Mapping a json string to an object with jackson will throw MismatchedInputException

Question

I have a simple class

public class AuthenticationToken {

    public String token;

    public AuthenticationToken(String token) {
        this.token = token;
    }

    public String getToken() {
        return token;
    }

    public void setToken(String token) {
        this.token = token;
    }
}

With jackson I am trying to map an string to this object like this

private String input = "{\"token\":\"adf\"}";


@Test
public void whenJsonString_ThenCreateAuthenticationObject() throws IOException {

    ObjectMapper jsonMapper = new ObjectMapper();
    AuthenticationToken tokenObject = jsonMapper.readValue(input, AuthenticationToken.class);
    assertThat(tokenObject).isNotNull();
}

But it throws the following exception

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `foo.AuthenticationToken` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)
at [Source: (String)"{"token":"adf"}"; line: 1, column: 2]

I tried to annotate the property in my AuthenticationToken as a @JsonProperty but this also resulted in in this exception.

Answer 1

Annotate the class constructor with @JsonCreator

Marker annotation that can be used to define constructors and factory methods as one to use for instantiating new instances of the associated class.

public class AuthenticationToken {
    public String token;

    @JsonCreator
    public AuthenticationToken(@JsonProperty("token") final String token) {
        this.token = token;
    }

    public String getToken() {
        return token;
    }

    public void setToken(String token) {
        this.token = token;
    }
}

Answer 2

Jackson will by default expect an " empty " constructor and will automatically fill your Object by the getters and setters that are provided for each field.

So removing the arguments of your constructor will already solve your problem:

public class AuthenticationToken {

    public String token;

    public AuthenticationToken() {}

    public String getToken() {
        return token;
    }

    public void setToken(String token) {
        this.token = token;
    }
}

You could also just add an additional empty constructor if you want to keep your current one as it is. Tested both options for your Test Case, both work fine.