要持久化类型为List的字段的实体,最聪明的方法是什么?
Command.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);
}
}
这段代码产生:
> 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:
> ...
发布于 2018-05-21 19:43:36
如果有人正在寻找另一种解决方案,将字符串列表作为一个字段存储在数据库中,下面是我解决这个问题的方法。创建一个转换器,如下所示:
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,您的列表将被存储为foo;bar;foobar
,并且在您的Java对象中,您将获得一个包含这些字符串的列表。
发布于 2008-11-13 15:34:47
这个答案是在JPA2实现之前做出的,如果你使用的是JPA2,请参阅上面的ElementCollection答案:
模型对象中的对象列表通常被认为是与另一个对象的"OneToMany“关系。但是,字符串(本身)不是一对多关系的允许客户端,因为它没有ID。
因此,您应该将String列表转换为包含ID和String的参数类JPA对象列表。
或者,您可以将列表转换为@瞬态,并向您的类添加另一个字段(argStorage),该字段可以是VARCHAR()或CLOB。然后需要添加3个函数:其中2个函数相同,应该将字符串列表转换为单个字符串(在argStorage中),以便于分隔。用@PrePersist和@PreUpdate注释这两个函数(每个函数做同样的事情)。最后,添加第三个函数,该函数将argStorage再次拆分为字符串列表,并将其注释为@PostLoad。这将使您的CLOB在您存储命令时使用字符串进行更新,并在您将其存储到数据库之前保持argStorage字段的更新。
我仍然建议做第一种情况。这是以后建立真正的关系的很好的实践。
发布于 2017-08-09 00:58:50
我们也可以使用这个。
@Column(name="arguments")
@ElementCollection(targetClass=String.class)
private List<String> arguments;
https://stackoverflow.com/questions/287201
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