JPA returning collection of type PersistentBag instead of ArrayList

[Nilesh Soni]

Hi All,
I have 2 entities (Category and Products) having one to many relationship between them. My architecture is having Spring (in business Layer) and am using Hibernate under the hood of JPA. I am using annotations for all my configurations. Please see the attached code.

Inside the finder method of a DAO, I am finding (EntityManager.find()) the parent entity. The problem is that the child property of this parent (which is a collection of type ArrayList) is actually returned as PersistentBag.

What should I do so that the getter method of my parent will return me collection of type ArrayList?

   

ProductCategory.java

@Entity
@Table(name="PRODUCT_CATEGORY")
public class ProductCategory extends AbstractVO implements Serializable {
    private static final long serialVersionUID = 1L;

    private Long        id;
    private String      categoryName ;
    private List<ProductMaster>  products = new ArrayList<ProductMaster>() ;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="CATEGORY_ID", unique=true, nullable=false)
    public Long getId() {
        return id;
    }

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

    @Column(name="NAME")
    public String getCategoryName() {
        return categoryName;
    }

    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }

    @org.hibernate.annotations.CollectionOfElements (targetElement=ProductMaster.class)
    @OneToMany(cascade=CascadeType.REMOVE, mappedBy="productCategory", targetEntity=ProductMaster.class,
        fetch=FetchType.EAGER)
    public List<ProductMaster> getProducts() {
        return products;
    }

    public void setProducts(List<ProductMaster> products) {
        this.products = products;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (id != null ? id.hashCode() : 0);
        //hash = id ;
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof ProductCategory)) {
            return false;
        }
        ProductCategory other = (ProductCategory) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
        //if (object == null || (other.id != this.id) ) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "com.nilesh.model.ProductCatetory[id=" + id + ", " +
                "categoryName=" + categoryName + ", " +
                "products=" + products + ", " +
                "]";
    }

}
//------------------

ProductMaster.java

@Entity
@Table(name="PRODUCT_MASTER")
public class ProductMaster extends AbstractVO implements Serializable
{
    private static final long serialVersionUID = 1L;

    private Long                id;
    private String              productName ;
    private float               prize ;
    private ProductCategory     productCategory ;

    public ProductMaster() {
        this.productCategory = new ProductCategory() ;
    }

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="PRODUCT_ID", columnDefinition="NUMERIC", nullable=false, unique=true)
    public Long getId() {
        return id;
    }

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

    @ManyToOne(cascade={CascadeType.PERSIST, CascadeType.MERGE} , targetEntity=ProductCategory.class,
        optional=false, fetch=FetchType.EAGER)
    @JoinColumn(name="CATEGORY_ID", nullable=false)
    public ProductCategory getProductCategory() {
        return productCategory;
    }

    public void setProductCategory(ProductCategory productCategory) {
        this.productCategory = productCategory;
    }

    @Column(name="NAME",  columnDefinition="VARCHAR", precision=30, scale=0, nullable=false, unique=false)
    public String getProductName() {
        return productName;
    }

    public void setProductName(String productName) {
        this.productName = productName;
    }

    @Column(name="PRIZE", columnDefinition="DECIMAL", precision=11, scale=2, nullable=true, unique=false)
    public float getPrize() {
        return prize;
    }

    public void setPrize(float prize) {
        this.prize = prize;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (id != null ? id.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof ProductMaster)) {
            return false;
        }
        ProductMaster other = (ProductMaster) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "com.nilesh.model.ProductMaster[id=" + id + ", " +
                "productName=" + productName + ", " +
                "prize=" + prize + ", " +
                "selected=" + selected + ", " +
                //"category=" + productCategory + ", " +
                "]";
    }

//----------------------
Finally the method in DAO which is returning collection of PersistentBag type rather than ArrayList.

    private List<ProductMaster> findProductMasterEntities(boolean all,int maxResults, int firstResult, Long id)
    {
        final String METHODNAME = "findProductMasterEntities" ;

        List<ProductMaster> l = null ;
        
        ProductCategory category = entityManager.find(ProductCategory.class, id) ;
        System.out.println("products size " + category.getProducts().size());
        l = category.getProducts() ;
        return l ;
    }

Do anyone has any idea?

Regards.

[Benjamin Winterberg]

Hibernate uses own List implementations dependening on your mapping. While mapping java.util.List without specifying @IndexColumn, hibernate uses bag semantic for this relation. This results in PersistentBags. You should define the index column to get list semantic (PersistentList).

Check out the reference documentation for further details.

[Angel Taveras]

Hello, basically the hibernate need to know what objects are loaded, if they are lazy loaded, the connection to the database and the session associated with the loaded objects among a lot of other stuff that the ArrayList can't hold and don't need to have. That why it uses a proprietary implementation of java.util.List.

Regards,

[Nilesh Soni]

Ben and Angel,
First of all thanks for quick reply.

After reading post from both of you, I am having some more questions for which I am not able to google proper answers.

1) For getting PersistentList instead of PersistenBag, we have to implement IndexColumn annotation and hence we have to create an extra column into the table. Why on earth, such kind of weird requirement? What if I don't want to have a dedicated column in DB for such solution?

2) Is it that hibernate will always going to return collection of PersistentXXX type only whenever we will find an Entity (having a member variable as a Collection) using EntityManager.find() method?
If I will implement the solution as specified by Ben, still I will get PersistentList, while my requirement is an ArrayList. So onus is on me to do the conversion as follows:

private List<ProductMaster> findProductMasterEntities(boolean all, int maxResults, int firstResult, Long id)
    {
        List<ProductMaster> l = null ;

        ProductCategory category = entityManager.find(ProductCategory.class, id) ;
        System.out.println("products size " + category.getProducts().size());
        l = new ArrayList(category.getProducts()) ;
        return l ;
    }

I hope I am not asking for too much.

Regards.

[Benjamin Winterberg]

Nilesh Soni wrote:1) For getting PersistentList instead of PersistenBag, we have to implement IndexColumn annotation and hence we have to create an extra column into the table. Why on earth, such kind of weird requirement? What if I don't want to have a dedicated column in DB for such solution?

List semantic means that hibernate persists the lists index of each element. Because of that you have to define the column where the index will be persisted.

Personally, I would avoid bag semantic because it has drawbacks. You have to take a look at the generated SQL while using bag semantic. E.g. when you remove 1 element from a collection mapped with bag semantic, the generated SQL first deletes ALL elements from the collection and than inserts ALL elements except the removed one.

I for one would avoid bag semantic and use list or set semantic instead.

Relating your question with ArrayList: Maybe Im wrong, but AFAIK hibernate never uses ArrayList automatically. You have to change the List implementation by yourself.