JPA: Saving List of values as comma separated values

Question

I am receiving a simple list of values part of JSON request which I want to save as comma separated values. Tried using following but it did not work.

@Column(nullable = true)
@GeneratedValue(strategy = GenerationType.AUTO)
private ArrayList<String> services = new ArrayList<String>() ;

and

@Column(nullable = true)
@ElementCollection(targetClass = String.class)
private List<String> services = new ArrayList<String>() ;

@ElementCollection threw an exception saying table services does not exist.

Answer 1

As mentioned by others in the comments an AttributeConverter works pretty well. This one uses Jackson to serialize as a JSON array. I recommend JSON since it cleanly handles delimiter escaping, nulls, quotes, etc.:

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

    private static final TypeReference<List<String>> TypeRef = new TypeReference<List<String>>(){};

    @Override
    public String convertToDatabaseColumn (List<String> attribute) {
        if (attribute == null) {
            return null;
        }
        try {
            return ObjectMapperFactory.getInstance().writeValueAsString(attribute);
        }
        catch (IOException ex) {
            throw new UncheckedIOException(ex);
        }
    }

    @Override
    public List<String> convertToEntityAttribute (String dbData) {
        if (dbData == null) {
            return null;
        }
        try {
            return ObjectMapperFactory.getInstance().readValue(dbData, TypeRef);
        }
        catch (IOException ex) {
            throw new UncheckedIOException(ex);
        }
    }
}

I've used this class and it works well in most cases. One caveat I've found is that using this converter can confuse some JPA criteria queries, because it expects a type List on the entity, but finds a String in the db.

Answer 2

The @ElementCollection requires a table to store multiples rows of values,

So you could define as a String column and join/explode in getters and setters, like this

private String services;

public setServices(String services[]) //Can be Array or List
{
     // this.services = Iterate services[] and create a comma separated string or Use ArrayUtils
}

public String[] getServices() //Can be Array or List
{
    // services.split(",") to get a list of Strings, then typecast/parse them to Strings before returning or use Arrays.asList(arguments.split(","));
}