In my Spring MVC 3 based application I had recently implemented a few Converters for some of my JPA based data objects. It started with one, then another, and so on. By the time I got around to adding my fourth converter to the spring configuration file I knew it was time to pull it out and abstract it a bit. Thankfully, Spring allows you to implement a ConverterFactory that is responsible for creating the converters for some types.

Each of my entities extend an abstract base class that looks basically like this

@MappedSuperclass
public abstract class DataObjectAbstract<K extends Serializable>
        implements DataObject<K>
{
    protected transient String[] excludedEqualsFields = new String[] { "key", "version" };

    @Version
    protected int version;

    @Override
    public boolean equals( Object that )
    {
        return EqualsBuilder.reflectionEquals( this, that, excludedEqualsFields );
    }

    @Override
    public int hashCode()
    {
        return HashCodeBuilder.reflectionHashCode( this, excludedEqualsFields );
    }

    @Override
    public String toString()
    {
        return ToStringBuilder.reflectionToString( this, ToStringStyle.MULTI_LINE_STYLE );
    }
}

The DataObject interface simply declares a getKey and setKey method.

So, in my Spring MVC Controller methods I was originally accepting a String or Long, then using my own data access objects to lookup the entities I needed. The next iteration in my implementation was to implement the Converters as I mentioned above. That was very simple and worked well, but having many data objects I didn’t want to copy that implementation over and over again. This is where the ConverterFactory comes in. Here’s my implementation:

@Component
public class DataObjectConverterFactory
        implements ConverterFactory<String, DataObject<Long>>
{
    @PersistenceContext
    EntityManager em;

    @Override
    public <T extends DataObject<Long>> Converter<String, T> getConverter( Class<T> type )
    {
        return new GenericLongKeyedDataObjectConverter<T>( type, em );
    }
}

The ConverterFactory interface is basically as simple as the Converter interface. The Class<T> type parameter to the getConverter method tells us what type we are going to convert to.  One option from here is to have a big nasty if/else statement with a bunch of instanceof methods that create a new Converter.  I thought about doing this and passing in the appropriate data access object and performing the lookup.  That would be only two classes and then I could convert all of my DataObjects, but I didn’t like the idea of a bajillion instanceof statements.  So you can see I implemented a GenericLongKeyedDataObjectConverter which takes the target type and the EntityManager as a parameters.  Here’s the implementation of the generic converter class:

/**
 * A generic converter used for converting from a string representation of an entity key to the DataObject itself.
 *
 * @param <T> The type that is to be converted to.
 */
public class GenericLongKeyedDataObjectConverter<T extends DataObject<Long>>
        implements Converter<String, T>
{
    private Class<T> type;
    private EntityManager em;

    /**
     *
     * @param type An instance of Class for the type being converted to
     * @param em EntityManager used to perform the lookup.
     */
    public GenericLongKeyedDataObjectConverter( Class<T> type, EntityManager em )
    {
        this.type = type;
        this.em = em;
    }

    @Override
    public T convert( String stringKey )
    {
        Long key = Long.parseLong( stringKey );
        return em.find( type, key );
    }
}

An extremely simple parameterized class implementation of the Converter interface. Here, with no use of instanceof, I’m creating the appropriate converter implementation for all of my persisted classes.  If you have a group of objects that you want converted and they all inherit from a base class, a ConverterFactory may be a better solution than implementing a bunch of converters manually.

Finally, here’s the bean xml configuration:

<bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
    <property name="converters">
        <list>
            <ref bean="dataObjectConverterFactory" />
        </list>
    </property>
</bean>

Notice that we reference the dataObjectConverterFactory bean, but I never defined it in my xml config.  That’s because I used the @Component annotation on my implementation class.