How can you use Builder Pattern for entities with JPA

Question

I read that it's useful to use builder pattern when you have a class with a lot of parameters. I wonder how you can implement an entity using builder pattern. It would be great if you can provide sample code.

Answer 1

Of course it is possible, you just have to provide a (possibly nested) Builder for every Entity.

Here is a working example:

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class FluentEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    private String someName;
    private int someNumber;
    private boolean someFlag;

    protected FluentEntity(){}

    private FluentEntity(String someName, int someNumber, boolean someFlag) {
        this.someName = someName;
        this.someNumber = someNumber;
        this.someFlag = someFlag;
    }

    public long getId() {
        return id;
    }

    public String getSomeName() {
        return someName;
    }

    public int getSomeNumber() {
        return someNumber;
    }

    public boolean isSomeFlag() {
        return someFlag;
    }

    public static FluentEntityBuilder builder() {
        return new FluentEntityBuilder();
    }

    public static class FluentEntityBuilder {

        private String someName;
        private int someNumber;
        private boolean someFlag;

        public FluentEntityBuilder setSomeName(final String someName) {
            this.someName = someName;
            return this;
        }

        public FluentEntityBuilder setSomeNumber(final int someNumber) {
            this.someNumber = someNumber;
            return this;
        }

        public FluentEntityBuilder setSomeFlag(final boolean someFlag) {
            this.someFlag = someFlag;
            return this;
        }

        public FluentEntity build() {
            return new FluentEntity(someName, someNumber, someFlag);
        }

    }

}

The code to use it would be this:

FluentEntity entity = FluentEntity.builder().setSomeName(someName).setSomeNumber(someNumber)
                .setSomeFlag(someFlag).build();

Just keep in mind that you have to exclude auto-generated fields like the primary key (in this example id) if you have some.

If you want to get rid of the "boilerplate" code for creating Builder classes for every Entity I would recommend a convenience library, something like Lombok. Then you will get your Builders (and even more) by just annotating your Entites, maybe it costs a little extra work to exclude the id fields.

You should take a look at Project Lombok

Nevertheless, here is some code to test this Builder (implemented with Spring Boot and Hibernate).

The repository:

import org.springframework.data.repository.CrudRepository;

import com.example.model.FluentEntity;

public interface FluentEntityRepository extends CrudRepository<FluentEntity, Long> {

}

And here are some tests:

import static org.hamcrest.CoreMatchers.is;
import static org.hamcrest.CoreMatchers.notNullValue;
import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.greaterThan;

import java.util.stream.StreamSupport;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.transaction.annotation.Transactional;

import com.example.model.FluentEntity;

@RunWith(SpringRunner.class)
@Transactional
@SpringBootTest
public class FluentEntityRepositoryTests {

    @Autowired
    private FluentEntityRepository fluentEntityRepository;

    @Test
    public void insertAndReceiveFluentEntityCreatedWithBuilder() {
        final String someName = "name";
        final int someNumber = 1;
        final boolean someFlag = true;

        FluentEntity entity = FluentEntity.builder().setSomeName(someName).setSomeNumber(someNumber)
                .setSomeFlag(someFlag).build();

        entity = fluentEntityRepository.save(entity);
        assertThat("Entity did not get a generated Id!", entity.getId(), greaterThan(-1L));
        assertThat("Entity name did not match!", entity.getSomeName(), is(someName));
        assertThat("Entity number did not match!", entity.getSomeNumber(), is(someNumber));
        assertThat("Entity flag did not match!", entity.isSomeFlag(), is(someFlag));
    }

    @Test
    public void insertSomeAndReceiveFirst() {
        fluentEntityRepository.save(FluentEntity.builder().setSomeName("A").setSomeNumber(1).setSomeFlag(true).build());
        fluentEntityRepository
                .save(FluentEntity.builder().setSomeName("B").setSomeNumber(2).setSomeFlag(false).build());
        fluentEntityRepository.save(FluentEntity.builder().setSomeName("C").setSomeNumber(3).setSomeFlag(true).build());

        final Iterable<FluentEntity> findAll = fluentEntityRepository.findAll();
        assertThat("Should get some iterable!", findAll, notNullValue());

        final FluentEntity fluentEntity = StreamSupport.stream(findAll.spliterator(), false).findFirst().get();
        assertThat("Should get some entity!", fluentEntity, notNullValue());
    }

}