JPA

Overview

Java Persistence API (JPA) is a POJO-based persistence specification. It offers object-relational mapping solution to enterprise Java applications. We will demonstrate examples on how to integrate a popular and commonly used combination: JPA & Spring. In our example project, we use a popular JPA 2.0 implementation, Hibernate 4.0.0.Final. We will also talk about solutions against well-known LazyInitializationException.

Integrate with Spring and JPA

Data Access Object (DAO) pattern is a good practice to implement a persistence layer. This pattern encapsulates persistence API in a DAO object and exposes the DAO object's interface to a business object to perform persistence operations relating to a particular persistent entity.

According to Hibernate EntityManager User Guide's suggestion , we should apply entitymanager-per-request pattern (aka. Open Session in View pattern) to manage entity manager. This pattern needs an interceptor to create a new EntityManager when a request is sent to the server and a DAO object would use the same EntityManger to perform persistence operation. We then close the EntityManager before a response is sent to the client. The challenge here is how to implement this pattern. The good news is that we can achieve this through Spring's OpenEntityManagerInViewFilter and dependency injection.

Applying this pattern also solves a common LazyInitializationException problem most developers may encounter when using a lazy fetching strategy. In brief, EntityManager is usually closed after a DAO object has performed an operation under entitymanager-per-operation pattern. Those persistent objects queried by EntityManager become detached after associated EntityManagers are closed. If we access a detached object's lazy-loaded collection when rendering the view, we will get an error message like LazyInitializationException: no session or session was closed. This problem is essentially identical to a lazy-loading problem in Hibernate. As we apply entitymanager-per-request pattern, an EntityManager is kept open when a View accessing a lazy-loaded collection. Those objects queried by EntityManager become detached later (after the interceptor closes the EntityManager) and as a result, the problem mentioned previously is now resolved.

Configuration

The minimal Maven dependencies you need are:

    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-web</artifactId>
      <version>3.0.7.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.hibernate</groupId>
      <artifactId>hibernate-entitymanager</artifactId>
      <version>4.0.0.Final</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-orm</artifactId>
      <version>3.0.7.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>cglib</groupId>
      <artifactId>cglib</artifactId>
      <version>2.2</version>
      <scope>compile</scope>
    </dependency>

Note: If you don't use Maven, please refer to JPA vendor's documentation to know which JAR file you need.

Our example project's Spring configuration is for non-managed environment.

Spring configuration

	<!-- omit headers -->

	<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource" >
		<property name="driverClassName" value="org.hsqldb.jdbcDriver" />
		<property name="url" value="jdbc:hsqldb:file:data/store" />
		<property name="username" value="sa" />
		<property name="password" value="" />
	</bean>
	
	<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean">
		<property name="persistenceUnitName" value="order"/>
	</bean>

	<bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
		<property name="entityManagerFactory" ref="entityManagerFactory" />
	</bean>

	<tx:annotation-driven />
	
	<context:component-scan base-package="org.zkoss.reference.developer.jpa" />

OpenEntityManagerInViewFilter

To apply entitymanager-per-request pattern, we can use Spring provided OpenEntityManagerInViewFilter instead of writing our own one. Make sure filter mapping's url-pattern covers all pages that access lazy-loaded entities. If you don't want this filter intercepting all pages, be sure to include ZK AU request path (/zkau/*) in url-pattern as your event handling methods (or command methods) might also access lazy-loaded objects.

Configure OpenEntityManagerInViewFilter in web.xml

	<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>

DAO Implemnetation

In the DAO, we can easily retrieve EntityManager by Spring's dependency inject without writing any utility class. Spring's declarative transaction management and rollback rule also reduces our work.

DAO empowered by Spring

@Repository
public class SpringOrderDao {

	@PersistenceContext
	private EntityManager em;
	
	@Transactional(readOnly=true)
	public List<Order> queryAll() {
		Query query = em.createQuery("from Order as o");
		List<Order> result = query.getResultList();
		return result;
	}

	@Transactional
	public Order save(Order newOrder){
		em.persist(newOrder);
		em.flush();
		return newOrder;
	}
	
	//...
}

• Line 4: Spring will inject EntityManager created by OpenEntityManagerInViewFilter.

Lazy Initialization Issue among AU Requests

We apply open session in view pattern to keep an EntityManager open after a page is rendered, this makes a ZUL that accesses a lazy-loaded collection to be rendered normally when you visit the ZUL at first request. However, if your event handling methods (or command methods) access lazy-loaded collection of another detached object, you still get LazyInitializationException when a user interacts with the ZUL. This is because even though the filter opens an EntityManager for each request, the detached objects don't attach to the EntityManager automatically. There are two solutions for this problem.

1. set fetching strategy to eagerly fetch.
2. Re-query detached object manually.

If you don't have large amount of data, you can choose the first solution; just change your fetching strategy to eager for one-to-many mapping.

We will demonstrate the second solution here using an example assuming that we have a "Order Viewer" system where we can view an order and its details. The upper Listbox contains a list of orders and the lower Grid contains the details the selected order. One order may contain many order items (one-to-many mapping), and we set order items collection to lazy fetching . When we select an order, the Grid displays details of the selected order which means accessing a lazy-loaded collection.

Order and OrderItem

@Entity
@Table(name="orders")
public class Order {

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	private Long id;
	private String status = PROCESSING;
	private String description;

	@OneToMany(mappedBy="orderId", fetch=FetchType.LAZY)
	private List<OrderItem> items = new ArrayList<OrderItem>();

	//other codes...
}

We set Listbox to select the first row of the orders as default. When the ZUL accesses the selected order's lazy-loaded items collection, JPA can load it successfully with the help of OpenEntityManagerInViewFilter because EntityManager is still open. However, if we click the second row which accesses a detached Order object's items collection, we should re-load the Order object with JPA EntityManager or we'll get LazyInitializationException.

source of above screen

In order to reload a detached object, we pass the selected order to DAO object and reload it.

Reload selected object

public class SpringOrderViewModel {

	@WireVariable
	private SpringOrderDao springOrderDao;

	private List<Order> orders ;
	private Order selectedItem;
	
	@Init
	public void init(){
		orders = springOrderDao.queryAll();
		if (!orders.isEmpty()){
			setSelectedItem(orders.get(0));
		}
	}
	
	public Order getSelectedItem() {
		if (selectedItem!=null){
			selectedItem = springOrderDao.reload(selectedItem);
			//you could replace the item in model list with persistent one
		}
		return selectedItem;
	}
	
	//omit other codes
}

• Line 12: Initialize the Listbox selection with the Order object at index 0 of orders.
• Line 17: Re-query the selectedItem.

We reload the detached Order objects from the database, this will make the detached object attach to an EntityManager. Then, when we access the lazy-loaded collection (items), JPA can retrieve the collection for us. After doing so, we can eliminate LazyInitializationException.

Reload detached object

@Repository
public class SpringOrderDao {

	@PersistenceContext
	private EntityManager em;

	//omit other codes

	@Transactional(readOnly=true)
	public Order reload(Order order){
		return em.find(Order.class, order.getId());
	}


}

Lazy Initialization Issue Under Render on Demand

Some AU requests cannot be interfered by developers like "Render On Demand" request. The rendering request is handled implicitly by a component itself. Under this situation, if a component needs to render some data from a detached object's lazy-loaded collection, developers won't have a chance to reload detached objects during rendering to avoid LazyInitailizationException. Let's use an example to explain this situation.

Assume we have a Listbox, it only displays 10 rows by default and it'a not in "page" mold. One of its columns display a lazy-loaded collection's size (each.items.size() )of an Order object.

Listbox that accesses lazy-loaded property

	<window title="" border="normal" width="600px" apply="org.zkoss.bind.BindComposer"
		viewModel="@id('vm') @init('org.zkoss.reference.developer.jpa.vm.RodViewModel')">
		Contain a customized model that reload lazy-loaded collection from database
		<listbox model="@load(vm.orderListModel)" rows="10">
			<listhead>
				<listheader label="ID" width="50px" />
				<listheader label="Description" />
				<listheader label="Item Count" width="150px" />
			</listhead>
			<template name="model">
				<listitem>
					<listcell label="@load(each.id) " />
					<listcell label="@load(each.description)" />
					<listcell label="@load(each.items.size())" />
				</listitem>
			</template>
		</listbox>
...

If we just pass a Java List object to be the model of the Listbox, when a user scrolls down to view other rows, ZK will send AU request to retrieve data for those un-rendered rows. Listbox will try to access lazy-loaded collection but objects in the list are already detached, and we will get LazyInitailizationException. During this rendering process, developers will not be notified and cannot interfere this process to reload detached objects. One solution is to implement a custom ListModel for the component.

We demonstrate 2 implementations here for your reference. The first one is simpler but less efficient; it re-queries each detached object when a component requests it.

Reloaded ListModel

public class OrderListModel extends AbstractListModel<Order>{

	private SpringOrderDao orderDao;
	List<Order> orderList = new LinkedList<Order>();
	
	public OrderListModel(List<Order> orders,OrderDao orderDao){
		this.orderList = orders;
		this.orderDao = orderDao;
	}
	
	@Override
	public Order getElementAt(int index) {
		//throw a runtime exception if orderDao does not find target object
		Order renewOrder = orderDao.reload(orderList.get(index));
		return renewOrder;
	}

	@Override
	public int getSize() {
		return orderList.size();
	}
}

• Line 1: We extend AbstractListModel to build our list model for it to handle selection for us, but we have to override Order's equals() and hashCode().
• Line 14: Re-query the detached object by its id and return a persistent one.

The second one is more complicated but more efficient; it re-queries a one page size data each time and stores as a cache in an execution. If the cache has the object that the component requests, it returns the one in cache without re-querying it again.

Lived ListModel

public class LiveOrderListModel extends AbstractListModel<Order>{

	private SpringOrderDao orderDao;
	private Integer totalSize;	
	private int pageSize = 30;
	private static final String CACHE_KEY= LiveOrderListModel.class+"_cache";
	
	public LiveOrderListModel(OrderDao orderDao){
		this.orderDao = orderDao;
	}

	/**
	 * query one page size of entity for one execution a time. 
	 */
	@Override
	public Order getElementAt(int index) {
		Map<Integer, Order> cache = getCache();

		Order targetOrder = cache.get(index);
		if (targetOrder == null){
			//if cache doesn't contain target object, query a page starting from the index
			List<Order> pageResult = orderDao.findAll(index, pageSize);
			int indexKey = index;
			for (Order o : pageResult ){
				cache.put(indexKey, o);
				indexKey++;
			}
		}else{
			return targetOrder;
		}

		//get the target after query from database
		targetOrder = cache.get(index);
		if (targetOrder == null){
			//if we still cannot find the target object from database, there is inconsistency in the database
			throw new RuntimeException("Element at index "+index+" cannot be found in the database.");
		}else{
			return targetOrder;
		}
	}

	private Map<Integer, Order> getCache(){
		Execution execution = Executions.getCurrent();
		//we only reuse this cache in one execution to avoid accessing detached objects.
		//our filter opens a session during a HTTP request
		Map<Integer, Order> cache = (Map)execution.getAttribute(CACHE_KEY);
		if (cache == null){
			cache = new HashMap<Integer, Order>();
			execution.setAttribute(CACHE_KEY, cache);
		}
		return cache;
	}
	
	@Override
	public int getSize() {
		if (totalSize == null){
			totalSize = orderDao.queryAllSize().intValue();
		}
		return totalSize; 
	}
}

• Line 16: If the cache doesn't contain target Order, we query one page size of Order starting from the index because passed index doesn't always increase sequentially.
• Line 42: The getElementAt(int) will be invoked multiple times during an execution. In order to avoid using a cache of detached objects, we make the cache as an attribute of an execution which is dropped after being handled.

Get Example Source Code

All source code used in this chapter can be found here.