Does the @Transactional Annotation Merge New Transactions into Existing Transactions?

Discover how the `@Transactional` annotation works in Spring, especially regarding transaction management across multiple requests and connections.
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Understanding the @Transactional Annotation in Spring

In the world of Java development, especially when using Spring Boot with frameworks like Hibernate or JdbcTemplate, managing transactions properly is crucial to maintain data consistency and integrity. One common question that arises is whether the @Transactional annotation merges new transactions into existing transactions, particularly when dealing with multiple concurrent requests. Let's break this down to clarify how Spring handles transactions in multi-threaded environments.

The Context: What is @Transactional?

The @Transactional annotation is a powerful feature in Spring that allows developers to define the scope of a single database transaction. By default, this annotation uses the propagation level Propagation.REQUIRED. This means that:

If an existing transaction is present, the method will execute within that transaction.

If no transaction exists, a new transaction will be created.

This behavior leads to interesting dynamics, especially when a controller method initiates multiple calls to the database in rapid succession.

The Scenario: Multiple Requests in a Short Timeframe

Let's say you have a controller method that accepts user input and calls a repository layer to perform database transactions. Here’s a simplified view of what your code looks like:

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

If you hit your API endpoint /hello with different data (say data1 and data2) before the first request has finished, the question is: Will there be a single transaction, or will there be two separate independent transactions?

The Answer: Transactions Per Thread

The simple answer is that you will have a transaction per thread. In the Spring web environment (like Tomcat), each incoming request is generally handled in a separate thread. Thus:

First API call with data1 starts a transaction in one thread.

Second API call with data2 starts a new transaction in another thread.

This means that each request is independent and does not interfere with the other, leading to two separate transactions based on your setup.

Understanding JdbcTemplate and Connection Management

Now, the next part of your inquiry involves how JdbcTemplate manages connections within transactions, particularly in the context of connection pools like Hikari. When using the @Transactional annotation, there’s a critical underlying operation regarding connection management:

Connection Pooling and JdbcTemplate

Connection Pooling: When you use a database connection pool, such as HikariCP, the pool provides connections to your application. If configured with a pool size (e.g., 5 connections), Spring can allocate from this pool as needed.

Transaction Connection Handling: When a transaction is started with @Transactional, it will usually borrow a connection from the pool. For example, let’s say it picks up connectionId1554.

Ensuring Connection Consistency: Spring ensures that all database operations conducted within that transaction use the same connection (connectionId1554) throughout its lifecycle. This is crucial for maintaining transactional integrity, ensuring that multiple database calls within the same transaction are executed against the same connection.

Conclusion

In summary, the @Transactional annotation is adept at managing transaction boundaries on a per-thread basis in a Spring application. Each request will have its own transaction, and Spring’s transaction management, coupled with connection pooling, ensures that the same connection is used for all operations within that transaction.

Final Thoughts

Understanding how Spring handles transactions with @Transactional can greatly enhance your ability to