1. Overview
In this tutorial, we’ll go over the most common Jackson Exceptions — JsonMappingException and UnrecognizedPropertyException.
Finally, we’ll briefly discuss Jackson “No such method” errors.
2. JsonMappingException: Can Not Construct Instance Of
2.1. The Problem
First, let’s take a look at JsonMappingException: Can Not Construct Instance Of.
This exception is thrown if Jackson can’t create an instance of the class, which happens if the class is abstract or it is just an interface.
Here we’ll try to deserialize an instance from class Zoo that has a property animal with abstract type Animal:
public class Zoo {
public Animal animal;
public Zoo() { }
}
abstract class Animal {
public String name;
public Animal() { }
}
class Cat extends Animal {
public int lives;
public Cat() { }
}
When we try to deserialize a JSON String to Zoo instance, it throws the JsonMappingException: Can Not Construct Instance Of:
@Test(expected = JsonMappingException.class)
public void givenAbstractClass_whenDeserializing_thenException()
throws IOException {
String json = "{"animal":{"name":"lacy"}}";
ObjectMapper mapper = new ObjectMapper();
mapper.reader().forType(Zoo.class).readValue(json);
}
This is the full exception:
com.fasterxml.jackson.databind.JsonMappingException:
Can not construct instance of org.baeldung.jackson.exception.Animal,
problem: abstract types either need to be mapped to concrete types,
have custom deserializer,
or be instantiated with additional type information
at
[Source: {"animal":{"name":"lacy"}}; line: 1, column: 2]
(through reference chain: org.baeldung.jackson.exception.Zoo["animal"])
at c.f.j.d.JsonMappingException.from(JsonMappingException.java:148)
2.2. Solutions
We can solve the problem with a simple annotation — @JsonDeserialize on the abstract class:
@JsonDeserialize(as = Cat.class)
abstract class Animal {...}
Note that if we have more than one subtype of the abstract class, we should consider including subtype information as shown in the article Inheritance With Jackson.
3. JsonMappingException: No Suitable Constructor
3.1. The Problem
Now let’s look at the common JsonMappingException: No Suitable Constructor found for type.
This exception is thrown if Jackson can’t access the constructor.
In the following example, class User doesn’t have a default constructor:
public class User {
public int id;
public String name;
public User(int id, String name) {
this.id = id;
this.name = name;
}
}
When we try to deserialize a JSON String to User, JsonMappingException: No Suitable Constructor Found is thrown:
@Test(expected = JsonMappingException.class)
public void givenNoDefaultConstructor_whenDeserializing_thenException()
throws IOException {
String json = "{"id":1,"name":"John"}";
ObjectMapper mapper = new ObjectMapper();
mapper.reader().forType(User.class).readValue(json);
}
And this is the full exception:
com.fasterxml.jackson.databind.JsonMappingException:
No suitable constructor found for type
[simple type, class org.baeldung.jackson.exception.User]:
can not instantiate from JSON object (need to add/enable type information?)
at [Source: {"id":1,"name":"John"}; line: 1, column: 2]
at c.f.j.d.JsonMappingException.from(JsonMappingException.java:148)
3.2. The Solution
To solve this problem, we just add a default constructor:
public class User {
public int id;
public String name;
public User() {
super();
}
public User(int id, String name) {
this.id = id;
this.name = name;
}
}
Now when we deserialize, the process will work just fine:
@Test
public void givenDefaultConstructor_whenDeserializing_thenCorrect()
throws IOException {
String json = "{"id":1,"name":"John"}";
ObjectMapper mapper = new ObjectMapper();
User user = mapper.reader()
.forType(User.class).readValue(json);
assertEquals("John", user.name);
}
4. JsonMappingException: Root Name Does Not Match Expected
4.1. The Problem
Next, let’s take a look at JsonMappingException: Root Name Does Not Match Expected.
This exception is thrown if the JSON doesn’t match exactly what Jackson is looking for.
For example, the main JSON could be wrapped:
@Test(expected = JsonMappingException.class)
public void givenWrappedJsonString_whenDeserializing_thenException()
throws IOException {
String json = "{"user":{"id":1,"name":"John"}}";
ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE);
mapper.reader().forType(User.class).readValue(json);
}
This is the full exception:
com.fasterxml.jackson.databind.JsonMappingException:
Root name 'user' does not match expected ('User') for type
[simple type, class org.baeldung.jackson.dtos.User]
at [Source: {"user":{"id":1,"name":"John"}}; line: 1, column: 2]
at c.f.j.d.JsonMappingException.from(JsonMappingException.java:148)
4.2. The Solution
We can solve this problem using the annotation @JsonRootName:
@JsonRootName(value = "user")
public class UserWithRoot {
public int id;
public String name;
}
When we try to deserialize the wrapped JSON, it works correctly:
@Test
public void
givenWrappedJsonStringAndConfigureClass_whenDeserializing_thenCorrect()
throws IOException {
String json = "{"user":{"id":1,"name":"John"}}";
ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE);
UserWithRoot user = mapper.reader()
.forType(UserWithRoot.class)
.readValue(json);
assertEquals("John", user.name);
}
5. JsonMappingException: No Serializer Found for Class
5.1. The Problem
Now let’s take a look at JsonMappingException: No Serializer Found for Class.
This exception is thrown if we try to serialize an instance while its properties and their getters are private.
We’ll try to serialize a UserWithPrivateFields:
public class UserWithPrivateFields {
int id;
String name;
}
When we try to serialize an instance of UserWithPrivateFields, JsonMappingException: No Serializer Found for Class is thrown:
@Test(expected = JsonMappingException.class)
public void givenClassWithPrivateFields_whenSerializing_thenException()
throws IOException {
UserWithPrivateFields user = new UserWithPrivateFields(1, "John");
ObjectMapper mapper = new ObjectMapper();
mapper.writer().writeValueAsString(user);
}
And this is the full exception:
com.fasterxml.jackson.databind.JsonMappingException:
No serializer found for class org.baeldung.jackson.exception.UserWithPrivateFields
and no properties discovered to create BeanSerializer
(to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) )
at c.f.j.d.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:59)
5.2. The Solution
We can solve this problem by configuring the ObjectMapper visibility:
@Test
public void givenClassWithPrivateFields_whenConfigureSerializing_thenCorrect()
throws IOException {
UserWithPrivateFields user = new UserWithPrivateFields(1, "John");
ObjectMapper mapper = new ObjectMapper();
mapper.setVisibility(PropertyAccessor.FIELD, Visibility.ANY);
String result = mapper.writer().writeValueAsString(user);
assertThat(result, containsString("John"));
}
Or we can use the annotation @JsonAutoDetect:
@JsonAutoDetect(fieldVisibility = Visibility.ANY)
public class UserWithPrivateFields { ... }
Of course, if we do have the option to modify the source of the class, we can also add in getters for Jackson to use.
6. JsonMappingException: Can Not Deserialize Instance Of
6.1. The Problem
Next, let’s take a look at JsonMappingException: Can Not Deserialize Instance Of.
This exception is thrown if the wrong type is used while deserializing.
In this example, we are trying to deserialize a List of User:
@Test(expected = JsonMappingException.class)
public void givenJsonOfArray_whenDeserializing_thenException()
throws JsonProcessingException, IOException {
String json
= "[{"id":1,"name":"John"},{"id":2,"name":"Adam"}]";
ObjectMapper mapper = new ObjectMapper();
mapper.reader().forType(User.class).readValue(json);
}
And here’s the full exception:
com.fasterxml.jackson.databind.JsonMappingException:
Can not deserialize instance of
org.baeldung.jackson.dtos.User out of START_ARRAY token
at [Source: [{"id":1,"name":"John"},{"id":2,"name":"Adam"}]; line: 1, column: 1]
at c.f.j.d.JsonMappingException.from(JsonMappingException.java:148)
6.2. The Solution
We can solve this problem by changing the type from User to List
@Test
public void givenJsonOfArray_whenDeserializing_thenCorrect()
throws JsonProcessingException, IOException {
String json
= "[{"id":1,"name":"John"},{"id":2,"name":"Adam"}]";
ObjectMapper mapper = new ObjectMapper();
List<User> users = mapper.reader()
.forType(new TypeReference<List<User>>() {})
.readValue(json);
assertEquals(2, users.size());
}
7. JsonMappingException: Cannot Deserialize Value of Type java.lang.String From Object Value
Before diving deep into the details, let’s try to understand what the exception means.
The stack trace of the exception says it all: “*Cannot deserialize value of type `java.lang.String` from Object value (token `JsonToken.START_OBJECT`)*“. It means that Jackson fails to deserialize an object into a String instance.
7.1. Reproducing the Exception
The most typical cause of this exception is mapping a JSON object into a String instance.
For instance, let’s consider the Person class:
public class Person {
private String firstName;
private String lastName;
private String contact;
// standard getters and setters
}
As shown above, we declared the contact field with the String type.
Now, let’s see what happens if we deserialize this JSON string:
{
"firstName":"Azhrioun",
"lastName":"Abderrahim",
"contact":{
"email":"[email protected]"
}
}
Jackson fails with the stack trace:
Cannot deserialize value of type `java.lang.String` from Object value (token `JsonToken.START_OBJECT`)
at [Source: (String)"{"firstName":"Azhrioun","lastName":"Abderrahim","contact":{"email":"[email protected]"}}"; line: 1, column: 59] (through reference chain: com.baeldung.exceptions.Person["contact"])
...
Let’s confirm this using a test case:
@Test
public void givenJsonObject_whenDeserializingIntoString_thenException() throws IOException {
final String json = "{\"firstName\":\"Azhrioun\",\"lastName\":\"Abderrahim\",\"contact\":{\"email\":\"[email protected]\"}}";
final ObjectMapper mapper = new ObjectMapper();
Exception exception = assertThrows(JsonMappingException.class, () -> mapper.reader()
.forType(Person.class)
.readValue(json));
assertTrue(exception.getMessage()
.contains("Cannot deserialize value of type `java.lang.String` from Object value (token `JsonToken.START_OBJECT`)"));
}
As we can see, we tried to deserialize the JSON object: “contact”:{“email”: “[email protected]”}}” into the contact property which is String. Hence the exception.
7.2. The Solution
The easiest solution would be mapping every JSON object to a Java object and not to a simple String object.
So, let’s create another class Contact to denote the JSON object“contact”:{“email”: “[email protected]”}}”:
public class Contact {
private String email;
// standard getter and setter
}
Next, let’s rewrite the Person class to change the contact type from String to Contact:
public class PersonContact {
private String firstName;
private String lastName;
private Contact contact;
// standard getters and setters
}
Now, we’re going to add another test case to verify that everything works as excepted:
@Test
public void givenJsonObject_whenDeserializingIntoObject_thenDeserialize() throws IOException {
final String json = "{\"firstName\":\"Azhrioun\",\"lastName\":\"Abderrahim\",\"contact\":{\"email\":\"[email protected]\"}}";
final ObjectMapper mapper = new ObjectMapper();
PersonContact person = mapper.reader()
.forType(PersonContact.class)
.readValue(json);
assertEquals("[email protected]", person.getContact().getEmail());
}
8. UnrecognizedPropertyException
8.1. The Problem
Now let’s see the UnrecognizedPropertyException.
This exception is thrown if there is an unknown property in the JSON String while deserializing.
We’ll try to deserialize a JSON String with extra property “checked“:
@Test(expected = UnrecognizedPropertyException.class)
public void givenJsonStringWithExtra_whenDeserializing_thenException()
throws IOException {
String json = "{"id":1,"name":"John", "checked":true}";
ObjectMapper mapper = new ObjectMapper();
mapper.reader().forType(User.class).readValue(json);
}
This is the full exception:
com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException:
Unrecognized field "checked" (class org.baeldung.jackson.dtos.User),
not marked as ignorable (2 known properties: "id", "name"])
at [Source: {"id":1,"name":"John", "checked":true}; line: 1, column: 38]
(through reference chain: org.baeldung.jackson.dtos.User["checked"])
at c.f.j.d.exc.UnrecognizedPropertyException.from(
UnrecognizedPropertyException.java:51)
8.2. The Solution
We can solve this problem by configuring the ObjectMapper:
@Test
public void givenJsonStringWithExtra_whenConfigureDeserializing_thenCorrect()
throws IOException {
String json = "{"id":1,"name":"John", "checked":true}";
ObjectMapper mapper = new ObjectMapper();
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
User user = mapper.reader().forType(User.class).readValue(json);
assertEquals("John", user.name);
}
Or we can use the annotation @JsonIgnoreProperties:
@JsonIgnoreProperties(ignoreUnknown = true)
public class User {...}
9. JsonParseException: Unexpected Character (”’ (code 39))
9.1. The Problem
Next, let’s discuss JsonParseException: Unexpected character (”’ (code 39)).
This exception is thrown if the JSON String to be deserialized contains single quotes instead of double quotes.
We’ll try to deserialize a JSON String containing single quotes:
@Test(expected = JsonParseException.class)
public void givenStringWithSingleQuotes_whenDeserializing_thenException()
throws JsonProcessingException, IOException {
String json = "{'id':1,'name':'John'}";
ObjectMapper mapper = new ObjectMapper();
mapper.reader()
.forType(User.class).readValue(json);
}
Here’s the full exception:
com.fasterxml.jackson.core.JsonParseException:
Unexpected character (''' (code 39)):
was expecting double-quote to start field name
at [Source: {'id':1,'name':'John'}; line: 1, column: 3]
at c.f.j.core.JsonParser._constructError(JsonParser.java:1419)
9.2. The Solution
We can solve this by configuring the ObjectMapper to allow single quotes:
@Test
public void
givenStringWithSingleQuotes_whenConfigureDeserializing_thenCorrect()
throws JsonProcessingException, IOException {
String json = "{'id':1,'name':'John'}";
JsonFactory factory = new JsonFactory();
factory.enable(JsonParser.Feature.ALLOW_SINGLE_QUOTES);
ObjectMapper mapper = new ObjectMapper(factory);
User user = mapper.reader().forType(User.class)
.readValue(json);
assertEquals("John", user.name);
}
10. Jackson NoSuchMethodError
Finally, let’s quickly discuss the Jackson “No such method” errors.
When java.lang.NoSuchMethodError Exception is thrown, it’s usually because we have multiple (and incompatible) versions of Jackson jars on our classpath.
This is the full exception:
java.lang.NoSuchMethodError:
com.fasterxml.jackson.core.JsonParser.getValueAsString()Ljava/lang/String;
at c.f.j.d.deser.std.StringDeserializer.deserialize(StringDeserializer.java:24)
11. Conclusion
In this article, we did a deep dive into the most common Jackson problems — exceptions and errors — looking at the potential causes and at the solutions for each one.
The implementation of all these examples and code snippets can be found on GitHub. This is a Maven-based project, so it should be easy to import and run as it is.