public class Project {
    // other properties

    @JsonIgnore
    @ManyToMany(mappedBy = "projectsMemberOf",
            cascade = {CascadeType.PERSIST, CascadeType.MERGE},
            fetch = FetchType.LAZY
    )
    public List<User> getProjectMembers() {
        return projectMembers;
    }

    public void setProjectMembers(List<User> projectMembers) {
        this.projectMembers = projectMembers;
    }
}

public class User {
    // other properties
    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinTable(name = "userProjects",
            joinColumns = @JoinColumn(name = "userId", referencedColumnName = "userId"),
            inverseJoinColumns = @JoinColumn(name = "projectId", referencedColumnName = "projectId"),
            foreignKey = @ForeignKey(name = "FK_userProjects_userId"),
            inverseForeignKey = @ForeignKey(name = "FK_userProjects_projectId")
    )
    public List<Project> getProjectsMemberOf() {
        return projectsMemberOf;
    }

    private void setProjectsMemberOf(List<Project> projectsMemberOf) {
        this.projectsMemberOf = projectsMemberOf;
    }   
}

        project.getProjectMembers().add(user);
        user.getProjectsMemberOf().add(project);
        em.merge(project);

        project.getProjectMembers().remove(user);
        user.getProjectsMemberOf().remove(project);
        em.merge(project);

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof User)) return false;

        User that = (User) o;
        return getUsername().equals(that.getUsername());
    }

    @Override
    public int hashCode() {
        return getUsername().hashCode();
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Project)) return false;

        Project project = (Project) o;

        return getProjectId() != 0 && getProjectId() == project.getProjectId();
    }

    @Override
    public int hashCode() {
        return 31;
    }

em.merge(user)