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This guide shows how to handle transactions in Spring Boot using the H2 database to ensure consistent data operations.

1. Add Dependencies

In pom.xml: 
<dependency>
  <groupId>com.h2database</groupId>
  <artifactId>h2</artifactId>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

2. Configure H2 Database

In application.properties: 
spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driver-class-name=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=
spring.jpa.hibernate.ddl-auto=update

3. Create Entity

import jakarta.persistence.*;

@Entity
public class Account {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String owner;
    private Double balance;
}

4. Repository

import org.springframework.data.jpa.repository.JpaRepository;

public interface AccountRepository extends JpaRepository<Account, Long> {
}

5. Service with Transactions

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

@Service
public class AccountService {

    private final AccountRepository repo;

    public AccountService(AccountRepository repo) {
        this.repo = repo;
    }

    @Transactional
    public void transfer(Long fromId, Long toId, Double amount) {
        Account from = repo.findById(fromId).orElseThrow();
        Account to = repo.findById(toId).orElseThrow();

        from.setBalance(from.getBalance() - amount);
        to.setBalance(to.getBalance() + amount);

        repo.save(from);
        repo.save(to);
    }
}
Here, @Transactional ensures both updates happen together, or none at all.

6. Test Transaction Rollback

If an exception occurs inside transfer(), the transaction will rollback and balances stay unchanged.

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