@Entity
@Table(name = "article")
public class Article implements Serializable {
 
	private static final long serialVersionUID = -5352780230114105874L;
	private Long id;
	private String title;
	private String description;
	private Account account;
	private Timestamp timestamp;
	private Set<Category> categories = new HashSet<Category>();

	@ManyToMany(mappedBy = "articles", fetch = FetchType.LAZY)
	public Set<Category> getCategories() {
		return categories;
	}

@Entity
@Table(name = "category")
public class Category implements Serializable {
 
	private static final long serialVersionUID = 2889640646104742628L;
	private Long id;
	private String title;
	private List<Article> articles = new ArrayList<Article>();
 
	@ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
	@JoinTable(name = "cat_art", joinColumns = { @JoinColumn(name = "category_id") }, inverseJoinColumns = { @JoinColumn(name = "article_id") })
	public List<Article> getArticles() {
		return articles;
	}

Article article = new Article();
		Account account = new Account();
		account.setId(1L);
		
		article.setTitle(articleDTO.getTitle());
		article.setDescription(articleDTO.getDescription());
		article.setAccount(account);
		
		Category cat = categoryService.getCategoryById(1L);
		article.getCategories().add(cat);
		cat.getArticles().add(article);
		
		articleService.addArticle(article);
		categoryService.addCategory(cat);

Hibernate: select category0_.id as id1_3_0_, category0_.title as title2_3_0_ from category category0_ where category0_.id=?
 
Hibernate: select articles0_.category_id as category1_3_0_, articles0_.article_id as article_2_2_0_, article1_.id as id1_1_1_, article1_.account_id as account_5_1_1_, article1_.description as descript2_1_1_, article1_.time as time3_1_1_, article1_.title as title4_1_1_ from cat_art articles0_ inner join article article1_ on articles0_.article_id=article1_.id where articles0_.category_id=?
 
Hibernate: insert into article (account_id, description, time, title) values (?, ?, ?, ?)