Question

I’m using ormlite for database interaction in java. Works fine.

Now I want to create a generic entity class. Perhaps I’ll start with a bit of code to explain it.

AbstractId is something like a base class for all ids:

public abstract class AbstractId {
    public AbstractId(String id) {
        this.id = id;    
    }

    public String value() {
        return this.id;
    }
}

Extended to:

   public class SampleId extends AbstractId {
        public SampleId(String id) {
            super(id);
        }
    }

Now I have a abstract base entity to save some typing 😉

public abstract class AbstractEntity<T> {

    @DatabaseField(id = true, columnName = "id")
    private T      id;

    @DatabaseField(columnName = "name", canBeNull = false)
    private String name;

    public AbstractEntity() {
    }
}

which is extended by a sample entity:

@DatabaseTable(tableName = "sample")
public class SampleEntity<T extends AbstractId> extends AbstractEntity<T> {

    @DatabaseField(canBeNull = false, columnName = "key")
    private String otherValue;

    public SampleEntity() {
    }
}

So how can i persist T? The entity instance is created by

SampleEntity<SampleId> entity = new SampleEntity<>() // (*)

I tried to create a new BaseDataType and setting the DatabaseField to use this as persisterClass:

public class AbstractIdPersister extends BaseDataType {

    public AbstractIdPersister(SqlType sqlType, Class<?>[] classes) {
        super(sqlType, classes);
    }

    @Override
    public Object parseDefaultString(FieldType fieldType, String defaultStr)
            throws SQLException {
        return null;
    }

    @Override
    public Object javaToSqlArg(FieldType fieldType, Object obj) {
        AbstractId abstractId = (AbstractId) obj;
        return abstractId.value();
    }

    @Override
    public Object sqlArgToJava(FieldType fieldType, Object sqlArg, int columnPos)
            throws SQLException {            
        // HOW DO I GET THE GENERIC TYPE DEFINED ABOVE (*)     
    }

    @Override
    public Object resultToSqlArg(FieldType fieldType, DatabaseResults results,
            int columnPos) throws SQLException {
        return results.getString(columnPos);
    }

}

Didn’t work … of course..

Does anyone has any idea how to solve this problem? Thank you!

Answer 1

So given your hierarchy, ORMLite is going to have a problem when it needs to create an instance of your abstract class from the associated database field. That’s what the sqlArgToJava method is trying to do. In the DATE_LONG type, for example, the date is stored in the database as a long time milliseconds, so the sqlArgToJava method is:

public Object sqlArgToJava(FieldType fieldType, Object sqlArg, int columnPos) {
    // create a Date object
    return new Date((Long) sqlArg);
}

Obviously there is no way to create an instance of AbstractId. You are going to have to make a persister for each of the subclasses instead.

If you are good with reflection then you could do something like the following but it’s a hack for sure. I think creating a persister for each of your concrete classes is the way to go.

try {
    // find the type of the field, and look for a String constructor
    return fieldType.getType().getConstructor(String.class).newInstance(sqlArg);
} catch (Exception e) {
    throw SqlExceptionUtil.create("could not call the string constructor on "
        + fieldType.getType(), e);
}