Modelmapper throws java.lang.IllegalArgumentException: object is not an instance of declaring class

[wrssmi]

Following Testcode:

public class MapTest{

    public static void main(String[] args) {
        ModelMapper modelMapper = new ModelMapper();
        modelMapper.addMappings(new PropertyMap<A, B>() {
            @Override
            protected void configure() {
                map().setDiff(ServiceUtil.getDaysBetween(source.getFrom(), source.getTo()));
            }
        });
        A a = new A(LocalDate.now(), LocalDate.now());
        B b = modelMapper.map(a, B.class);
    }

    public static int getDaysBetween(LocalDate von, LocalDate bis) {
        if (von != null && bis != null) {
            return Period.between(von, bis).getDays();
        }
        return 0;
    }
}

with following Objects:

public class A
{
    LocalDate from;

    LocalDate to;

    public A(LocalDate from, LocalDate to) {
        this.from = from;
        this.to = to;
    }

    public LocalDate getFrom() {
        return from;
    }

    public void setFrom(LocalDate from) {
        this.from = from;
    }

    public LocalDate getTo() {
        return to;
    }

    public void setTo(LocalDate to) {
        this.to = to;
    }
}
public class B
{

    int diff;

    public B()
    {

    }

    public void setDiff(int diff) {
        this.diff = diff;
    }

    public int getDiff() {
        return diff;
    }
}

throws following Exception:
Exception in thread "main" org.modelmapper.MappingException: ModelMapper mapping errors:

1. Error mapping A to B

1 error
at org.modelmapper.internal.Errors.throwMappingExceptionIfErrorsExist(Errors.java:374)
at org.modelmapper.internal.MappingEngineImpl.map(MappingEngineImpl.java:69)
at org.modelmapper.ModelMapper.mapInternal(ModelMapper.java:497)
at org.modelmapper.ModelMapper.map(ModelMapper.java:340)
at MapTest.main(MapTest.java:31)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
Caused by: org.modelmapper.MappingException: ModelMapper mapping errors:

1. Failed to get value from A.getTo()

1 error
at org.modelmapper.internal.Errors.toMappingException(Errors.java:258)
at org.modelmapper.internal.PropertyInfoImpl$MethodAccessor.getValue(PropertyInfoImpl.java:100)
at org.modelmapper.internal.MappingEngineImpl.resolveSourceValue(MappingEngineImpl.java:189)
at org.modelmapper.internal.MappingEngineImpl.propertyMap(MappingEngineImpl.java:156)
at org.modelmapper.internal.MappingEngineImpl.typeMap(MappingEngineImpl.java:131)
at org.modelmapper.internal.MappingEngineImpl.map(MappingEngineImpl.java:92)
at org.modelmapper.internal.MappingEngineImpl.map(MappingEngineImpl.java:60)
... 8 more
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.modelmapper.internal.PropertyInfoImpl$MethodAccessor.getValue(PropertyInfoImpl.java:95)
... 13 more

[chhsiao90]

@wrssmi - It seems that PropertyMap only support simple getter/setter method.
The implementation about how a PropertyMap work is based on java reflection and it cannot handle for calculation or converting.

The name PropertyMap implied that it was only used for mapping,
So I think you should use converters for converting!

[wrssmi]

Thank you for that hint. following is working.

public static void main(String[] args) {
        ModelMapper modelMapper = new ModelMapper();
        modelMapper.addMappings(new PropertyMap<A, B>() {
            @Override
            protected void configure() {
                using(new Converter<A, Integer>() {
                    public Integer convert(MappingContext<A, Integer> context) {
                        return context.getSource() == null ? null : Period.between(context.getSource().getFrom(),
                              context.getSource().getTo()).getDays();
                    }
                }).map(source).setDiff(0);
            }
        });
        A a = new A(LocalDate.now(), LocalDate.now().plusDays(10));
        B b = modelMapper.map(a, B.class);
        System.out.println(b.diff);
    }

[chhsiao90]

Wow, the code that used PropertyMap with converter looks great!
Hadn't used that before.

[mbm56767]

you can simply put the conversion logic outside of property map and simply use the computed value inside mappings. This way things will be easy to maintain.