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Taking JPA for a Test Drive


by Samudra Gupta

A case study in the use and deployment of the Java Persistence API (JPA)

Published November 2006

The Summer 2006 release of the EJB 3.0 specification delivers a much simplified but more powerful EJB framework that demonstrates a marked preference for annotations over traditional EJB 2.x deployment descriptors. Annotations, introduced in J2SE 5.0, are modifiers that can be used in classes, fields, methods, parameters, local variables, constructors, enumerations, and packages. Annotation use is highlighted in a myriad of new EJB 3.0 features such as Plain Old Java Object-based EJB classes, dependency injection of EJB manager classes, the introduction of interceptors or methods that can intercept other business method invocations, and a greatly enhanced Java Persistence API (JPA).

To illustrate the concepts of JPA, let's walk through a real-life example. Recently, my office had to implement a tax registration system. Like most systems, it had its own complexities and challenges. Because its particular challenge concerned data access and object-relational mapping (ORM), we decided to test-drive the new JPA while implementing the system.

We faced several challenges during the project:

  • Relationships exist among entities use in the application.
  • The application supports complex searches across relational data.
  • The application must ensure data integrity.
  • The application validates data before persisting it.
  • Bulk operations are required.

The Data Model

First let's review a reduced version of our relation data model, which will be sufficient to explain the nuances of JPA. From a business perspective, a main applicant submits a tax registration application. The applicant may have zero or more partners. Applicant and Partner must specify two addresses viz. registered address and the trading address. The main applicant also must declare and describe any penalties he's received in past.

Figure 1

Defining Entities. We defined the following entities by mapping them to individual tables:

Entity Tabled Mapped To
Registration REGISTRATION
Party PARTY
Address ADDRESS
Penalty PENALTY
CaseOfficer CASE_OFFICER

Table 1.
Entity-table mappings

Identifying the entities to map the database tables and columns was easy. Here's a simplified example of the Registration entity. (I'll introduce additional mappings and configurations for this entity later.)

@Entity
@Table(name="REGISTRATION")

public class Registration implements Serializable{

    @Id
    private int id;

    @Column(name="REFERENCE_NUMBER")
    private String referenceNumber;

    

   ..........
    }

For us, the main benefit of using JPA entities was that we felt as if we were coding routine Java classes: The complicated lifecycle methods were gone. We could use annotations to assign persistence features to the entities. We found that we didn't need another extra layer of Data Transfer Objects (DTO) and that we could re-use the entities to move between layers. Suddenly, data became more mobile.

Supporting Polymorphism. Looking at our data model, we noted that we used the PARTY table to store both Applicant and Partner records. These records shared some common attributes but also had individual attributes.

We wanted to model this model in an inheritance hierarchy. With EJB 2.x, we could use only one Party entity bean and then create Applicant or Partner objects based on the party type by implementing the logic within the code. JPA, on the other hand, let us specify the inheritance hierarchy at the entity level.

We decided to model the inheritance hierarchy with an abstract Party entity and two concrete entities, Partner and Applicant:

@Entity
@Table(name="PARTY_DATA")
@Inheritance(strategy= InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="PARTY_TYPE")

public abstract class Party  implements Serializable{

    @Id
    protected int id;

    @Column(name="REG_ID")
    protected int regID;


   
    protected String name;

   .........

 }

The two concrete classes, Partner and Applicant, will now inherit from the abstract Party class.

@Entity
@DiscriminatorValue("0")
public class Applicant extends Party{

    @Column(name="TAX_REF_NO")
    private String taxRefNumber;

    @Column(name="INCORP_DATE")
    private String incorporationDate;

  ........
                               
}

If the party_type column has a value of 0, the persistence provider will return an instance of Applicant entity; if the value is 1, it will return an instance of Partner entity.

Building relationships. The PARTY table in our application data model contains a foreign-key column (reg_id) to the REGISTRATION table. In this structure, the Party entity becomes the owning side of the entity or the source of the relationship because it's where we specify the join column. Registration becomes the target of the relationship.

With every ManyToOne relationship, it's more likely that the relationship is bi-directional; that is, there will also be a OneToMany relationship between two entities. The table below shows our relationship definitions:

>

Relationship Owning Side Multiplicity/Mapping
Registration->CaseOfficer CaseOfficer OneToOne
Registration->Party Party ManyToOne
Party->Address Address ManyToOne
Party->Penalty Penalty ManyToOne
Reverse Side of Relationship
Registration->CaseOfficer
OneToOne
Registration->Party
OneToMany
Party->Address
OneToMany
Party->Penalty
OneToMany

Table 2.
The relationships

public class Registration  implements Serializable{
....

    @OneToMany(mappedBy = "registration")
    private Collection<Party> parties;

....
}

public abstract class Party  implements Serializable{
....
    @ManyToOne
    @JoinColumn(name="REG_ID")
    private Registration registration;
....

Note: The mappedBy element indicates that the join column is specified at the other end of the relationship.

Next, we had to consider the behavior of the relationships as defined by the JPA specification and implemented by persistence providers. How did we want to fetch the related data—EAGER or LAZY? We looked at the default FETCH types for relationships as defined by JPA, and then added an extra column to Table 2 to include our findings:

Relationship Owning Side Multiplicity/Mapping Default FETCH Type
Registration->CaseOfficer CaseOfficer OneToOne EAGER
Party->Registration Party ManyToOne EAGER
Address->Party Address ManyToOne EAGER
Penalty->Party Penalty ManyToOne EAGER




Reverse Side of Relationship
Registration->Party
OneToMany LAZY
Party->Address
OneToMany LAZY
Party->Penalty
OneToMany LAZY

Table 3. Setting default FETCH types

Looking at the business requirement, it appeared that when we obtained Registration details, we'd always have to display the details of the Party associated with that registration. With the FETCH type set to LAZY, we'd have to make repetitive calls to the database to obtain data. This implied that we'd get better performance if we changed the Registration->Party relationship to the FETCH type EAGER. With that setting, the persistence provider would return the related data as part of a single SQL.

Similarly, when we displayed the Party details on screen we needed to display its associated Address(es). Thus, it also made sense to change the Party-Address relationship to use the EAGER fetch type.

On the other hand, we could set the Party->Penalty relationship FETCH type to LAZY since we didn't need to display the details of penalties unless a user requested them. If we used the EAGER fetch type, for m number of parties having n number of penalties each, we'd end up loading m*n number of Penalty entities, which would result in an unnecessarily large object graph and degrade performance.

public class Registration  implements Serializable{

    @OneToMany(mappedBy = "registration", fetch = FetchType.EAGER)
    private Collection<Party> parties;

 .....
}

public abstract class Party implements Serializable{

       @OneToMany (mappedBy = "party", fetch = FetchType.EAGER)
    private Collection<Address> addresses;

    @OneToMany (mappedBy = "party", fetch=FetchType.LAZY)
    private Collection<Penalty> penalties;

 .....
}

Accessing lazy relationships. When considering using a lazy loading approach, consider the scope of the persistence context. You can choose between an EXTENDED persistence context or a TRANSACTION scoped persistence context. An EXTENDED persistence context stays alive between transactions and acts much like a stateful session bean.

Because our application isn't conversational, the persistence context didn't have to be durable between transactions; thus, we decided to use the TRANSACTION scoped persistence context. However, this posed a problem with lazy loading. Once an entity is fetched and the transaction has ended, the entity becomes detached. In our application, trying to load any lazily loaded relationship data will result in an undefined behavior.

In most cases, when caseworkers retrieve a registration data back, we don't need to display penalty records. But for managers, we need to additionally display the penalty records. Considering that most of the times, we need NOT display the penalty records, it would make no sense to change the relationship FETCH type to EAGER. Instead, we can trigger the lazy loading of the relationship data by detecting when a manager is using the system. This will make the relationship data available even when the entity is detached and can be accessed later. The example below explains the concept:

Registration registration = em.find(Registration.class, regID);

     Collection<Party> parties = registration.getParties();
     for (Iterator<Party> iterator = parties.iterator(); iterator.hasNext();) {
         Party party = iterator.next();
         party.getPenalties().size();

     }
     return registration;
               

In the above example, we just invoke the size() methods of the penalties collection of the Party entity. This does the trick and triggers the lazy loading and all the collections will be populated and available even when the Registration entity is detached. (Alternatively, you can use a special feature of JP-QL called FETCH JOIN, which we shall examine later in the article.)

Relationships and Persistence

Next, we had to consider how the relationships behaved in the context of persisting data. In essence, if we made any changes to the relationship data, we wanted to do so at the object level and have the changes persisted by the persistence provider. In JPA, we can use CASCADE types to control the persistence behavior.

There are four CASCADE types defined within JPA:

  • PERSIST: When the owning entity is persisted, all its related data is also persisted.
  • MERGE: When a detached entity is merged back to an active persistence context, all its related data is also merged.
  • REMOVE: When an entity is removed, all its related data is also removed.
  • ALL: All the above applies.

Creating an entity. We decided that in all cases, when we created a new parent entity, we wanted all its related child entities to also be persisted automatically. This made coding easier: We just had to set the relationship data correctly and we didn't need to invoke the persist() operation on each entity separately. This means coding is easier this way, as we just have had to set the relationship data correctly and we don't didn't need to invoke the persist() operation on each entity separately.

So, cascade type PERSIST was the most attractive option for us. We refactored all our relationship definitions to use it.

Updating an entity It's common to obtain data within a transaction and then make changes to the entities outside the transaction and persist the changes. For example, in our application, users could retrieve an existing registration and change the address of the main applicant. When we obtain an existing Registration entity and thus, all of its related data within a particular transaction, the transaction ends there and the data is sent to the presentation layer. At this point, the Registration and all other related entity instances become detached from the persistence context.

In JPA, to persist the changes on a detached entity, we use the EntityManager's merge() operation. Also, in order to propagate the changes to the relationship data, all the relationship definitions must include CASCADE type MERGE along with any other CASCADE type defined in the relationship mapping's configuration.

With this background, we made sure that we specified the correct CASCADE types for all relationship definitions.

Removing an entity. Next, we had to determine what would happen when we deleted or removed certain entities. For example, if we deleted a Registration, we could safely delete all the Party(s) associated with that Registration. But the reverse is not true. The trick here is to avoid any unwanted removal of entities by cascading the remove() operation on the relationship. As you'll see in the next section, such operations may not succeed because of referential integrity constraints.

We concluded that in a clear parent-child relationship such as Party and Address or Party and Penalty, which follow OnetoMany relationships, it's safe to specify the CASCADE type REMOVE only on the parent (ONE) side of the relationship. We then refactored the relationship definitions accordingly.

                               
public abstract class Party implements Serializable{

   @OneToMany (mappedBy = "party", fetch = FetchType.EAGER, cascade =
                                
{CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE}) private Collection<Address> addresses; @OneToMany (mappedBy = "party", fetch=FetchType.LAZY, cascade =
{CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE}) private Collection<Penalty> penalties; ..... }

Managing Relationships

According to JPA, managing relationships is the sole responsibility of the programmer. Persistence providers don't assume anything about the state of relationship data, so they don't attempt to manage the relationship.

Given this fact, we re-examined our strategy to manage relationships and pinpoint potential problem areas. We discovered the following:

  • If we try to set a relationship between a parent and the child, and the parent no longer exists in the database (perhaps it was removed by another user), this will lead to data integrity issues.
  • If we try to delete a parent record without first removing its child record, a referential integrity will be violated.

Thus, we mandated the following coding guidelines:

  • If we obtain an entity and its related entities in a transaction, change the relationship outside the transaction, and then try to persist the changes within a new transaction, it's best to re-fetch the parent entity.
  • If we try to delete a parent record without deleting the child records, we must set the foreign-key field for all children to NULL before removing the parent.

Consider the OneToOne relationship between CaseWorker and Registration. When we delete a particular registration, we don't delete the caseworker; thus, we need to set the reg_id foreign key to null before we delete any registration.

@Stateless
public class RegManager {
.....

public void deleteReg(int regId){
        Registration reg = em.find(Registration.class, regId);
        CaseOfficer officer =reg.getCaseOfficer();
        officer.setRegistration(null);
        em.remove(reg);
    }
}

Data Integrity

When a user is viewing a registration record, another user might be making changes to the same application. If the first user then makes additional changes to the application, he risks unknowingly overwriting it with old data.

To address this issue, we decided to use "optimistic locking." In JPA, entities can define a versioning column, which we can use to implement the optimistic locking.

public class Registration  implements Serializable{


    @Version

    private int version;
.....
}

The persistence provider will match the in-memory value of the version column with that of the database. If the values are different, the persistence provider will report an exception.

Validation

When we say that the main applicant must have at least one address and the address must at least contain the first line and the post code, we're applying a business rule across the Party and Address entities. However, if we say that each address line must always be less than 100 characters, this validation is intrinsic to the Address entity.

In our application, we decided to implement the cross object/business rules type validations into the Session Bean layer because that's where most of the workflow and process-oriented logic is coded. However, we placed the intrinsic validations within the entities. Using JPA, we could associate any method to a lifecycle event for an entity.

The following example validates that an Address line can contain no more than 100 characters and invokes this method before the Address entity is persisted (with a @PrePersist annotation). Upon failure, this method will throw a business exception (which extends from RuntimeException class) to the caller, which can then be used to pass a message to the user.

public class Address  implements Serializable{
.....
    @PrePersist
   public void validate() 
       if(addressLine1!=null && addressLine1.length()>1000){
           throw new ValidationException("Address Line 1 is longer than 1000 chars.");
       }
   }

Search

Our tax registration application offered a search facility to find details about a particular registration, its parties, and other details. Providing an efficient search facility involved many challenges such as writing efficient queries and implementing pagination for browsing a large result list. JPA specifies a Java Persistence Query Language (JP-QL) to be used with entities in order to implement data access. It's a major improvement over EJB 2.x EJB QL. We successfully used JP-QL to provide an efficient data access mechanism.

Queries

In JPA, we had options for creating queries dynamically or defining static queries. These static or named queries support parameters; the parameter values are assigned at runtime. Because the scope of our queries was fairly well-defined, we decided to use named queries with parameters. Named queries are also more efficient as the persistence provider can cache the translated SQL queries for future use.

Our application provided a simple use case for this: when a user enters an application reference number to retrieve registration details. We provided a named query on the Registration entity as follows:

@Entity
@Table(name="REGISTRATION")
@NamedQuery(name="findByRegNumber", query = "SELECT r FROM REGISTRATION r WHERE r.appRefNumber=?1")
public class Registration implements Serializable{
.....
}

For example, one search requirement within our application required special attention: a report query to retrieve all the parties with their total penalty amounts. Since the application allows parties to exist without penalties, a simple JOIN operation wouldn't list parties without any penalty. To overcome this, we used the OUTER JOIN facility of JP-QL. We could also use the GROUP BY clause to add up the penalties. We added another named query in the Party entity as follows:

                               
@Entity
@Table(name="PARTY_DATA")
@Inheritance(strategy= InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="PARTY_TYPE")

@NamedQueries({@NamedQuery(name="
                                
generateReport", 
                           query=" SELECT NEW com.ssg.article.ReportDTO(p.name, SUM(pen.amount))
                                
FROM Party p LEFT JOIN p.penalties pen GROUP BY p.name""), @NamedQuery(name="bulkInactive", query="UPDATE PARTY P SET p.status=0 where p.registrationID=?1")}) public abstract class Party { ..... }

Notice in the above example of named query "generateReport" that we have instantiated a new ReportDTO object within the query itself. This is nonetheless a very powerful feature of JPA.

Can We Do It in Bulk?

In our application, an officer can retrieve a registration and make it inactive. In this case, we should set all the associated Party to the Registration as inactive, as well. This typically means setting the Status column in the PARTY table to 0. To improve performance, we'd use batch updates rather than execute individual SQLs for each Party.

Fortunately, JPA provides a way to do this:

@NamedQuery(name="bulkInactive", query="UPDATE PARTY p SET p.status=0 where p.registrationID=?1")
public abstract class Party implements Serializable{
.....
}

Note: A Bulk operation issues SQL directly to the database, which means the persistence context isn't updated to reflect the changes. When using an extended persistence context that lasts beyond a single transaction, cached entities may contain stale data.

Getting It Early.

Another challenging requirement was selective data display. For example, if a manager searched for registrations, we needed to display all the penalties that the registered parties recorded. This information is not otherwise available to a normal caseworker. for some registrations, we needed to display all the penalties that the registered parties recorded. This information is not otherwise available to a normal caseworker.

The relationship between a Party and Penalty is OneToMany. As mentioned earlier, the default FETCH type for this is LAZY. But to meet this search selective display requirement, it made sense to fetch the Penalty details as a single SQL to avoid multiple SQL calls.

The FETCH Join feature in JP-QL came to our rescue. If we wanted to temporarily override the LAZY fetch type, we could use Fetch Join. However, if this was used frequently, it would be wise to consider refactoring the FETCH type to EAGER.

                               
@NamedQueries({@NamedQuery(name="generateReport",
                           query=" SELECT NEW com.ssg.article.ReportDTO(p.name, SUM(pen.amount))
                                
FROM Party p LEFT JOIN p.penalties pen GROUP BY p.name""), @NamedQuery(name="bulkInactive", query="UPDATE PARTY P SET p.status=0 where p.registrationID=?1"), @NamedQuery(name="getItEarly", query="SELECT p FROM Party p JOIN FETCH p.penalties")}) public abstract class Party { ..... }

Conclusion

Overall, JPA made coding for persistence much simpler. We found it feature-rich and quite efficient. Its rich query interface and greatly improved query language made it much easier to cope with complex relational scenarios. Its inheritance support helped us sustain the logical domain model at the persistence level and we could reuse the same entities across layers. With all its benefits, JPA is the clear choice for the future.


Samudra Gupta is an independent Java/J2EE Consultant in United Kingdom. He has more than nine years of experience in building Java/J2EE applications for public sector, retail and national security systems. He is the author of Pro Apache Log4j (Apress, 2005) and actively contributes articles to Javaboutique, Javaworld, and Java Developers Journal. When not programming, he loves to play contract bridge and ten-pin bowling.