What are transaction isolation levels?

Transaction isolation levels in Java (via JDBC) determine how much a transaction should be isolated from changes made by other concurrent transactions. They control which anomalies of parallel transaction execution can occur.

Standard isolation levels (according to ANSI SQL):

• TRANSACTION_READ_UNCOMMITTED:

  • The lowest level.
  • One transaction can see uncommitted changes of another transaction (dirty read).
  • Non-repeatable read and phantom read are also possible.
  • Provides maximum performance by minimal locking.

• TRANSACTION_READ_COMMITTED:

  • A transaction sees only committed changes of other transactions.
  • Dirty reads are impossible.
  • Non-repeatable read and phantom read are possible.
  • Often used as the default.

• TRANSACTION_REPEATABLE_READ:

  • A transaction sees only changes committed before it started. Re-reading the same data within a transaction will yield the same values.
  • Dirty read and non-repeatable read are impossible.
  • Phantom reads are possible (inserting a new row visible upon re-query with a predicate).

• TRANSACTION_SERIALIZABLE:

  • The highest level.
  • Transactions are executed sequentially, as if they were run one after another.
  • All anomalies of parallel execution are excluded (dirty read, non-repeatable read, phantom read).
  • Ensures maximum consistency at the cost of significant performance reduction due to strong locking.

In JDBC, the isolation level can be set for a connection:

// Getting a connection
Connection connection = DriverManager.getConnection(url, user, password);

// Setting the isolation level
connection.setTransactionIsolation(Connection.TRANSACTION_READ_COMMITTED);

// Disabling auto-commit before starting a transaction
connection.setAutoCommit(false);

try {
    // Performing database operations

    // Committing the transaction
    connection.commit();
} catch (SQLException e) {
    // Rolling back the transaction in case of error
    connection.rollback();
    throw e;
} finally {
    // Restoring auto-commit and closing the connection
    connection.setAutoCommit(true);
    if (connection != null) {
        connection.close();
    }
}

Choosing an isolation level depends on data consistency requirements and application performance. Lower levels increase concurrency but may lead to anomalies. Higher levels guarantee greater consistency but may reduce performance.