@Entity(name = "news")
public class News implements Serializable {
    private Long id;
    private Date date;
    private String text;
    private String author;
    private Set<HashTag> hashTags = new HashSet<HashTag>(0);
    private byte[] image;

    public News() {
    }

    public News(Date date, String text, String author, Set<HashTag> hashTags, byte[] image) {
        this.date = date;
        this.text = text;
        this.author = author;
        this.hashTags = hashTags;
        this.image = image;
    }

    @Id
    @Column(name = "news_id", nullable = false)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "NewsIDGenerator")
    @SequenceGenerator(name = "NewsIDGenerator", sequenceName = "SEQ_NEWS_ID")
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @Temporal(TemporalType.DATE)
    @DateTimeFormat(pattern = "DD/MM/YYYY")
    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }

    @Lob
    @Type(type = "org.hibernate.type.TextType")
    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    @ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @Column(name = "hashTags")
    @JoinTable(name = "news_hashTag",
            joinColumns = @JoinColumn(name = "news_id", nullable = false),
            inverseJoinColumns = @JoinColumn(name = "hashtag_id", nullable = false))
    public Set<HashTag> getHashTags() {
        return hashTags;
    }

    public void setHashTags(Set<HashTag> hashTags) {
        for (HashTag hashTag: hashTags) {
            hashTag.addNews(this);
        }
        this.hashTags = hashTags;
    }

    @Lob
    public byte[] getImage() {
        return image;
    }

    public void setImage(byte[] image) {
        this.image = image;
    }
}

@Entity(name = "hashTag")
public class HashTag implements Serializable {
    private Long id;

    private String name;
    private Set<News> news = new HashSet<News>(0);

    public HashTag() {
    }

    public HashTag(String name) {
        this.name = name;
    }

    @Id
    @Column(name = "hashtag_id", nullable = false)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "HashTagIDGenerator")
    @SequenceGenerator(name = "HashTagIDGenerator", sequenceName = "SEQ_HASHTAG_ID")
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @ManyToMany(fetch = FetchType.LAZY, mappedBy = "hashTags")
    @Column(name = "news")
    public Set<News> getNews() {
        return news;
    }

    public void setNews(Set<News> news) {
        this.news = news;
    }
}

{
   "text": "Unbelievable news!",
   "author": "Unknown",
   "hashTags": [
      {
         "name": "hashTag_1"
      },
      {
         "name": "hashTag_2"
      }
      ]
}

   @RequestMapping(method = RequestMethod.POST, produces = {"application/json"}, consumes = {"application/json"})
    @ResponseStatus(HttpStatus.OK)
    public @ResponseBody String addNews(@RequestBody String jsonString) {
        if (LoggedUser.isAdmin()) {
            News news = new News(jsonString);
            if (news.getText() != null) {
                if (news.getDate() == null) {
                    news.setDate(new Date());
                }
                news.setHashTags(news.getHashTags()); // just set for each hashTag link to news
                newsService.save(news);
                return news.toJson();
            }
        }
        return null;
    }

news.setHashTags(news.getHashTags()); // just set for each hashTag link to news

public void addHashTags(HashTag hashTag) {
        hashTag.addNews(this);
        this.hashTags.add(hashTag);
}

    @ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @Column(name = "hashTags")
    @JoinTable(name = "news_hashTag",
            joinColumns = @JoinColumn(name = "news_id", nullable = false),
            inverseJoinColumns = @JoinColumn(name = "hashtag_id", nullable = false))
    public Set<HashTag> getHashTags() {
        return hashTags;
    }