Multi tool use

hibernate - Persisting a composition interface of strategy pattern

I have the following class structure:

public abstract class Creature{
private String name;
//strategy pattern composition
private SkillInterface skill;
}

public interface SkillInterface {
void attack();
}

public class NoSkill implements SkillInterface {
@Override
public void attack() {
//statements
}
}


My goal is to persist Creature objects at one table in database. Subclasses of SkillInterface are without any fields. As they determine the behaviour, I want to convert selected SkillInterface class name to a String, as I only need to persist the classname of the current skill strategy of creature, with a String like skill.getClass().getSimpleName(). I tried to implement it with @Converter annotation, using AttributeConverter class to convert SkillInterface to String and save, but always had mapping exceptions. I want to be able to save it as String and retrieve as SkillInterface object.

But how can I implement it with Hibernate? Or do I have a design mistake?

2 Answers
2

You can use a proxy field to this for you like below:

abstract class Creature {
@Column
private String name;
// strategy pattern composition
private SkillInterface skill;

@Column
private String skillName;

public String getSkillName() {
return skill.getClass().getSimpleName();
}

public void setSkillName(String skillName) {
//ignore
}
}


Ok looks like I have found a basic solution that can be used to persist Strategy Pattern interfaces implementations. I used a @Converter annotation and a AttributeConverter class to convert strategy class names to column while saving to database and cast the retrieved String back to strategy class as following:

@Entity
public class Creature {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int id;

@Convert(converter = SkillConverter.class)
private SkillInterface skill;
}

public class SkillConverter implements AttributeConverter<SkillInterface,String> {
@Override
public String convertToDatabaseColumn(SkillInterface skill) {
return skill.getClass().getSimpleName().toLowerCase();
}

@Override
public SkillInterface convertToEntityAttribute(String dbData) {
//works as a factory
if (dbData.equals("noskill")) {
return new NoSkill();
} else if (dbData.equals("axe")) {
return new Axe();
}
return null;
}
}

public interface SkillInterface {
public String getSkill();

void attack();
}


public class NoSkill implements SkillInterface{
public String getSkill() {
return getClass().getSimpleName();
}

@Override
public void attack() {
//strategy statements
}
}


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