How to enable strict type parsing for jackson?

Question

Jackson 1.9.9 is somewhat inconsistent in what it parses into scalar values (bool, int, string). Any array or object type fails but you can put any scalar type into a string. For bool 0 and not 0 are mapped to false/true. int attributes accept only numbers.

public class Foo { public String s; public boolean b; public int i; }

ObjectMapper mapper = new ObjectMapper();
System.out.println(mapper.readValue("{\"s\":\"abc\"}", Foo.class).s); // "abc"
System.out.println(mapper.readValue("{\"s\":true}", Foo.class).s); // "true"
System.out.println(mapper.readValue("{\"s\":123}", Foo.class).s); // "123"
System.out.println(mapper.readValue("{\"b\":123}", Foo.class).b); // true
System.out.println(mapper.readValue("{\"b\":0}", Foo.class).b); // false
System.out.println(mapper.readValue("{\"b\":\"abc\"}", Foo.class).b); // fails with JsonMappingException
System.out.println(mapper.readValue("{\"i\":\"abc\"}", Foo.class).i); // fails with JsonMappingException
System.out.println(mapper.readValue("{\"i\":true}", Foo.class).i); // fails with JsonMappingException
System.out.println(mapper.readValue("{\"s\":[]}", Foo.class).s); // fails with JsonMappingException
System.out.println(mapper.readValue("{\"s\":{}}", Foo.class).s); // fails  with JsonMappingException
System.out.println(mapper.readValue("{\"b\":[]}", Foo.class).b); // fails with JsonMappingException
System.out.println(mapper.readValue("{\"b\":{}}", Foo.class).b); // fails  with JsonMappingException
System.out.println(mapper.readValue("{\"i\":[]}", Foo.class).i); // fails with JsonMappingException
System.out.println(mapper.readValue("{\"i\":{}}", Foo.class).i); // fails  with JsonMappingException

Is there a strict mode for Jackson so that I get an error if someone passes for example a boolean value into a String attribute?

I am using this in a JAX-RS project and this makes error reporting based on exceptions thrown by Jackson somewhat difficult since I get most of the errors but not all of them. I would like to avoid getting a raw ObjectNode and checking everything manually. If a caller passes a boolean for a string then I would like to tell him that since this is most likely a programming error.

Answer 1

FWIW, if you come across this much later, like I did, ALLOW_COERCION_OF_SCALARS will help somewhat by reducing the number of conversions that Jackson will do you your behalf, although it's far from a complete strict mode; many type coercions will still be performed, but some will be prevented. The linked documentation gets into some of the details.

Answer 2

Jackson data-binding has grown over time to accept more automatic coercions, and although there are some features (DeserializationConfig.Feature.FAIL_ON_NULL_FOR_PRIMITIVES) to enforce stricter checks, not many have been requested.

Given this, your best is to register custom JsonDeserializer for types you want to be stricter on, like boolean/Boolean. You can implement stricter checks there.

In addition, you can request feature(s) for stricter limitations: I think this does make sense for some use cases, esp. if looser conversions can hide real problems.