Querydsl fetchCount() & fetch() NullPointerException, Connection is closed - spring-boot

Question

My goal is to implement dao method within pagination, sorting and filtering. For pagination I need get firstly count and then set offset & limit and get result (so get just one "page" from database):
#Slf4j
#Repository
public class UserJdbcRepository {
private final SQLQueryFactory queryFactory;
#Autowired
public UserJdbcRepository(DataSource dataSource) {
Configuration configuration = new Configuration(new OracleTemplates());
configuration.setExceptionTranslator(new SpringExceptionTranslator());
this.queryFactory = new SQLQueryFactory(configuration, dataSource);
}
public Page<User> findAll(BooleanExpression predicate, Pageable pageable) {
QUser u = new QUser("u");
SQLQuery<Tuple> sql = queryFactory
.select(u.userId, // omitted)
.from(u)
.where(predicate);
long count = sql.fetchCount();
List<Tuple> results = sql.fetch();
// Conversion List<Tuple> to List<User> omitted
return new PageImpl<>(users, pageable, count);
}
}
fetchCount() is executed correctly, but fetch() is throwing NullPointerException:
java.lang.NullPointerException: null
at com.querydsl.sql.AbstractSQLQuery.fetch(AbstractSQLQuery.java:502) ~[querydsl-sql-4.4.0.jar:na]
From debug I found that root cause is in com.querydsl.sql.AbstractSQLQuery:
java.sql.SQLException: Connection is closed
If I create second (the same as first one) query sql2, then it is working (of course):
SQLQuery<Tuple> sql2 = queryFactory
.select(... // same as first one)
long count = sql.fetchCount();
List<Tuple> results = sql2.fetch();
My question is if connection should be really closed after fetchCount() is called? Or do I have some misconfiguration?
I have SpringBoot 2.4.5; spring-data-commons 2.5.0; Oracle driver ojdbc8 21.1.0.0; QueryDSL 4.4.0
<dependency>
<groupId>com.querydsl</groupId>
<artifactId>querydsl-sql</artifactId>
<version>${querydsl.version}</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>com.querydsl</groupId>
<artifactId>querydsl-sql-spring</artifactId>
<version>${querydsl.version}</version>
<scope>compile</scope>
</dependency>
<plugin>
<groupId>com.querydsl</groupId>
<artifactId>querydsl-maven-plugin</artifactId>
<version>${querydsl.version}</version>
<executions>
<execution>
<goals>
<goal>export</goal>
</goals>
</execution>
</executions>
<configuration>
<jdbcDriver>oracle.jdbc.OracleDriver</jdbcDriver>
<jdbcUrl>jdbc:oracle:thin:#//localhost:1521/XE</jdbcUrl>
<jdbcUser>user</jdbcUser>
<jdbcPassword>password</jdbcPassword>
<sourceFolder>${project.basedir}/src/main/java</sourceFolder>
<targetFolder>${project.basedir}/src/main/java</targetFolder>
<packageName>org.project.backend.repository.querydsl</packageName>
<schemaToPackage>true</schemaToPackage>
<schemaPattern>project</schemaPattern>
<tableNamePattern>
// omitted
</tableNamePattern>
</configuration>
<plugin>

Answer 1

Issue is caused by missconfiguration. QueryDSL doc contains Spring integration section, where is mentioned that SpringConnectionProvider must be used. So I changed my constructor and it is working now as expected:
#Autowired
public UserJdbcRepository(DataSource dataSource) {
Configuration configuration = new Configuration(new OracleTemplates());
configuration.setExceptionTranslator(new SpringExceptionTranslator());
// wrong: this.queryFactory = new SQLQueryFactory(configuration, dataSource);
Provider<Connection> provider = new SpringConnectionProvider(dataSource);
this.queryFactory = new SQLQueryFactory(configuration, provider);
}
I also found there is useful method fetchResults() containing count for pagination purpose (so not needed to explicitly call fetchCount()):
public Page<User> findAll(BooleanExpression predicate, Pageable pageable) {
QUser u = new QUser("u");
SQLQuery<Tuple> sql = queryFactory
.select(u.userId, // omitted)
.from(u)
.where(predicate);
sql.offset(pageable.getOffset());
sql.limit(pageable.getPageSize());
QueryResults<Tuple> queryResults = sql.fetchResults();
long count = queryResults.getTotal();
List<Tuple> results = queryResults.getResults();
// Conversion List<Tuple> to List<User> omitted
return new PageImpl<>(users, pageable, count);
}