Thursday, March 06, 2014

World's smallest DAO

The world's smallest Java DAO (with heavy lifting provided by Spring Framework):

public class SimpleDAO {
    private final DataSourceTransactionManager transactionManager;

    public SimpleDAO(DataSourceTransactionManager transactionManager) {
        this.transactionManager = transactionManager;
    }

    public <T> T dao(Dao<T> dao) {
        return dao.using(transactionManager);
    }

    public interface Dao<T> {
        default T using(DataSourceTransactionManager transactionManager) {
            return new TransactionTemplate(transactionManager).execute(
                    status -> on(new JdbcTemplate(transactionManager.getDataSource()), status));
        }

        T on(JdbcTemplate jdbcTemplate, TransactionStatus status);
    }
}

Usage:

class InviteesDAO {
    private final SimpleDAO transact;

    InviteesDAO(DatabaseTransactionManager transactionManager) {
        transact = new SimpleDAO(tranactionManager);
    }

    List<String> getInvitees() {
        return transact.dao((jdbcTemplate, status) -> jdbcTemplate.queryForList(
            "SELECT username FROM invitees", String.class));
    }

    void invite(String username) {
        transact.dao((jdbcTemplate, status) -> jdbcTemplate.update(
            "INSERT INTO invitees (username) VALUES (?)", username));
    }
}

UPDATE: Demonstrate with composition rather than inheritance.

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