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## org.neo4j.graphdb.event
Interface TransactionEventHandler<T>

Type Parameters:
  `T` - The type of a state object that the transaction handler can use to pass information from the `beforeCommit(TransactionData)` event dispatch method to the `afterCommit(TransactionData, Object)` or `afterRollback(TransactionData, Object)` method, depending on whether the transaction succeeded or failed.

    public interface TransactionEventHandler<T>

An event handler interface for Neo4j Transaction events. Once it has been registered at a `GraphDatabaseService` instance it will receive events about what has happened in each transaction which is about to be committed. Handlers won't get notified about transactions which hasn't performed any write operation or won't be committed (either if `Transaction.success()` hasn't been called or the transaction has been marked as failed, `Transaction.failure()`.

Right before a transaction is about to be committed the `beforeCommit(TransactionData)` method is called with the entire diff of modifications made in the transaction. At this point the transaction is still running so changes can still be made. However there's no guarantee that other handlers will see such changes since the order in which handlers are executed is undefined. This method can also throw an exception and will, in such a case, prevent the transaction from being committed.

If `beforeCommit(TransactionData)` is successfully executed the transaction will be committed and the `afterCommit(TransactionData, Object)` method will be called with the same transaction data as well as the object returned from `beforeCommit(TransactionData)`. This assumes that all other handlers (if more were registered) also executed `beforeCommit(TransactionData)` successfully.

If `beforeCommit(TransactionData)` isn't executed successfully, but instead throws an exception the transaction won't be committed and a `TransactionFailureException` will (eventually) be thrown from `Transaction.finish()`. All handlers which at this point have had its `beforeCommit(TransactionData)` method executed successfully will receive a call to `afterRollback(TransactionData, Object)`.

Author:
  Tobias Ivarsson, Mattias Persson

Method Summary
`void` | `afterCommit(TransactionData data, T state)` Invoked after the transaction has been committed successfully.
`void` | `afterRollback(TransactionData data, T state)` Invoked after the transaction has been rolled back if committing the transaction failed for some reason.
`T` | `beforeCommit(TransactionData data)` Invoked when a transaction is about to be committed.

Method Detail

### beforeCommit

    T beforeCommit(TransactionData data)
                   throws Exception

  Invoked when a transaction is about to be committed. If this method throws an exception the transaction will be rolled back and a `TransactionFailureException` will be thrown from `Transaction.finish()`. The transaction is still open when this method is invoked, making it possible to perform mutating operations in this method. This is however highly discouraged. Changes made in this method are not guaranteed to be visible by this or other `TransactionEventHandler`s.

  Parameters:
    `data` - the changes that will be committed in this transaction.
  Returns:
    a state object (or `null`) that will be passed on to `afterCommit(TransactionData, Object)` or `afterRollback(TransactionData, Object)` of this object.
  Throws:
    `Exception` - to indicate that the transaction should be rolled back.

### afterCommit

    void afterCommit(TransactionData data,
                     T state)

  Invoked after the transaction has been committed successfully.

  Parameters:
    `data` - the changes that were committed in this transaction.
    `state` - the object returned by `beforeCommit(TransactionData)`.

### afterRollback

    void afterRollback(TransactionData data,
                       T state)

  Invoked after the transaction has been rolled back if committing the transaction failed for some reason.

  Parameters:
    `data` - the changes that were committed in this transaction.
    `state` - the object returned by `beforeCommit(TransactionData)`.

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