Atomicity defines how the card handles the contents of persistent storage after a stop, failure, or fatal exception during an update of a single object or class field or array component. If power is lost during the update, the applet developer shall be able to rely on what the field or array component contains when power is restored.

The Java Card platform guarantees that any update to a single persistent object or class field will be atomic. In addition, the Java Card platform provides single component level atomicity for persistent arrays. That is, if the smart card loses power during the update of a data element (field in an object, class or component of an array) that shall be preserved across CAD sessions, that data element shall be restored to its previous value.

Some methods also guarantee atomicity for block updates of multiple data elements. For example, the atomicity of the Util.arrayCopy method guarantees that either all bytes are correctly copied or else the destination array is restored to its previous byte values.

An applet might not require atomicity for array updates. The Util.arrayCopyNonAtomic method is provided for this purpose. It does not use the transaction commit buffer even when called with a transaction in progress.