Domain Model in Hibernate

Domain Model in Hibernate

The application domain model is the central character in an ORM. Domain Model are the classes we would be mapping in our application. These domain model classes needs to follow the Plain Old Java Object (POJO) / JavaBean programming model in order to make Hibernate works best.

Applications using Hibernate uses its proprietary XML mapping file format for this purpose. With JPA, most of this information is now defined in a way that is portable across ORM/JPA providers using annotations and/or standardized XML format.

Mapping types

Hibernate understands both the Java and JDBC representations of application data. The ability to read/write this data from/to the database is the function of a Hibernate type. A type, is an implementation of the org.hibernate.type.Type interface.

To help understand the type categorizations, we take an example of a simple table and domain model that we wish to map.

create table ContactInfo (     id integer not null,     firstName varchar(255),     lastName varchar(255),     middleName varchar(255),     notes varchar(255),     starred boolean not null,     website varchar(255),     primary key (id) )

@Entity(name = "ContactInfo") public static class ContactInfo {     @Id     private Integer id;     private Name name;     private String notes;     private URL website;     private boolean starred;     //Getters and setters }

@Embeddable public class Name {     private String firstName;     private String middleName;     private String lastName;     // getters and setters }

In the broadest sense, Hibernate categorizes types into two groups:

• Value types
• Entity types

Value types

A value type is a piece of data that does not define its own lifecycle. It is, owned by an entity, which defines its lifecycle. All the state of an entity is made up entirely of value types. These state fields or JavaBean properties are termed persistent attributes. The persistent attributes of the ContactInfo class are value types.

Value types are further classified into three sub-categories:

Basic types

In mapping the ContactInfo table, all attributes except for name would be basic types.

Embeddable types

The name attribute is an example of an embeddable type.

Collection types

Collection types are also a distinct category among value types.

Entity types

Entities, exist independently of other objects whereas values do not. Entities are domain model classes which correlate to rows in a database table, using a unique identifier. Entities exist independently and define their own lifecycle. The ContactInfo class is an example of an entity. 

Hibernate Architecture Overview

Hibernate Architecture Overview

Hibernate, is an ORM solution, which sits between the Java application data access layer and the Relational Database. The Java application makes use of the Hibernate APIs to load, store, query, etc its domain data. Hibernate implements the Java Persistence API (JPA) specifications. 

SessionFactory (org.hibernate.SessionFactory)

SessionFactory  is a thread-safe representation of the mapping of the application domain model to a database. SessionFactory acts as a factory for org.hibernate.Session instances. The EntityManagerFactory is the JPA equivalent of a SessionFactory.

A SessionFactory is very expensive to create, so, for any given database, the application should have only one associated SessionFactory. The SessionFactory maintains services that Hibernate uses across all Sessions such as second level caches, connection pools, transaction system integrations, etc.

Session (org.hibernate.Session)

A single-threaded, short-lived object conceptually modeling a "Unit of Work". In JPA nomenclature, the Session is represented by an EntityManager.

The Hibernate Session wraps a JDBC java.sql.Connection and acts as a factory for org.hibernate.Transaction instances. 

Transaction (org.hibernate.Transaction)

A single-threaded, short-lived object used by the application to demarcate individual physical transaction boundaries. 

Spring AOP – Advice

Spring AOP – Advice

Spring AOP Advice is the action taken by an aspect at a particular join point. Different types of advice include 

• Around advice
• Before advice
• After advice

Many AOP frameworks, including Spring, model an advice as an interceptor, maintaining a chain of interceptors around the join point.

Types of advice:

• Before advice: Advice that executes before a join point, but which does not have the ability to prevent execution flow proceeding to the join point.
  
  
• After returning advice: Advice to be executed after a join point completes normally.
  
  
• After throwing advice: Advice to be executed if a method exits by throwing an exception.
  
  
• After (finally) advice: Advice to be executed regardless of the means by which a join point exits, normal or exceptional return.
  
  
• Around advice: Advice that surrounds a join point such as a method invocation. This is the most powerful kind of advice. Around advice can perform custom behavior before and after the method invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the advised method execution by returning its own return value or throwing an exception.

Aspect Oriented Programming with Spring

Aspect Oriented Programming with Spring

Aspects enable the modularization of concerns such as transaction management that cut across multiple types and objects. One of the key components of Spring is the AOP framework. While the Spring IoC container does not depend on AOP, meaning you do not need to use AOP if you don't want to, AOP complements Spring IoC to provide a very capable middleware solution. Aspect-Oriented Programming requires breaking down program logic into distinct parts called concerns. 

The functions that span multiple points of an application are called cross-cutting concerns and these cross-cutting concerns are conceptually separate from the application's business logic. Examples of aspects are logging, auditing, declarative transactions, security, caching, etc. Spring AOP module provides interceptors to intercept an application. For example, when a method is executed, you can add extra functionality before or after the method execution.

AOP is used in the Spring Framework to

• Provide declarative enterprise services, especially as a replacement for EJB declarative services. 

• Allow users to implement custom aspects, complementing their use of OOP with AOP.

AOP Terminologies

Before we start working with AOP, let us become familiar with the AOP concepts and terminology. These terms are not specific to Spring, rather they are related to AOP.

• Aspect: a modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in Java EE applications. In Spring AOP, aspects are implemented using regular classes  or regular classes annotated with the @Aspect annotation.

• Join point: a point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.

• Advice: action taken by an aspect at a particular join point. Different types of advice include "around," "before" and "after" advice

• Pointcut: a predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut.

• Introduction: declaring additional methods or fields on behalf of a type. Spring AOP allows you to introduce new interfaces to any advised object. 

• Target object: object being advised by one or more aspects. Also referred to as the advised object. 

• AOP proxy: an object created by the AOP framework in order to implement the aspect contracts. In the Spring Framework, an AOP proxy will be a JDK dynamic proxy or a CGLIB proxy.

• Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time, load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.

Spring Dependencies

Spring Dependencies

An application will have certain objects that work together to present what the end-user sees as a coherent application. We can define a number of bean definitions that are stand-alone, each to themselves, to a fully realized application where objects work (or collaborate) together to achieve some goal.

Injecting Dependencies

It becomes evident upon usage that code gets much cleaner when the DI principle is applied, and reaching a higher grade of decoupling is much easier when beans do not look up their dependencies, but are provided with them.

Setter Injection

Setter-based DI is realized by calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

public class SimpleMovieLister { // the SimpleMovieLister has a dependency on the MovieFinder     private MovieFinder movieFinder; // a setter method so that the Spring container can 'inject' a MovieFinder     public void setMovieFinder         (MovieFinder movieFinder) {       this.movieFinder = movieFinder;     } }

Constructor Injection

Constructor-based DI is realized by invoking a constructor with a number of arguments, each representing a collaborator. Additionally, calling a static factory method can be considered almost equivalent.

public class SimpleMovieLister {     // the SimpleMovieLister has a dependency on the MovieFinder     private MovieFinder movieFinder;     // a constructor so that the Spring container can 'inject' a MovieFinder     public SimpleMovieLister        (MovieFinder movieFinder) {         this.movieFinder = movieFinder;     }     }

Constructor or Setter DI?

·         Setter based DI makes objects of that class responsive to being re-configured (or re-injected) at some later time.

·         Constructor-injection supplying all of an object's dependencies means that that object is never returned to client (calling) code in a less than totally initialized state.