為什么在restapi中多次參考外鍵

我想通過One to Many and Many to One Bidirectional relationship mapping但同時請求在插入7460行后保留資料時將一些資料存盤到資料庫中。我可以谷歌搜索但沒有找到合適的解決方案。

下面是我的代碼：

物體

public class Author {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer author_id;
    private String name;
    private String language;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "author")
    private Set<Book> book;

    // getter setter
}

public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String title;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author_id")
    private Author author;

    // getter setter
}

服務

@Override
public Author insertAuthor(Author author) {
        Set<Book> bookList = new HashSet<>();
        
        Book book = new Book();
        book.setTitle(book.getTitle());
        
        bookList.add(book);
        
        book.setAuthor(author);
        author.setBook(bookList);
        
        return authorRepo.save(author);
}

控制器

@RestController
public class PojoController {
    
    @Autowired
    private PojoService pojoService;
    
    @RequestMapping(value="/book", method = RequestMethod.POST)
    public Author addBookCourse(@RequestBody Author author) {
        return this.pojoService.insertAuthor(author);
    }
}

要求

{
    "language": "english",
    "name": "Banjamin franklin",
    "book": [{
        "title": "Theory Of Everything"
    },
    {
        "title": "A Brief Story Of Time"
    }]
}

輸出

{
    "author_id": 1,
    "name": "Banjamin franklin",
    "language": "english",
    "book": [
        {
            "id": 1,
            "title": null,
            "author": {
                "author_id": 1,
                "name": "Banjamin franklin",
                "language": "english",
                "book": [
                    {
                        "id": 1,
                        "title": null,
                        "author": {
                            "author_id": 1,
                            "name": "Banjamin franklin",
                            "language": "english",
                            "book": [
                                {
                                    "id": 1,
                                    "title": null,
                                    "author": {
                                        "author_id": 1,
                                        "name": "Banjamin franklin",
                                        "language": "english",
                                        "book": [
                                            {
                                                "id": 1,
                                                "title": null,
                           more 7460 line
                           .......
                           .......
                           .......
{
    "timestamp": "2021-11-30T10:25:03.957 00:00",
    "status": 200,
    "error": "OK",
    "trace": "org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Infinite recursion (StackOverflowError); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Infinite recursion (StackOverflowError) (through reference chain: com.rest.RestApiPojo.Entity.Author[\"book\"]->org.hibernate.collection.internal.PersistentSet[0]->com.rest.RestApiPojo.Entity.Book[\"author\"]->com.rest.RestApiPojo.Entity.Author[\"book\"]->org.hibernate.collection.internal.PersistentSet[0]->com.rest.RestApiPojo.Entity.Book[\"author\"]
}    

uj5u.com熱心網友回復：

您需要使用@JsonManagedReference，并@JsonBackReference允許杰克遜更好地處理之間的關系Author和Book：

public class Author {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer author_id;
    private String name;
    private String language;

    @JsonManagedReference
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "author")
    private Set<Book> book;

    // getter setter
}

public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String title;

    @JsonBackReference
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author_id")
    private Author author;

    // getter setter
}

您還有其他選擇（例如使用@JsonIdentityInfo）來處理這個無限遞回問題，但這是最常見的解決方案。您可以在以下在線資源https://www.baeldung.com/jackson-bidirectional-relationships-and-infinite-recursion查看所有其他可能的方法。

此外，在您的服務中，您正在創建一個全新的服務Book并將其標題設定為book.setTitle(book.getTitle());，這基本上什么都不做。事實上，你甚至不需要做你在那里做的大部分事情，因為Book實體已經在Author，你只需要按如下方式設定Author每個Book實體：

@Override
public Author insertAuthor(Author author) {
    for (Book book : author.getBook()) {
        book.setAuthor(author);
    }
    
    return authorRepo.save(author);
}

最后，考慮將book屬性更改為Author，books因為它包含多本書（之后您需要調整代碼）。

標籤：弹簧靴 休眠

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