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## javax.xml.bind.annotation
Annotation Type XmlIDREF

    @Retention(value=RUNTIME)
    @Target(value={FIELD,METHOD})
    public @interface XmlIDREF

Maps a JavaBean property to XML IDREF.

To preserve referential integrity of an object graph across XML serialization followed by a XML deserialization, requires an object reference to be marshalled by reference or containment appropriately. Annotations `@XmlID` and `@XmlIDREF` together allow a customized mapping of a JavaBean property's type by containment or reference.

Usage

The `@XmlIDREF` annotation can be used with the following program elements:

- a JavaBean property
- non static, non transient field

See "Package Specification" in javax.xml.bind.package javadoc for additional common information.

The usage is subject to the following constraints:

- If the type of the field or property is a collection type, then the collection item type must contain a property or field annotated with `@XmlID`.
- If the field or property is single valued, then the type of the property or field must contain a property or field annotated with `@XmlID`.

  Note: If the collection item type or the type of the property (for non collection type) is java.lang.Object, then the instance must contain a property/field annotated with `@XmlID` attribute.

- This annotation can be used with the following annotations: `XmlElement`, `XmlAttribute`, `XmlList`, and `XmlElements`.

Example: Map a JavaBean property to `xs:IDREF` (i.e. by reference rather than by containment)

       //EXAMPLE: Code fragment
       public class Shipping {
           @XmlIDREF public Customer getCustomer();
           public void setCustomer(Customer customer);
           ....
        }

       <!-- Example: XML Schema fragment -->
       <xs:complexType name="Shipping">
         <xs:complexContent>
           <xs:sequence>
             <xs:element name="customer" type="xs:IDREF"/>
             ....
           </xs:sequence>
         </xs:complexContent>
       </xs:complexType>

Example 2: The following is a complete example of containment versus reference.

        // By default, Customer maps to complex type xs:Customer
        public class Customer {

            // map JavaBean property type to xs:ID
            @XmlID public String getCustomerID();
            public void setCustomerID(String id);

            // .... other properties not shown
        }

       // By default, Invoice maps to a complex type xs:Invoice
       public class Invoice {

           // map by reference
           @XmlIDREF public Customer getCustomer();
           public void setCustomer(Customer customer);

          // .... other properties not shown here
       }

       // By default, Shipping maps to complex type xs:Shipping
       public class Shipping {

           // map by reference
           @XmlIDREF public Customer getCustomer();
           public void setCustomer(Customer customer);
       }

       // at least one class must reference Customer by containment;
       // Customer instances won't be marshalled.
       @XmlElement(name="CustomerData")
       public class CustomerData {
           // map reference to Customer by containment by default.
           public Customer getCustomer();

           // maps reference to Shipping by containment by default.
           public Shipping getShipping();

           // maps reference to Invoice by containment by default.
           public Invoice getInvoice();
       }

       <!-- XML Schema mapping for above code frament -->

       <xs:complexType name="Invoice">
         <xs:complexContent>
           <xs:sequence>
             <xs:element name="customer" type="xs:IDREF"/>
             ....
           </xs:sequence>
         </xs:complexContent>
       </xs:complexType>

       <xs:complexType name="Shipping">
         <xs:complexContent>
           <xs:sequence>
             <xs:element name="customer" type="xs:IDREF"/>
             ....
           </xs:sequence>
         </xs:complexContent>
       </xs:complexType>

       <xs:complexType name="Customer">
         <xs:complexContent>
           <xs:sequence>
             ....
           </xs:sequence>
           <xs:attribute name="CustomerID" type="xs:ID"/>
         </xs:complexContent>
       </xs:complexType>

       <xs:complexType name="CustomerData">
         <xs:complexContent>
           <xs:sequence>
             <xs:element name="customer" type="xs:Customer"/>
             <xs:element name="shipping" type="xs:Shipping"/>
             <xs:element name="invoice"  type="xs:Invoice"/>
           </xs:sequence>
         </xs:complexContent>
       </xs:complexType>

       <xs:element name"customerData" type="xs:CustomerData"/>

       <!-- Instance document conforming to the above XML Schema -->
        <customerData>
           <customer customerID="Alice">
               ....
           </customer>

           <shipping customer="Alice">
               ....
           </shipping>

           <invoice customer="Alice">
               ....
           </invoice>
       </customerData>

Example 3: Mapping List to repeating element of type IDREF

         // Code fragment
         public class Shipping {
             @XmlIDREF
             @XmlElement(name="Alice")
                 public List customers;
         }

         <!-- XML schema fragment -->
         <xs:complexType name="Shipping">
           <xs:sequence>
             <xs:choice minOccurs="0" maxOccurs="unbounded">
               <xs:element name="Alice" type="xs:IDREF"/>
             </xs:choice>
           </xs:sequence>
         </xs:complexType>

Example 4: Mapping a List to a list of elements of type IDREF.

         //Code fragment
         public class Shipping {
             @XmlIDREF
             @XmlElements(
                 @XmlElement(name="Alice", type="Customer.class")
                  @XmlElement(name="John", type="InternationalCustomer.class")
             public List customers;
         }

         <!-- XML Schema fragment -->
         <xs:complexType name="Shipping">
           <xs:sequence>
             <xs:choice minOccurs="0" maxOccurs="unbounded">
               <xs:element name="Alice" type="xs:IDREF"/>
               <xs:element name="John" type="xs:IDREF"/>
             </xs:choice>
           </xs:sequence>
         </xs:complexType>

Since:
  JAXB2.0
See Also:
  `XmlID`

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