How to deserialize JSON file starting with an array in Jackson?

Question

I have a Json file that looks like this:

[
    { \"field\":\"val\" },
....
]

I have Java object representing single object and collection o

Answer 1

Try

   mapper.readValue(in, ObjectClass[].class);

Where ObjectClass is something like:

  public class ObjectClass {
    String field;

    public ObjectClass() { }

    public void setField(String value) {
      this.field = value;
    }
  }

Note: in your posted version of the Objects class, you're declaring a Collection of Lists (i.e. a list of lists), which is not what you want. You probably wanted a List<ObjectClass>. However, it's much simpler to just do YourObject[].class when deserializing with Jackson, and then converting into a list afterwards.

Answer 2

You can directly get a list through the following way:

List<ObjectClass> objs =  
    mapper.readValue(in, new TypeReference<List<ObjectClass>>() {});

Answer 3

Passing an array or List type works, as @dmon answered.

For sake of completeness, there is also an incremental approach if you wanted to read contents one-by-one:

Iterator<Objects> it = mapper.reader(Objects.class).readValues(in);
while (it.hasNext()) {
  Objects next = it.next();
  // ... process it
}

This is useful if you have huge lists or sequences of objects; either with enclosing JSON Array, or just root-level values separated by spaces or linefeeds.