Home > Articles > Programming > Java

This chapter is from the book

This chapter is from the book

3.6 Reading an Object

After an instance has been made persistent and its fields stored in the datastore, it can be retrieved again, either within the same application or by a different application.

There are three primary ways of finding an instance in the datastore with JDO—by navigation, via an Extent, or by a Query.

3.6.1 Reading by navigation

Retrieving an instance by navigation is simple. Extending the previous example, this code snippet taken from ReadByNavigationExample.java shows how the Author instance can be retrieved again in the next transaction simply by using it:


String name = author.getName();
   System.out.println("Author's name is '" + name + "'.");

Underneath the covers, the JDO implementation retrieves the fields for the Author instance from the datastore. The output would be as follows:

Author's name is 'Keiron McCammon'.

Using navigation to retrieve persistent objects from the datastore also applies when one persistent object is referenced from another. Essentially, anytime a persistent object is referenced, the JDO implementation retrieves the instance's fields from the datastore, if it has not already done so within the current transaction.

A common question often arises from the previous example: Why do the fields of the Author instance have to be retrieved from the datastore again after it was initially created?

The answer has to do with transactions. After each transaction ends (either by a commit or rollback), all in-memory persistent objects become hollow. This is because the representation of a persistent object in the datastore could be updated by another application after the transaction ends, making any in-memory values out of sync. For this reason, JDO's default behavior is to clear the fields of each persistent object and retrieve them again if used within the next transaction, thereby ensuring that the in-memory instance remains consistent with the datastore.

JDO has some advanced options that change this default behavior and allow instances to be retained over transaction boundaries or even accessed outside of a transaction altogether. See Chapter 5 for more details.

3.6.2 Reading using an extent

What happens if another application wants to access the new Author instance? Obviously, this application won't have a reference to which it can navigate. In this case, the application can use an Extent to find all Author instances.


An Extent represents a collection of all instances in the datastore of a particular persistence-capable class and can consist of just instances of the class or instances of all subclasses. An application can get an Extent for a class from a PersistenceManager and then use an Iterator to iterate through all instances, or it can use the Extent as input to a query.

By default, it is possible to get an Extent for any persistence-capable class. If an Extent is not required, then as a potential optimization, it is possible to specify explicitly that an extent is not required in the JDO metadata for the class. Depending on the underlying datastore, this may or may not make any difference.

An application can get an Extent using the getExtent() method on PersistenceManager:

public Extent getExtent(Class pc, boolean subclasses)

The subclasses flag determines whether the returned Extent represents instances of the specified class only or includes instances of subclasses as well. If true, then it includes instances of all subclasses as well.

The iterator() method on Extent can be used to get an Iterator that iterates through all the instances in the Extent:

public Iterator iterator()

The following code snippet taken from ReadByExtentExample.java shows how an Extent can be used to iterate through the instances of the Author class:


Extent extent = pm.getExtent(Author.class, true);
   Iterator authors = extent.iterator();
   while (authors.hasNext()) {
   Author author = (Author) authors.next();
   String name = author.getName();
   System.out.println("Author's name is '" + name + "'.");

It is a good practice to use the close() or closeAll() method on Extent when an iterator has been used. The close() method closes an individual iterator, whereas closeAll() closes all iterators retrieved from the Extent:

public void close(Iterator iterator)
public void closeAll()

Depending on the underlying datastore, this may free resources that were allocated when the Iterator was created (a cursor, for example).

3.6.3 Reading using a query

An Extent is useful if an application wants to retrieve all instances of a class in no particular order. However, if an application is looking for a particular instance that has certain field values, an Extent is not very useful. In this case, a query is needed to find an instance based on the values of its fields.

JDO defines a query language called JDOQL (JDO Query Language). JDOQL is essentially a simplified version of the Java syntax for an "if" statement. The PersistenceManager is used to create a Query instance, and once created, a Query can be executed to return a collection of matching persistent objects. There are approximately nine methods on PersistenceManager to create a Query instance; the most straightforward requires only a Java class and a filter:

public Query newQuery(Class cln, String filter)

The class specifies the persistence-capable class that the query is for, and the filter, which is just a string, specifies which instances of the class should be returned.

A Query can also be created using an Extent. A query created using a Java class is equivalent to a Query created using an Extent whose subclasses property is true. This means that a Query created from a Java class may include instances of the specified class and subclass (which is generally the desired behavior).

If an application specifically wants to restrict the query to instances of a given class only, then it can create an Extent whose subclasses property is false and use the Extent to create the Query. When executed, the Query includes only instances of the class itself and ignores subclasses.

The basic syntax for a filter is as follows:

<field> <operator> <constant>

where <field> is the name of the field as declared in the Java class, <operator> is one of the standard Java comparison operators (==, !=, <, >, and so on), and <constant> is the value to be compared against. Much more sophisticated queries than this can be defined; see Chapter 6 for details.

The following code snippet taken from ReadByQueryExample.java shows how to use a query to find a previously created Author by name. For simplicity, it assumes that at least one author with the given name is in the datastore:


Query query =
   pm.newQuery(Author.class, "name == \"Keiron McCammon\"");
   Collection result = (Collection) query.execute();
   Author author = (Author) result.iterator().next();
   String name = author.getName();
   System.out.println("Author's name is '" + name + "'.");

The output would be as follows:

Author's name is 'Keiron McCammon'.

The execute() method on Query is used to execute the query and return the result. A half dozen ways of executing a query exist. The simplest is this:

public Object execute()

The execute() methods all return java.lang.Object, which must be cast to a java.util.Collection type for JDOQL queries. This collection is the set of instances that matched the specified filter. It can be iterated through in the normal manner to retrieve the persistent objects themselves.

The execute() methods on Query return java.lang.Object rather than java.util.Collection directly to allow for implementation-specific extensions. JDO requires that JDOQL be supported; however, an implementation can optionally support alternative query languages (maybe SQL, OQL, or something proprietary). In this case, the result of executing a query may not be a java.util.Collection; instead, it needs to be cast to something specific to the query language being used.

It is a good practice to use the close() method on Query to release the result returned from a call to execute():

public Object close(Object result)

Depending on the underlying datastore, this may free resources that were allocated during the execution of the query (a database cursor, for example).

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.


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.


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.


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.


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


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


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.


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.


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