Querydsl

Type-safe queries for Java

Timo Westkämper@timowest

www.querydsl.com

Before Querydsl

● Queries as strings within code

TypedQuery<Person> query = em.createQuery( "select person from Person person " +

"where person.firstName = ?1", Person.class); query.setParameter(1, "Max"); List<Person> persons = query.getResultList();

● Must remember query syntax, domain classes, properties and relationships● Syntax reference always at hand● Domain model/schema reference at hand● High cognitive overhead● Error-prone

Before Querydsl

● Dynamic query building by string concatenation● Very hard with multiple joins, ordering and complex

conditionals depending on actual parameters

StringBuilder where = new StringBuilder(); if (firstName != null) where.append("person.firstName = :firstName"); ... TypedQuery<Person> query = entityManager.createQuery( "select person from Person person where " + where, Person.class); if (firstName != null) query.setParameter("firstName", firstName); ... List<Person> persons = query.getResultList();

Before Querydsl

● Query syntax validation by execution is slow and breaks the flow

● Each back-end has its own query language and API● SQL-like for JPA and JDO, but not for MongoDB

and Lucene● Verbose parameter binding by name or position to

parameter placeholders of a prepared statement● Or risk injection attack if parameters are directly

concatenated to query

Before Querydsl

● Hibernate Criteria API as an alternative? ● Better for dynamic queries and has easier

parameter binding, but...● Lacking expressivity, unintuitive, verbose,

cognitive overhead for schema if not for syntax, not type-safe, slow validation...

● Hibernate with three query languages to master with different focuses and expressivity

Querydsl

● Domain model specific type-safe query language● Compile time query validation ● Instant feedback on query errors

● Compact and intuitive fluent syntax● Syntactically close to SQL● Great for dynamic query building

● Supports multiple back-ends and query languages with consistent query API● JPA/Hibernate, JDO, SQL, Lucene, Mongodb...● Once you know basics of one language, you know

basics of all SQL-like Querydsl languages

Querydsl

● Autocomplete with Java IDEs● No need to remember exact syntax● No need to remember property names

● Better support for domain model refactoring● When domain changes, queries show compile

errors● Autocomplete helps fixing those

● Developed for real-life projects, e.g. Balancion and Cyclos

● Business friendly license (Apache 2.0)

Querydsl

● Development started in 2007 with public releases since 2008

● Querydsl statistics:● Approximately 24 000 LOC● Test Code coverage about 75% (target 80%)● Sonar reports● FindBugs with extra annotations (@Nullable)

● Discussions about standardisations● JDO/DataNucleus started with Querydsl...

Querydsl usage

● Create your variablesQPerson.person // default variablenew QPerson("myPerson") // custom variable

● Create your queryJPAQuery, HibernateQuery, SQLQueryImpl etc

● Populate your queryfrom, where, groupBy, having, orderBy

● Get the resultscount, iterate, list, uniqueResult

Querydsl usage

● All expressions can be reused, immutables with caching – except BooleanBuilder and a few others

● Queries, sub queries and BooleanBuilder are stateful builder with cascading methods

Overview of JPAQuery signature

fromQuery sources

innerJoin, join, leftJoin, fullJoin, onJoin elements join(source, alias) [.on(source.prop.eq(alias.prop))]

whereQuery filters, varargs for intersection (and)and(), or(), allOf(), anyOf()

Overview of JPAQuery signature

groupByGroup by arguments in varargs form

havingHaving filter of the "group by” as an varags array of Predicate expressions.

orderByOrdering of the result as an varargs array of order expressions. asc() and desc() on numeric, string and other comparable expression

limit, offset, restrictPaging of the result Limit for max results and Offset for skipping rows and Restrict for defining both in one call

Overview of JPAQuery signature

listGet the results as a typed List

listResultsGet the results as a typed List and total row count for paging

iterate Get the results as a typed Iterator

countGet the row count as a long

uniqueResultGet a typed single row result

Simple example

QPerson person = QPerson.person;JPAQuery query = new JPAQuery(entityManager);List<Person> persons = query.from(person) .where( person.firstName.eq("John"), person.lastName.eq("Doe")) .list(person);

=>

select person from com.acme.Person personwhere person.firstName eq = ?1 and person.lastName = ?2

Order

// Get persons ordered by last name and first name (desc)query.from(person)

.orderBy(person.lastName.asc(), person.firstName.desc())

.list(person);

=>

select person from Person person order by person.lastname asc, person.firstName desc

Order

// Get persons ordered by women firstquery.from(person)

.orderBy(person.gender.when(Gender.FEMALE).then(0).otherwise(1).asc())

.list(person);

=>

select person from Person personorder by case person.gender = Gender.FEMALE then 0 else 1 end asc

Grouping

// Get person counts grouped by last namequery.from(person) .groupBy(person.lastName) .list(person.lastName, person.count());

=>

select person.lastName, count(person) from Person persongroup by person.lastName

Subqueries

//Get persons with max child countQPerson parent = new QPerson("parent");query.from(person) .where(person.children.size().eq( new JPASubQuery().from(parent)

.uniqueResult(parent.children.size().max()) )).list(person);

=>

select person from Person personwhere person.children.size() = ( select max(parent.children.size()) from Person parent)

Constructor projection

// DTO class with @QueryProjection constructor annotationpublic class PersonInfo { long id; String name; @QueryProjection public PersonInfo(long id, String name) { this.id = id; this.name = name; }}

// List PersonInfo DTOs List<PersonInfo> infos = query.from(person)

.list(new QPersonInfo(person.id, person.lastName.concat(", ”).concat(person.firstName)));

Tuple projection

// List ages of personsList<Tuple> tuples = query.from(person).list(new QTuple( person.lastName, person.firstName, person.yearOfBirth));

for (Tuple tuple : tuples){ // Typed access to mapped query results! String name = tuple.get(person.firstName) + " " + tuple.get(person.lastName); int age = tuple.get(person.yearOfBirth) - getCurrentYear(); System.out.println(name + " is " + age + " years");}

BooleanBuilder

● Helper for building complex Boolean expressions dynamically

BooleanBuilder nameDisjunction = new BooleanBuilder();for (String name : names) { nameDisjunction.or(person.firstName.like(name)); nameDisjunction.or(person.lastName.like(name));}query.where(nameDisjunction);

Update

// Set firstName of all Does to Johnlong updatedRowCount = new JPAUpdateClause(getEntityManager(), person) .set(person.firstName, "John") .where(person.lastName.eq("Doe")) .execute();

=>

update Person personset person.firstName = ?1where person.lastName = ?2

Delete

// Delete all John Doeslong updatedRowCount = new JPADeleteClause(getEntityManager(), person) .where(person.lastName.eq("Doe"), person.firstName.eq("John")) .execute();

=>

delete Person personwhere person.lastName = ?1 and person.firstName = ?2

Querydsl extensions

● Customize the code generation● @QueryType(PropertyType.NONE)

● Non searchable● @QueryType(PropertyType.SIMPLE)

● Equality comparisons only (eq, ne, in)● Custom query classes

● Extend abstract super classes and preserve fluent API

● Custom expressions● Static delegate methods with @QueryDelegate● Template expressions for e.g. custom SQL

functions

Querydsl extensions

● Query serialization can be customized● Works for JPA, JDO and SQL● SQL dialects● Overriding default templates (e.g.

String#startsWith with like or regexp or...)● Expression DSL can be replaced

● E.g. Querydsl for Scala● Custom back-ends

● Lucene (10 classes) + Mongodb (6 classes)

Delegate methods

public class MyQueryExtensions { @QueryDelegate(Date.class) public static NumberExpression<Integer> yearAndMonth(DateTimePath<Date> date) { return date.year().multiply(100).add(date.month()); }}

=>

package ext.java.util;...public class QDate extends DateTimePath<java.util.Date> {... public NumberExpression<Integer> yearAndMonth() { return MyQueryExtensions.yearAndMonth(this); }}

Template expressions

// ilike query.from(person).where(BooleanTemplate.create("{0} ilike {1}”, person.lastName, ConstantImpl.create("P%"))).list(person);

=>

select person from Person personwhere person.lastName ilike ?1

Custom query classes

public class PersonQuery extends AbstractJPAQuery<PersonQuery> { final QPerson person = QPerson.person; public PersonQuery(EntityManager em) { super(em); from(person); } public PersonQuery nameMatches(String name) { return where(person.firstName.like(name) .or(person.lastName.like(name))); }}

JPA 2.0 Criteria vs Querydsl

● JPA 2 Criteria is the standard for type-safe queries in JPA, but Querydsl is in our opinion superior in many ways● Easier and less verbose syntax● Customizable● Supports multiple back-ends – not just JPA

● JPA has a difficult to use static query-model● Verbose property paths● Operations via builder object

● Inverse order: “equals property value” vs. “property equals value”● Broken flow

Criteria example

// All possible pairs of single males and femalesCriteriaQuery<Person> query = builder.createQuery(Person.class);Root<Person> men = query.from( Person.class );Root<Person> women = query.from( Person.class );Predicate menRestriction = builder.and( builder.equal( men.get( Person_.gender ), Gender.MALE ), builder.equal( men.get( Person_.relationshipStatus ), RelationshipStatus.SINGLE ));Predicate womenRestriction = builder.and( builder.equal( women.get( Person_.gender ), Gender.FEMALE ), builder.equal( women.get( Person_.relationshipStatus ), RelationshipStatus.SINGLE ));query.where( builder.and( menRestriction, womenRestriction ) );

Querydsl example

// All possible pairs of single males and femalesJPAQuery query = new JPAQuery(entityManager);QPerson men = new QPerson("men");QPerson women = new QPerson("women");query.from(men, women).where(

men.gender.eq(Gender.MALE),men.relationshipStatus.eq(RelationshipStatus.SINGLE),women.gender.eq(Gender.FEMALE),women.relationshipStatus.eq(RelationshipStatus.SINGLE));

SQL

● Pretty similar to JPA/Hibernate● No deep paths over relations though● No implicit joins

SQLTemplates templates = new MySQLTemplates();...

SQLQuery query = new SQLQueryImpl(connection, templates);

query.from(person);query.innerJoin(parent).on(parent.id.eq(person.parent.id));

● Shortcut for joins with foreign keys

query.innerJoin(person.parentFK, parent);

SQL

● Maven plugin for generating query model● Support for special SQL constructs and extensions

● Databases supported include● MySQL● PostgreSQL● Oracle● MS SQL Server● H2● HSQLDB● Derby● SQLite● CUBRID

SQL extensions

● Sub class of AbstractSQLQuery● e.g. OracleQuery with connectByPrior

● Template expressions● Direct addition of “flags”

SQLInsertClause insert = new SQLInsertClause(connection, templates, person);insert.addFlag(Position.START_OVERRIDE, "replace into ");

JPA/Hibernate Maven Integration

<build><plugins><plugin> <groupId>com.mysema.maven</groupId> <artifactId>maven-apt-plugin</artifactId> <version>1.0.3</version> <executions> <execution> <goals><goal>process</goal></goals> <configuration> <outputDirectory>target/generated-sources/java</outputDirectory> <processor>com.mysema.query.apt.jpa.JPAAnnotationProcessor</processor> </configuration> </execution> </executions></plugin></plugins></build>

SQL Maven Integration

<build><plugins><plugin> <groupId>com.mysema.querydsl</groupId> <artifactId>querydsl-maven-plugin</artifactId> <version>${querydsl.version}</version> <executions><execution> <goals><goal>export</goal></goals> </execution></executions> <configuration> <jdbcDriver>org.apache.derby.jdbc.EmbeddedDriver</jdbcDriver> <jdbcUrl>jdbc:derby:target/demoDB;create=true</jdbcUrl> <!—- optional elements : namePrefix, jdbcUser, jdbcPassword, schemaPattern, tableNamePattern --> <packageName>com.myproject.domain</packageName> <targetFolder>${project.basedir}/target/generated-sources/java</targetFolder> </configuration> <dependencies><dependency> <!—- jdbc driver dependency --> <groupId>org.apache.derby</groupId> <artifactId>derby</artifactId> <version>${derby.version}</version> </dependency></dependencies></plugin></plugins></build>

What does Mysema offer for Querydsl?

● Free public support● GitHub Issues● Querydsl Google Group● Mysema Blog

● Consulting services● User support● Custom extensions and integration● Training

Querydsl support from other companies

● VMware uses Querydsl in Spring Data for the following backends● JPA● SQL/JDBC● MongoDB● Neo4j

● Spring Data is a good option if you want to use repositories in Spring with Querydsl support

Questions?

Thanks!

Timo Westkämper@timowest

www.querydsl.comwww.mysema.com