Home > Articles > Programming > Java

Getting Started with JDO

Learn what transparent persistence is all about, and what it means when a Java class is persistence-capable. In this chapter from Core Java Data Objects, you'll see how to create, read, update, and delete persistent objects, and explore how JDO supports the full Java object model: basic types, references, collection classes, and inheritance.
This chapter is from the book

This chapter is from the book

"The expert at anything was once a beginner."

Hayes

Using JDO to build an application that creates, reads, updates, and deletes persistent instances of Java classes is easy and requires only some basic knowledge about how JDO works and how to use it. Armed with this knowledge, you can develop your first JDO application and persist instances of Java classes transparently in a datastore. This chapter is a guide to getting started with JDO, providing an understanding of how JDO works and how to use the basic APIs, and exploring some of the more advanced concepts related to using JDO.

This chapter covers these topics:

  • How JDO is able to transparently persist instances of Java classes.

  • The basic JDO interfaces and how they are related.

  • How to define a Java class that can be used with a JDO implementation.

  • How to connect to a datastore.

  • How to create, read, update, and delete persistent objects.

  • The types of fields, system classes, collection classes, and inheritance supported by JDO.

  • How to handle exceptions within an application.

  • The concept of object identity.

  • The different types of identity that can be used.

  • How concurrency control is enforced between multiple applications.

The examples for this chapter can be downloaded from the Internet at www.corejdo.com and are located in the com.corejdo.examples.chapter3 package. In many cases, the code snippets shown are simplified versions of the actual classes to allow the examples to focus only on the relevant concepts.

3.1 How Does JDO Work?

The goal of the JDO is to allow a Java application to transparently store instances of any user-defined Java class in a datastore and retrieve them again, with as few limitations as possible. This book refers to the instances that JDO stores and retrieves as persistent objects. From the application perspective, these persistent objects appear as regular, in-memory Java objects. However, the fields of these instances are actually stored in some underlying datastore persistently—all without any explicit action on behalf of the application.

JDO has nothing to do with where methods are executed; it does not provide a means of remote method invocation à la RMI and EJB, nor does it store and execute methods in some datastore. JDO simply specifies how the fields of a persistent object should be managed in-memory, being transparently stored to and retrieved from an underlying datastore. With JDO, methods are invoked on a persistent object by an application as per any regular in-memory Java object. Figure 3-1 provides a schematic of how JDO works.

03fig01.gifFigure 3-1. JDO runtime environment.




The JDO implementation and the application run together in the same JVM. The application delegates to the JDO implementation to retrieve the fields of persistent objects as needed. The JDO implementation tracks modifications to the fields and writes these changes back to the datastore at the end of the transaction. The JDO implementation is responsible for mapping the fields of the persistent objects to and from memory and the underlying datastore.

JDO achieves transparency of access by defining a contract to which a class must adhere. Any class that implements this contract can then be used with any JDO implementation. JDO requires that a JDO implementation ensure that any class that adheres to the JDO persistence-capable contract can be used with any JDO implementation, without recompilation.

The ability to run a JDO application with any JDO implementation is akin to using JDBC, a JDBC application can be run "as is" using JDBC drivers from different vendors and even using different relational databases. In fact, JDO is somewhat better than this, because with JDBC an application is still prone to differences in SQL support across different databases. With JDO, SQL is not directly exposed. Although a JDO runtime may itself use JDBC to access a relational database as its datastore, it is the responsibility of the JDO implementation to resolve the differences in SQL support across databases.

Even better, unlike SQL, a JDO application can work "as is" across different types of databases, not just relational: object databases, flat-files, and so on. All that is required is a JDO implementation that supports the datastore.

The JDO specification defines the persistence-capable contract as a Java interface, called PersistenceCapable, and a programming style that the class implementation must follow. A class that adheres to this contract is referred to as being "persistence-capable."

A class is said to be persistence-capable if its instances can be stored in a datastore by a JDO implementation. However, just because a class is persistence-capable doesn't mean that all its instances have to be persistent; it just means the option is there. Whether a particular instance is persistent depends on the application. It's similar to Java serialization. Just because a class implements the Serializable interface doesn't mean that all its instances have to be serialized.

However, the intention of JDO is not to expect the developer to worry about making a class persistence-capable; it's a tedious job better left to tooling.

You can create a persistence-capable class in three main ways:

  • Source code generation: With this method, the source code for a class is generated from scratch. This approach works well if the object model is defined in a modeling tool and is being automatically generated, or the datastore schema already exists and the object model can be generated from it. Tools supplied by the JDO implementation would be used to generate source code adhering to the persistence-capable contract. The drawback of this approach is that it won't work for existing classes and won't appeal to those who like to write their own code.

  • Source code preprocessing: With this method, existing source code is preprocessed and updated. This approach works well if the source code for a class is available. Tools supplied by the JDO implementation would be used to read the original source code and update it to adhere to the persistence-capable contract. The drawback of this approach is that it won't work unless the original source code is available, but it does have the benefit that a developer can write his or her own source code. Typically, the preprocessing is a precompilation step in the build process, and the generated code may be kept to aid in debugging.

  • Byte code enhancement: With this method, the compiled Java byte code for a class is enhanced directly. This approach works well even if the source code is not available. Tools supplied by the JDO implementation would be used to read a class file and insert additional byte code directly to make the class adhere to the persistence-capable contract. This approach has the benefit of being completely transparent to the developer, and the enhancement is simply a post-compilation step in the build process. Although the JDO specification requires that an enhanced class still function correctly when debugged against the original source code, some developers may be distrustful if they can't see the actual code for what has been changed (although they could, of course, always decompile the enhanced class file afterward).

Byte code enhancement is the approach used by the JDO reference implementation available from SUN Microsystems, and the enhancement tool is available for any developer to use. Some JDO implementations may provide their own enhancement tools also. Figure 3-2 provides a schematic of how the byte code enhancement process works.

03fig02.gifFigure 3-2. The byte code enhancement process.




The Java classes are compiled using a Java compiler to generate class files. The byte code enhancement tool reads the class files along with the JDO metadata for the classes (this metadata is explained in Section 3.3.1) and either updates the existing class files or creates new ones. The "enhanced" class files are then loaded by a JVM along with the JDO implementation and the application. The application can then use JDO to store instances of the persistence-capable classes in the datastore.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020