource Routing with Spring @Transactional

I was inspired by Carl Papa's use of aspects with the Spring Framework to determine the DataSource to use (either read-write or read-only). So, I'm writing this post.

I must admit that I have long been familiar with Spring's AbstractRoutingDataSource. But I did not have a good idea where it can be used. Thanks to Carl and team, and one of their projects. Now, I know a good use case.

@Transactional

With Spring, read-only transactions are typically marked with annotations.

public class ... {
    @Transactional(readOnly=true)
    public void ...() {...}

    @Transactional // read-write
    public void ...() {...}
}

To take advantage of this, we use Spring's TransactionSynchronizationManager to determine if the current transaction is read-only or not.

AbstractRoutingDataSource

Here, we use Spring's AbstractRoutingDataSource to route to the read-only replica if the current transaction is read-only. Otherwise, it routes to the default which is the master.

public class ... extends AbstractRoutingDataSource {
    @Override
    protected Object determineCurrentLookupKey() {
        if (TransactionSynchronizationManager
                .isCurrentTransactionReadOnly() ...) {
            // return key to a replica
        }
        return null; // use default
    }
    ...
}

Upon using the above approach, we found out that the TransactionSynchronizationManager is one step behind because Spring will have already called DataSource.getConnection() before a synchronization is established. Thus, a LazyConnectionDataSourceProxy needs to be configured as well.

As we were discussing this, we figured if there was another way to determine if the current transaction is read-only or not (without resorting to LazyConnectionDataSourceProxy). So, we came up with an experimental approach where an aspect captures the TransactionDefinition (from the @Transactional annotation, if any) as a thread-local variable, and an AbstractRoutingDataSource that routes based on the captured information.

The relevant source code can be found on GitHub. Thanks again, Carl! BTW, Carl is also an award-winning movie director. Wow, talent definitely knows no boundaries.

Spring-managed Transactions Explained - Part 2 (JPA)

In the first part of the series, I showed how transactions work in plain-vanilla JDBC. And then I showed how Spring manages JDBC-based transactions. In this second part of the series, I'll show how transactions work in plain-vanilla JPA first. And then show how Spring manages JPA-based transactions.

Funds Transfer

To help illustrate transactions, I'll be using the same case study of transferring funds from one bank account to another. Here, we show code snippets of debit, credit, and transfer methods.

... class BankAccountService {
  public void transfer(MonetaryAmount amount, ...) {
    debit(amount, ...);
    credit(amount, ...);
    ...
  }
  public void credit(MonetaryAmount amount, AccountId accountId) {
    ...
  }
  public void debit(MonetaryAmount amount, AccountId accountId) {
    ...
  }
  ...
}

JPA Transactions

In plain-vanilla JPA, transactions are started by calling getTransaction().begin() on the EntityManager. The code snippet below illustrates this.

import javax.persistence.*;
...
EntityManagerFactory emf = ...;
EntityManager em = emf.createEntityManager();
try {
  em.getTransaction().begin();
  // make changes through entities
  em.getTransaction().commit();
  ...
} catch(Exception e) {
  em.getTransaction().rollback();
  throw e;
} finally {
  em.close();
}

Technically, the EntityManager is in a transaction from the point it is created. So calling begin() is somewhat redundant. Until begin() is called, certain operations such as persist, merge, remove cannot be called. Queries can still be performed (e.g. find()).

Objects that were returned from queries can be changed. Although the JPA specification is somewhat unclear on what will happen to these changes when no transaction has been started.

Now, let's apply JPA to the funds transfer case study.

We defined a BankAccount entity to handle debit() and credit() behavior.

import javax.persistence.*;

@Entity
... class BankAccount {
  @Id ...;
  ...
  public void debit(MonetaryAmount amount) {...}
  public void credit(MonetaryAmount amount) {...}
  ...
}

We add an EntityManagerFactory to BankAccountService to enable the creation of EntityManagers when needed.

import javax.persistence.*;

... class BankAccountService {
  private EntityManagerFactory emf; // injected via constructor
  ...
  public void transfer(MonetaryAmount amount, ...) ... {
    EntityManager em = emf.createEntityManager();
    try {
      em.getTransaction().begin();
      BankAccount fromAccount = em.find(BankAccount.class, ...);
      BankAccount toAccount = em.find(BankAccount.class, ...);
      fromAccount.debit(amount);
      toAccount.credit(amount);
      em.getTransaction().commit();
      ...
    } catch(Exception e) {
      em.getTransaction().rollback();
      // handle exception (possibly rethrowing it)
    } finally {
      em.close();
    }
  }
  public void credit(MonetaryAmount amount, AccountId ...) ... {
    EntityManager em = emf.createEntityManager();
    try {
      em.getTransaction().begin();
      BankAccount theAccount = em.find(BankAccount.class, ...);
      theAccount.credit(amount);
      em.getTransaction().commit();
      ...
    } catch(Exception e) {
      em.getTransaction().rollback();
      // handle exception (possibly rethrowing it)
    } finally {
      em.close();
    }
  }
  public void debit(MonetaryAmount amount, AccountId ...) ... {
    EntityManager em = emf.createEntityManager();
    try {
      em.getTransaction().begin();
      BankAccount theAccount = em.find(BankAccount.class, ...);
      theAccount.debit(amount);
      em.getTransaction().commit();
      ...
    } catch(Exception e) {
      em.getTransaction().rollback();
      // handle exception (possibly rethrowing it)
    } finally {
      em.close();
    }
  }
}

Spring-managed JPA Transactions

The transfer, credit, and debit methods could sure use a template class (something like a JdbcTemplate) to remove all the boilerplate code. Spring previously provided a JpaTemplate class, but was deprecated as of Spring 3.1, in favor of native EntityManager usage (typically obtained through @PersistenceContext).

So, let's do just that — use EntityManager obtained through @PersistenceContext.

import javax.persistence.*;

... class BankAccountService {
  @PersistenceContext
  private EntityManager em;
  ...
  public void transfer(MonetaryAmount amount, ...) ... {
    try {
      em.getTransaction().begin();
      BankAccount fromAccount = em.find(BankAccount.class, ...);
      BankAccount toAccount = em.find(BankAccount.class, ...);
      fromAccount.debit(amount);
      toAccount.credit(amount);
      em.getTransaction().commit();
      ...
    } catch(Exception e) {
      em.getTransaction().rollback();
      // handle exception (possibly rethrowing it)
    }
  }
  public void credit(MonetaryAmount amount, AccountId ...) ... {
    try {
      em.getTransaction().begin();
      BankAccount theAccount = em.find(BankAccount.class, ...);
      theAccount.credit(amount);
      em.getTransaction().commit();
      ...
    } catch(Exception e) {
      em.getTransaction().rollback();
      // handle exception (possibly rethrowing it)
    }
  }
  public void debit(MonetaryAmount amount, AccountId ...) ... {
    try {
      em.getTransaction().begin();
      BankAccount theAccount = em.find(BankAccount.class, ...);
      theAccount.debit(amount);
      em.getTransaction().commit();
      ...
    } catch(Exception e) {
      em.getTransaction().rollback();
      // handle exception (possibly rethrowing it)
    }
  }
}

Our code is a little bit simpler. Since we didn't create an EntityManager, we don't have to close it. But we are still calling getTransaction().begin(). Is there a better way? And how does an EntityManager get injected into the object in the first place?

From my previous post in this series, the astute reader is probably already thinking of having Spring do the work for us. And rightfully so!

EntityManager and @PersistenceContext

We tell Spring to inject an EntityManager from the EntityManagerFactory by adding a PersistenceAnnotationBeanPostProcessor (either through XML <bean>, or simply using a Java-based configuration via @Configuration classes loaded via AnnotationConfigApplicationContext).

• When using XML-based configuration, a PersistenceAnnotationBeanPostProcessor is transparently activated by the <context:annotation-config /> element. And this element also gets transparently activated by <context:component-scan />.
• When using Java-based @Configuration, the AnnotationConfigApplicationContext is used. And with it, annotation config processors are always registered (one of which is the aforementioned PersistenceAnnotationBeanPostProcessor).

By adding a single bean definition, the Spring container will act as a JPA container and inject an EnitityManager from your EntityManagerFactory.

JPA and @Transactional

Now that we have an EntityManager, how can we tell Spring to begin transactions for us?

We tell Spring to start transactions by marking methods as @Transactional (or mark the class as @Transactional which makes all public methods transactional). This is consistent with the way Spring enables transactions with JDBC.

import javax.persistence.*;
import org.springframework.transaction.annotation.Transactional;

@Transactional
... class BankAccountService {
  @PersistenceContext
  private EntityManager em;
  ...
  public void transfer(MonetaryAmount amount, ...) ... {
      BankAccount fromAccount = em.find(BankAccount.class, ...);
      BankAccount toAccount = em.find(BankAccount.class, ...);
      fromAccount.debit(amount);
      toAccount.credit(amount);
  }
  public void credit(MonetaryAmount amount, AccountId ...) ... {
      BankAccount theAccount = em.find(BankAccount.class, ...);
      theAccount.credit(amount);
  }
  public void debit(MonetaryAmount amount, AccountId ...) ... {
      BankAccount theAccount = em.find(BankAccount.class, ...);
      theAccount.debit(amount);
  }
}

Wow, that was nice! Our code just got a lot shorter.

And just as explained in the first part of this series, when Spring encounters this annotation, it proxies the object (usually referred to as a Spring-managed bean). The proxy starts a transaction (if there is no on-going transaction) for methods that are marked as @Transactional, and ends the transaction when the method returns successfully.

A call to debit() will use a transaction. A separate call to credit() will use a transaction. But what happens when a call to transfer() is made?

Since the transfer() method is marked as @Transactional, Spring will start a transaction. This same transaction will be used for calls to debit() and credit(). In other words, debit(amount) and credit(amount) will not start a new transaction. It will use the on-going transaction (since there is one).

But wait! How does Spring know when to inject a proper entity manager? Is it only injected when a transactional method is invoked?

Shared EntityManager

In one of my training classes, I tried the following to better understand how Spring injects an EntityManager via @PersistenceContext. And I believe it will help others too. So, here's what I tried:

import javax.persistence.*;
import org.springframework.transaction.annotation.Transactional;
import org.springframework.beans.factory.InitializingBean;

@Transactional
... class BankAccountService implements InitializingBean {
  @PersistenceContext
  private EntityManager em;
  ...
  @Override
  public void afterPropertiesSet() {
    System.out.println(em.toString());
  }
  ...
}

An output of something like this was displayed on the console after the application context started.

Shared EntityManager proxy for target factory [...]

So what is this shared entity manager?

When the application context starts, Spring injects a shared entity manager. The shared EntityManager will behave just like an EntityManager fetched from an application server's JNDI environment, as defined by the JPA specification. It will delegate all calls to the current transactional EntityManager, if any; otherwise, it will fall back to a newly created EntityManager per operation.

Going back to our question. Spring doesn't inject the right entity manager at the right time. It always injects a shared entity manager. But this shared entity manager is transaction-aware. It delegates to the current transactional EntityManager, if there is an on-going transaction.

Conclusion

This concludes the two-part series. I hope that by starting off with the plain-vanilla versions of JDBC and JPA (sans DAOs and repositories), I was able to make it clearer as to how Spring is able to manage transactions behind the scenes. And that by having a clearer idea as to what Spring is doing behind the scenes, you can troubleshoot better, understand why you get an TransactionRequiredException saying "No transactional EntityManager available", and add better fixes to your applications.

Now, it's time for a cold one.

Spring-managed Transactions Explained - Part 1 (JDBC)

I've been meaning to write about this for quite some time now. In the training courses I've been privileged to conduct, I've noticed that course participants have had the most difficulty trying to understand how Spring manages transactions. In the first part of this series, I'll start by showing how transactions work in plain-vanilla JDBC. And then show how Spring manages JDBC-based transactions.

In the second part of this series, I'll show how transactions work in plain-vanilla JPA. And then show how Spring manages JPA-based transactions.

Update: The second part of this series has just been posted.

Funds Transfer

To help illustrate transactions, I'll be using the often used case study of transferring funds from one bank account to another. Here, we show code snippets of debit, credit, and transfer methods.

... class BankAccountService {
  public void transfer(MonetaryAmount amount, ...) {
    debit(amount, ...);
    credit(amount, ...);
    ...
  }
  public void credit(MonetaryAmount amount, AccountId accountId) {
    ...
  }
  public void debit(MonetaryAmount amount, AccountId accountId) {
    ...
  }
  ...
}

By default a Connection object is in auto-commit mode, which means that it automatically commits changes after executing each statement. If auto-commit mode has been disabled, the method commit() must be called explicitly in order to commit changes; otherwise, database changes will not be saved.

JDBC Transactions

In plain-vanilla JDBC, transactions are started by setting the Connection's auto-commit mode to false (or to manual commit mode). Once the Connection's auto-commit is set to false, subsequent database changes are not committed until a call to commit() is made. The changes brought about by the SQL operations will not be seen by other connections until the changes are committed. The code snippet below illustrates this.

import java.sql.*;
...
try (Connection conn = dataSource.getConnection()) {
  conn.setAutoCommit(false);
  ... // SQL operations here (inserts, updates, deletes, etc)
  // database changes will not be saved until commit
  conn.commit(); // save changes (all-or-nothing)
} catch (SQLException e) {
  // handle exception
  conn.rollback(); // rollback changes
}

Now, let's apply this to the funds transfer case study.

The transfer() method is now overloaded with an additional connection parameter.

import java.sql.*;
...
... class BankAccountService {
  public void transfer(MonetaryAmount amount, ...)
      throws SQLException {...}
  public void transfer(
        MonetaryAmount amount, ..., Connection conn)
      throws SQLException {...}
  ...
}

The method that does not accept a connection, creates a new connection, and calls the same method that accepts a connection parameter. The method that accepts a connection, uses it to carry out debit and credit operations.

import java.sql.*;

... class BankAccountService {
  public void transfer(MonetaryAmount amount, ...)
      throws SQLException {
    try (Connection conn = dataSource.getConnection()) {
      conn.setAutoCommit(false);
      transfer(amount, ..., conn);
      conn.commit(); // save changes (all-or-nothing)
    } catch (SQLException e) {
      // handle exception
      conn.rollback(); // rollback changes
    }
  }
  public void transfer(
        MonetaryAmount amount, ..., Connection conn)
      throws SQLException {
    debit(amount, ..., conn);
    credit(amount, ..., conn);
    ...
  }
  public void credit(MonetaryAmount amount, AccountId accountId)
      throws SQLException {...}
  public void credit(MonetaryAmount amount, AccountId accountId,
      Connection conn) throws SQLException {...}
  public void debit(MonetaryAmount amount, AccountId accountId)
      throws SQLException {...}
  public void debit(MonetaryAmount amount, AccountId accountId,
      Connection conn) throws SQLException {...}
  ...
}

The debit() and credit() methods are also overloaded. Just like the transfer() methods, the one that does not accept a connection parameter, creates a new connection, and calls the one that accepts a connection object. The method that accepts a connection, uses it to carry out SQL operations.

import java.sql.*;

... class BankAccountService {
  public void transfer(MonetaryAmount amount, ...)
      throws SQLException {
    ... transfer(amount, ..., conn); ...
  }
  public void transfer(
        MonetaryAmount amount, ..., Connection conn)
      throws SQLException {
    debit(amount, ..., conn);
    credit(amount, ..., conn);
    ...
  }
  public void credit(MonetaryAmount amount, AccountId accountId)
      throws SQLException {
    try (Connection conn = dataSource.getConnection()) {
      conn.setAutoCommit(false);
      credit(amount, accountId, conn);
      conn.commit(); // save changes (all-or-nothing)
    } catch (SQLException e) {
      // handle exception
      conn.rollback(); // rollback changes
    }
  }
  public void credit(MonetaryAmount amount, AccountId accountId,
      Connection conn) throws SQLException {
    ... = conn.prepareStatement(...);
    ... // SQL operations for credit
  }
  public void debit(MonetaryAmount amount, AccountId accountId)
      throws SQLException {
    try (Connection conn = dataSource.getConnection()) {
      conn.setAutoCommit(false);
      debit(amount, accountId, conn);
      conn.commit(); // save changes (all-or-nothing)
    } catch (SQLException e) {
      // handle exception
      conn.rollback(); // rollback changes
    }
  }
  public void debit(MonetaryAmount amount, AccountId accountId,
      Connection conn) throws SQLException {
    ... = conn.prepareStatement(...);
    ... // SQL operations for debit
  }
  ...
}

Notice that we'll need to pass connection objects all over the place. Yes, this looks ugly. But stay with me for a few more minutes and read on.

A pattern emerges from these methods. Whenever a database transaction needs to be made, a connection is either provided or established, then used to carry out SQL operations (or database changes), and the changes are committed (or rolled back) by the method that established the connection.

The method accepting a connection object gives up control of the transaction to its caller (as in the case of credit(amount) calling credit(amount, connection)). The scope of transactions can span one method call, or several methods calls (as in the case of transfer(amount) calling debit(amount, connection) and credit(amount, connection)).

As already mentioned, this looks ugly and error-prone. The connection object is passed all over the place. So, how can we improve on this?

Spring-managed JDBC Transactions

First, we'll use Spring's JdbcTemplate to help us deal with JDBC boilerplate code and SQLExceptions. Second, we'll use Spring's declarative transaction management.

When we start using a JdbcTemplate inside BankAccountService, we no longer need to handle connections explicitly, and we can remove those overloaded methods. The connection object actually gets hidden in some thread local object.

import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.transaction.annotation.Transactional;

@Transactional
... class BankAccountService {
  private JdbcTemplate jdbcTemplate; // injected via constructor
  ...
  public void transfer(MonetaryAmount amount, ...) {
    ...
  }
  public void credit(MonetaryAmount amount, AccountId accountId) {
    jdbcTemplate.update(...); // SQL operations for credit
    ...
  }
  public void debit(MonetaryAmount amount, AccountId accountId) {
    jdbcTemplate.update(...); // SQL operations for debit
    ...
  }
  ...
}

How does JdbcTemplate get a Connection?

For each JdbcTemplate method, it retrieves a connection from the given DataSource that is provided (either as constructor argument, or as property setDataSource(DataSource)). After the JdbcTemplate method returns, the connection is closed. Without transactions, the connection's auto-commit mode remains true. The good thing is, JdbcTemplate is transaction-aware. That means, it will participate in the on-going transaction, if there is one (i.e. use the on-going transaction's connection).

So, how do we set the connection's auto-commit mode to false, so that a transaction can be started and span more than one method call to JdbcTemplate?

To use transactions, we do not set the connection's auto-commit mode to false. Instead, we tell Spring to start transactions. In turn, Spring will retrieve a connection, set its auto-commit mode to false, and keep using this same connection until the transaction is completed (either with all changes saved/committed, or all changes rolled back).

So, how do we tell Spring to start transactions, and what does @Transactional do?

As you might have guessed, the @Transactional annotation tells Spring when to start a transaction (by setting the connection's auto-commit mode to false).

We tell Spring to start transactions by marking methods as @Transactional (or mark the class as @Transactional which makes all public methods transactional). And for Spring to start noticing @Transactional annotations, we'll need to add <tx:annotation-driven /> in our XML-based configuration, or add @EnableTransactionManagement in our Java-based @Configuration.

When Spring encounters this annotation, it proxies the object (usually referred to as a Spring-managed bean). The proxy starts a transaction (if there is no on-going transaction) for methods that are marked as @Transactional, and ends the transaction when the method returns successfully.

Since JdbcTemplate is transaction-aware, it knows how to get the on-going transaction's connection (if one exists), and not create a new one.

...
import org.springframework.transaction.annotation.Transactional;

@Transactional
... class BankAccountService {
  ...
  public void transfer(MonetaryAmount amount, ...) {
    ...
    debit(amount, ...);
    credit(amount, ...);
    ...
  }
  public void credit(MonetaryAmount amount, AccountId accountId) {...}
  public void debit(MonetaryAmount amount, AccountId accountId) {...}
  ...
}

So, a call to debit() will use a transaction. A separate call to credit() will use a transaction. But what happens when a call to transfer() is made?

Since the transfer() method is marked as @Transactional, Spring will start a transaction. This same transaction will be used for calls to debit() and credit(). In other words, debit(amount) and credit(amount) will not start a new transaction. It will use the on-going transaction (since there is one).

Here's another good blog post to better understand Transactions, Caching and AOP in Spring.

One Thing Good Spring Developers Know

In my recent training sessions on the (core) Spring Framework, I was asked, "If there was one thing that a (Java) Spring developer should know, what should that be?" That question caught me off guard. Yes, the (core) Spring Framework does cover a lot of areas (e.g. beans, configuration, aspect-oriented programming, transactions). And it was difficult for me to point out just one thing. I ended up mentioning everything that we covered in our (3 day) training course.

If there was one thing that a (Java) Spring developer should know, what should that be?

As I gave that question more thought, I began to think about the most important one. I ended up thinking of how Spring uses aspects to add behavior to managed objects (usually called beans) as the most important. This is how the Spring Framework supports transactions, security, scope, Java-based configuration, among others. And I'm sharing my thoughts here in this post.

I ended up thinking of how Spring uses aspects to add behavior to managed objects (usually called beans) as the most important.

ORM and Lazy Loading Exceptions

Most developers who use some form of ORM have encountered an exception that signifies that child entities could not be loaded (e.g. LazyInitializationException).

Some developers who have encountered this would use an "open session in view" (OSIV) pattern to keep the session open and prevent this exception from happening. But I find this to be an overkill. Worse, some developers consider the "open session in view" pattern to be the only solution. A possible underlying cause for this misconception could be that the developer is probably not armed with the knowledge of using the Spring Framework effectively to keep the ORM session open longer.

In the case of JPA, the "open entity manager in view" pattern will create an entity manager at the beginning of the request, bind it to the request thread, and close it when the response is completed.

So, if not the OSIV pattern, what would be a better solution?

The short answer is to use the Spring Framework to keep the session open for the duration that you need it (e.g. @Transactional). Keep on reading as I'll provide a longer answer.

Services and Repositories

In a layered architecture, a typical design pattern is to define a domain or application service (usually defined as an interface) to provide business functionality (e.g. start using a shopping cart, adding items to that shopping cart, searching for products). Domain and application service implementations would typically delegate the retrieval/persistence of domain entities to repositories.

Presentation Layer

Business Layer

Data Access (or Infrastructure) Layer

Repositories (or data access objects) are also defined as interfaces to retrieve/persist domain entities (i.e. provide ORM and CRUD access). Naturally, repository implementations use ORM libraries (e.g. JPA/Hibernate, myBATIS) to retrieve and persist domain entities. With this, it uses the ORM framework's classes to connect to the persistent store, retrieve/persist the entity, and close the connection (called session in Hibernate). There's no problem of lazy loading failures at this point.

The problem of lazy loading failures occur when the service retrieves a domain entity using the repository, and wants to load child entities (after the repository method has returned). By the time the repository returns the domain entity, the ORM session gets closed. Because of this, attempts to access/load child entities in the domain service cause an exception.

The code snippets below illustrate how a lazy loading exception can occur when the child items of an order entity is lazily loaded after being returned by the repository.

@Entity
public class Order {
    @OneToMany // defaults to FetchType.LAZY
    private List<OrderItem> items;
    …
    public List<OrderItem> getItems() {…}
}

public class SomeApplicationServiceImpl implements SomeApplicationService {
    private OrderRepository orderRepository;
    …
    @Override
    public void method1(…) {
        …
        order = orderRepository.findById(...);
        order.getItems(); // <-- Lazy loading exception occurs!
        …
    }
    …
}

public class OrderRepositoryImpl implements OrderRepository {
    @PersistenceContext
    private EntityManager em;
    …
    @Override
    public Order findById(...) {...}
    …
}

The repository implementation explicitly uses JPA for its ORM (as illustrated with the use of an EntityManager).

At this point, some developers may opt to use eager fetch to prevent the lazy initialization exception. Telling the ORM to eagerly fetch the child items of an order entity will work. But sometimes, we don't need to load the child items. And eagerly loading this might be unnecessary overhead. It would be great to only load it when we need it.

To prevent the lazy initialization exception (and not be forced to eagerly fetch), we'll need to keep the ORM session open until the calling service method returns. In Spring, it can be as simple as annotating the service method as @Transactional to keep the session open. I find that this approach is better than using "open session in view" pattern (or being forced to use eager fetching), since it keeps the session open only for the duration that we intend it to be.

public class SomeApplicationServiceImpl implements SomeApplicationService {
    private OrderRepository orderRepository;
    …
    @Override
    @Transactional // <-- open the session (if it's not yet open)
    public void method1(…) {
        …
        order = orderRepository.findById(...);
        order.getItems(); // <-- Lazy loading exception should not happen
        …
    }
    …
}

Domain Entities in the Presentation Layer

Even after keeping the ORM session open in the service layer (beyond the repository implementation objects), the lazy initialization exception can still occur when we expose the domain entities to the presentation layer. Again, because of this, some developers prefer the OSIV approach, since it will also prevent lazy initialization exceptions in the presentation layer.

But why would you want to expose domain entities in the presentation layer?

From experience, I've worked with teams who prefer to expose domain entities in the presentation layer. This usually leads to anemic domain model, since presentation layer frameworks need a way to bind input values to the object. This forces domain entities to have getter and setter methods, and a zero-arguments constructor. Having getters and setters will make invariants difficult to enforce. For simple domains, this is workable. But for more complex domains, a richer domain model would be preferred, as it would be easier to enforce invariants.

In a richer domain model, the objects that represent the presentation layer input/output values are actually data transfer objects (DTOs). They represent inputs (or commands) that are carried out in the domain layer. With this in mind, I prefer to use DTOs and maintain a richer domain model. Thus, I don't really run into lazy initialization exceptions in the presentation layer.

Aspects to add behavior to managed objects

Spring intercepts calls to these @Transactional annotated methods to ensure that an ORM session is open.

Transactions (or simply keeping an ORM session open) are not the only behavior provided using aspects. There's security, scope, Java-based configuration, and others. Knowing that the Spring Framework uses aspects to add behavior is one of the key reasons why we let Spring manage the POJOs that we develop.

Conclusion

There you go. That for me is the one most important thing that a Spring Framework developer should know when using the core. Now that I've given my opinion as to what is the one most important thing, how about you? What do you think is the one most important thing to know when tackling Core Spring. Cheers!