ZK Essentials/Chapter 10: JPA Integration

This article is out of date, please refer to http://books.zkoss.org/zkessentials-book/master/ for more up to date information.

Overview

In previous chapters, we mimic a database with a static list as follows:

public class TodoListServiceChapter6Impl implements TodoListService {

	static int todoId = 0;
	static List<Todo> todoList = new ArrayList<Todo>();  
	static{
		todoList.add(new Todo(todoId++,"Buy some milk",Priority.LOW,null,null));
		todoList.add(new Todo(todoId++,"Dennis' birthday gift",Priority.MEDIUM,dayAfter(10),null));
		todoList.add(new Todo(todoId++,"Pay credit-card bill",Priority.HIGH,dayAfter(5),"$1,000"));
	}
	
	/** synchronized is just because we use static userList in this demo to prevent concurrent access **/
	public synchronized List<Todo>getTodoList() {
		List<Todo> list = new ArrayList<Todo>();
		for(Todo todo:todoList){
			list.add(Todo.clone(todo));
		}
		return list;
	}
	
...
}

• Line 4, 12: Store objects in a static list and perform all data operation on it.

Originally we perform all persistence operations on the list, but we will replace this part with a real database and a persistence framework, JPA.

Java Persistence API (JPA) is a POJO-based persistence specification. It offers an object-relational mapping solution to enterprise Java applications. In this chapter, we don't create new applications but re-write data persistence part based on chapter 9 with JPA. We will create a simple database with HSQL and implement a persistence layer using the DAO (Data Access Object) pattern to encapsulate all database related operations. We also have to annotate all entity classes that will be stored in the database with JPA annotations. To make the example close to a real application, we keep using the Spring framework and demonstrate how to integrate Spring with JPA.

Source Code

As we mentioned in the Chapter 2, our source code has 3 branches in github. The source code of this chapter's example belongs to a different branch in the github: chapter10.

We don't create new examples in this chapter, but we add 2 DAO classes written in JPA under the package org.zkoss.essentials.services.impl.

Configuration

Maven

When using a database, JPA, and integration of JPA and Spring, we should add following dependencies based on chapter 9's configuration: (We add spring-web and cblib for Spring framework which is explained in chapter 9.)

	<properties>
		<zk.version>6.5.1</zk.version>
		<maven.build.timestamp.format>yyyy-MM-dd</maven.build.timestamp.format>
		<packname>-${project.version}-FL-${maven.build.timestamp}</packname>
		<spring.version>3.1.2.RELEASE</spring.version>
	</properties>

...
		<!-- Spring 3 dependencies -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-web</artifactId>
			<version>${spring.version}</version>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-orm</artifactId>
			<version>${spring.version}</version>
		</dependency>
		<dependency>
			<groupId>cglib</groupId>
			<artifactId>cglib</artifactId>
			<version>2.2.2</version>
		</dependency>
		<!-- JPA(Hibernate) and HSQL dependencies -->
		<dependency>
	  		<groupId>org.hibernate</groupId>
	  		<artifactId>hibernate-entitymanager</artifactId>
	  		<version>4.0.0.Final</version>
	  	</dependency>
		<dependency>
			<groupId>org.hsqldb</groupId>
			<artifactId>hsqldb</artifactId>
			<version>2.2.6</version>
		</dependency>
...

• Line 5, 16~17: Spring provides a module to integrate several ORM (Object Relation Mapping) frameworks, integrating JPA requires this dependency.
• Line 27~28: There are various implementations of JPA specification, we choose Hibernate's one which is the most popular.
• Line 32~33: For using HSQL, we should add its JDBC driver.

Persistence Unit Configuration

We should describe persistence unit configuration in an XML file called persistence.xml and we need to specify name, transaction-type, and properties. The properties are used by persistence provider (Hibernate) to establish database connection and setup vendor specific configurations.

<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="2.0">

	<persistence-unit name="myapp" transaction-type="RESOURCE_LOCAL">
		<provider>org.hibernate.ejb.HibernatePersistence</provider>
		<properties>
			<property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect" />
			<property name="hibernate.connection.driver_class" value="org.hsqldb.jdbcDriver" />
			<property name="hibernate.connection.username" value="sa" />
			<property name="hibernate.connection.password" value="" />
			<property name="hibernate.show_sql" value="true" />
			<property name="hibernate.connection.url" value="jdbc:hsqldb:file:data/store" />
			<property name="hibernate.hbm2ddl.auto" value="create" />
		</properties>
	</persistence-unit>
</persistence>

• Line 4: The persistence unit name myapp will be used later in Spring configuration.

Deployment Descriptor

You aren't required to add any special configuration for JPA to work. Here we add a Spring provided OpernEntityMangerInViewFilter to resolve an issue caused by lazy-fetching in one-to-many mapping. Please refer to Developer's Reference for this issue in more detail.

Extracted from web.xml

...
	<!-- Spring configuration -->
	<!-- Initialize spring context -->
	<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>
	<!-- Enable webapp Scopes-->
	 <listener>
    	<listener-class>org.springframework.web.context.request.RequestContextListener</listener-class>
	</listener>

	<filter>
		<filter-name>OpenEntityManagerInViewFilter</filter-name>
		<filter-class>org.springframework.orm.jpa.support.OpenEntityManagerInViewFilter</filter-class>
	</filter>
	<filter-mapping>
		<filter-name>OpenEntityManagerInViewFilter</filter-name>
		<url-pattern>/*</url-pattern>
	</filter-mapping>
...

Entity Annotation

Before storing objects into a database, we should specify OR (object-relation) mapping for Java classes with meta data. After JDK 5.0, we can specify OR mapping as annotations instead of XML files. JPA supports configuration by exception which means that it defines default for most cases of application and users only need to override the configuration value when it is exception to the default, not necessary.

First, annotate the class with @Entity to turn it into an entity, and annotate the member field for primary key with @Id. All other annotations are optional and we use them to override default values.

Todo class used in todo-list management

@Entity
@Table(name="apptodo")
public class Todo implements Serializable, Cloneable {


	@Id
	@GeneratedValue(strategy=GenerationType.AUTO)
	Integer id;
	
	boolean complete;
	
	@Column(nullable=false,length=128)
	String subject;
	
	@Column(nullable=false,length=128)
	@Enumerated(EnumType.ORDINAL)
	Priority priority;
	
	@Temporal(TemporalType.TIMESTAMP)
	Date date;
	
	String description;

	//omit getter, setter, hashCode(), equals() and other methods

}

Spring Beans Configuration

Spring JPA , available under the org.springframework.orm.jpa package, offers integration support for Java Persistence API. According to Spring framework reference, there are three options for JPA setup. We choose the simplest one.

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:context="http://www.springframework.org/schema/context"
	xmlns:tx="http://www.springframework.org/schema/tx"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="
        http://www.springframework.org/schema/beans     
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/context 
        http://www.springframework.org/schema/context/spring-context-3.0.xsd
        http://www.springframework.org/schema/tx
		http://www.springframework.org/schema/tx/spring-tx-3.0.xsd">
 
 	<context:component-scan base-package="org.zkoss.essentials" />
 
 	<!-- jpa(hibernate) configuration -->
	<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean">
		<property name="persistenceUnitName" value="myapp"/>
	</bean>
	
	<bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
		<property name="entityManagerFactory" ref="entityManagerFactory" />
	</bean>

	<tx:annotation-driven />
</beans>

• Line 16: The simplest Spring setup for JPA is to add a LocalEntityManagerFactoryBean which create a EntityManagerFactory for simple deployment environments and specify its persistenceUnitName property.
• Line 17: Set property persistenceUnitName with the name we specified in persistence.xml.
• Line 20,24: Enable Spring's declarative transaction management.

DAO Implementation

The Data Access Object (DAO) pattern is a good practice to implement a persistence layer and it encapsulates data access codes from the business tier. A DAO object exposes an interface to a business object and performs persistence operation relating to a particular persistent entity. Now, we can implement persistence operations like CRUD in a DAO class with JPA and EntityManager injected by Spring.

@Repository
public class TodoDao {

	@PersistenceContext
	private EntityManager em;

	@Transactional(readOnly=true)
	public List<Todo> queryAll() {
		Query query = em.createQuery("SELECT t FROM Todo t");
		List<Todo> result = query.getResultList();
		return result;
	}

	...

	@Transactional
	public Todo save(Todo todo){
		em.persist(todo);
		return todo;
	}

	@Transactional
	public Todo update(Todo todo){
		todo = em.merge(todo);
		return todo;
	}

	@Transactional
	public void delete(Todo todo){
		Todo r = get(todo.getId());
		if(r!=null){
			em.remove(r);
		}
	}
}

• Line 1: We register TodoDao as a Spring bean with @Repository because it is a DAO class according to Spring's suggestion.
• Line 4: As Spring manages our entity manager factory, it can understand @PersistenceContext and inject a transaction scope EntityManager for us. Hence, we don't need to create EntityManager by our own.
• Line 7: We have enabled Spring's declarative transaction management so that we can apply @Transactional on a methods.

After completing DAO classes, we can inject them to our service class with Spring's @Autowired because they are all Spring beans.

@Service("todoListService")
@Scope(value="singleton",proxyMode=ScopedProxyMode.TARGET_CLASS)
public class TodoListServiceImpl implements TodoListService {

	@Autowired
	TodoDao dao;
	
	public List<Todo>getTodoList() {
		return dao.queryAll();
	}
	
	...
}

Completing the above steps, we have created a dependency relationship among the controller, service, and persistence classes as follows:

Each of these classes encapsulates cohesive functions and has decoupled relationships with others. You can easily expand the architecture by adding more classes or create dependencies between two layers.

You can visit http://localhost:8080/essentials to see the result.

Conclusion

Our book ends here. But the adventure toward ZK just begins. This book opens you a door and introduces you some basic concepts and usages of ZK. You can start to deploy ZK to your server according to ZK Installation Guide which contains information you will use in deploying to web servers. For further development help, the ZK_Developer's Reference contains complete references for various topics in developing ZK based applications. ZUML Reference describes syntax and EL expression used in a zul. If you want to know details of a component, please refer to the ZK Component Reference. ZK also provides lots of configuration, you can take a look at ZK Configuration Reference.

We hope you can make use of what you learn here to obtain an even greater knowledge of ZK.