Question

I have a JSON array that I want to make sure only contains strings. Jackson implicitly casts integers and dates to strings. I want to make sure that every element in the JSON array is in fact a string.

    Object[] badModuleArray = new Object[]{"case", 1, 2, "employee", new Date()};

    ObjectMapper objectMapper = new ObjectMapper();
    String jsonModules = objectMapper.writeValueAsString(badModuleArray);

    try
    {
        TypeFactory typeFactory = TypeFactory.defaultInstance();
        mapper.readValue(modules, typeFactory.constructCollectionType(List.class, String.class));
    }
    catch(IOException e)
    {
        logger.error("something other than strings in JSON object");

    }

In the above example, I want the ObjectMapper to not cast integer, dates, etc to Strings. I want an exception thrown if each element in the JSON array is not a string. Is this possible?

Answer 1

Jackson is casting each object to a string because you’ve told it that you want a List<String>.

Instead, ask Jackson for a List<Object> and examine the contents of the List yourself to throw an error if any of them are not a String:

List list = objectMapper.readValue(jsonModules, typeFactory.constructCollectionType(List.class, Object.class));
for (Object item : list) {
    System.out.println(item + " is a: " + item.getClass());
    if (!(item instanceof String)) {
        System.out.println("Not a string!");
    }
}

For JSON of ["case",1,2,"employee",1358444552861] I get:

case is a: class java.lang.String
1 is a: class java.lang.Integer
Not a string!
2 is a: class java.lang.Integer
Not a string!
employee is a: class java.lang.String
1358444552861 is a: class java.lang.Long
Not a string!