Expected BEGIN_OBJECT but was STRING

The fix

Read the column and the path, then fix what they point at. If the body is not JSON, log it and find out why you got it. If a field has the wrong type, change the field in your class, as in the fixed code.

{
  "id": 1,
  "customer": "Ada"
}

public class Order {
  public long id;
  public Customer customer;
}

public class Customer {
  public String name;
}

Order order = gson.fromJson(json, Order.class);

public class Order {
  public long id;
  public String customer;
}

Order order = gson.fromJson(json, Order.class);

How the message appears

The first line is the example on this page. The rest are the same message for other bodies: an HTML page or plain text, a JSON string, a list where the class wants one object, an object where it wants a list, and the reverse of this page, an object in a field that is a string. Every one of them is followed by the link on the last line.

What causes it

1. 1

   line 1 column 1 path $: the body is not JSON

   Column 1 and path $ mean the first character of the body is not JSON. It is an HTML error page or a plain-text error such as error: upstream timeout. Gson reads leniently by default, so it takes that text as a string and says your class needs an object. A strict Gson reader calls the same kind of text malformed JSON instead. Log the body and look at the status code.

2. 2

   but was STRING at column 2 path $: the body is a JSON string

   The whole body is one JSON string, and the object you wanted is inside it with every quote escaped. That means the JSON was encoded twice. The sender turned an object into a string and then put the string through JSON again. Fix the sender so it sends the object.

3. 3

   path $.customer: one field has the wrong type

   A name after the $ is the field. In this example the JSON has "customer": "Ada", a string, and your class has public Customer customer, so Gson wants an object there. Change the field to String, as in the fixed code, or get the sender to send an object. The reverse gives Expected a string but was BEGIN_OBJECT with the path of the field.

4. 4

   BEGIN_ARRAY: a list where the class wants one object, or the reverse

   If the JSON starts with [ and you asked for Order.class, Gson says Expected BEGIN_OBJECT but was BEGIN_ARRAY. If it starts with { and you asked for a List<Order>, it says Expected BEGIN_ARRAY but was BEGIN_OBJECT. Look at the first character of the body and at the type you asked for.

5. 5

   It happens in Retrofit too

   Retrofit with the Gson converter hands the response body to Gson, so a Retrofit call can throw this as well. The cause is the same. Log the raw body and compare it with your class. Retrofit was not tested here.

Worth knowing