将实体中的列表转换为数据库中的单个字符串列 [英] Convert list in entity to single string column in database

问题描述

我的数据库中有一个 VARCHAR 字段，该字段的值为 val1,val2,val3.

I have a VARCHAR field in my database, and the value of this field is val1,val2,val3.

是否可以将其设置为使用逗号作为拆分分隔符的实体的 ArrayList 属性?

Is it possible to set this into an ArrayList<String> attribute of an entity using comma as split delimiter?

推荐答案

如果你使用JPA 2.1，那么你可以创建一个AttributeConverter:

If you use JPA 2.1, then you can create an AttributeConverter:

@Converter
public class StringListConverter implements AttributeConverter<List<String>, String> {

  @Override
  public String convertToDatabaseColumn(List<String> list) {
    // Java 8
    return String.join(",", list); 
    // Guava
    return Joiner.on(',').join(list); 
  }

  @Override
  public List<String> convertToEntityAttribute(String joined) {
    return new ArrayList<>(Arrays.asList(joined.split(",")));
  }

}

您可以在您的实体中使用此转换器:

You can use this converter in your entity:

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

在 JPA 2.1 之前，您可以手动执行此操作:

For before JPA 2.1 you could do this by hand:

@Entity
private MyEntity {
  ...
  private String strings;

  public List<String> getStrings() {
    return Arrays.asList(strings.split(","));
  }

  public void setStrings(List<String> list) {
    strings = String.join(",", list);
  }
}

我将 Arrays.asList 包装在转换器中的 ArrayList 中，因为结果存储在属性中，对该列表的任何更改都将写回数据库- 因此我需要一个可更改的列表(我不能在 Arrays.asList 的结果中添加任何内容).在 2.1 之前 解决方案中，结果与属性无关，并且可变列表不会与属性同步.

I wrap Arrays.asList in an ArrayList in the converter, because the result is stored in the attribute and any change to that list will be written back to the database - thus I need a changeable list (I can't add anything to the result of Arrays.asList). In the before 2.1 solution the result is not connected with the attribute and a changeable list would not be synchronized with the attribute.

要查询在此类属性中包含特定项目的实体，请参阅我的回答 这里

To query for an entity that contains a specific item in such an attribute, see my answer here

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