Understanding the EntityManager refresh() Method: Why It Resets Instead of Refreshes?

Описание: Discover the behavior of the `EntityManager` `refresh()` method in Hibernate and JPA. Learn about transaction isolation levels and how to address issues where `refresh()` resets instead of refreshing data.
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Understanding the EntityManager refresh() Method: Why It Resets Instead of Refreshes?

If you're working with Hibernate and JPA, you may encounter a situation where the em.refresh(person) method does not behave as you would expect. Instead of refreshing the entity with the latest value from the database, it resets the changes made to the entity in memory.

This behavior can be puzzling, especially when you see outputs that contradict your expectations.

Let's delve deeper into the problem and the solution.

The Problem: Unexpected Behavior of refresh()

Consider the following code snippet:

[[See Video to Reveal this Text or Code Snippet]]

This code raises an important question: Why does refresh() return the old value rather than the updated one?

The Solution: Transaction Isolation Level

To understand this behavior, it’s crucial to recognize the role of transaction isolation levels in database management. The default isolation level for many databases is Repeatable Read, which means:

During a transaction, if a value is read, the transaction will always return that value unless modified within the same transaction.

Changes made by other transactions are not visible until the transaction is completed.

This isolation can lead to confusion, as fresh updates from the database might not appear to be reflected when calling refresh().

Steps to Solve the Problem

To see changes made by other transactions while inside your transaction, you need to change the isolation level to READ_COMMITTED. This isolation level allows transactions to see changes made by others if they are committed before your transaction completes. To implement this, add the following property to your Hibernate configuration:

[[See Video to Reveal this Text or Code Snippet]]

Clarifying Definitions

To further elucidate the situation, it helps to understand some key concepts:

Repeatable Read: Within a transaction, the database will see the same value unless it's modified within that transaction.

Hibernate Flush: Changes you make (like person.setAge(24)) are not visible to the database until they are flushed. Flushing occurs automatically under certain conditions—like committing the transaction or executing a query—but not when calling refresh().

Hibernate Refresh: This method reads the entity data from the database and updates the session or first-level cache with that data.

Summary of Behavior

Here’s a concise recap of the points contributing to this issue:

When you modify the age with setAge(), that change isn't automatically reflected in the database until you flush.

Concurrent modifications to the entity from another transaction are not visible because of the isolated nature of transactions (unless using READ_COMMITTED).

Thus, calling refresh() does not ‘see’ the change from the other transaction; it sees the last committed value from the database, giving you 23 instead of the expected 25.

Conclusion

Understanding the interaction between transaction isolation levels and the EntityManager methods can greatly aid in resolving issues related to data visibility in Hibernate. By adjusting the isolation level to READ_COMMITTED, you’ll be able to ensure that your transactions properly reflect the most current data, avoiding the confusion that arises when refresh() resets rather than refreshes.

For further queries or additional assistance, feel free to share your experiences or ask questions in the comments below!