Why Querydsl always requires connection to be transactional? - spring

Question

Recently I tried a new tool for db access called Querydsl in my Spring Boot app, here is how I configure the context in #Configuration class:
#Bean
public com.querydsl.sql.Configuration querydslConfiguration() {
SQLTemplates templates = OracleTemplates.builder().build();
com.querydsl.sql.Configuration configuration = new com.querydsl.sql.Configuration(templates);
configuration.setExceptionTranslator(new SpringExceptionTranslator());
return configuration;
}
#Bean
public SQLQueryFactory queryFactory(DataSource dataSource) {
Provider<Connection> provider = new SpringConnectionProvider(dataSource);
return new SQLQueryFactory(querydslConfiguration(), provider);
}
My query is a quite simple select:
fun detailedEntityByIds(ids: Set<String>): List<DetailedEntity> {
val qDetails = QTContainerDetails.tContainerDetails
return sqlQueryFactory.select(qDetails).from(qDetails)
.where(qDetails.id.`in`(ids))
.fetch().map { mapper.qDslEntToModel(it) }
}
Then I faced with was the following exception:
java.lang.IllegalStateException: Connection is not transactional
I quickly found this question: [QueryDSL/Spring]java.lang.IllegalStateException: Connection is not transactional with an advice to use #Transactional for solving this problem.
Why does Querydsl requires connections to be transactional? I used to put #Transactional on a service layer methods where I really need it. Now Querydsl 'forces' me to put it on a whole DAO class, because looks like it is required for every Querydsl query.

Answer 1

From the Javadoc
/**
* {#code SpringConnectionProvider} is a Provider implementation which provides a transactionally bound connection
*
* <p>Usage example</p>
* <pre>
* {#code
* Provider<Connection> provider = new SpringConnectionProvider(dataSource());
* SQLQueryFactory queryFactory = SQLQueryFactory(configuration, provider);
* }
* </pre>
*/
The reason is for resource management. You don't have access to the underlying JDBC implementation. The transaction will close ResultSets, Statements, Connections, etc. Without a transaction, every connection would be left open, the connection pool saturated, the database running out of resources, etc.
If you want to manage your own resources, you could write your own Provider<Connection> and pass in the DataSource E.G.
private static Provider<Connection> getConnection(DataSource dataSource) {
return () -> org.springframework.jdbc.datasource.DataSourceUtils.getConnection(dataSource);
}