Lombok toBuilder() 方法是否创建字段的深层副本

Question

我toBuilder()在对象实例上使用来创建构建器实例，然后使用 build 方法来创建新实例。原始对象有一个列表，新对象是否引用了同一个列表或它的副本？@Getter@Setter@AllArgsConstructorpublic class Library {    private List<Book> books;    @Builder(toBuilder=true)    public Library(final List<Book> books){         this.books = books;    }}Library lib2  = lib1.toBuilder().build();lib2 书籍会引用与 lib1 书籍相同的列表吗？

Answer 1

是的，@Builder(toBuilder=true)注解不执行对象的深层复制，只复制字段的引用。

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

您可以使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

实际上你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

它可以很容易地包装在一些实用方法中，以便在整个项目中使用，比如ConvertUtils.clone(rollingStones, Band.class)