Cannot construct instance of - Jackson

Question

I am using Jackson and I m having problems  when I try to deserialize an Object I get the following error   com fasterxml jackson databind JsonMappingException       Can not construct instance of net MyAbstractClass       problem  abstract types either need to be mapped to concrete types           have custom deserializer  or be instantiated with additional type information   I am having problems in the attribute    JsonTypeInfo use   JsonTypeInfo Id NAME  include   JsonTypeInfo As WRAPPER OBJECT  property     id    JsonSubTypes    JsonSubTypes Type value   MyAbstractClass class  name    MyAbstractClass       ManyToOne private MyAbstractClass object    Could anyone help me

User · Answer

Your  JsonSubTypes declaration does not make sense  it needs to list implementation  sub-  classes  NOT the class itself  which would be pointless   So you need to modify that entry to list sub-class es  there are  or use some other mechanism to register sub-classes  SimpleModule has something like addAbstractTypeMapping

User · Answer

In your concrete example the problem is that you don t use this construct correctly   JsonSubTypes    JsonSubTypes Type value   MyAbstractClass class  name    quot MyAbstractClass quot        JsonSubTypes Type should contain the actual non-abstract subtypes of your abstract class  Therefore if you have  abstract class Parent and the concrete subclasses Ch1 extends Parent and Ch2 extends Parent Then your annotation should look like   JsonSubTypes               JsonSubTypes Type value   Ch1 class  name    quot ch1 quot               JsonSubTypes Type value   Ch2 class  name    quot ch2 quot       Here name should match the value of your  discriminator    JsonTypeInfo use   JsonTypeInfo Id NAME   include   JsonTypeInfo As WRAPPER OBJECT   property    quot type quot    in the property field  here it is equal to type  So type will be the key and the value you set in name will be the value  Therefore  when the json string comes if it has this form      quot type quot    quot ch1 quot     quot other quot   quot fields quot     Jackson will automatically convert this to a Ch1 class  If you send this      quot type quot    quot ch2 quot     quot other quot   quot fields quot     You would get a Ch2 instance

User · Answer

You cannot instantiate an abstract class  Jackson neither  You should give Jackson information on how to instantiate MyAbstractClass with a concrete type   See this answer on stackoverflow  Jackson JSON library  how to instantiate a class that contains abstract fields  And maybe also see Jackson Polymorphic Deserialization

User · Answer

For me there was no default constructor defined for the POJOs I was trying to use  creating default constructor fixed it   public class TeamCode         Expose     private String value       public String getValue             return value               public TeamCode                  public TeamCode String value            this value   value              Override     public String toString             return  TeamCode                      value      value                                          public void setValue String value            this value   value

User · Answer

You need to use a concrete class and not an Abstract class while deserializing  if the Abstract class has several implementations then  in that case  you can use it as below-      JsonTypeInfo  use   JsonTypeInfo Id NAME  include   JsonTypeInfo As PROPERTY  property    type        JsonSubTypes           Type value   Bike class  name    bike            Type value   Auto class  name    auto            Type value   Car class  name    car              public abstract class Vehicle              fields  constructors  getters  setters

[java] Cannot construct instance of - Jackson

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