3. Java Persistence API. 4. Entity Relationships презентация

Basics
A foreign key can be one or more columns that reference a unique key, usually the primary key, in another table
A foreign key and the primary parent key it references must have the same number and type of fields
Foreign keys represents relationships from a column or columns in one table to a column or columns in another table
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Victor Mozharsky

Multiplicity in Entity Relationships
Multiplicities are of the following types:
Many-to-one: Multiple instances of an entity can be related to a single instance of the other entity
One-to-one: Each entity instance is related to a single instance of another entity
One-to-many: An entity instance can be related to multiple instances of the other entities
Many-to-many: The entity instances can be related to multiple instances of each other.
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Victor Mozharsky

CM Database Schema
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Victor Mozharsky

Many-to-One Mappings
A many-to-one mapping is defined by annotating the attribute in the source entity (the attribute that refers to the target entity) with the @ManyToOne annotation
A @JoinColumn(name=Fkname) annotation discribes a foreing key of a relationship
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Victor Mozharsky

Exercise: Read a Payment
Create a project for read payment data by payment id with a merchant name instead of merchant id
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Victor Mozharsky

CM Database Schema
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Victor Mozharsky

Payment Entity (1 of 2)
@Entity
public class Payment {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private double charge;
private double total;
private String goods;
private java.sql.Date dt;
@ManyToOne
@JoinColumn(name="merchantId")
private Merchant merchant;
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Victor Mozharsky

Payment Entity (2 of 2)
public Merchant getMerchant(){ return merchant; }
public void setMerchant(Merchant value){
merchant = value;
}
. . . . . . . . . . . . . . . .
public String getStringForPrint() {
String txt = "id = " + id + "; date = ";
SimpleDateFormat dtFrm = new SimpleDateFormat("dd.MM.yyyy");
txt += dtFrm.format(dt) + "; merchant = " + merchant.getName();
txt += "; total = " + total + "; charge = " + charge;
return txt;
}
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Victor Mozharsky

DAO & Service Interfaces
public interface PaymentDao {
public List
findByMerchantId(int id);
public double getPaymentSum();
public Payment findById(int id);
}
public interface PaymentService {
public List
findByMerchantId(int id);
public double getPaymentSum();
public Payment findById(int id);
}
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Victor Mozharsky

PaymentDaoImpl Class
public Payment findById(int id){
return em.find(Payment.class, id);
}
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Victor Mozharsky

Main Class
@SuppressWarnings("resource")
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
PaymentService paymentService = context.getBean(PaymentService.class);
Payment p = paymentService.findById(2);
System.out.println(p.toString());
}
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Victor Mozharsky

Exercise: Read a Payment
See P351ReadPayment project for the full text
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Victor Mozharsky

Exercise: Group Payments
Modify P237Grouping project using entities from the previous project
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Victor Mozharsky

PaymentDaoImp Class
public List
getTotalReport(){
String txt = "SELECT new com.bionic.edu.Result (p.merchant.name, count(p), SUM(p.chargePayed))";
txt += "FROM Payment p GROUP BY p.merchant.name";
TypedQuery
query = em.createQuery(txt, Result.class);
return query.getResultList();
}
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Victor Mozharsky

Main Class
@SuppressWarnings("resource")
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
PaymentService paymentService = context.getBean(PaymentService.class);
List
list = paymentService.getTotalReport();
for(Result r: list)
System.out.format("%1$25s %2$3d %3$8.2f \n", r.getName(), r.getCount(), r.getSum());
}
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Victor Mozharsky

Exercise: Group Payments
See P352GroupPayment project for the full text
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Victor Mozharsky

One-to-Many Mappings
An entity instance can be related to multiple instances of the other entities
One-to-many relationships use the @OneToMany annotation on the corresponding persistent property or field
The mappedBy element is needed to refer to the attribute annotated by ManyToOne in the corresponding entity:
@OneToMany(mappedBy=“attribute”)
A one-to-many association needs to map the collection of entities
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Victor Mozharsky

Exercise: Read a Merchant’s Payments
Create a project for read payment data that correspond to a merchant given by its id
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Victor Mozharsky

CM Database Schema
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Victor Mozharsky

Merchant Entity
@Entity
public class Merchant {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private String name;
private double charge;
private int period;
private double total;
@OneToMany(mappedBy="merchant")
private Collection
payments;
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Victor Mozharsky

Main Class (1 of 2)
@SuppressWarnings("resource")
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
MerchantService merchantService = context.getBean(MerchantService.class);
List
list = merchantService.getSortedByNeedToPay();
for(Merchant m: list){
System.out.println("=======================");
System.out.println(m.getName());
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Victor Mozharsky

Main Class (2 of 2)
System.out.println(" ");
Collection
payments = m.getPayments();
for (Payment p : payments) {
System.out.println(p.toString());
}
}
}
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Victor Mozharsky

Exercise: Read a Merchant’s Payments
See P353MerchantPayments project for the full text
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Victor Mozharsky

Many-to-Many Mappings
The entity instances can be related to multiple instances of each other
Many-to-many relationships use the @ManyToMany annotation on the corresponding persistent property or field
We must use a third table to associate the two entity types (join table)
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Victor Mozharsky

@JoinTable Annotation
@JoinTable(name=“table name”,
joinColumns=@JoinColumn(name=“c1Id”),
inverseJoinColumns=
@JoinColumn(name=“c2Id”))
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Victor Mozharsky

Exercise: Read Customer’s Merchants
Create a project to read data about merchants whose goods were bought by a given customer
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Victor Mozharsky

Customer Entity (1 of 2)
@Entity
public class Customer {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private java.sql.Date maturity;
private String ccType;
private String ccNo;
private String eMail;
private String address;
private String name;
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Victor Mozharsky

Customer Entity (2 of 2)
@ManyToMany
@JoinTable(name="Payment", joinColumns=@JoinColumn(name="customerId"),
inverseJoinColumns=@JoinColumn(name="merchantId"))
private Collection
merchants;
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Victor Mozharsky

Main Class
@SuppressWarnings("resource")
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
CustomerService customerService = context.getBean(CustomerService.class);
Customer customer = customerService.findById(1);
if (customer != null){
System.out.println(customer.toString());
Collection
merchants = customer.getMerchants();
for (Merchant m : merchants) { System.out.println(m.getName());}}}
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Victor Mozharsky

Exercise: Read Customer’s Merchants
See P354CustAndMerch project for the full text
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Victor Mozharsky

Lazy Relationships
The fetch mode can be specified on any of the four relationship mapping types:
@ManyToOne(fetch=FetchType.LAZY)
Lazy loading can essential enhance performance
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Victor Mozharsky

One-to-One Mappings
Each entity instance is related to a single instance of another entity
One-to-one relationships use the @OneToOne annotation on the corresponding persistent property or field
Example of One-To-One mapping: Employee and ParkingSpace entities
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Victor Mozharsky

Embedded Objects (1 of 2)
An embedded object is one that is dependent on an entity for its identity
It has no identity of its own, but is merely part of the entity state
The state of the embedded object is stored with the rest of the entity state in the database row, with no distinction between the state in the Java entity and that in its embedded object
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Victor Mozharsky

Embedded Objects (2 of 2)
We can share the same embedded object type with other entities that also have the same internal representation
Example: the home address of the employee (contains Street, City, State, ZipCode fields)
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Victor Mozharsky

Embedded Type
An embedded type is marked as such by adding the @Embeddable annotation to the class definition
Its fields and properties will be persistable as part of an entity
We might also want to define the access type of the embeddable object so it is accessed the same way regardless of which entity it is embedded in
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Victor Mozharsky

Address Embedded Type
@Embeddable
@Access(AccessType.FIELD)
public class Address {
private String street;
private String city;
private String state;
private String zipCode;
// ...
}
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Victor Mozharsky

Using an Embedded Object
@Entity
public class Employee {
@Id private int id;
private String name;
private long salary;
@Embedded
private Address address;
// ...
}
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Victor Mozharsky

Reuse of Embedded Type
An Address class could be reused in both Employee and Company entities
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Victor Mozharsky

Database Schema Generation
Automatically generation the tables and database schema for a persistence unit can be done through the "eclipselink.ddl-generation" persistence unit property
The tables and constraints will be generated for all of the classes defined in that persistence unit.
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Victor Mozharsky

eclipselink.ddl-generation Values
create-tables – if the table already exists then it will not be dropped or replaced, the existing table will be used
drop-and-create-tables - will first drop the existing table, and then create the new table. Note that this will lose all of the data in the tables when they are dropped!!
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Victor Mozharsky

Schema Generation Practice
One of the complaints around schema generation is that you can’t specify everything that you need to be able to finely tune the schema.
There are too many differences between databases and too many different settings to try to put in options for every database type.
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Victor Mozharsky