@Entity
@Table(name = "PRODUCTS")
public class Product implements Serializable {
	private static final long serialVersionUID = 1L;
	@Id
	@GeneratedValue
	@Column(name = "ID_PRODUCT")
	private int id;
	@Column(name = "name")
	private String name;

	@ManyToOne
	@JoinColumn(name = "ID_CATEGORY", nullable = false)
	@Cascade({CascadeType.MERGE, CascadeType.SAVE_UPDATE})
	private Category category;
	
	@ManyToOne
	@JoinColumn(name = "ID_PRODUCER", nullable = false)
	@Cascade({CascadeType.MERGE, CascadeType.SAVE_UPDATE})
	private Producer producer;
		
	@Column(name = "price")
	private int price;
			
	@ManyToMany(fetch = FetchType.LAZY, mappedBy = "products")
	private Set<User> users = new HashSet<User>(0);

@Entity
@Table(name = "CATEGORY")
public class Category implements Serializable {
	private static final long serialVersionUID = 1L;
	@Id
	@GeneratedValue
	@Column(name = "ID")
	private int id;
	@Column(name = "name")
	private String name;
	@OneToMany(fetch = FetchType.LAZY, mappedBy = "category")
	private Set<Product> products = new HashSet<Product>(0);

@Entity
@Table(name = "Producer")
public class Producer implements Serializable {
	private static final long serialVersionUID = 1L;
	@Id
	@GeneratedValue
	@Column(name = "ID")
	private int id;
	@Column(name = "name")
	private String name;
	@Column(name = "links")
	private String links;

	@OneToMany(fetch = FetchType.LAZY, mappedBy = "producer")
	private Set<Product> products = new HashSet<Product>(0);

<form:form name='updateForm' commandName="product"
			onsubmit="return validate(this); " method="POST">
			<fieldset>


				<div class="form-group">
					<label class="control-label">Name</label>
					<div class="controls">
						<form:input path="name" />
					</div>
				</div>

				<div class="form-group">
					<label class="control-label">Category</label>
					<div class="controls">
						<select id="CategoryService" name="category" class="span3">
							<c:forEach var="cat" varStatus="status" items="${listCategories}">
								<option value="${cat.id}">${cat.name}</option>
							</c:forEach>
						</select>
					</div>
				</div>
				<div class="form-group">
					<label class="control-label">Producer</label>
					<div class="controls">
						<select id="ProducerService" name="producer" class="span3">
							<c:forEach var="prod" varStatus="status" items="${listProducers}">
								<option value="${prod.id}">${prod.name}</option>
							</c:forEach>
						</select>
					</div>
				</div>
				
				<div class="form-group">
					<label class="control-label">Price</label>
					<div class="controls">
						<form:input path="price" />
					</div>
				</div>
				
				<div class="form-actions">
					<button id="updateButton" name="updateButton"
						class="btn btn-primary" type="submit">Update</button>

				</div>

			</fieldset>
		</form:form>

@RequestMapping(value = "/products/{id}/update.html", method = RequestMethod.GET)
	public String updateCourse(Model model, HttpSession session, HttpServletRequest request,
			@PathVariable("id") Integer productId) {
		
		model.addAttribute("Product", (Product) productService.findProductById(productId));
		model.addAttribute("listCategories", categoryService.getAllCategories());
		model.addAttribute("listProducers", producerService.getAllProducers());
		Product findedProduct = (Product) productService.findProductById(productId);
		model.addAttribute(findedProduct);
		return "update";

	}

	@RequestMapping(value = "/products/{id}/update.html", method = RequestMethod.POST)
	public String updateCoursePost(Model model, HttpSession session, HttpServletRequest request,
			@PathVariable("id") Integer productId, @Valid Product product, BindingResult result)
			throws AddressException, Exception {
	
		
		product = productService.updateProduct(product);
	        log.info(product.getName()+"  is updated");
		return "redirect:/products.html";

	}

@ManyToOne
	@JoinColumn(name = "ID_category", nullable = false)
	@Cascade({CascadeType.MERGE, CascadeType.SAVE_UPDATE})
	private Category category;
	
	@ManyToOne
	@JoinColumn(name = "ID_producer", nullable = false)
	@Cascade({CascadeType.MERGE, CascadeType.SAVE_UPDATE})
	private Producer producer;

product.setCategory(categoryService.findCategoryByID(3));
product.setProducer(producerService.findProducerById(3));

<div class="form-group">
		<label class="control-label">Category</label>
		<div class="controls">
			<select  name="category" class="span3">
						<c:forEach var="cat" items="${listCategories}">
						<option value="${cat.id}">${cat.name}</option>
					</c:forEach>
			</select>
	</div>
</div>

product.setCategory(categoryService.findCategoryByID(3));
product.setProducer(producerService.findProducerById(3));

@RequestMapping(value = "/products/{id}/update.html", method = RequestMethod.POST)
public String updateCoursePost(...) throws AddressException, Exception {
    org.springframework.util.Assert.notNull(product.getCategory());
    org.springframework.util.Assert.notNull(product.getCategory().getId());
    product = productService.updateProduct(product);
    log.info(product.getName()+"  is updated");
    return "redirect:/products.html";
}

org.springframework.util.Assert.notNull(product.getCategory(), "Category is NULL");
org.springframework.util.Assert.notNull(product.getCategory().getId(), "Category.ID is NULL");

<select  name="category" class="span3">
	<c:forEach var="cat" items="${listCategories}">
		<option value="${cat}">${cat.name}</option>
	</c:forEach>
</select>

@OneToMany(fetch = FetchType.LAZY,mappedBy = "category")
	private Set<Product> products = new HashSet<Product>(0);

@ManyToOne
	@JoinColumn(name = "ID_category", nullable = true)
	private Category category;

@RequestMapping(value = "/products/{id}/update.html", method = RequestMethod.POST)
public String updateCoursePost(...) throws AddressException, Exception {
	product.setCategory(...);
	product = productService.updateProduct(product);
	log.info(product.getName()+"  is updated");
	return "redirect:/products.html";
}