Add the following dependencies to your Maven project and make sure that the Maven 2 repo of Mysema Source (http://source.mysema.com/maven2/releases) is accessible from your POM if the version cannot yet be found in other public Maven repos :

<dependency>
  <groupId>com.mysema.querydsl</groupId>
  <artifactId>querydsl-apt</artifactId>
  <version>0.5.4</version>
  <scope>provided</scope>
</dependency>        
    
<dependency>
  <groupId>com.mysema.querydsl</groupId>
  <artifactId>querydsl-jdoql</artifactId>
  <version>0.5.4</version>
</dependency>

And now, configure the Maven APT plugin which generates the query types used by Querydsl :

<project>
  <build>
    <plugins>
      ...
      <plugin>
        <groupId>com.mysema.maven</groupId>
        <artifactId>maven-apt-plugin</artifactId>
        <version>0.2.0</version>
        <executions>
          <execution>
            <goals>
              <goal>process</goal>
            </goals>
            <configuration>
              <outputDirectory>target/generated-sources/java</outputDirectory>
              <processor>com.mysema.query.apt.jdo.JDOAnnotationProcessor</processor>
            </configuration>
          </execution>
        </executions>
      </plugin>
    ...
    </plugins>
  </build>
</project>

The JDOAnnotationProcessor finds domain types annotated with the javax.jdo.annotations.PersistenceCapable annotation and generates Querydsl query types for them. Run mvn eclipse:eclipse or clean install and you will get your Query types generated into target/generated-sources/java.

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

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 :

@PersistenceCapable
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");

QCustomer reflects all the properties of the original type Customer as public fields. The firstName field can be accessed like this

customer.firstName;

The generated Querydsl query types are based on a builtin expression type system. The Querydsl expression archetypes are presented in more detailed on this page : [Querydsl expressions].

For the JDOQL-module JDOQLQueryImpl is the main Query implementation. It is instantiated like this :

PersistenceManager pm;
JDOQLQuery query = new JDOQLQueryImpl (pm);

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

QCustomer customer = QCustomer.customer;
JDOQLQuery query = new JDOQLQueryImpl (pm);
Customer bob = query.from(customer)
  .where(customer.firstName.eq("Bob"))
  .uniqueResult(customer);
query.close();

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 just use the JDOQLQuery interface like this :

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

Use the the cascading methods of the JDOQLQuery method like this

from : Define the query sources here, the first argument becomes the main source and the others are treated as variables.

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 varargs array of EBoolean 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

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

Grouping can be done in the following form

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

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

   
QCat cat = QCat.cat;
// delete all cats
new JDOQLDeleteClause(pm, cat).execute();
// delete all cats with kittens
new JDOQLDeleteClause(pm, cat).where(cat.kittens.isNotEmpty()).execute();  

The second parameter of the JDOQLDeleteClause 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.

To create a subquery you create a JDOQLSubQuery 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 result and list for a list result.

query().from(department)
    .where(department.employees.size().eq(
        new JDOQLSubQuery().from(d).unique(AggregationFunctions.max(d.employees.size()))
     )).list(department);  

represents the following native JDOQL query

SELECT this FROM com.mysema.query.jdoql.models.company.Department
WHERE this.employees.size() == 
(SELECT max(d.employees.size()) FROM com.mysema.query.jdoql.models.company.Department d)

Another example

query().from(employee)
    .where(employee.weeklyhours.gt(
        new JDOQLSubQuery().from(employee.department.employees, e)
        .where(e.manager.eq(employee.manager))
        .unique(AggregationFunctions.avg(e.weeklyhours))
    )).list(employee);

which represents the following native JDOQL query

SELECT this FROM com.mysema.query.jdoql.models.company.Employee 
WHERE this.weeklyhours > 
(SELECT avg(e.weeklyhours) FROM this.department.employees e WHERE e.manager == this.manager)