Convert list in entity to single string column in database

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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(",")));
  }

}

You can use this converter in your entity:

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

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);
  }
}

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.

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

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