Modelmapper: How to apply custom mapping when source object is null?

Question

Lets assume I have class MySource:

public class MySource {
    public String fieldA;
    public String fieldB;

    public MySource(String A, String B) {
       this.fieldA = A;
       this.fieldB = B;
    }
}

and I would like to translate it to object MyTarget:

public class MyTarget {
    public String fieldA;
    public String fieldB;
}

using default ModelMapper settings I can achieve it in the following way:

ModelMapper modelMapper = new ModelMapper();
MySource src = new MySource("A field", "B field");
MyTarget trg = modelMapper.map(src, MyTarget.class); //success! fields are copied

It can happen however, that MySource object will be null. In this case, MyTarget will be also null:

    ModelMapper modelMapper = new ModelMapper();
    MySource src = null;
    MyTarget trg = modelMapper.map(src, MyTarget.class); //trg = null

I would like to specify custom mapping in such a way, that (pseudo code):

MySource src != null ? [perform default mapping] : [return new MyTarget()]

Does anybody have idea how to write an appropriate converter to achieve that?

Answer 1

It is not possible using ModelMapper directly because ModelMapper map(Source, Destination) method checks if source is null, in that case it throws an exception...

Take a look to ModelMapper Map method implementation:

public <D> D map(Object source, Class<D> destinationType) {
    Assert.notNull(source, "source"); -> //IllegalArgument Exception
    Assert.notNull(destinationType, "destinationType");
    return mapInternal(source, null, destinationType, null);
  }

Solution

I propose extends ModelMapper class and override map(Object source, Class<D> destinationType) like this:

public class MyCustomizedMapper extends ModelMapper{

    @Override
    public <D> D map(Object source, Class<D> destinationType) {
        Object tmpSource = source;
        if(source == null){
            tmpSource = new Object();
        }

        return super.map(tmpSource, destinationType);
    }
}

It checks if source is null, in that case it initializes then it calls super map(Object source, Class<D> destinationType).

Finally, you could use your customized mapper like this:

public static void main(String args[]){
    //Your customized mapper
    ModelMapper modelMapper = new MyCustomizedMapper();

    MySource src = null;
    MyTarget trg = modelMapper.map(src, MyTarget.class); //trg = null
    System.out.println(trg);
}

The output would be a new MyTarget():

Output Console: NullExampleMain.MyTarget(fieldA=null, fieldB=null)

Thus it is initialized.