Selecting a Primary Key - 2

Transcription

1 Software Architectures and Methodologies A.A 2016/17 Java Persistence API Mapping, Inheritance and Performance Primary Key Dipartimento di Ingegneria dell'informazione Laboratorio Tecnologie del Software Università di Firenze 1 Selecting a Primary Key - 1 You can specify a key generation strategy with the strategy attribute of annotation Try: different values AUTO ENTITY SEQUENCE (AUTO for Hibernate) TABLE Selecting a Primary Key - table="sequence_table", generator="table_generator") public Long getid() { return id; void setid(long id) { this.id = id; 2 3

2 Equals & Uuid equals() method, by default, checks whether two references refer to the same inmemory instance on the Java heap Equals - 1 em.persist(user); em.clear(); User user2 = em.find(user.class, 1L); 4 5 equals() method should be overridden. It is reflexive, symmetric, transitive, consistent For any non-null reference value x, x.equals(null) shoul d return false Equals - 2 private Long public boolean equals(object obj) { if (this == obj) return true; if (obj == null) if (!(obj instanceof User)) User otheruser = (User)obj; return id.equals(otheruser.id); Try: with and withouth equals() method. Equals - 3 em.persist(user); em.clear(); User user2 = em.find(user.class, 1L); //equals su id //default 6 7

3 Equals - 4 id is an generated property set by the persistence provider when persisting the entity User user2 = user; user2.setusername( user"); 8 Equals - 5 Try: compare two transient object. User user2 = user; user2.setusername( user"); // Exception in thread "main" java.lang.nullpointerexception con equals su id 9 How do we check equality for two transient objects? uuid public class User{ private Long id; private String uuid; public User( String uuid ) { super(); this.uuid = uuid; protected User() { super(); public static User create User() { return new User( UU.randomUU().toString() ); private Long public boolean equals(object obj) { if (this == obj) return true; if (obj == null) if (!(obj instanceof User)) if(uuid == null) User otheruser = (User)obj; return uuid.equals( otheruser.getuuid() ); Enum & Embedding 10 11

4 Enum - 1 Enum - 2 Let s assume that we want to add a Role enumerated type for our User, to specify the role that he has in the application Try: with and without annotation and compare results on the database public enum Role { ADMIN; User user password: String firstname: String lastname: String dateofbirth: Date bio: String profilepic: byte[] <<enumerated>> Role ADMIN private Role role; public Role getrole() { return role; public void setrole(role role) { this.role = role; private Role public Role getrole() { return role; public void setrole(role role) { this.role = role; Embedded - 1 Embedded - 2 To map an embedded class with annotations, you can declare a particular property in the owning entity class You map the dependent component class with annotation Try: add an embedded Address and experiment results annotation User user password: String firstname: String lastname: String dateofbirth: Date bio: String profilepic: byte[] getfullname(): String getage(): int <<enumerated>> Role <<embeddable>> Address city: String zipcode: String street: String ADMIN 14 15

5 Embedded - 3 Embedded vs public class Address { protected String city; protected String street; protected String protected Zipcode public class Zipcode { protected String zip; protected String protected Long id; private String protected Address public class Address { protected String city; protected String street; protected String protected Zipcode public class Zipcode { protected String zip; protected String protected Long id; private column=@column(name="addr_zip")) protected Address address; 16 There are two important limitations to classes mapped as components. shared references aren t possible Second, there is no elegant way to represent a null reference to an Address. Hibernate represents a null component as null values in all mapped columns of the component. If you store a component object with all null property values, Hibernate returns a null component when the owning entity object is retrieved from the database. 17 Mapping Class Inheritance Inheritance Let s add another class to our model to represent an item of our online store The item can be a physical item (with limited storage quantity) or a downloadable item (a song, movie..) 18 19

6 Mapping Class Inheritance There are four differentapproachesin JPA to represent inheritancehierarchy: Table per concrete class with implicit polimorfism Table per concrete class Table per class hierarchy Table per subclass URI (1) Table per concrete class with implicit polymorphism DOWNLOADABLE_ITEM PRODUCT public abstract class { public class extends { public class extends { (2) Table per concrete class (3) Table per class hierarchy (Single public abstract class { public class extends public abstract class { public class extends { DOWNLOADABLE_ITEM URI PRODUCT QUANTITY public class extends { ITEM_TYPE QUANTITY URI ITEM public class extends { 22 23

7 (4) Table per subclass (Joined public abstract class { public class extends { public class extends { Choosing Mapping Strategy for Class Inheritance Table per concrete class with implicit polimorfism: if you don t require polymorphic associations or queries. Table per class hierarchy: if subclasses declare relatively few properties and application is readdominant. Table per subclass: if you do require polymorphic associations or queries, and subclasses declare many properties. Table per concrete class: if you have wide or deep inheritance hierarchy and if joins are more expensive than unions Choosing Mapping Strategy for Class Inheritance Normalization saves space, but space is cheap Normalization simplifies updates, but reads are more common Performance The data is present in the same table so there is no need for any joins, hence the selects are very fast. A single table with all the required data allows much more efficient index usage. If the columns are indexed properly, then results can be filtered and sorted by utilizing the same index. While in the case of a normalized table, since the data would be spread out in different tables, this would not be possible. 26 try it @Inheritance(strategy=InheritanceType. ) GenerationType.TABLE <<enumerated>> Role ADMIN User user password: String uuid: String GenerationType.ENTITY <<embeddable>> PersonalData firstname: String lastname: String dateofbirth: Date bio: String profilepic: equals(): boolean -homeaddress GenerationType.SEQUENCE -workaddress getfullname(): String getage(): int <<embeddable>> Address city: String zipcode: String street: String 27