Expected behavior

public B toB(A source) {

 if ( source == null ) {
     return null;
 }
 String field1 = null;
 String field2 = null;
 String field3 = null;

 field1 = source.getfield1();
 field2 = source.getfield2();
 field3 = source.getfield3();

 B result = new B(field1, field2, field3 );
 return result;

}

Actual behavior

public B toB(A source) { if ( source == null ) { return null; } B result = new B(); return result; }

Steps to reproduce the problem

Create two data classes:

data class A( val field1: String?, val field2: String?, val field3: String? )

data class B( val field1: String?, val field2: String?, val field3: String? )

Define mapper class:

@Mapper(componentModel = "spring")
abstract class TestMapper {
abstract fun toB(source: A): B
}

Check the generated mappings.

NOTE: if at least one non-nullable property is added, it works as expected.

MapStruct Version

1.4.2.Final