What is propagation and transaction dissemination in Java?

Propagation of transactions in Java defines how the transactional context is propagated between methods. When calling a transactional method from another method (transactional or not), the propagation rule determines whether to use an existing transaction, create a new one, or behave differently.

The most common propagation rules in the Spring Framework are:

• REQUIRED: Use the existing transaction if one exists. If not, create a new one. This is the default rule.
• SUPPORTS: Use the existing transaction if available. If not, execute without a transaction.
• MANDATORY: Use the existing transaction. If none exists, throw an exception.
• REQUIRES_NEW: Always create a new, independent transaction. If an existing transaction is active, it is suspended.
• NOT_SUPPORTED: Execute without a transaction. If an existing transaction is active, it is suspended.
• NEVER: Execute without a transaction. If an existing transaction is active, throw an exception.
• NESTED: If an active transaction exists, create a "nested" transaction (savepoint). If not, behave like REQUIRED. Nested transactions are not supported by all databases.

Example of usage in Spring:

// Example of using @Transactional annotation with propagation
@Transactional(propagation = Propagation.REQUIRES_NEW)
public void createUserWithIndependentOperation(User user) {
    // Logic for creating a user

    performIndependentOperation(); // This method will run in a new transaction
}

@Transactional(propagation = Propagation.REQUIRES_NEW)
public void performIndependentOperation() {
    // Logic for independent operation
}

The choice of propagation rule depends on the required atomicity and isolation of operations. Incorrect usage can lead to undesirable behavior, such as loss of changes or deadlocks.