如何在 JPA 中保留 List<String> 类型的属性？

Question

获取具有 List 类型字段的实体的最聪明方法是什么？

命令.java

 package persistlistofstring;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import javax.persistence.Basic;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Persistence;

@Entity
public class Command implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    Long id;
    @Basic
    List<String> arguments = new ArrayList<String>();

    public static void main(String[] args) {
        Command command = new Command();

        EntityManager em = Persistence
                .createEntityManagerFactory("pu")
                .createEntityManager();
        em.getTransaction().begin();
        em.persist(command);
        em.getTransaction().commit();
        em.close();

        System.out.println("Persisted with id=" + command.id);
    }
}

---

此代码产生：

 > Exception in thread "main" javax.persistence.PersistenceException: No Persistence provider for EntityManager named pu: Provider named oracle.toplink.essentials.PersistenceProvider threw unexpected exception at create EntityManagerFactory:
> oracle.toplink.essentials.exceptions.PersistenceUnitLoadingException
> Local Exception Stack:
> Exception [TOPLINK-30005] (Oracle TopLink Essentials - 2.0.1 (Build b09d-fcs (12/06/2007))): oracle.toplink.essentials.exceptions.PersistenceUnitLoadingException
> Exception Description: An exception was thrown while searching for persistence archives with ClassLoader: sun.misc.Launcher$AppClassLoader@11b86e7
> Internal Exception: javax.persistence.PersistenceException: Exception [TOPLINK-28018] (Oracle TopLink Essentials - 2.0.1 (Build b09d-fcs (12/06/2007))): oracle.toplink.essentials.exceptions.EntityManagerSetupException
> Exception Description: predeploy for PersistenceUnit [pu] failed.
> Internal Exception: Exception [TOPLINK-7155] (Oracle TopLink Essentials - 2.0.1 (Build b09d-fcs (12/06/2007))): oracle.toplink.essentials.exceptions.ValidationException
> Exception Description: The type [interface java.util.List] for the attribute [arguments] on the entity class [class persistlistofstring.Command] is not a valid type for a serialized mapping. The attribute type must implement the Serializable interface.
>         at oracle.toplink.essentials.exceptions.PersistenceUnitLoadingException.exceptionSearchingForPersistenceResources(PersistenceUnitLoadingException.java:143)
>         at oracle.toplink.essentials.ejb.cmp3.EntityManagerFactoryProvider.createEntityManagerFactory(EntityManagerFactoryProvider.java:169)
>         at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:110)
>         at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:83)
>         at persistlistofstring.Command.main(Command.java:30)
> Caused by:
> ...

原文由 Andrea Francia 发布，翻译遵循 CC BY-SA 4.0 许可协议

Answer 1

使用一些 JPA 2 实现：它添加了一个 @ElementCollection 注释，类似于 Hibernate 注释，它完全满足您的需要。 这里 有一个例子。

编辑

正如下面评论中提到的，正确的 JPA 2 实现是

javax.persistence.ElementCollection

@ElementCollection
Map<Key, Value> collection;

请参阅： http ://docs.oracle.com/javaee/6/api/javax/persistence/ElementCollection.html

原文由 Thiago H. de Paula Figueiredo 发布，翻译遵循 CC BY-SA 3.0 许可协议

Answer 2

如果有人正在寻找一种替代解决方案，将字符串列表作为一个字段存储在数据库中，那么我就是这样解决的。像这样创建一个转换器：

 import java.util.Arrays;
import java.util.List;

import javax.persistence.AttributeConverter;
import javax.persistence.Converter;

import static java.util.Collections.*;

@Converter
public class StringListConverter implements AttributeConverter<List<String>, String> {
    private static final String SPLIT_CHAR = ";";

    @Override
    public String convertToDatabaseColumn(List<String> stringList) {
        return stringList != null ? String.join(SPLIT_CHAR, stringList) : "";
    }

    @Override
    public List<String> convertToEntityAttribute(String string) {
        return string != null ? Arrays.asList(string.split(SPLIT_CHAR)) : emptyList();
    }
}

现在像这样在你的实体上使用它：

 @Convert(converter = StringListConverter.class)
private List<String> yourList;

在数据库中，您的列表将存储为 foo;bar;foobar ，在您的 Java 对象中，您将获得包含这些字符串的列表。

原文由 Jonck van der Kogel 发布，翻译遵循 CC BY-SA 4.0 许可协议