Detached entities in Java

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Detached entities
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Entity instances become unmanaged and detached when a transaction scope or extended persistence context ends. An interesting side effect is that detached entities can be serialized and sent across the network to a remote client. The client can make changes remotely to these serialized object instances and send them back to the server to be merged back and synchronized with the database. This behavior is very different from the EJB 2.1 entity model, where entities are always managed by the container. In EJB 2.1, applications using entity beans always had a proxy to the entity bean; in EJB 3.x, we work with concrete instances of plain Java classes. For EJB 2.1 developers, this behavior will seem strange at first, since you are used to the container managing every aspect of the entity. You ll find that after you get used to the new EJB 3.x model, your application code actually shrinks and is easier to manage.
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The reason we can eliminate code is very simple to illustrate. EJB 2.1 code often used the Value Object Pattern (often called Data Transfer Objects). The idea of this pattern was that the entity bean exposed a method that copied its entire state into an object that could be serialized to remote clients (like a Swing application) that needed access to the entity s state:
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// EJB 2.1 Entity bean class public class CustomerBean implements javax.ejb.EntityBean { CustomerValueObject getCustomerVO() { return new CustomerValueObject(getFirstName(), getLastName(), getStreet(), getCity(), getState, getZip()); }
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This is exactly the kind of plumbing we earlier deemed evil when discussing the benefits of using a Container and Application Server. Application code, armed with the right tools, should be free of this kind of error-prone and excessive noise. Also, it is very expensive to make a remote method call to an entity bean from a client. If the client had to call getFirstName(), getLastName(), etc., to get information about a customer it was displaying, performance would suffer. This is where the Value Object Pattern came in. EJB 3.x eliminates the need for this pattern because persistent objects become value objects automatically when they are detached from a persistent context. One side effect we encounter in dealing with detached entities revolves around entity relationships, which we ll discuss later.
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Packaging a Persistence Unit
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An EntityManager maps a fixed set of classes to a particular database. This set of classes is called a persistence unit. Before you can even think about creating or querying entities with an entity manager, you must learn how to package a persistence unit for use within a Java SE (regular Java application) or Java EE (application server) environment. A persistence unit is defined in a persistence.xml file, which is described by the JPA2 specification in section 8.2.1. This file is a required deployment descriptor for the Java Persistence specification. A persistence.xml file can define one or more persistence units. The JAR file or directory that contains a META-INF/persistence.xml file is called the root of the persistence unit, and this may be: An EJB JAR file The WEB-INF/classes directory of a WAR file A JAR file in the WEB-INF/lib directory of a WAR file A JAR file in an EAR library directory An application client JAR file
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The structure of one of these JAR files may look like the following:
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/META-INF/ /META-INF/persistence.xml /org/ /org/example/ /org/example/entity/ /org/example/entity/Employee.class
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The persistence.xml deployment descriptor defines the identities and configuration properties of each persistence unit described within it. Each persistence unit must have an identity, although the empty string is a valid name. The set of classes that belong to the persistence unit can be specified, or you can opt for the persistence provider to scan the JAR file automatically for the set of classes to deploy as entities. When scanning is used, the persistence provider will look at every class file within the JAR to determine whether it is annotated with the @javax.persistence.Entity annotation, and if it is, it will add it to the set of entities that must be mapped. Each persistence unit is tied to one and only one data source. In Java SE environments, vendor-specific configuration must be used to define and configure these data sources. In Java EE environments, specific XML elements define this association. The root of the persistence.xml XML schema is the <persistence> element, which contains one or more <persistence-unit> elements. Each <persistence-unit> has two attributes: name (required) and transaction-type (optional). The subelements of <persistence-unit> are <description> (optional), <provider> (optional), <jta-datasource> (optional), <non-jta-data-source> (optional), <mapping-file> (optional), <jarfile> (optional), <class> (optional), <properties> (optional), and <exclude-unlistedclasses> (optional). Here s an example of a persistence.xml file:
< xml version="1.0" encoding="UTF-8" > <persistence xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd" version="2.0"> <persistence-unit name="nameOfMyPu"> <jta-data-source>java:/DataSourceNameInJndi</jta-data-source> <properties> <!-- Standard and Provider-specific config may go here --> <property name="myprovider.property.name" value="someValue"/> </properties> </persistence-unit> </persistence>
The name attribute defines the name by which the unit will be referenced. This name is used by injection annotations and XML deployment descriptor elements to reference this unit. This attribute is required.
The transaction-type attribute defines whether you want your persistence unit to be managed by and integrated with Java EE transactions (JTA) or you want to use the resource local (RESOURCE_LOCAL) javax.persistence.EntityTransaction API to manage the integrity of your EntityManager instances. This attribute defaults to JTA in Java EE environments and to RESOURCE_LOCAL in SE environments. The <description> element is really just a comment describing the given persistence unit and is not required. The <provider> element is the fully qualified name of a class that implements the javax.persistence.PersistenceProvider interface. In Java EE and SE environments, the persistence implementation is pluggable: your vendor provides an appropriate implementation. Usually, you do not have to define this element and can rely on the default value. If you are using JTA or RESOURCE_LOCAL persistence units, you will probably define a <jta-data-source> or <non-jta-data-source> element, respectively. These elements specify a vendor-specific identity of a particular data source. Usually, this string is the global JNDI name for referencing the data source. If neither is defined, then a vendorprovided default will be used. The <properties> element defines the set of vendor-specific attributes passed to the persistence provider. They specify configuration that is specific to a vendor implementation. Since there is no registry or JNDI service within Java SE, this is usually how vendors configure data sources, instead of using the <jta-data-source> and <non-jtadata-source> elements.
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