code: private MapperInstance getInstance( JType mappedType, JClassType classType, boolean isSerializers ) throws UnableToCompleteException {
          int nbParam = 0;
          if ( null != mappedType.isGenericType() && (!isSerializers || !typeOracle.isEnumSupertype( mappedType )) ) {
              nbParam = mappedType.isGenericType().getTypeParameters().length;
          }
  
          // we first look at static method
          for ( JMethod method : classType.getMethods() ) {
              // method must be public static, return the instance type and take no parameters
              if ( method.isStatic() && null != method.getReturnType().isClassOrInterface()
                      && classType.isAssignableTo( method.getReturnType().isClassOrInterface() )
                      && method.getParameters().length == nbParam && method.isPublic() ) {
                  MapperType[] parameters = getParameters( mappedType, method, isSerializers );
                  if ( null == parameters ) {
                      continue;
                  }
  
                  return new MapperInstance( classType, method, parameters );
              }
          }
  
          // then we search the default constructor
          for ( JConstructor constructor : classType.getConstructors() ) {
              if ( constructor.isPublic() && constructor.getParameters().length == nbParam ) {
                  MapperType[] parameters = getParameters( mappedType, constructor, isSerializers );
                  if ( null == parameters ) {
                      continue;
                  }
  
                  return new MapperInstance( classType, constructor, parameters );
              }
          }
  
          logger.log( Type.ERROR, "Cannot instantiate the custom serializer/deserializer " + classType.getQualifiedSourceName() + ". "
                  + "Check the page https://github.com/nmorel/gwt-jackson/wiki/Custom-serializers-and-deserializers for more details." );
          throw new UnableToCompleteException();
      }
docs: Search a static method or constructor to instantiate the mapper and return a {@link String} calling it.