EJB 3.0 Entity Manager

Entities are classes that need to be persisted; their state is stored outside the application, typically in a relational database. Unlike session beans, entities do not have business logic other than validation. As well as storing such entities, we want to query, update, and delete them.

also read:

The EJB 3.0 specification recognizes that many applications have the above persistence needs without requiring the services (security, transactions) of an application server EJB container. Consequently the persistence aspects of EJB 3.0 have been packaged as a separate specifi cation—the Java Persistence API (JPA). JPA does not assume we have a container and can even be used in a Java SE (Standard Edition) application. As well as persistence, JPA deals with Object/Relational Mapping and Queries, these are covered in Chapters 4 and 5 respectively. Most of our examples assume that the persistence engine exists within an EJB 3 container such as GlassFish or JBoss. In Chapter 6 we shall show examples of persistence outside a container.

Successful standalone object-relational mapping products such as open source Hibernate and proprietary Oracle Toplink have implemented these persistence technologies for a number of years. Creators of Oracle Toplink and Hibernate have been infl uential in the development of the JPA specifi cation. So, readers who are familiar with either Hibernate or Toplink will recognize similarities between the JPA and those products. Furthermore, under the covers, the GlassFish application server implements the JPA using Toplink and the JBoss application server uses Hibernate. These are pluggable defaults however, so it is possible to implement the JPA in GlassFish using Hibernate for example.

The JPA can be regarded as a higher level of abstraction sitting on top of JDBC. Under the covers the persistence engine converts JPA statements into lower level JDBC statements.

EJB 3.0 Entities

In JPA, any class or POJO (Plain Old Java Object) can be converted to an entity with very few modifications. The following listing shows an entity Customer.java with attributes id, which is unique for a Customer instance, and firstName and lastName.

package ejb30.entity;
import javax.persistence.Entity;
import javax.persistence.Id;
@Entity
public class Customer implements java.io.Serializable {
private int id;
private String firstName;
private String lastName;
public Customer() {}
@Id
public int getId() { return id; }
public void setId(int id) { this.id = id; }
public String getFirstname() { return firstName; }
public void setFirstname(String firstName) {
this.firstName = firstName;
}
public String getLastname() { return lastName; }
public void setLastname(String lastName) {
this.lastName = lastName;
}
public String toString() {
return "[Customer Id =" + id + ",first name=" +
firstName + ",last name=" + lastName + "]";
}
}

The class follows the usual JavaBean rules. The instance variables are non-public and are accessed by clients through appropriately named getter and setter accessor methods. Only a couple of annotations have been added to distinguish this entity from a POJO. Annotations specify entity metadata. They are not an intrinsic part of an entity but describe how an entity is persisted or, as we shall see in Chapter 4, how an entity is related to other entities. The @Entity annotation indicates to the persistence engine that the annotated class, in this case Customer, is an entity. The annotation is placed immediately before the class defi nition and is an example
of a class level annotation. We can also have property-based and fi eld-based annotations, as we shall see.

The @Id annotation specifi es the primary key of the entity. The id attribute is a primary key candidate. Note that we have placed the annotation immediately before the corresponding getter method, getId(). This is an example of a property-based annotation. A property-based annotation must be placed immediately before the corresponding getter method, and not the setter method. Where property-based annotations are used, the persistence engine uses the getter and setter methods to access and set the entity state.

An alternative to property-based annotations are fi eld-based annotations. We show an example of these later in this chapter. Note that all annotations within an entity, other than class level annotations, must be all property-based or all fi eld-based.
The final requirement for an entity is the presence of a no-arg constructor. Our Customer entity also implements the java.io.Serializable interface. This is not essential, but good practice because the Customer entity has the potential of becoming a detached entity. Detached entities must implement the Serializable interface. We will discuss detached entities in Chapter 6.

At this point we remind the reader that, as throughout EJB 3.0, XML deployment descriptors are an alternative to entity metadata annotations.

Comparison with EJB 2.x Entity Beans

An EJB 3.0 entity is a POJO and not a component, so it is referred to as an entity and not an entity bean. In EJB 2.x the corresponding construct is an entity bean component with the same artifacts as session beans, namely an XML deployment descriptor fi le, a remote or local interface, a home or localhome interface, and the bean class itself. The remote or local interface contains getter and setter method defi nitions. The home or local interface contains defi nitions for the create() and findByPrimaryKey() methods and optionally other fi nder method defi nitions. As with session beans, the entity bean class contains callback methods such as ejbCreate(), ejbLoad(), ejbStore(), ejbRemove(), ejbActivate(), ejbPassivate(), and setEntityContext().

The EJB 3.0 entity, being a POJO, can run outside a container. Its clients are always local to the JVM. The EJB 2.x entity bean is a distributed object that needs a container to run, but can have clients from outside its JVM. Consequently EJB 3 entities are more reusable and easier to test than EJB 2.x entity beans.

In EJB 2.x we need to decide whether the persistence aspects of an entity bean are handled by the container (Container Managed Persistence or CMP) or by the application (Bean Managed Persistence or BMP).

In the case of CMP, the entity bean is defi ned as an abstract class with abstract getter and setter method defi nitions. At deployment the container creates a concrete implementation of this abstract entity bean class.

In the case of BMP, the entity bean is defi ned as a class. The getter and setter methods need to be coded. In addition the ejbCreate(), ejbLoad(), ejbStore(),ejbFindByPrimaryKey(), and any other fi nder methods need to be coded using JDBC.

Mapping an Entity to a Database Table

We can map entities onto just about any relational database. GlassFish includes an embedded Derby relational database. If we want GlassFish to access another relational database, Oracle say, then we need to use the GlassFish admin console to set up an Oracle data source. We also need to refer to this Oracle data source in the persistence.xml fi le. We will describe the persistence.xml fi le later in this chapter. These steps are not required if we use the GlassFish default Derby data source. All the examples in this book will use the Derby database.

EJB 3 makes heavy use of defaulting for describing entity metadata. In this section we describe a few of these defaults.

First, by default, the persistence engine maps the entity name to a relational table name. So in our example the table name is CUSTOMER. If we want to map the Customer entity to another table we will need to use the @Table annotation as we shall see later in this chapter.

By default, property or fi elds names are mapped to a column name. So ID, FIRSTNAME, and LASTNAME are the column names corresponding to the id, firstname, and lastname entity attributes. If we want to change this default behavior we will need to use the @Column annotation as we shall see later in this chapter.

JDBC rules are used for mapping Java primitives to relational datatypes. So a String will be mapped to VARCHAR for a Derby database and VARCHAR2 for an Oracle database. An int will be mapped to INTEGER for a Derby database and NUMBER for an
Oracle database.

The size of a column mapped from a String defaults to 255, for example VARCHAR(255) for Derby or VARCHAR2(255) for Oracle. If we want to change this column size then we need to use the length element of the @Column annotation as we shall see later in this chapter.

Most persistence engines, including the GlassFish default persistence engine, Toplink, have a schema generation option, although this is not required by the JPA specifi cation. In the case of GlassFish, if a fl ag is set when the application is deployed to the container, then the container will create the mapped table in the database. Otherwise the table is assumed to exist in the database.

What EJB 3.0 Developer Guide Covers

  • Chapter 1 Introduction to EJB 3—A general overview of the Java EE architecture including EJB container services, the JPA persistence engine, and initial installation of the GlassFish application server.
  • Chapter 2 Session Beans—Creation of a session bean and its client and examples of running it from the application client container. Exploring the program directory structure. Packaging and deploying a session bean. A look at the stateless and stateful session beans lifecycle.
  • Chapter 3 Entities—Exploring EJB 3.0 entities. How to package and deploy entities and map an entity to a database table. Metadata defaults and how to override them. Generating primary keys.
  • Chapter 4 Object/Relational Mapping—One-to-one, one-to-many and many-to-many associations. Default object/relational mapping values and how to override them. Object/relational inheritance mapping and additional annotations.
  • Chapter 5 JPQL (Java Persistence Query Language)—Looking at different groups of queries including aggregate functions, queries with relationships, subqueries, queries with parameters and named queries. JPQL joins and functions are also explained.
  • Chapter 6 Entity Manager—Looks in detail at the entity manager. Covers both the EJB 3 container-managed and application-managed entity managers.
  • Chapter 7 Transactions—ACID properties and isolation levels. Container-managed transactions. Bean-managed transactions.
  • Chapter 8 Messaging—Looks at the JMS (Java Messaging Service) API. Examples of queue producer and queue consumers. Topic producer and consumers. Message driven beans: their activation configuration properties, lifecycles and transactions.
  • Chapter 9 EJB Timer Service—Examples of single and interval events. Timer interface methods. Timers and transactions.
  • Chapter 10 Interceptors—Covers interceptor methods, classes and interceptor communication as well as default interceptors.
  • Chapter 11 Implementing EJB 3.0 Web Services—An overview of web service concepts and the web service definition language (WSDL). Creating and deploying a Java application as a web service. Creating and deploying an EJB session bean as a web service. Creating a web service client.
  • Chapter 12 EJB 3.0 Security—A look at security, GlassFish authentication, declarative and programmatic EJB authorization and Web Tier authentication and authorization.
  • The Appendix shows EJB 3.0 annotations described in this book with their corresponding packages.

Introducing the EntityManager

We will use a remote stateless session bean for business operations on the Customer entity. We will call our session bean interface BankService. In later chapters, we will add more entities and business operations that are typically used by a banking application. At this stage our banking application deals with one entity—Customer, and business methods addCustomer() and findCustomer(), which respectively add a customer to the database and retrieve a customer entity given the customers identifier. The interface listing follows:

package ejb30.session;
import javax.ejb.Remote;
import ejb30.entity.Customer;
@Remote
public interface BankService {
void addCustomer(int custId, String firstName,
String lastName);
Customer findCustomer(int custId);
}

The code below shows the session bean implementation, BankServiceBean:

package ejb30.session;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import ejb30.entity.Customer;
import javax.persistence.PersistenceContext;
@Stateless
public class BankServiceBean implements BankService {
@PersistenceContext(unitName="BankService")
private EntityManager em;
public Customer findCustomer(int custId) {
return ((Customer)
em.find(Customer.class, custId));
}
public void addCustomer(int custId, String firstName,
String lastName) {
Customer cust = new Customer();
cust.setId(custId);
cust.setFirstname(firstName);
cust.setLastname(lastName);
em.persist(cust);
}
}

The EntityManager is a service provided by the persistence engine which provides methods for persisting, fi nding, querying, removing, and updating entities. The set of managed entity instances is known as a persistence context. It is important to stress that a persistence context is associated with a managed set of entity instances or objects. If we have, say, 10 clients accessing the bean, we will have 10 persistence contexts. Associated with an EntityManager is a persistence unit which specifi es configuration information. The statement:

@PersistenceContext(unitName="BankService")
private EntityManager em;

defi nes an EntityManager instance em with an associated persistence context and a persistence unit named BankService. The actual persistence unit confi guration is specifi ed in a persistence.xml fi le which we shall see shortly.

Actually, behind the scenes, the @PersistenceContext annotation causes the container to:

  • Use the EntityManagerFactory to create an instance of the EntityManager
  • Bind the EntityManager instance to the BankService persistence unit
  • Inject the EntityManager instance into the em field.

Now let’s take a look at the addCustomer() method. First, the statement

Customer cust = new Customer();

creates, as expected, an instance of the Customer entity. However, at this stage the entity instance is not managed. The instance is not yet associated with a persistence context. Managed entity instances are also referred to as attached instances and unmanaged instances as detached instances.

The next few statements invoke the Customer setter methods in the usual way. The statement:

em.persist(cust);

invokes the EntityManager.persist() method. At this stage the entity instance is not necessarily written immediately to the database. Rather, the entity instance becomes managed or attached and associated with a persistence context. The EntityManager manages the synchronization of a persistence context with the database. There may be more setter methods in our method after the persist()
statement. The EntityManager is unlikely to update, or fl ush, the database after each setter as database write operations are expensive in terms of performance. More likely the entity state would be kept in a cache and fl ushed to the database at the end of the current transaction. The latest point at which the EntityManager will fl ush to the database is when the transaction commits, although the EntityManager may chose to fl ush sooner.

By default, a transaction starts when a method is invoked, and is committed on leaving a method. We will discuss transactions further in Chapter 7.

Now let’s look at the findCustomer() method. The statement:

return ((Customer) em.find(Customer.class, custId));

invokes the EntityManager.find() method. This method retrieves an entity by its primary key. The parameters of find() are the entity class and a primary key, which is an Object type, which uniquely identifi es the entity. In our example we use the variable custId as the primary key (the autoboxing feature of Java SE 5 converts the supplied int type to an Object type).

The EntityManager service also provides a query language, JPQL, which we will cover in Chapter 5. Furthermore the EntityManager provides entity management methods, such as merging detached entities. At this stage the reader might think that entities should always be in an attached (managed) state, however we shall see examples in Chapter 6 where entities do exist in a detached state.

The following listing is an example of how a client might invoke the BankServicemethods to create and then find an entity.

package ejb30.client;
import javax.naming.Context;
import javax.naming.InitialContext;
import ejb30.session.*;
import ejb30.entity.Customer;
import javax.ejb.EJB;
public class BankClient {
@EJB
private static BankService bank;
public static void main(String[] args) {
try {
int custId = 0;
String firstName = null;
String lastName = null;
try {
custId = Integer.parseInt(args[0]);
firstName = args[1];
lastName = args[2];
} catch (Exception e) {
System.err.println(
"Invalid arguments entered, try again");
System.exit(0);
}
// add customer to database
bank.addCustomer(custId, firstName, lastName);
Customer cust = bank.findCustomer(custId);
System.out.println(cust);
} catch (Throwable ex) {
ex.printStackTrace();
}
}
}

The code is fairly straightforward. The variables custId, firstName, and lastName are initialized to parameters supplied to the main method when it is invoked. The client uses the BankService.addCustomer() method to create and add a Customer entity to the database. The BankService.findCustomer() method is then invoked to retrieve the entity just created.

We now return to the persistence unit which we referred to in the @PersistenceContext annotation that we used in the BankService session bean. The actual confi guration is specifi ed in an XML file, persistence.xml, which is shown below:

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

Again we make heavy use of defaults here. A persistence.xml fi le is required whenever an application makes use of EntityManager services, even if we rely entirely on defaults. In our case we have just used the name element to defi ne the BankService persistence unit. Note the name must match the unitName used earlier in the @PersistenceContext annotation. There are a number of persistence unit elements, in particular regarding transactions and data sources. We will return to these elements later in the book.

Packaging and Deploying Entities

As we have introduced a number of new source code artifacts that were not present in the previous chapter on session beans, the packaging process will be a little different. First we look at the program directory structure for our sample application.

Building the Application

B ecause we are persisting entities from an EJB container we need to place the persistence.xml fi le in the META-INF directory within the EJB module, BankService.jar. The package-ejb target in the Ant build fi le does this:

<target name="package-ejb" depends="compile">
<jar jarfile="${build.dir}/BankService.jar">
<fileset dir="${build.dir}">
<include name="ejb30/session/**" />
<include name="ejb30/entity/**" />
</fileset>
<metainf dir="${config.dir}">
<include name="persistence.xml" />
</metainf>
</jar>
</target>

We can use the jar –tvf command to examine the contents of the JAR file:

C:\EJB3Chapter03\glassfish\lab1\build>jar –tvf BankService.jar
...META-INF/
...META-INF/MANIFEST.MF
...ejb30/
...ejb30/entity/
...ejb30/session/
...ejb30/entity/Customer.class
...ejb30/session/BankService.class
...ejb30/session/BankServiceBean.class
...META-INF/persistence.xml

This is only one of a number of ways to package the persistence unit. Entities can be persisted not only from an EJB container but also from a web container or from a Java SE application running outside the container. We shall see an example of Java SE persistence in Chapter 6. Consequently the persistence unit (entity classes together with the persistence.xml fi le) can be placed in WEB-INF/classes
directory of a WAR fi le or in a Java SE application client jar file.

Alternatively we can package a persistence unit into can then be added to the WEB-INF/lib directory file or in the library directory of an EAR fi le. Adding module limits its scope to that module. If we place is visible to all modules within the application.

In our example we want the database tables to be created at deploy time, and dropped when we undeploy an application from the container. We modify the deploy target in our Ant build fi le, adding a createtables=true clause:

<target name="deploy">
<exec executable="${glassfish.home}/bin/asadmin"
failonerror="true"
vmlauncher="false">
<arg line="deploy --user admin --passwordfile
adminpassword --createtables=true
${build.dir}/BankService.ear"/>
</exec>
</target>
Similarly we add a droptables=true clause for the undeploy target.
<target name="undeploy">
<exec executable="${glassfish.home}/bin/asadmin"
failonerror="true"
vmlauncher="false">
<arg line="undeploy --user admin --passwordfile
adminpassword --droptables=true BankService"/>
</exec>
</target>

Note that before we can deploy BankService within GlassFish, we need to start up the embedded Derby database. We do this from the command-line with the asadmin utility: C:\> asadmin start-database

Field-Based Annotations

Recall we used a property-based annotation for the primary key @Id in the Customer entity. This results in the persistence engine using getter and setter methods to access and set the entity state. In this section we will modify the Customer entity to use fi eld-based, rather than property-based annotations. The following listing demonstrates this:

@Entity
public class Customer implements java.io.Serializable {
@Id
private int id;
private String firstName;
@Basic
private String lastName;
public Customer() {};
public Customer(int id, String firstName,
String lastName){
this.id = id;
this.firstName = firstName;
this.lastName = lastName;
}
public int getId() { return id; }
public void setId(int id) { this.id = id; }
public String getFirstname() { return firstName; }
public void setFirstname(String firstName) {
this.firstName = firstName;
}
public String toString() {
return "[Customer Id =" + id + ",first name=" +
firstName + ",last name=" + lastName + "]";
}
}

Note that we have placed the primary key @Id annotation immediately before the field declaration. If we use a fi eld-based annotation then all other annotations, other than class level annotations such as @Entity, must also be fi eld-based annotations. In the case of fi eld-based annotations the persistence engine will use refl ection to access and set the entity state. Getter and setter methods may be present in the entity but they are ignored by the persistence engine. To demonstrate this point we have removed the getLastName() and setLastName() methods from the Customer entity.

We have seen that attributes other than the primary key are mapped to a database by default. We call these mappings basic mappings. A basic mapping is used where the attribute is a Java primitive type or any Serializable type such as a String. We can
explicitly fl ag these mappings using the @Basic annotation. We have done this for the lastName field.

Generating Primary Keys

Upto now we have relied on the application to set an entity’s primary key by supplying a primary key identifi er as a parameter. However, we may want to relieve the application of this responsibility and use automatically generated keys. JPA provides the Table, Sequence, Auto, and Identity strategies for generating primary keys.

Table Strategy

With this strategy the persistence engine uses a relational database table from which the keys are generated. This strategy has the advantage of portability; we can use it with any relational database. Here is an example of how we would specify a table generation strategy for the id attribute of the Customer entity:

@TableGenerator(name="CUST_SEQ",
table="SEQUENCE_TABLE",
pkColumnName="SEQUENCE_NAME",
valueColumnName="SEQUENCE_COUNT")
@Id
@GeneratedValue(strategy=GenerationType.TABLE,
generator="CUST_SEQ")
public int getId() { return id; }

First we use the @TableGenerator annotation to specify the table used for key generation. This annotation can be placed on the primary key attribute, as we have done here, or on the entity class.

The name element in our example CUST_SEQ, identifi es the generator. The table element is the name of the table that stores the generated values. In our case we have chosen SEQUENCE_TABLE as the table name. There is a default table element, but this is dependent on the persistence engine being used. In the case of GlassFish, this table name is SEQUENCE. The pkColumnName element is the name of the primary key column in the sequence table. We have chosen SEQUENCE_NAME as the column name. Again the default is persistence engine dependent. In the case of GlassFish this value is SEQ_NAME. The valueColumnName element is the name of the column that stores the last key value generated. The default is persistence engine dependent. In the case of GlassFish this value is SEQ_COUNT.

One element of @TableGenerator for which we assumed the default is pkColumnValue. This is the String value that is entered in the pkColumnName column. The default is persistence engine dependent. In the case of GlassFish, this value is set to the name element of @TableGenerator. In our example this is CUST_SEQ. So SEQUENCE_TABLE will initially have the following row present:

SQL> SELECT * FROM SEQUENCE_TABLE;
SEQUENCE_NAME SEQUENCE_COUNT
------------- --------------
CUST_SEQ 0

If we want to store several sequences in the same sequence table, for example a separate sequence for each entity, then we need to manually specify the pkColumnValue element. The following statement sets the pkColumnValue to

CUSTOMER_SEQ:
@TableGenerator(name="CUST_SEQ",
table="SEQUENCE_TABLE",
pkColumnName="SEQUENCE_NAME",
valueColumnName="SEQUENCE_COUNT",
pkColumnValue="CUSTOMER_SEQ")

We should mention two other @TableGenerator elements: initialValue and allocationSize. initialValue is the initial value assigned to the primary key sequence. The default value is 0. The sequence is incremented by a value of 1. The allocationSize is the cache size into which the persistence engine reads from the sequence table. The default value is 50.

The @GeneratedValue annotation specifi es the key generation strategy with the strategy element. In our case we have chosen the TABLE strategy. The default strategy is AUTO which we describe later in this chapter. The generator element provides the name of the primary key generator. In our case this is CUST_SEQ and must match the name element of the @TableGenerator annotation. The
@GeneratedValue annotation is a fi eld-based or property-based annotation, so must be present immediately before the primary key fi eld or getter method.

Sequence Strategy

Some databases, such as Oracle, have a built-in mechanism called sequences for generating keys. To invoke such a sequence we need to use the @SequenceGenerator annotation. For example:

@SequenceGenerator(name="CUST_SEQ",
sequenceName="CUSTOMER_SEQUENCE")

As with the @TableGenerator annotation, the name element identifi es the generator. The sequenceName element identifi es the database sequence object. initialValue is the initial value assigned to the primary key sequence. The default differs from the @TableGenerator equivalent, and is equal to 1. The allocationSize is the cache size into which the persistence engine reads from the sequence. The default value is 50.

The @SequenceGenerator annotation can be placed on the primary key attribute or on the entity class.

As with Table generated sequences, we use the @GeneratedValue annotation to specify the generation strategy. For example,

@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE,
generator="CUST_SEQ")

This time we have specifi ed the SEQUENCE strategy. The name of the primary key generator is CUST_SEQ, and this must match the name element of the @SequenceGenerator annotation. Remember the @GeneratedValue annotation is a fi eld-based or property-based annotation, so it must be present immediately before the primary key fi eld or getter method.

Identity Strategy

Some databases, such as Microsoft SQL Server, use an identity column for generating keys. To use this we specify the IDENTITY strategy in the @GeneratedValue annotation:

@GeneratedValue(strategy=GenerationType.IDENTITY)

Note that there is no generator element that we have for the Table and Sequence strategies.

Auto Strategy

The final strategy is the AUTO strategy. With this strategy the persistence engine selects the strategy. In the case of GlassFish the TABLE strategy is selected. We can specify an AUTO strategy either explicitly:

@GeneratedValue(strategy=GenerationType.AUTO)
or implicitly:
@GeneratedValue
as the default strategy is AUTO.

Overriding Metadata Defaults

In this section we return to the Customer entity and override some of the default mapping options.

First we want to use the CLIENT table with columns CUSTOMER_ID, FIRST_NAME and LAST_NAME to store Customer entities. The primary key is CUSTOMER_ID. The table defi nition is:

,code>
CLIENT
CUSTOMER_ID NUMBER(38) PRIMARY KEY
FIRST_NAME VARCHAR(30) NOT NULL
LAST_NAME VARCHAR(30) NOT NULL

The modified listing for Customer.java is shown below:

@Entity
@Table(name = "CLIENT")
public class Customer implements java.io.Serializable {
private int id;
private String firstName;
private String lastName;
public Customer() {};
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name = "CUSTOMER_ID")
public int getId() { return id; }
public void setId(int id) { this.id = id; }
@Column(name = "FIRST_NAME")
public String getFirstname() { return firstName; }
public void setFirstname(String firstName) {
this.firstName = firstName;
}
@Column(name = "LAST_NAME")
public String getLastname() { return lastName; }
public void setLastname(String lastName) {
this.lastName = lastName;
}
public String toString() {
return "[Customer Id =" + id + ",first name=" +
firstName + ",last name=" + lastName + "]";
}
}

Note we have used the @Table annotation to specify the table name as CLIENT. The default table name is the entity name, CUSTOMER in our example. The catalog element specifi es the database catalog where the table is located. Many database systems, Derby included, do not support the concept of a catalog. So in our example we can leave this element out. The schema element specifi es the database schema where the table is located. The default is persistence provider specifi c, in the case of the GlassFish APP is the default schema for the embedded Derby database. Again, in our example, we rely on the default.

We have explicitly specifi ed an AUTO primary key generation strategy:

@Id
@GeneratedValue(strategy=GenerationType.AUTO)

Recall that GlassFish selects the TABLE strategy in this case.

Recall that by default column names are set to attribute names. To override this we
use the @Column annotation. For example,

@Column(name = "CUSTOMER_ID")
public int getId() { return id; }

will map the id attribute to the CUSTOMER_ID column. In a similar fashion we map
the firstName and lastName attributes to the FIRST_NAME and LAST_NAME
columns respectively.

Summary

Entities are classes that need to be persisted, usually in a relational database. The persistence aspects of EJB 3 have been packaged as a separate specifi cation, the Java Persistence API (JPA), so that applications that do not need EJB container services can still persist their entities to a relational database. Persistence services are handled by a persistence engine. In this chapter we make use of the Toplink persistence engine that comes bundled with the GlassFish application server.

Any Java class, or POJO, can be converted to an entity using metadata annotations. We described by means of an example the default rules for mapping an entity to a relational database table.

We introduced the EntityManager service, which provides methods for persisting, fi nding, querying, removing and updating entities. We saw examples of the EntityManager.persist() and EntityManager.find() methods. We introduced the concept of a persistence context, which is the set of managed entity instances. We looked at Ant scripts for packaging and deploying an application which uses entities.

We examined strategies for generating primary keys. Finally we looked at examples of overriding default rules for mapping entities to relational tables.

Comments

comments

About Krishna Srinivasan

He is Founder and Chief Editor of JavaBeat. He has more than 8+ years of experience on developing Web applications. He writes about Spring, DOJO, JSF, Hibernate and many other emerging technologies in this blog.

Comments

  1. Rocky Dayaramani says:

    Hi Krishna Sir,

    The article is very good and very much clear. But I would also like to read on the chapters that you have mentioned in this article in the “What EJB 3.0 developer guide covers?” section.

    I would be even more thankful if you please provide the links of those chapters as I dint find them in the J2EE section in your website.

    Thank you.

Trackbacks

Speak Your Mind

*