EJB 3 Entity Relationships

Transcription

1 Berner Fachhochschule Technik und Informatik EJB 3 Entity Relationships Course Multi Tier Business Applications with Java EE Prof. Dr. Eric Dubuis Berner Fachhochschule Biel

2 Content What are relationships? Relationship types Relationship annotations Navigability of relationships An example Fetch strategies Cascading of Entity Manager operations May 6, 2008 MTA with Java EE / Entity Relationships 2

3 What Are Entity Relationships? Entity relationships are persistent associations between EJB entities. Example (short notation): ProductDescription (1)-----(*) Product Example, UML notation: ProductDescription -description : String -baseprice : float //... The O/R mapping might look like: 1 * +products getter and setter in class //... ProductDescription Product -partnumber : String PRODUCT_DESCRIPTION ID DESCRIPTION BASEPRICE PRODUCT ID PARTNUMBER PD_ID May 6, 2008 MTA with Java EE / Entity Relationships 3

4 Types of Entity Relationships one-to-one A a 1 can also denote: B Navigability unidirectional bidirectional A B A B 1 one-to-many A * B A B * many-to-one A 1 B A B A B * many-to-many A B A B A B * Relationships are polymorphic. 7 combinations! May 6, 2008 MTA with Java EE / Entity Relationships 4

5 Relationship Annotations One of the following relationship annotations must be added to the persistent property or field: You'll use the following types for collections: java.util.collection java.util.set java.util.list java.util.map An empty collection (and not null) should be returned if the many side is empty. interfaces can be initialized with concrete types, e.g. List<Product> p = new ArrayList<Product>(); May 6, 2008 MTA with Java EE / Entity Relationships 5

6 Relationship Annotations (cont'd) Note: If the persistent property or persistent field is a collection-type property or field then it is necessary to specify the entity that is the target of the relationship. There are two possibilities: Person * 1 +employees Company by parameterized private Set<Person> employees; by specifying the target entity with the = Person.class) private Set employees; May 6, 2008 MTA with Java EE / Entity Relationships 6

7 Relationship Annotations (cont'd) Note: The targetentity element in relationship annotations must also be used if entities implement Java interfaces denoting the business concepts. «interface» Customer 1 * «interface» Order For example: Customer Entity Order = OrderEntity.class) private List<Order> orders; May 6, 2008 MTA with Java EE / Entity Relationships 7

8 Direction (Navigability) of Relationships Relationships can be: bidirectional unidirectional You must know which side is the owning side... Characteristics of a bidirectional relationship: has an owning side has an inverse side A B Characteristics of a unidirectional relationship: has only an owning side The owning side determines the updates to the relationship in the database. Bidirectional relationships between managed entities will be persisted based on references held by the owning side of the relationship. [EJB Persistence, 3.2.3] May 6, 2008 MTA with Java EE / Entity Relationships 8

9 Rules For Bidirectional Relationships [EJB Persistence, 2.1.7] Reason for mappedby : if there are 2 or more relationships between the same entities Person * owns > * lives > House * 1 The inverse side must refer to its owning side by use of mappedby element @ManyToOne, annotation. The many side of one-to-many / many-to-one bidirectional relationships must be the owning side, hence the mappedby element cannot be specified on annotation. For one-to-one bidirectional relationships, the owning side corresponds to the side that contains the corresponding foreign key. For many-to-many bidirectional relationships either side may be the owning side. Additional mapping annotations (e.g., column, table) may be specified. The persistence provider handles the object-relational mapping of the relationships. May 6, 2008 MTA with Java EE / Entity Relationships 9

10 Field or Property Annotations for Relationships Relationships are another kind of fields or properties. Given the field or property strategy of an entity you must use one of the following kinds of annotations for a relationship: field type field: private Address address; Person public Address getaddress() { return address; public void setaddress(address a) { address = a; 1 1 Address property: private Address public Address getaddress() { return address; public void setaddress(address a) { address = a; May 6, 2008 MTA with Java EE / Entity Relationships 10

11 Example: Bidirectional One-to-Many Relationship Recall the preceding example (short notation): ProductDescription (1)-----(*) Product Recall: OneToMany, ManyToOne ==> owning side is always the many side UML notation: ProductDescription -description : String -baseprice : float // * inverse +products owning Product -partnumber : String //... May 6, 2008 MTA with Java EE / Entity Relationships 11

12 Fragment of Product Entity 1 0..* public class Product implements private int id; Product // Details, e.g., no-arg constructor, omitted, except: public Product(ProductDescription pd) { productdescription = pd; // This class is the owner of the relationship. // [EJB Persistence, private ProductDescription productdescription;... public ProductDescription getproductdescription() { return this.productdescription; Must match... (next slide) public void setproductdescription(productdescription pd) { this.productdescription = pd; May 6, 2008 MTA with Java EE / Entity Relationships 12

13 Fragment of ProductDescription Entity ProductDescription 1 public class ProductDescription implements private int id; Product // details, e.g., no-arg constructor, omitted... // This is the inverse side of the relationship. // [EJB Persistence, (mappedby = "productdescription") private Collection<Product> products;... with this (previous slide) public Collection<Product> getproducts() { return products; public void setproducts(collection<product> products) { this.products = products; May 6, 2008 MTA with Java EE / Entity Relationships 13

14 A Session Bean as Manager A factory method for creating a ProductDescription public class ProductManagerBean implements ProductManager private EntityManager em;... // Business method, called by a remote client: public ProductDescription createproductdescription(string text, BigDecimal price) { ProductDescription pd = new ProductDescription(text, price); this.em.persist(pd); return pd;... // continued on the next slide... May 6, 2008 MTA with Java EE / Entity Relationships 14

15 A Session Bean as Manager A factory method for creating Product entities: // in class ProductManagerBean public void createproducts(productdescription pd, int howmany) { Product p = null; for (int i = 0; i < howmany; i++) { // Complete: p = new Product(pd); this.em.persist(p); Constructor initializes relationship field // continued on next slide... May 6, 2008 MTA with Java EE / Entity Relationships 15

16 A Session Bean as Manager Given the id, the following methods return the corresponding entities: // in class ProductManagerBean By default, product entities public ProductDescription will not be getproductdescriptionbyid(object id) { fetched. return this.em. find(productdescription.class, id); public Product getproductbyid(object id) { return this.em. find(product.class, id);... By default, product description entities will be fetched, too. May 6, 2008 MTA with Java EE / Entity Relationships 16

17 A Remote Client An excerpt of a remote client might look like: // In some client-side class: private ProductManager manager =...; public ProductDescription createproductdescription(string text, BigDecimal price) { return manager.createproductdescription(text, price); public void createproduct(productdescription pd, int howmany) { manager.createproduct(pd, howmany); Notice the navigability of the relationship public void printproduct(object pid) { Product p = manager.getproductbyid(pid); System.out.println(p.getPartNumber() + "," + p.getproductdescription().getdescription() + "," + p.getproductdescription().getbaseprice()); May 6, 2008 MTA with Java EE / Entity Relationships 17

18 Navigability from the Inverse Side of a One-To-Many Relationship A client can first obtain a product description, and then the collection of associated products: // In some client-side class: public void printproducts(object pdid) { ProductDescription pd = manager.getproductdescriptionbyid(pdid); System.out.println(pd.getDescription()+ ":"); for (Product p: pd.getproducts()) { System.out.println(p.getPartNumber()); To get a ProductDescription entity having all its Product entities of the oneto-many relationship requires a FetchType.EAGER loading strategy at the inverse side of the relationship, see next slide. May 6, 2008 MTA with Java EE / Entity Relationships 18

19 Eager Fetching at the Inverse You'll have to modify the inverse side of the one-to-many relationship to eagerly fetch the public class ProductDescription implements private int id; // This is the inverse side of the relationship. // [EJB Persistence, ] // Eager productdescription, fetch = FetchType.EAGER) private Collection<Product> products;... The fetch strategy FetchType.EAGER implies that then loading the ProductDescription entity, all its related Product entities are loaded as well. ProductDescription 1 0..* Product May 6, 2008 MTA with Java EE / Entity Relationships 19

20 Fetch Strategy The FetchType enum defines strategies for fetching data from the database: public enum FetchType { LAZY, EAGER ; The EAGER strategy is a requirement on the persistence provider runtime that data must be eagerly fetched. The LAZY strategy is a hint to the persistence provider runtime that data should be fetched lazily when it is first accessed. Relationship Type EAGER LAZY EAGER LAZY May 6, 2008 MTA with Java EE / Entity Relationships 20

21 Cascading Persist, Merge, Remove, and Refresh Operations The cascade element specifies the set of cascadable operations that are propagated to the associated entity. The operations that are cascadable are defined by the CascadeType enum: public enum CascadeType { ALL, PERSIST, MERGE, REMOVE, REFRESH ; REMOVE can be applied in The value OneToOne cascade=all and OneToMany is equivalent to cascade={persist, MERGE, REMOVE, REFRESH. Cascade Strategy By default, no operation is cascaded May 6, 2008 MTA with Java EE / Entity Relationships 21

22 Cascading Persist, Merge, Remove, and Refresh Operations (cont'd) Given the Person and Address public class Person... // owning side private Address public class Address... {... You cascade an operation by specifying the cascade public class Person... = ALL) // owning side private Address address; In a session bean, a Person entity and its Address entity are updated: // in a session private EntityManager em; public void updateperson(person p) { this.em.merge(p); If p contains a modified address, it is updated, too! May 6, 2008 MTA with Java EE / Entity Relationships 22

23 Non-Optional Relationships If a relationship cannot be null then you public class Person... optional = false) // owning side private Address address; The element value optional = false denotes that a Person entity must always have an associated Address entity in the persistent store, that is, at the time of a COMMIT performed by the container. May 6, 2008 MTA with Java EE / Entity Relationships 23

24 Summary of Elements in Relationship Annotations Element Name targetentity cascade fetch optional mappedby Type Class CascadeType[] FetchType boolean String Remarks Non-generic collections; interfaces for business abstractions Default: none Inverse side only; not May 6, 2008 MTA with Java EE / Entity Relationships 24

25 Dereferencing Non-Fetched Collections at the Client If a client dereferences the collection of a detached entity whose items were not previously fetched when an exception is raised: detached ProductDescription 1 0..* getproductdescription Product LAZY assumed :ProductDescription :ProductDescription p1:product px:product Collection Proxy p2:product pn:product May 6, 2008 MTA with Java EE / Entity Relationships 25

26 References [EJB] JSR 220: Enterprise JavaBeans, Version 3.0 EJB 3.0 Simplified API [EJB Core] JSR 220: Enterprise JavaBeans, Version 3.0 EJB Core Contracts and Requirements [EJB Persistence] JSR 220: Enterprise JavaBeansTM,Version 3.0 Java Persistence API May 6, 2008 MTA with Java EE / Entity Relationships 26