Deserialize String as List of Strings

I have a POJO that contains the following attributes

public class Example {
  @JsonProperty("inputFoo")
  private String foo
  @JsonProperty("inputBar")
  private String bar
  @JsonProperty("inputBaz")
  @JsonDeserialize(using = MyDeserializer.class)
  private Set<String> baz
}

The JSON that I am working with to represent this data currently represents the baz attribute as a single string:

{"inputFoo":"a", "inputBar":"b", "inputBaz":"c"}

I am using the Jackson ObjectMapper to attempt to convert the JSON to my POJO. I know that the input baz String from the JSON wont map cleanly to the Set that I am trying to represent it as, so I defined a custom Deserializer:

public class MyDeserializer extends StdDeserializer<Set<String>> {
   public MyDeserializer(){}

   public MyDeserializer(Class<?> vc) {
      super(vc);
   }

   public Set<String> deserialize(JsonParser p, DeserializationContext cxt) throws IOException, JsonProcessingException {
      String input = p.readValueAs(String.class);
      Set<String> output = new HashSet<>();
      if(input != null) {
         output.add(input);
      }
      return output;
   }
}

I am getting an IllegalArgumentException referencing the "inputBaz" attribute, which I can provide details on. Does anyone see any obvious issue with my deserializer implementation? Thanks

You do not need to implement custom deserialiser, use ACCEPT_SINGLE_VALUE_AS_ARRAY feature. It works for sets as well:

> Feature that determines whether it is acceptable to coerce non-array
> (in JSON) values to work with Java collection (arrays,
> java.util.Collection) types. If enabled, collection deserializers will
> try to handle non-array values as if they had "implicit" surrounding
> JSON array. This feature is meant to be used for
> compatibility/interoperability reasons, to work with packages (such as
> XML-to-JSON converters) that leave out JSON array in cases where there
> is just a single element in array. Feature is disabled by default.

See also:

• com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of java.util.ArrayList out of START_OBJECT token

Replace the 2 constructors with this no-arg constructor:

public MyDeserializer() {
	super(TypeFactory.defaultInstance().constructCollectionType(Set.class, String.class));
}

ACCEPT_SINGLE_VALUE_AS_ARRAY as suggested is a good option.

Maybe your actual problem is more complicated but if not you could also try @JsonCreator instead of custom deserializer. Like:

public class Example {

	@JsonCreator
	public Example(@JsonProperty("inputFoo") String foo,
			@JsonProperty("inputBar") String bar,
			@JsonProperty("inputBaz") String strBaz) {
		this.foo = foo;
		this.bar = bar;
		this.baz = new HashSet<>();
		baz.add(strBaz);
	}

	private String foo;
	private String bar;
	private Set<String> baz;
}

Just to show that in more general case you might avoid implementing custom deserializer with @JsonCreator also but still make some simple conversions.