如何使用構造模式用於具有JPA的實體

Question

我讀到，當您有一個具有大量參數的類時，使用構建器模式很有用。我想知道如何使用構建器模式來實現一個實體。如果您可以提供示例代碼，那將會很棒。

Answer 1

當然，這是可能的，你只需要爲每個實體提供一個（可能是嵌套的）Builder。

下面是一個工作示例：

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); 
     } 

    } 

} 

使用它會是這樣的代碼：

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

請記住，你必須排除像主鍵自動生成的字段（這個例子的案例是id）如果你有一些。

如果你想擺脫爲每個實體創建Builder類的「樣板」代碼，我會推薦一個類似lombok的便利庫。然後，您只需註釋您的Entites即可獲得Builders（甚至更多），也許需要額外的工作才能排除id字段。

你應該看看Project Lombok

儘管如此，這裏是一些代碼來測試這個生成器（與Spring引導和Hibernate實現）。

存儲庫：

import org.springframework.data.repository.CrudRepository; 

import com.example.model.FluentEntity; 

public interface FluentEntityRepository extends CrudRepository<FluentEntity, Long> { 

} 

這裏有一些測試：

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 an 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()); 
    } 

}