ConverterLongToDate.java

package me.falci.planoB.converter;

import java.util.Calendar;
import java.util.Date;

import org.springframework.core.convert.converter.Converter;

/**
 * @author Falci
 *
 */
public class ConverterLongToDate implements Converter<Long, Date> {

	@Override
	public Date convert(Long timestamp) {
		Calendar calendar = Calendar.getInstance();
		calendar.setTimeInMillis(timestamp);
		
		return calendar.getTime();
	}

}

ConverterTest.java

/**
 * 
 */
package me.falci.planoB;

import static org.junit.Assert.assertEquals;

import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

import me.falci.planoB.converter.ConverterLongToDate;

import org.junit.Before;
import org.junit.Test;
import org.springframework.core.convert.ConverterNotFoundException;
import org.springframework.core.convert.support.DefaultConversionService;

/**
 * @author Falci
 *
 */
public class ConverterTest {
	
	private DefaultConversionService conversionService;

	@Before
	public void init() {
		conversionService = new DefaultConversionService();
	}
	
	@Test
	public void testConversaoSimples() {
		assertEquals(new Long(123L), conversionService.convert("123", Long.class));
		assertEquals(new Double(12.34), conversionService.convert("12.34", Double.class));
		
		assertEquals(new Long(123L), conversionService.convert(123D, Long.class));
	}
	
	@Test(expected=ConverterNotFoundException.class)
	public void testConversorNaoRegistrado() {
		conversionService.convert(1398654000000L, Date.class);
	}
	
	@Test
	public void testConversaoDeLongParaData() {
		conversionService.addConverter(new ConverterLongToDate());
		
		Date time = new GregorianCalendar(2014, Calendar.APRIL, 28).getTime();
		Long timeAsLong = time.getTime();
		
		assertEquals(time, conversionService.convert(timeAsLong, Date.class));
	}

}