Generic way to update pojos via getters and setters

问题

Let's say I have POJO with getters and setters of different types. I want to write some generic algorithm for updating data from one to another based on just defining getters and setters via lambdas. I'm trying to create it this way

private static final Map<Function<Entity, Object>, BiConsumer<Entity, Object>> ACCESSORS = new HashMap
        <Function<Entity, Object>, BiConsumer<Entity, Object>>() {{
    put(Entity::getAreaCode, Entity::setAreaCode);
}});

Then I go through all entries applying target entity to them and if result of getter is not null then I want to apply corresponding setter for other entity.

But it won't work because Object is not castable to String. And I want to use it for different types not only String but also Integers, etc...

Is is solvable in some simple approach without creating special converter and associate it with every entry?

回答1:

Use something like

private static final List<BiConsumer<Entity,Entity>> ACCESSORS =
    Collections.unmodifiableList(Array.asList(
        (src,dst) -> dst.setAreaCode(src.getAreaCode()),
        (src,dst) -> dst.setOtherProperty(src.getOtherProperty())
        /* etc */
));

Then, you can loop over the list and apply each operation to two entities, like

static final void copyAll(Entity src, Entity dst) {
    ACCESSORS.forEach(op -> op.accept(src, dst));
}

The key point is that the actual property value type is handled within each BiConsumer but is not part of the generic signature anymore and therefore doesn’t need to be declared for ACCESSORS. It’s even more efficient, as it can handle primitive data types without boxing overhead.

The Map wasn’t an appropriate data structure for this task anyway, as for these functions, no meaningful lookup could be performed, so this is a data structure is only intended to be iterated over.

You can integrate the “copy only when non-null” logic with a generic helper method:

private static final List<BiConsumer<Entity,Entity>> ACCESSORS =
    Collections.unmodifiableList(Arrays.asList(
        copyWhenNonNull(Entity::getAreaCode, Entity::setAreaCode),
        copyWhenNonNull(Entity::getOtherProperty, Entity::setOtherProperty)
        /* etc */
));
private static <E,V> BiConsumer<E,E> copyWhenNonNull(
    Function<? super E, ? extends V> getter, BiConsumer<? super E, ? super V> setter) {
    return (src,dst) -> {
        V value = getter.apply(src);
        if(value != null) setter.accept(dst, value);
    };
}

The copyAll method doesn’t change. This even allows mixing unconditional copying of properties which can never be null with conditional copying.

回答2:

I know you already have an answer but for anyone needing something like this in the future: I have developed a small library around this context - datus.

Here is an example which shows some of its features:

class Person {
  //getters + setters omitted for brevity
  private String firstName;
  private String lastName;
}

class PersonDTO {
  //getters + setters + empty constructor omitted for brevity
  private String firstName;
  private String lastName;
}

  //the mutable API defines a mapping process by multiple getter-setter steps
  Mapper<Person, PersonDTO> mapper = Datus.forTypes(Person.class, PersonDTO.class).mutable(PersonDTO::new)
      .from(Person::getFirstName).into(PersonDTO.setFirstName)
      .from(Person::getLastName)
      .given(Objects::nonNull, ln -> ln.toUpperCase()).orElse("fallback")
      .into(PersonDTO::setLastName)
      .from(/*...*/).into(/*...*/)
      .build();

  Person person = new Person();
person.setFirstName("firstName");
    person.setLastName(null);
    PersonDTO personDto = mapper.convert(person);
/*
    personDto = PersonDTO [
        firstName = "firstName",
        lastName = "fallback"
    ]
*/
    person.setLastName("lastName");
    personDto = mapper.convert(person);
/*
    personDto = PersonDTO [
        firstName = "firstName",
        lastName = "LASTNAME"
    ]
*/

来源：https://stackoverflow.com/questions/54294416/generic-way-to-update-pojos-via-getters-and-setters