Add the following dependencies to your Maven project :

<dependency>
  <groupId>com.mysema.querydsl</groupId>
  <artifactId>querydsl-apt</artifactId>
  <version>${querydsl.version}</version>
  <scope>provided</scope>
</dependency>    
    
<dependency>
  <groupId>com.mysema.querydsl</groupId>
  <artifactId>querydsl-jpa</artifactId>
  <version>${querydsl.version}</version>
</dependency>

<dependency>
  <groupId>org.slf4j</groupId>
  <artifactId>slf4j-log4j12</artifactId>
  <version>1.6.1</version>
</dependency>   

And now, configure the Maven APT plugin :

<project>
  <build>
  <plugins>
    ...
    <plugin>
      <groupId>com.mysema.maven</groupId>
      <artifactId>apt-maven-plugin</artifactId>
      <version>1.0.6</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>
</project>

The JPAAnnotationProcessor finds domain types annotated with the javax.persistence.Entity annotation and generates query types for them.

If you use Hibernate annotations in your domain types you should use the APT processor com.mysema.query.apt.hibernate.HibernateAnnotationProcessor instead.

Run clean install and you will get your Query types generated into target/generated-sources/java.

If you use Eclipse, run mvn eclipse:eclipse to update your Eclipse project to include target/generated-sources/java as a source folder.

Now you are able to construct JPQL query instances and instances of the query domain model.

Place the jar files from the full-deps bundle on your classpath and use the following tasks for Querydsl code generation :

    <!-- APT based code generation -->
    <javac srcdir="${src}" classpathref="cp">
      <compilerarg value="-proc:only"/>      
      <compilerarg value="-processor"/>
      <compilerarg value="com.mysema.query.apt.jpa.JPAAnnotationProcessor"/>
      <compilerarg value="-s"/>
      <compilerarg value="${generated}"/>
    </javac>
    
    <!-- compilation -->
    <javac classpathref="cp" destdir="${build}">      
      <src path="${src}"/>
      <src path="${generated}"/>
    </javac>    

Replace src with your main source folder, generated with your folder for generated sources and build with your target folder.

If you are using Hibernate with an XML based configuration, you can use the XML metadata to create your Querydsl model.

com.mysema.query.jpa.hibernate.HibernateDomainExporer provides the functionality for this :

HibernateDomainExporter exporter = new HibernateDomainExporter(
  "Q",                     // name prefix
  new File("target/gen3"), // target folder
  configuration);          // instance of org.hibernate.cfg.Configuration 

exporter.export();

The HibernateDomainExporter needs to be executed within a classpath where the domain types are visible, since the property types are resolved via reflection.

All JPA annotations are ignored, but Querydsl annotations such as @QueryInit and @QueryType are taken into account.

To create queries with Querydsl you need to instantiate variables and Query implementations. We will start with the variables.

Let's assume that your project has the following domain type :

@Entity
public class Customer {
    private String firstName;
    private String lastName;

    public String getFirstName(){
        return firstName;
    }

    public String getLastName(){
        return lastName;
    }

    public void setFirstName(String fn){
        firstName = fn;
    }

    public void setLastName(String ln)[
        lastName = ln;
    }
}

Querydsl will generate a query type with the simple name QCustomer into the same package as Customer. QCustomer can be used as a statically typed variable in Querydsl queries as a representative for the Customer type.

QCustomer has a default instance variable which can be accessed as a static field :

QCustomer customer = QCustomer.customer;

Alternatively you can define your own Customer variables like this :

QCustomer customer = new QCustomer("myCustomer");

The Querydsl JPA module supports both the JPA and the Hibernate API.

To use the Hibernate API you use HibernateQuery instances for your queries like this :

// where session is a Hibernate session
HibernateQuery query = new HibernateQuery (session); 

If you are using the JPA API instead, you can instantiate a JPAQuery like this :

// where entityManager is a JPA EntityManager   
JPAQuery query = new JPAQuery (entityManager); 

Both HibernateQuery and JPAQuery implement the JPQLQuery interface.

To retrieve the customer with the first name Bob you would construct a query like this :

QCustomer customer = QCustomer.customer;
HibernateQuery query = new HibernateQuery (session);
Customer bob = query.from(customer)
  .where(customer.firstName.eq("Bob"))
  .uniqueResult(customer);

The from call defines the query source, the where part defines the filter and uniqueResult defines the projection and tells Querydsl to return a single element. Easy, right?

To create a query with multiple sources you use the query like this :

QCustomer customer = QCustomer.customer;
QCompany company = QCompany.company;
query.from(customer, company);    

And to use multiple filters use it like this

query.from(customer)
    .where(customer.firstName.eq("Bob"), customer.lastName.eq("Wilson"));   

Or like this

query.from(customer)
    .where(customer.firstName.eq("Bob").and(customer.lastName.eq("Wilson")));

In native JPQL form the query would be written like this :

from Customer as customer
    where customer.firstName = "Bob" and customer.lastName = "Wilson"

If you want to combine the filters via "or" then use the following pattern

query.from(customer)
    .where(customer.firstName.eq("Bob").or(customer.lastName.eq("Wilson")));

Querydsl supports the following join variants in JPQL : inner join, join, left join and full join. Join usage is typesafe, and follows the following pattern :

  
QCat cat = QCat.cat;
QCat mate = new QCat("mate");
QCate kitten = new QCat("kitten");
query.from(cat)
    .innerJoin(cat.mate, mate)
    .leftJoin(cat.kittens, kitten)
    .list(cat);

The native JPQL version of the query would be

from Cat as cat
    inner join cat.mate as mate
    left outer join cat.kittens as kitten

Another example

 
query.from(cat)
    .leftJoin(cat.kittens, kitten)
    .with(kitten.bodyWeight.gt(10.0))
    .list(cat);

With the following JPQL version

  
from Cat as cat
    left join cat.kittens as kitten
    with kitten.bodyWeight > 10.0  

Use the the cascading methods of the JPQLQuery interface like this

from : Define the query sources here.

innerJoin, join, leftJoin, fullJoin, with : Define join elements using these constructs. For the join methods the first argument is the join source and the second the target (alias).

where : Define the query filters, either in varargs form separated via commas or cascaded via the and-operator.

groupBy : Define the group by arguments in varargs form.

having : Define the having filter of the "group by" grouping as an varags array of Predicate expressions.

orderBy : Define the ordering of the result as an varargs array of order expressions. Use asc() and desc() on numeric, string and other comparable expression to access the OrderSpecifier instances.

limit, offset, restrict : Define the paging of the result. Limit for max results, offset for skipping rows and restrict for defining both in one call.

The syntax for declaring ordering is

 
QCustomer customer = QCustomer.customer;
query.from(customer)
    .orderBy(customer.lastName.asc(), customer.firstName.desc())
    .list(customer);

which is equivalent to the following native JPQL

  
from Customer as customer
    order by customer.lastName asc, customer.firstName desc

Grouping can be done in the following form

 
query.from(customer)
    .groupBy(customer.lastName)
    .list(customer.lastName);

which is equivalent to the following native JPQL

  
select customer.lastName
    from Customer as customer
    group by customer.lastName

Delete clauses in Querydsl JPA follow a simple delete-where-execute form. Here are some examples :

   
QCustomer customer = QCustomer.customer;
// delete all customers
new HibernateDeleteClause(session, customer).execute();
// delete all customers with a level less than 3
new HibernateDeleteClause(session, customer).where(customer.level.lt(3)).execute();  

The second parameter of the HibernateDeleteClause constructor is the entity to be deleted. The where call is optional and the execute call performs the deletion and returns the amount of deleted entities.

For JPA based Delete usage, use the JPADeleteClause instead.

Update clauses in Querydsl JPA follow a simple update-set/where-execute form. Here are some examples :

   
QCustomer customer = QCustomer.customer;
// rename customers named Bob to Bobby
new HibernateUpdateClause(session, customer).where(customer.name.eq("Bob"))
    .set(customer.name, "Bobby")
    .execute();  

The second parameter of the HibernateUpdateClause constructor is the entity to be updated. The set invocations define the property updates in SQL-Update-style and the execute call performs the Update and returns the amount of updated entities.

For JPA based Update usage, use the JPAUpdateClause instead.

To create a subquery you create a HibernateSubQuery instance, define the query parameters via from, where etc and use unique or list to create a subquery, which is just a type-safe Querydsl expression for the query. unique is used for a unique (single) result and list for a list result.

QDepartment department = QDepartment.department;
QDepartment d = new QDepartment("d");
query.from(department)
    .where(department.employees.size().eq(
        new HibernateSubQuery().from(d).unique(d.employees.size().max())
     )).list(department);

Another example

QEmployee employee = QEmployee.employee;
QEmployee e = new QEmployee("e");
query.from(employee)
    .where(employee.weeklyhours.gt(
        new HibernateSubQuery().from(employee.department.employees, e)
        .where(e.manager.eq(employee.manager))
        .unique(e.weeklyhours.avg())
    )).list(employee);

For JPA based sub query usage, use the JPASubQuery instead.

If you need to do tune the original Query before the execution of the query you can expose it like this :

HibernateQuery query = new HibernateQuery(session);
org.hibernate.Query hibQuery = query.from(employee).createQuery(employee);
hibQuery.setResultTransformer(someTransformer);
List results = hibQuery.list();

Querydsl supports Native SQL in Hibernate via the HibernateSQLQuery class.

To use it, you must generate Querydsl query types for your SQL schema. This can be done for example with the following Maven configuration :

<plugin>
  <groupId>com.mysema.querydsl</groupId>
  <artifactId>querydsl-maven-plugin</artifactId>
  <version>${project.version}</version>
  <executions>
    <execution>
      <goals>
        <goal>export</goal>
      </goals>
    </execution>            
  </executions>
  <configuration>
    <jdbcDriver>org.apache.derby.jdbc.EmbeddedDriver</jdbcDriver>
    <jdbcUrl>jdbc:derby:target/demoDB;create=true</jdbcUrl>
    <packageName>com.mycompany.mydomain</packageName>
    <targetFolder>${project.basedir}/target/generated-sources/java</targetFolder> 
  </configuration>
  <dependencies>
    <dependency>
      <groupId>org.apache.derby</groupId>
      <artifactId>derby</artifactId>
      <version>${derby.version}</version>
    </dependency>
  </dependencies>
</plugin>

When the query types have successfully been generated into the location of your choice, you can use them in your queries.

Single column query :

// serialization templates
SQLTemplates templates = new DerbyTemplates(); 
// query types (S* for SQL, Q* for domain types)
SAnimal cat = new SAnimal("cat");   
SAnimal mate = new SAnimal("mate"); 
QCat catEntity = QCat.cat;          
 
HibernateSQLQuery query = new HibernateSQLQuery(session, templates);
List<String> names = query.from(cat).list(cat.name);

Query multiple columns :

query = new HibernateSQLQuery(session, templates);
List<Object[]> rows = query.from(cat).list(cat.id, cat.name);

Query all columns :

List<Object[]> rows = query.from(cat).list(cat.all());
 

Query in SQL, but project as entity :

query = new HibernateSQLQuery(session, templates);
List<Cat> cats = query.from(cat).orderBy(cat.name.asc()).list(catEntity);

Query with joins :

query = new HibernateSQLQuery(session, templates);
cats = query.from(cat)
    .innerJoin(mate).on(cat.mateId.eq(mate.id))
    .where(cat.dtype.eq("Cat"), mate.dtype.eq("Cat"))
    .list(catEntity);

Query and project into DTO :

query = new HibernateSQLQuery(session, templates);
List<CatDTO> catDTOs = query.from(cat)
    .orderBy(cat.name.asc())
    .list(ConstructorExpression.create(CatDTO.class, cat.id, cat.name));

If you are using the JPA API instead of the Hibernate API, then use JPASQLQuery instead of HibernateSQLQuery