JAXB & Collection Properties

In this post will examine the different options JAXB offers for representing collections in XML.  We will look at the following annotations:

• @XmlElement
• @XmlElementWrapper
• @XmlList
• @XmlList and @XmlAttribute
• @XmlList and @XmlValue

Java Model

For this example we will use the following model.   We will apply different JAXB annotations to observe the effect it has on the XML representation.

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import java.util.*; import javax.xml.bind.annotation.*;   @XmlRootElement @XmlAccessorType(XmlAccessType.FIELD) public class Customer {       private List<String> emailAddresses;           public Customer() {         emailAddresses = new ArrayList<String>();     }       public List<String> getEmailAddresses() {         return emailAddresses;     }       public void setEmailAddresses(List<String> emailAddresses) {         this.emailAddresses = emailAddresses;     }   }

Demo Code

The following code will be used to convert the Customer object to XML.  We will examine the impact of changing the metadata on the XML representation.

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import javax.xml.bind.JAXBContext; import javax.xml.bind.Marshaller;   public class Demo {       public static void main(String[] args) throws Exception {         JAXBContext jc = JAXBContext.newInstance(Customer.class);           Customer customer = new Customer();         customer.getEmailAddresses().add("janed@example.com");         customer.getEmailAddresses().add("jdoe@example.org");           Marshaller marshaller = jc.createMarshaller();         marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);         marshaller.marshal(customer, System.out);     } }

Default

By default each item in the collection will be marshalled to an XML element.

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<customer>     <emailAddresses>janed@example.com</emailAddresses>     <emailAddresses>jdoe@example.org</emailAddresses> </customer>

@XmlElement

We can control the name of the XML element a collection item is marshalled to by using the @XmlElement annotation.

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import java.util.*; import javax.xml.bind.annotation.*;   @XmlRootElement @XmlAccessorType(XmlAccessType.FIELD) public class Customer {       @XmlElement(name="email-address")     private List<String> emailAddresses;   }

The following is the corresponding XML output:

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<customer>     <email-address>janed@example.com</email-address>     <email-address>jdoe@example.org</email-address> </customer>

@XmlElementWrapper

Sometimes we want to add a grouping element to organize our collection data.  This is done using the @XmlElementWrapper annotation.

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import java.util.*; import javax.xml.bind.annotation.*;   @XmlRootElement @XmlAccessorType(XmlAccessType.FIELD) public class Customer {       @XmlElementWrapper(name="email-addresses")     @XmlElement(name="email-address")     private List<String> emailAddresses;   }

The following is the corresponding XML output:

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<customer>     <email-addresses>         <email-address>janed@example.com</email-address>         <email-address>jdoe@example.org</email-address>     </email-addresses> </customer>

@XmlList

We can also represent our collection data as space seperated text.  This is done using the @XmlList annotation.

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import java.util.*; import javax.xml.bind.annotation.*;   @XmlRootElement @XmlAccessorType(XmlAccessType.FIELD) public class Customer {       @XmlList     private List<String> emailAddresses;   }

The following is the corresponding XML output:

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<customer>     <emailAddresses>janed@example.com jdoe@example.org</emailAddresses> </customer>

@XmlList and @XmlAttribute

Since @XmlList allows us to represent a collection in a single piece of text it is also compatible with an XML attribute.

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import java.util.*; import javax.xml.bind.annotation.*;   @XmlRootElement @XmlAccessorType(XmlAccessType.FIELD) public class Customer {       @XmlList     @XmlAttribute     private List<String> emailAddresses;   }

The following is the corresponding XML output:

1 2	<customer     emailAddresses="janed@example.com jdoe@example.org"/>

@XmlList and @XmlValue

Since @XmlList allows us to represent a collection in a single piece of text it is also compatible with a single text node.

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import java.util.*; import javax.xml.bind.annotation.*;   @XmlRootElement @XmlAccessorType(XmlAccessType.FIELD) public class Customer {       @XmlList     @XmlValue     private List<String> emailAddresses;   }

The following is the corresponding XML output:

1	<customer>janed@example.com jdoe@example.org</customer>