Lombok Tobuilder()方法是否会创建字段的深层副本

Question

我在对象实例上使用toBuilder()来创建构建器实例，然后构建创建新实例的方法。原始对象具有列表，新对象是否引用了同一列表或副本？

@Getter
@Setter
@AllArgsConstructor
public class Library {
    private List<Book> books;
    @Builder(toBuilder=true)
    public Library(final List<Book> books){
         this.books = books;
    }
}

Library lib2  = lib1.toBuilder().build();

lib2书籍会与lib1书籍相同的列表？

Answer 1

是的， @Builder(toBuilder=true)注释不执行对象的深副本，而仅复制字段的引用。

List<Book> books = new ArrayList<>();
Library one = new Library(books);
Library two = one.toBuilder().build();
System.out.println(one.getBooks() == two.getBooks()); // true, same reference

Answer 2

您可以用一个简单的技巧手动制作集合的副本：

    List<Book> books = new ArrayList<>();
    Library one = new Library(books);
    Library two = one.toBuilder()
        .books(new ArrayList<>(one.getBooks))
        .build();
    System.out.println(one.getBooks() == two.getBooks()); // false, different refs

Answer 3

实际上可以做的是使用其他映射工具从现有对象创建一个新对象。

例如com.fasterxml.jackson.databind.ObjectMapper

    @AllArgsConstructor
    public static class Book
    {
        private String title;
    }
    @NoArgsConstructor
    @AllArgsConstructor
    @Getter
    public static class Library
    {
        private List<Book> books;
    }
    ObjectMapper objectMapper = new ObjectMapper(); //it's configurable
    objectMapper.configure( DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false );
    objectMapper.configure( SerializationFeature.FAIL_ON_EMPTY_BEANS, false );
    List<Book> books = new ArrayList<>();
    Library one = new Library( books );
    Library two = objectMapper.convertValue( one, Library.class );
    System.out.println( one.getBooks() == two.getBooks() ); // false, different refs

它可以轻松地用某种实用方法包裹，以在整个项目中使用，例如ConvertUtils.clone(rollingStones, Band.class)