Copy non-null properties from one object to another using BeanUtils or similar

I ended up using Spring BeanUtils library. Here is my working method:

import org.springframework.beans.BeanWrapper;
import org.springframework.beans.BeanWrapperImpl;

import java.lang.reflect.Field;
import java.util.Collection;

public class MyBeansUtil<T> {
    public T copyNonNullProperties(T target, T in) {
        if (in == null || target == null || target.getClass() != in.getClass()) return null;

        final BeanWrapper src = new BeanWrapperImpl(in);
        final BeanWrapper trg = new BeanWrapperImpl(target);

        for (final Field property : target.getClass().getDeclaredFields()) {
            Object providedObject = src.getPropertyValue(property.getName());
            if (providedObject != null && !(providedObject instanceof Collection<?>)) {
                trg.setPropertyValue(
                        property.getName(),
                        providedObject);
            }
        }
        return target;
    }
}

It works fine, but notice that it ignores fields that are collections. That's on purpose, I handle them separately.

You can create your own method to copy properties while ignoring null values.

public static String[] getNullPropertyNames (Object source) {
    final BeanWrapper src = new BeanWrapperImpl(source);
    java.beans.PropertyDescriptor[] pds = src.getPropertyDescriptors();

    Set<String> emptyNames = new HashSet<String>();
    for(java.beans.PropertyDescriptor pd : pds) {
        Object srcValue = src.getPropertyValue(pd.getName());
        if (srcValue == null) emptyNames.add(pd.getName());
    }
    String[] result = new String[emptyNames.size()];
    return emptyNames.toArray(result);
}

// then use Spring BeanUtils to copy and ignore null
public static void myCopyProperties(Object src, Object target) {
    BeanUtils.copyProperties(src, target, getNullPropertyNames(src))
}

Using BeanUtils and java8 we can achieve this:

BeanUtils.copyProperties(Object_source, Object_target, getNullPropertyNames(Object_source));

private String[] getNullPropertyNames(Object source) {
        final BeanWrapper wrappedSource = new BeanWrapperImpl(source);
        return Stream.of(wrappedSource.getPropertyDescriptors()).map(FeatureDescriptor::getName)
                .filter(propertyName -> wrappedSource.getPropertyValue(propertyName) == null).toArray(String[]::new);
    }