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JPA-based persistence infrastructure of the Open Application Standard Platform for Java (devon4j).
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package com.devonfw.module.jpa.dataaccess.api;

import java.util.Collection;

import com.devonfw.module.basic.common.api.RevisionMetadataType;
import com.devonfw.module.basic.common.api.entity.GenericEntity;
import com.devonfw.module.basic.common.api.reference.GenericIdRef;
import com.devonfw.module.basic.common.api.reference.Ref;

/**
 * Helper class for generic handling of persistence {@link GenericEntity entities} (based on
 * {@link javax.persistence.EntityManager}). In some cases it is required to access JPA features in a static way. E.g. a
 * common case is a setter in your {@link GenericEntity entity} for a
 * {@link com.devonfw.module.basic.common.api.reference.Ref reference} from an
 * {@link com.devonfw.module.basic.common.api.to.AbstractEto ETO} that can be achieved via the following code:
 *
 *  * @Entity
 * @Table("Foo")
 * public class FooEntity extends ApplicationPersistenceEntity implements Foo {
 *   @OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
 *   @JoinColumn(name = "bar")
 *   private BarEntity bar;
 *   ...
 *   @Override
 *   public void setBarId({@link com.devonfw.module.basic.common.api.reference.IdRef}{@literal } barId) {
 *     this.bar = {@link JpaHelper}.{@link JpaHelper#asEntity(Ref, Class) asEntity}(barId, BarEntity.class);
 *   }
 * }
 * 
 *
 * @since 3.0.0
 */
public class JpaHelper {

  /**
   * @param  generic type of the {@link GenericEntity entity}.
   * @param reference the {@link Ref} or {@code null}. Typically an
   *        {@link com.devonfw.module.basic.common.api.reference.IdRef}.
   * @param entityClass the {@link GenericEntity entity} {@link Class}.
   * @return the {@link GenericEntity entity} of the specified {@link Class} with the {@link Ref#getId() ID} from the
   *         given {@link GenericIdRef} or {@code null} if the given {@link Ref} is {@code null}.
   */
  public static  E asEntity(Ref reference, Class entityClass) {

    if (reference == null) {
      return null;
    } else {
      return JpaEntityManagerAccess.getEntityManager().getReference(entityClass, reference.getId());
    }
  }

  /**
   * @param  generic type of the input {@link GenericEntity entities} (most commonly the entity interface).
   * @param  generic type of the output {@link GenericEntity entities}.
   * @param input the {@link Collection} of {@link GenericEntity entities} (e.g.
   *        {@link com.devonfw.module.basic.common.api.to.AbstractEto ETOs}) to use as input.
   * @param entityClass the {@link Class} reflecting the {@link GenericEntity entity}.
   * @param output die {@link Collection} where to {@link Collection#add(Object) add} the {@link GenericEntity entities}
   *        corresponding to the input {@link GenericEntity entities}. Most probably {@link Collection#isEmpty() empty}
   *        but may also already contain entities so this method will add additional entities.
   */
  @SuppressWarnings("unchecked")
  public static , P extends E> void asEntities(Collection input,
      Class
 entityClass, Collection output) {

    for (E eto : input) {
      P entity;
      if (entityClass.isInstance(eto)) {
        entity = (P) eto;
      } else {
        entity = JpaEntityManagerAccess.getEntityManager().getReference(entityClass, eto.getId());
      }
      output.add(entity);
    }
  }

  /**
   * This is the newer implementation of RevisionMetadataType should be used with
   * {@link com.devonfw.module.basic.common.api.RevisionMetadata}
   *
   * @param revEntity die {@link AdvancedRevisionEntity}.
   * @return die Instanz von {@link RevisionMetadataType} bzw. {@code null} falls {@code revision} den Wert {@code null}
   *         hat.
   */
  public static RevisionMetadataType asRevisionMetaData(AdvancedRevisionEntity revEntity) {

    if (revEntity == null) {
      return null;
    }
    return new RevisionMetadataType(revEntity.getId(), revEntity.getDate(), revEntity.getUserLogin());
  }

}