my aim is to copy fields of one object into another, but only those that aren't null. I don't want to assign it explicitly. A more generic solution would be very useful and easier to maintain i.e. for implementing PATCH in REST API where you allow providing only specific fields.
I saw this similar thread and I'm trying to implement some of the ideas from here: Helper in order to copy non null properties from object to another ? (Java)
But the objects aren't altered in any way after the program execution.
So here are my example classes created for example:
class Person {
String name;
int age;
Pet friend;
public Person() {
}
public Person(String name, int age, Pet friend) {
this.name = name;
this.age = age;
this.friend = friend;
}
// getters and setters here
}
class Pet {
String name;
int age;
public Pet(String name, int age) {
this.name = name;
this.age = age;
}
// getters and setters here
}
Here is my overridden copyProperty method:
import org.apache.commons.beanutils.BeanUtilsBean;
import java.lang.reflect.InvocationTargetException;
public class MyBeansUtil extends BeanUtilsBean {
@Override
public void copyProperty(Object dest, String name, Object value)
throws IllegalAccessException, InvocationTargetException {
if(value == null) return;
super.copyProperty(dest, name, value);
}
}
... and here is the place I'm trying to test it on some examples:
public class SandBox {
public static void main(String[] args) {
Person db = new Person("John", 36, new Pet("Lucy", 3));
Person db2 = new Person("John", 36, new Pet("Lucy", 2));
Person db3 = new Person("John", 36, new Pet("Lucy", 4));
Person in = new Person();
in.age = 17;
in.name = "Paul";
in.friend = new Pet(null, 35);
Person in2 = new Person();
in2.name = "Damian";
Person in3 = new Person();
in3.friend = new Pet("Lup", 25);
try {
BeanUtilsBean notNull =new MyBeansUtil();
notNull.copyProperties(db, in);
notNull.copyProperties(db2, in2);
notNull.copyProperties(db3, in3);
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
}
Unfortunately, the original objects db, db1, db2 stay the same as they were. Am I doing something wrong here?
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))
}
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