1. Overview

In this short tutorial, we’ll shed light on how to fix the exception JsonMappingException: Can not deserialize instance of java.util.ArrayList from Object value (token `JsonToken.START_OBJECT`).

First, we’ll highlight the main cause of the exception. Then we’ll demonstrate how to reproduce it in practice, and finally, how to solve it.

2. Understanding the Exception

Typically, Jackson throws JsonMappingException to signal a fatal mapping error when deserializing a JSON string. Therefore, the stack trace “Can not deserialize instance of java.util.ArrayList” indicates that Jackson failed to map a JSON property into an instance of ArrayList.

In short, the most common cause of this exception is using curly braces “{…}” to denote a collection instead of brackets “[…]”.

3. Reproducing the Exception

Now that we know what causes Jackson to throw JsonMappingException, let’s see how to reproduce it using a practical example.

Let’s consider the Country class:

public class Country {

 private String name;
 private List<String> cities;

 // standard getters and setters
}

As we can see, a country is defined by a name and a list of cities.

Next, let’s pretend to use curly braces to define cities in a JSON string:

{
 "name": "Netherlands", 
 "cities": {"Amsterdam", "Tamassint"}
}

Now trying to deserialize the JSON string into an object of Country type will result in the following:

Cannot deserialize value of type `java.util.ArrayList<java.lang.String>` from Object value (token `JsonToken.START_OBJECT`)
at [Source: (String)"{"name":"Netherlands","cities":{"Amsterdam", "Tamassint"}}"; line: 1, column: 32] 
(through reference chain: com.baeldung.mappingexception.Country["cities"])
...

Finally, we’ll create a test case to confirm this:

@Test
public final void givenJsonWithInvalidList_whenDeserializing_thenThrowException() throws JsonParseException, IOException {
 String json = "{\"name\":\"Netherlands\",\"cities\":{\"Amsterdam\", \"Tamassint\"}}";
 ObjectMapper mapper = new ObjectMapper();

 Exception exception = assertThrows(JsonMappingException.class, () -> mapper.reader()
 .forType(Country.class)
 .readValue(json));

 assertTrue(exception.getMessage()
 .contains("Cannot deserialize value of type `java.util.ArrayList<java.lang.String>`"));
}

As shown above, Jackson fails with “Cannot deserialize value of type `java.util.ArrayList<java.lang.String>`”. 

The main reason here is that we used curly braces to represent a list of cities. For Jackson, {“Amsterdam”, “Tamassint”} isn’t a JSON array.

4. Fixing the Exception

The easiest way to avoid the exception is to use brackets instead of curly braces to define a collection of elements. So to solve the exception, we need to fix our JSON string first:

{ 
 "name": "Netherlands",
 "cities": ["Amsterdam", "Tamassint"]
}

Now let’s verify that everything works as excepted using a test case:

@Test
public final void givenJsonWithValidList_whenDeserializing_thenCorrect() throws JsonParseException, IOException {
 String json = "{\"name\":\"Netherlands\",\"cities\":[\"Amsterdam\", \"Tamassint\"]}";
 ObjectMapper mapper = new ObjectMapper();

 Country country = mapper.reader()
 .forType(Country.class)
 .readValue(json);

 assertEquals("Netherlands", country.getName());
 assertEquals(Arrays.asList("Amsterdam", "Tamassint"), country.getCities());
}

As we can see, the new JSON string is successfully deserialized into a Country object.

5. Conclusion

In this brief article, we discussed the main cause of the JsonMappingException: Can not deserialize instance of java.util.ArrayList from Object value (token `JsonToken.START_OBJECT`). Then we showcased how to produce the exception, and how to solve it.