For a longer description, you can read my Open Session In View Anti-Pattern article. Otherwise, here's a summary for why you shouldn't use Open Session In View.

Open Session In View takes a bad approach to fetching data. Instead of letting the business layer decide how it’s best to fetch all the associations that are needed by the View layer, it forces the Persistence Context to stay open so that the View layer can trigger the Proxy initialization.

The OpenSessionInViewFilter calls the openSession method of the underlying SessionFactory and obtains a new Session .

calls the method of the underlying and obtains a new . The Session is bound to the TransactionSynchronizationManager .

is bound to the . The OpenSessionInViewFilter calls the doFilter of the javax.servlet.FilterChain object reference and the request is further processed

calls the of the object reference and the request is further processed The DispatcherServlet is called, and it routes the HTTP request to the underlying PostController .

is called, and it routes the HTTP request to the underlying . The PostController calls the PostService to get a list of Post entities.

calls the to get a list of entities. The PostService opens a new transaction, and the HibernateTransactionManager reuses the same Session that was opened by the OpenSessionInViewFilter .

opens a new transaction, and the reuses the same that was opened by the . The PostDAO fetches the list of Post entities without initializing any lazy association.

fetches the list of entities without initializing any lazy association. The PostService commits the underlying transaction, but the Session is not closed because it was opened externally.

commits the underlying transaction, but the is not closed because it was opened externally. The DispatcherServlet starts rendering the UI, which, in turn, navigates the lazy associations and triggers their initialization.

starts rendering the UI, which, in turn, navigates the lazy associations and triggers their initialization. The OpenSessionInViewFilter can close the Session , and the underlying database connection is released as well.

At a first glance, this might not look like a terrible thing to do, but, once you view it from a database perspective, a series of flaws start to become more obvious.

The service layer opens and closes a database transaction, but afterward, there is no explicit transaction going on. For this reason, every additional statement issued from the UI rendering phase is executed in auto-commit mode. Auto-commit puts pressure on the database server because each statement must flush the transaction log to disk, therefore causing a lot of I/O traffic on the database side. One optimization would be to mark the Connection as read-only which would allow the database server to avoid writing to the transaction log.

There is no separation of concerns anymore because statements are generated both by the service layer and by the UI rendering process. Writing integration tests that assert the number of statements being generated requires going through all layers (web, service, DAO), while having the application deployed on a web container. Even when using an in-memory database (e.g. HSQLDB) and a lightweight web server (e.g. Jetty), these integration tests are going to be slower to execute than if layers were separated and the back-end integration tests used the database, while the front-end integration tests were mocking the service layer altogether.

The UI layer is limited to navigating associations which can, in turn, trigger N+1 query problems. Although Hibernate offers @BatchSize for fetching associations in batches, and FetchMode.SUBSELECT to cope with this scenario, the annotations are affecting the default fetch plan, so they get applied to every business use case. For this reason, a data access layer query is much more suitable because it can be tailored for the current use case data fetch requirements.

Last but not least, the database connection could be held throughout the UI rendering phase(depending on your connection release mode) which increases connection lease time and limits the overall transaction throughput due to congestion on the database connection pool. The more the connection is held, the more other concurrent requests are going to wait to get a connection from the pool.

So, either you get the connection held for too long, either you acquire/release multiple connections for a single HTTP request, therefore putting pressure on the underlying connection pool and limiting scalability.

Spring Boot

Unfortunately, Open Session in View is enabled by default in Spring Boot.

So, make sure that in the application.properties configuration file, you have the following entry:

spring.jpa.open-in-view=false