Add the following dependencies to your Maven project:

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

<dependency>
  <groupId>com.querydsl</groupId>
  <artifactId>querydsl-jdo</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 which generates the query types used by Querydsl:

<project>
  <build>
    <plugins>
      ...
      <plugin>
        <groupId>com.mysema.maven</groupId>
        <artifactId>apt-maven-plugin</artifactId>
        <version>1.1.3</version>
        <executions>
          <execution>
            <goals>
              <goal>process</goal>
            </goals>
            <configuration>
              <outputDirectory>target/generated-sources/java</outputDirectory>
              <processor>com.querydsl.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 query types for them.

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 JDO 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="${src}" classpathref="cp">
      <compilerarg value="-proc:only"/>
      <compilerarg value="-processor"/>
      <compilerarg value="com.querydsl.apt.jdo.JDOAnnotationProcessor"/>
      <compilerarg value="-s"/>
      <compilerarg value="${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.

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 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;

For the JDO-module JDOQuery is the main Query implementation. It is instantiated like this:

PersistenceManager pm = ...;
JDOQuery<?> query = new JDOQuery<Void>(pm);

For the examples of this chapter the queries are created via a JDOQueryFactory instance. JDOQueryFactory should be the preferred option to obtain JDOQuery instances.

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

QCustomer customer = QCustomer.customer;
Customer bob = queryFactory.selectFrom(customer)
                   .where(customer.firstName.eq("Bob"))
                   .fetchOne();

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

Alternatively you can express it also like this

QCustomer customer = QCustomer.customer;
Customer bob = queryFactory.select(customer).from(customer)
                   .where(customer.firstName.eq("Bob"))
                   .fetchOne();

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

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

And to use multiple filters use it like this

queryFactory.selectFrom(customer)
    .where(customer.firstName.eq("Bob"), customer.lastName.eq("Wilson"));

Or like this

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

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

queryFactory.selectFrom(customer)
    .where(customer.firstName.eq("Bob").or(customer.lastName.eq("Wilson")));

Use the the cascading methods of the JDOQuery class like this

select: Set the projection of the query. (Not necessary if created via query factory)

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

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

groupBy: Add group by arguments in varargs form.

having: Add having filters of the "group by" grouping as an varargs array of Predicate expressions.

orderBy: Add 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: Set 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;
queryFactory.selectFrom(customer)
    .orderBy(customer.lastName.asc(), customer.firstName.desc())
    .fetch();

Grouping can be done in the following form

queryFactory.select(customer.lastName).from(customer)
    .groupBy(customer.lastName)
    .fetch();

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

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

The second parameter of the JDODeleteClause 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 can use one of the factory methods of JDOExpressions and add the query parameters via from, where etc.

QDepartment department = QDepartment.department;
QDepartment d = new QDepartment("d");
queryFactory.selectFrom(department)
    .where(department.size.eq(JDOExpressions.select(d.size.max()).from(d))
    .fetch();

represents the following native JDO query

SELECT this FROM com.querydsl.jdo.models.company.Department
WHERE this.size ==
(SELECT max(d.size) FROM com.querydsl.jdo.models.company.Department d)
    

Another example

QEmployee employee = QEmployee.employee;
QEmployee e = new QEmployee("e");
queryFactory.selectFrom(employee)
    .where(employee.weeklyhours.gt(
        JDOExpressions.select(e.weeklyhours.avg())
                      .from(employee.department.employees, e)
                      .where(e.manager.eq(employee.manager)))
    .fetch();

which represents the following native JDO query

SELECT this FROM com.querydsl.jdo.models.company.Employee
WHERE this.weeklyhours >
(SELECT avg(e.weeklyhours) FROM this.department.employees e WHERE e.manager == this.manager)
    

Querydsl supports Native SQL in JDO via the JDOSQLQuery 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:

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

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");

JDOSQLQuery<?> query = new JDOSQLQuery<Void>(pm, templates);
List<String> names = query.select(cat.name).from(cat).fetch();

Query multiple columns:

query = new JDOSQLQuery<Void>(pm, templates);
List<Tuple> rows = query.select(cat.id, cat.name).from(cat).fetch();

Query all columns:

List<Tuple> rows = query.select(cat.all()).from(cat).fetch();
 

Query with joins:

query = new JDOSQLQuery<Void>(pm, templates);
cats = query.select(catEntity).from(cat)
    .innerJoin(mate).on(cat.mateId.eq(mate.id))
    .where(cat.dtype.eq("Cat"), mate.dtype.eq("Cat"))
    .fetch();

Query and project into DTO:

query = new JDOSQLQuery<Void>(pm, templates);
List<CatDTO> catDTOs = query.select(Projections.constructor(CatDTO.class, cat.id, cat.name))
    .from(cat)
    .orderBy(cat.name.asc())
    .fetch();