请注意，使用新的主要版本的QueryDSL（4.x）和querydsl-jpa可能需要进行一些更改。

在我们的项目之一，我们用QueryDSL用QueryDslPredicateExecutor<T>。

  public Predicate createPredicate(DataEntity dataEntity) {
    QDataEntity qDataEntity = QDataEntity.dataEntity;
    BooleanBuilder booleanBuilder = new BooleanBuilder();
    if (!StringUtils.isEmpty(dataEntity.getCnsiConsumerNo())) {
      booleanBuilder
        .or(qDataEntity.cnsiConsumerNo.contains(dataEntity.getCnsiConsumerNo()));
    }
    if (!StringUtils.isEmpty(dataEntity.getCnsiMeterNo())) {
      booleanBuilder.or(qDataEntity.cnsiMeterNo.contains(dataEntity.getCnsiMeterNo()));
    }

    return booleanBuilder.getValue();
  }

我们可以在存储库中使用它：

@Repository
public interface DataEntityRepository
  extends DaoRepository<DataEntity, Long> {

哪里DaoRepository是

@NoRepositoryBean
public interface DaoRepository<T, K extends Serializable>
  extends JpaRepository<T, K>,
  QueryDslPredicateExecutor<T> {
}

因为这样，你可以使用存储库谓词方法。

Iterable<DataEntity> results = dataEntityRepository.findAll(dataEntityPredicateCreator.createPredicate(dataEntity));

要获取QClasses，你需要在pom.xml中指定QueryDSL APT Maven插件。

  <build>
    <plugins>
      <plugin>
        <groupId>com.mysema.maven</groupId>
        <artifactId>maven-apt-plugin</artifactId>
        <version>1.0.4</version>
        <executions>
          <execution>
            <phase>generate-sources</phase>
            <goals>
              <goal>process</goal>
            </goals>
            <configuration>
              <outputDirectory>target/generated-sources</outputDirectory>
              <processor>com.mysema.query.apt.jpa.JPAAnnotationProcessor</processor>
            </configuration>
          </execution>
        </executions>
      </plugin>

依赖项是

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

或对于Gradle：

sourceSets {
    generated
}
sourceSets.generated.java.srcDirs = ['src/main/generated']
configurations {
    querydslapt
}
dependencies {
    // other deps ....
    compile "com.mysema.querydsl:querydsl-jpa:3.6.3"
    compile "com.mysema.querydsl:querydsl-apt:3.6.3:jpa"
}
task generateQueryDSL(type: JavaCompile, group: 'build', description: 'Generates the QueryDSL query types') {
    source = sourceSets.main.java
    classpath = configurations.compile + configurations.querydslapt
    options.compilerArgs = [
            "-proc:only",
            "-processor", "com.mysema.query.apt.jpa.JPAAnnotationProcessor"
    ]
    destinationDir = sourceSets.generated.java.srcDirs.iterator().next()
}

compileJava {
    dependsOn generateQueryDSL
    source generateQueryDSL.destinationDir
}

compileGeneratedJava {
    dependsOn generateQueryDSL
    classpath += sourceSets.main.runtimeClasspath
}

你可以使用Spring-data为你提供的即用型规范。并能够使用标准API以编程方式构建查询。要支持规范，你可以使用JpaSpecificationExecutor接口扩展存储库接口

public interface CustomerRepository extends SimpleJpaRepository<T, ID>, JpaSpecificationExecutor {

}

附加接口（JpaSpecificationExecutor）包含允许你以多种方式执行Specification的方法。

例如，findAll方法将返回与规范匹配的所有实体：

List<T> findAll(Specification<T> spec);

规范界面如下：

public interface Specification<T> {
     Predicate toPredicate(Root<T> root, CriteriaQuery<?> query,
            CriteriaBuilder builder);
}

好的，那么典型的用例是什么？规范可以轻松地用于在实体之上构建可扩展的谓词集，然后可以将其与JpaRepository组合并使用，而无需为每个所需的组合声明查询（方法）。这是一个示例：示例2.15。客户规格

public class CustomerSpecs {
    public static Specification<Customer> isLongTermCustomer() {
        return new Specification<Customer>() {
            public Predicate toPredicate(
                Root<Customer> root, CriteriaQuery<?> query,
                CriteriaBuilder builder) {
                LocalDate date = new LocalDate().minusYears(2);
                return builder.lessThan(root.get('dateField'), date);
            }
        };
    }

    public static Specification<Customer> hasSalesOfMoreThan(MontaryAmount value) {
        return new Specification<Customer>() {
            public Predicate toPredicate(
                Root<T> root, CriteriaQuery<?> query,
                CriteriaBuilder builder) {
                // build query here
            }
        };
    }
}

你在业务需求抽象级别上表达了一些标准，并创建了可执行的规范。因此，客户端可以使用以下规范：

List customers = customerRepository.findAll(isLongTermCustomer());

你还可以结合规范示例2.17。组合规格

    MonetaryAmount amount = new MonetaryAmount(200.0, Currencies.DOLLAR);
    List<Customer> customers = customerRepository.findAll(
        where(isLongTermCustomer()).or(hasSalesOfMoreThan(amount)));

如你所见，规范提供了一些粘合代码方法来链接和组合规范。因此，扩展数据访问层只是创建新的规范实现并将它们与现有的实现结合在一起的问题。

你可以创建复杂的规格，这是一个示例

public class WorkInProgressSpecification {
    public static Specification<WorkInProgress> findByCriteria(final SearchCriteria searchCriteria) {

        return new Specification<WorkInProgress>() {

            @Override
            public Predicate toPredicate(
                Root<WorkInProgress> root,
                CriteriaQuery<?> query, CriteriaBuilder cb) {

                List<Predicate> predicates = new ArrayList<Predicate>();

                if (searchCriteria.getView() != null && !searchCriteria.getView().isEmpty()) {
                    predicates.add(cb.equal(root.get("viewType"), searchCriteria.getView()));
                }
                if (searchCriteria.getFeature() != null && !searchCriteria.getFeature().isEmpty()) {
                    predicates.add(cb.equal(root.get("title"), searchCriteria.getFeature()));
                }
                if (searchCriteria.getEpic() != null && !searchCriteria.getEpic().isEmpty()) {
                    predicates.add(cb.equal(root.get("epic"), searchCriteria.getEpic()));
                }
                if (searchCriteria.getPerformingGroup() != null && !searchCriteria.getPerformingGroup().isEmpty()) {
                    predicates.add(cb.equal(root.get("performingGroup"), searchCriteria.getPerformingGroup()));
                }
                if (searchCriteria.getPlannedStartDate() != null) {
                    System.out.println("searchCriteria.getPlannedStartDate():" + searchCriteria.getPlannedStartDate());
                    predicates.add(cb.greaterThanOrEqualTo(root.<Date>get("plndStartDate"), searchCriteria.getPlannedStartDate()));
                }
                if (searchCriteria.getPlannedCompletionDate() != null) {
                    predicates.add(cb.lessThanOrEqualTo(root.<Date>get("plndComplDate"), searchCriteria.getPlannedCompletionDate()));
                }
                if (searchCriteria.getTeam() != null && !searchCriteria.getTeam().isEmpty()) {
                    predicates.add(cb.equal(root.get("agileTeam"), searchCriteria.getTeam()));
                }

                return cb.and(predicates.toArray(new Predicate[] {}));
            }
        };
    }
}