How to properly implement a Spring Converter? - spring

Question

I have a Money class with factory methods for numeric and String values. I would like to use it as a property of my input Pojos.
I created some Converters for it, this is the String one:
#Component
public class StringMoneyConverter implements Converter<String, Money> {
#Override
public Money convert(String source) {
return Money.from(source);
}
}
My testing Pojo is very simple:
public class MoneyTestPojo {
private Money value;
//getter and setter ommited
}
I have an endpoint which expects a Pojo:
#PostMapping("/pojo")
public String savePojo(#RequestBody MoneyTestPojo pojo) {
//...
}
Finally, this is the request body:
{
value: "100"
}
I have the following error when I try this request:
JSON parse error: Cannot construct instance of
br.marcellorvalle.Money (although at least one Creator
exists): no String-argument constructor/factory method to deserialize
from String value ('100'); nested exception is
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot
construct instance of br.marcellorvalle.Money (although at
least one Creator exists): no String-argument constructor/factory
method to deserialize from String value ('100')\n at [Source:
(PushbackInputStream); line: 8, column: 19] (through reference chain:
br.marcellorvalle.MoneytestPojo[\"value\"])",
If I change Money and add a constructor which receives a String this request works but I really need a factory method as I have to deliver special instances of Money on specific cases (zeros, nulls and empty strings).
Am I missing something?
Edit: As asked, here goes the Money class:
public class Money {
public static final Money ZERO = new Money(BigDecimal.ZERO);
private static final int PRECISION = 2;
private static final int EXTENDED_PRECISION = 16;
private static final RoundingMode ROUNDING = RoundingMode.HALF_EVEN;
private final BigDecimal amount;
private Money(BigDecimal amount) {
this.amount = amount;
}
public static Money from(float value) {
return Money.from(BigDecimal.valueOf(value));
}
public static Money from(double value) {
return Money.from(BigDecimal.valueOf(value));
}
public static Money from(String value) {
if (Objects.isNull(value) || "".equals(value)) {
return null;
}
return Money.from(new BigDecimal(value));
}
public static Money from(BigDecimal value) {
if (Objects.requireNonNull(value).equals(BigDecimal.ZERO)) {
return Money.ZERO;
}
return new Money(value);
}
//(...)
}

Answer 1

Annotating your factory method with #JsonCreator (from the com.fasterxml.jackson.annotation package) will resolve the issue:
#JsonCreator
public static Money from(String value) {
if (Objects.isNull(value) || "".equals(value)) {
return null;
}
return Money.from(new BigDecimal(value));
}
I just tested it, and it worked for me. Rest of your code looks fine except for the sample request (value should be in quotes), but I guess that's just a typo.
Update 1:
If you're unable to make changes to the Money class, I can think of another option - a custom Jackson deserializer:
public class MoneyDeserializer extends StdDeserializer<Money> {
private static final long serialVersionUID = 0L;
public MoneyDeserializer() {
this(null);
}
public MoneyDeserializer(Class<?> vc) {
super(vc);
}
#Override
public Money deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
JsonNode node = jp.getCodec().readTree(jp);
String value = node.textValue();
return Money.from(value);
}
}
Just register it with your ObjectMapper.

Answer 2

It seems that using the org.springframework.core.convert.converter.Converter only works if the Money class is a "#PathVariable" in the controller.
I finally solved it using the com.fasterxml.jackson.databind.util.StdConverter class:
I created the following Converter classes:
public class MoneyJsonConverters {
public static class FromString extends StdConverter<String, Money> {
#Override
public Money convert(String value) {
return Money.from(value);
}
}
public static class ToString extends StdConverter<Money, String> {
#Override
public String convert(Money value) {
return value.toString();
}
}
}
Then I annotated the Pojo with #JsonDeserialize #JsonSerialize accordingly:
public class MoneyTestPojo {
#JsonSerialize(converter = MoneyJsonConverters.ToString.class)
#JsonDeserialize(converter = MoneyJsonConverters.FromString.class)
private Money value;
//getter and setter ommited
}