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# Data Access Exception Translation
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Spring's DAO exception hierarchy provides consistent error handling across different data access technologies.
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## Core Exception Hierarchy
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```java { .api }
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public abstract class DataAccessException extends NestedRuntimeException {
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    public DataAccessException(String msg);
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    public DataAccessException(String msg, @Nullable Throwable cause);
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}
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// Key exception types
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public class DataIntegrityViolationException extends NonTransientDataAccessException {}
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public class DataRetrievalFailureException extends NonTransientDataAccessException {}
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public class DeadlockLoserDataAccessException extends PessimisticLockingFailureException {}
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public class DuplicateKeyException extends DataIntegrityViolationException {}
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public class OptimisticLockingFailureException extends ConcurrencyFailureException {}
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public class PessimisticLockingFailureException extends ConcurrencyFailureException {}
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public class QueryTimeoutException extends TransientDataAccessException {}
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```
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## DataAccessUtils
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```java { .api }
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public abstract class DataAccessUtils {
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    public static <T> T singleResult(@Nullable Collection<T> results) 
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        throws IncorrectResultSizeDataAccessException;
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    public static <T> T requiredSingleResult(@Nullable Collection<T> results) 
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        throws IncorrectResultSizeDataAccessException;
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    public static <T> T uniqueResult(@Nullable Collection<T> results) 
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        throws IncorrectResultSizeDataAccessException;
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}
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```
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## Usage Examples
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```java
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@Service
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public class UserService {
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    public User createUser(User user) {
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        try {
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            return userRepository.save(user);
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        } catch (DuplicateKeyException e) {
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            throw new UserAlreadyExistsException(user.getEmail());
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        } catch (DataIntegrityViolationException e) {
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            throw new InvalidUserDataException(e.getMessage());
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        }
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    }
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}
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```