Transaction management

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 In JDBC (Java Database Connectivity), transaction management refers to the process of controlling and managing the execution of multiple database operations as a single unit of work. A transaction ensures that either all operations succeed, or none of them do, maintaining the integrity of the database.

Basic Concepts of Transactions in JDBC:

  1. ACID Properties:

    • Atomicity: All operations within the transaction are treated as a single unit. If any operation fails, the entire transaction is rolled back.
    • Consistency: The database is always in a consistent state after a transaction, whether it succeeds or fails.
    • Isolation: Transactions are isolated from each other, meaning changes made in one transaction are not visible to others until the transaction is committed.
    • Durability: Once a transaction is committed, the changes are permanent, even if the system crashes.

In JDBC, rollback is used to undo any changes made to the database since the last commit(In the context of JDBC (Java Database Connectivity), a commit refers to the operation where changes made to a database are finalized and permanently saved.). It is typically used in transaction management when you want to ensure that if something goes wrong during a sequence of SQL statements, the database remains in a consistent state.

By default, JDBC commits every SQL statement immediately after it is executed. To enable transaction control (i.e., to control when commits and rollbacks happen), you need to disable auto-commit mode, begin a transaction, and manually commit or roll back the transaction.

Here is an example of how to use rollback in JDBC:

Steps for Using Rollback in JDBC

  1. Disable auto-commit mode.
  2. Execute a series of SQL statements.
  3. If everything is fine, commit the transaction.
  4. If an error occurs, rollback the transaction.

JDBC Rollback Example

import java.sql.*;

public class JDBCTransactionRollbackExample {

    private static final String DB_URL = "jdbc:postgresql://localhost:5432/mydatabase";
    private static final String USER = "postgres";
    private static final String PASSWORD = "root";
    
    public static void main(String[] args) {
        // Establish connection and perform transaction with rollback
        try (Connection conn = DriverManager.getConnection(DB_URL, USER, PASSWORD)) {

            // Disable auto-commit mode
            conn.setAutoCommit(false);
            System.out.println("Auto-commit is disabled, starting transaction...");

            try (Statement stmt = conn.createStatement()) {
                
                // 1. Execute some SQL operations
                String insertSQL1 = "INSERT INTO users (name, email) VALUES ('John Doe', 'john.doe@example.com')";
                stmt.executeUpdate(insertSQL1);
                System.out.println("First insert executed.");
                
                // Simulate an error during the second insert
                String insertSQL2 = "INSERT INTO users (name, email) VALUES ('Jane Doe', NULL)"; // NULL email
                stmt.executeUpdate(insertSQL2);
                System.out.println("Second insert executed.");
                
                // If everything is fine, commit the transaction
                conn.commit();
                System.out.println("Transaction committed successfully.");
                
            } catch (SQLException e) {
                // If an exception occurs, rollback the transaction
                System.out.println("Error occurred, rolling back transaction.");
                conn.rollback(); // Rollback the transaction to the last commit point
                e.printStackTrace();
            }

        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Explanation of the Code:

  1. Disable Auto-Commit Mode:

    conn.setAutoCommit(false);
    • This line disables auto-commit mode. By default, JDBC commits every transaction immediately after each statement is executed. With auto-commit disabled, you must manually control when to commit or roll back.

  2. Execute SQL Statements:

    • First, we execute an INSERT statement to add a user.
    • Then, we execute another INSERT statement, but this one is deliberately set to fail by using a NULL value for the email field, which might cause a database constraint violation (depending on the table's constraints).

  3. Commit the Transaction:

    conn.commit();
    • If no exceptions occur, the transaction is committed, making the changes permanent.

  4. Rollback the Transaction:

    conn.rollback();
    • If any exception occurs (e.g., trying to insert a NULL value for a field that should not be NULL), the transaction is rolled back. This undoes all changes made during the current transaction, and the database is restored to its state before the transaction started.

  5. Exception Handling:

    • The code inside the try block is where SQL operations are performed.
    • If any exception occurs, the catch block is triggered, and the transaction is rolled back to ensure data consistency.

  6. Transaction Management:

    • In case of a rollback, all database operations executed within the transaction (since the last commit) are undone. This ensures that your database doesn't end up in an inconsistent state due to partially completed transactions.

Summary of Key Methods:

  • conn.setAutoCommit(false): Disables auto-commit, so you can manage the transaction manually.
  • conn.commit(): Commits the transaction, making all changes permanent.
  • conn.rollback(): Rolls back the transaction, undoing any changes made during the transaction.

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