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Question

I am new to Java EE, using Spring Boot, JPA and Hibernate. I have two entities, Game and Genre with ManyToMany relationship here.

Game:


@Data
@Entity
@Table(name = "games")
public class Game implements Serializable {
    private static final long serialVersionUID = 1L;
    
    @Id
    @Column(name = "game_id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    
    @Column(name = "game_title")
    private String gameTitle;
    
    @Column(name = "price")
    private double price;
    
    @Column(name = "description")
    private String description;
    
    @Column(name = "game_picture")
    private String picture;
    
    @Column(name = "platform")
    private String platform;
    
    @Column(name = "release_date")
    private Date releaseDate;
    
    @ManyToMany
    @JoinTable(name = "game_genre", joinColumns = @JoinColumn(name = "game_id"), inverseJoinColumns = @JoinColumn(name = "genre_id"))
    private Set<Genre> genres;

    public String getGenreNames() {
        return genres.stream().map(Genre::getTitle).collect(Collectors.joining(", "));
    }
}

Genre:

@Data
@Entity
@Table(name = "genre")
public class Genre {

    @Id
    @Column(name = "genre_id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    @Column(name = "title")
    private String title;

    @ManyToMany(mappedBy = "genres", fetch = FetchType.EAGER)
    private Set<Game> games;

}

TestController

@Controller
public class TestController {

    @Autowired
    private GameService gameService;
    @Autowired
    private GenreService genreService;

    @GetMapping("/test")
    public String testPage(@RequestParam(name = "genreId", required = false) Integer genreId, Model model) {
        List<Game> gameList;
        if (genreId != null) {
            Genre genre = genreService.getGenreById(genreId);
            String genreTitle = genre.getTitle(); // Gọi getTitle() trực tiếp
            gameList = gameService.selectByGenre(List.of(genreTitle)); 
        } else {
            gameList = gameService.selectAll();
        }
        model.addAttribute("genreList", genreService.getAllGenres());
        model.addAttribute("gameList", gameList);
        return "test";
    }

test.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Product List</title>
</head>
<body>
    <form action="${pageContext.request.contextPath}/test">
        <select name="genreId">
            <option value="">Choose genre</option>
            <c:forEach var="genre" items="${genreList}">
                <option value="${genre.id}">${genre.title}</option>
            </c:forEach>
        </select>
        <button type="submit">Apply Filter</button>
    </form>

    <c:forEach var="game" items="${gameList}">
        <div class="Product row bg-light Product___">
            <img alt="${game.gameTitle}" src="${pageContext.request.contextPath}/image/${game.picture}" class="col-3" width="90%">
            <div class="Product_info col-9">
                <h5>${game.gameTitle}</h5>
                <p>Genres: ${game.genreNames}</p>
                <p>Price: ${game.price}</p>
                <form action="${pageContext.request.contextPath}/Product_info?id=${game.id}">
                    <button>More Information</button>
                </form>
            </div>
        </div>
    </c:forEach>
</body>
</html>

But somehow, only the last item of gameList displays the genres, other items have the genres as NULL. I have tried to execute the genres separately, but it’s still NULL.

I’m sorry if there is already an answer out there, I cannot find it after crawling in the Internet and tried to fix it for 3 days. Please help!

I expect to get the Game’s “genres” attribute as String (such as Action, Role-playing), but it returns nothing, except the last game.
This is the result
In the beginning, it’s NULL for all games, until I add this for Genre
@ManyToMany(mappedBy = "genres", fetch = FetchType.EAGER)

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Danh mục

How FetchType.EAGER and FetchType.LAZY works in JPA/Hibernate