jpa-criteria-api

The main goal of this project is to explore basic query features of JPA 2.1 specification:

• JPQL
• JPA Criteria API (typesafe with auto-generated static metamodels)

As a JPA provider we choose Hibernate to test some of HQL.

project structure

technologies used

• Flyway - constructs the database & provide datafeed in two migrations. Ensures data coherence by checking if schema in database is up to date ( by validating additional database table concerning migrations: flyway_schema_history).
  Reference: https://flywaydb.org/documentation/
• JPA 2.1
  Reference: Pro JPA 2 2nd Edition
  Reference: JSR 338: JavaTM Persistence API, Version 2.1
• Hibernate - provider of JPA 2.1 specification. We don't use EclipseLink despite the fact that it is reference implementation of JPA 2.1 because apart from JPQL we want to explore HQL as a more powerful query language.
  Reference: Java Persistence with Hibernate Second Edition
  Reference: Hibernate Interview Questions and Answers
• Jupiter & AssertJ - to test result sets (AssertJ has very convinient methods to comparing lists regardless order).

project description

• The main idea is to explore JPA Criteria API by writing a querys in JPQL or HQL, then to try re-write them in JPA Criteria API & check result-sets identity.
• Comparing equality of result sets with JUnit & hamcrest:

  import static org.hamcrest.Matchers.containsInAnyOrder;
  ...
  Assert.assertThat(entityManager.createQuery(cc_query)
              .getResultList(),
      containsInAnyOrder(jpql_query.getResultList().toArray()));
  

  but we decide to use Jupiter & AssertJ (more expressive):

  import static org.assertj.core.api.Assertions.assertThat;
  ...
  assertThat(entityManager.createQuery(cc_query).getResultList())
      .containsExactlyInAnyOrderElementsOf(jpql_query.getResultList());
  

project content

src/main/java: entities, utility classes & META-INF folder with persistence.xml
resources/db/migration: Flyway migrations as SQL scripts
test/java: showcase of JPQL, HQL & Criteria API with tests
target/generated-sources: static metamodels of entities

tests

• We have to classes with nearly the same content: Tests and TestsWithFullTypeSafe. The only difference is that in the first class we use strings to denote fields while in the letter we use static metamodels.

• Example:

  • From Tests:

    orderBy(cb.asc(cc_query_root.get("title")));
    

  • From TestsWithFullTypeSafe:

    orderBy(cb.asc(cc_query_root.get(Book_.title)));
    

• All methods are quite simple & straightforward use of Criteria API.
  The most interesting are:

  • getBookstoresWithMostExpensiveBook() - we show how to use subqueries
  • getBookstoresThatHaveTitle() - we show how to reference a FROM expression of the parent query in the FROM clause of a subquery
  • getBookstoresThatHaveAtLeastOneBookWrittenBy() - we show how to reference a JOIN expression of the parent query in the FROM clause of a subquery
  • countBooksByGenre() - we show how to use Tuple with aliases
  • getBookstoresWithCountBooksAndPriceAverage() - we show how to use multiselect with dedicated class

static metamodels

They are auto-generated by maven-compiler-plugin.
All you need to do is:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <configuration>
        <source>1.8</source>
        <target>1.8</target>
        <compilerArguments>
            <processor>org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor</processor>
        </compilerArguments>
    </configuration>
</plugin>

<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-jpamodelgen</artifactId>
    <version>5.2.17.Final</version>
    <scope>provided</scope>
</dependency>

Mark target/generated-sources as a Generated Sources Root.
In IntelliJ just left-click on target/generated-sources -> Mark Directory As -> Generated Sources Root.