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[jsr338-experts] Re: converters

  • From: Gordon Yorke <gordon.yorke@...>
  • To: jsr338-experts@...
  • Subject: [jsr338-experts] Re: converters
  • Date: Mon, 05 Mar 2012 17:41:05 -0400

The Convert annotation may be applied to an entity that extends a mapped
superclass to specify or override the conversion mapping for an inherited
basic or embedded attribute.
Overriding conversion through the Convert annotation adds complexity and duplicates an overriding facility that is already present in the Specification. It would be far clearer to add a "converter" attribute to the AttributeOverride annotation. There are already well defined rules for how AttributeOverrides are applied and these rules are applicable to the converter override as well. If the type needs to be overridden it is likely that an AttributeOverride will already be specified. If the override rules remain in the Convert annotations users may consider the "attribute" path is required for every MappedSuperclass or Embeddable.

So Linda has pointed out that the "column" attribute in AttributeOverride is of type @Column with it's own defaults. As such we are unable to make @Column optional in a clean way, so please ignore my suggestion on using @AttributeOverride for @Convert overrides.
Thanks,
Gordon

On 09/02/2012 9:57 PM, Gordon Yorke wrote:
Some comments inline:
--Gordon

On 2/6/2012 6:30 PM, Linda DeMichiel wrote:
Here's a more formal writeup of the converter proposal, which factors
in results of the recent discussion.

In conjunction with the consensus around extending the original
proposal to "autoapply" converters, I've also extended the syntax
to allow @Convert to be specified more broadly as well as to allow
for disabling autoapply converters.

Mike and I have already gone through the details here several times.
Please review and post to the group if you think we should make any
changes or if there are any corrections.

I have left as an open issue whether we should also add a capability
to add converters dynamically or whether we should defer this pending
further feedback from developers.

-Linda

---------------------------

Converters may be specified to provide conversion between the entity
attribute representation and the database representation for
attributes of basic types.  Converters may be used to convert
attributes defined by entity classes, mapped superclasses, or
embeddable classes.

The conversion of all basic types are supported except for the
following: Id attributes, version attributes, relationship attributes,
and attributes explicitly annotated (or designated via XML) as
Enumerated or Temporal.  Auto-apply converters will not be applied to
such attributes,
I understand the goal of this wording but this will be a barrier to us supporting such conversion in the future as any support would not be backward compatible. It should be suffecient to just have the list of supported and unsupported.
and applications that apply converters to such
attributes through use of the Convert annotation will not be portable.

The persistence provider runtime is responsible for invoking the
corresponding conversion method when loading the entity attribute from
the database and before storing the entity attribute state to the
database.  Instances of attribute values used within JPQL or criteria
queries (such as in comparisons, bulk updates, etc.) must be converted
to their database types by the provider before being sent to the
database for the query execution.  If the result of a JPQL or criteria
query includes one or more specific entity attributes that have been
designated for conversion, then the results must be converted to their
entity attribute representation before being returned.

An attribute converter must implement the javax.persistence.mapping.AttributeConverter interface.

/**
 * A class that implements this interface can be used to convert entity
 * attribute state into database column representation and back again.
 * Note that the X and Y types may be the same Java type.
 *
 * @param X  the type of the entity attribute
 * @param Y  the type of the database column
 */
public interface AttributeConverter<X,Y> {

    /**
     * Converts the value stored in the entity attribute into the data
     * representation to be stored in the database.
     *
     * @param attribute  the entity attribute value to be converted
     * @return  the converted data to be stored in the database column
     */
    public Y convertToDatabaseColumn (X attribute);

    /**
     * Converts the data stored in the database column into the value
     * to be stored in the entity attribute.
     * Note that it is the responsibility of the converter writer to
     * specify the correct dbData type for the corresponding column for
* use by the JDBC driver, i.e., persistence providers are not expected
     * to do such type conversion.
     *
     * @param dbData  the data from the database column to be converted
     * @return  the converted value to be stored in the entity attribute
     */
    public X convertToEntityAttribute (Y dbData);
}


A converter class must be annotated with the Converter annotation or
defined in the object/relational mapping descriptor as a converter.

/**
* Specifies that the annotated class is a converter and defines its scope
 */
@Target({Type})
@Retention(RUNTIME)
public @interface Converter {

   /**
    * If set to true, specifies that the converter will automatically
    * be applied to all mapped attributes of the specified
    * target type pair for the entities in the persistence unit,
"target type pair" is not well defined and implies the database column type is used to evaluate applicability of converters but it is only the attribute which is (and should be) considered. "target type" would be better.
* unless overridden by means of the Convert annotation (or XML equivalent).
    * In determining whether a converter is applicable to an attribute,
* the provider must treat primitive types and wrapper types as equivalent.
    *
    * Note that Id attributes, version attributes, relationship
    * attributes, and attributes explicitly annotated as Enumerated
    * or Temporal (or designated as such via XML) will not be converted.
    *
* If autoApply is false, only those attributes of the target type pair
* for which the Convert annotation (or corresponding XML element) has
    * been specified will be converted.
    *
    * The behavior is undefined if there are two converters defined for
    * the same target types and the Convert annotation is not used
    * to explicitly specify use of a converter.
    *
* Note that if autoApply is true, the Convert annotation may be used to
* override or disable auto-apply conversion on a per-attribute basis.
    */
   boolean autoApply() default false;
}


Type conversion may be specified at the level of individual attributes
by means of the Convert annotation.  The Convert annotation may also be
used to override or disable an auto-apply conversion.

The Convert annotation may be applied directly to an attribute of an
entity, mapped superclass, or embeddable class to specify conversion of
the attribute or to override the use of a converter that has been
specified as autoApply=true. When persistent properties are used, the Convert
annotation is applied to the getter method.

The Convert annotation may be applied to an entity that extends a mapped
superclass to specify or override the conversion mapping for an inherited
basic or embedded attribute.
Overriding conversion through the Convert annotation adds complexity and duplicates an overriding facility that is already present in the Specification. It would be far clearer to add a "converter" attribute to the AttributeOverride annotation. There are already well defined rules for how AttributeOverrides are applied and these rules are applicable to the converter override as well. If the type needs to be overridden it is likely that an AttributeOverride will already be specified. If the override rules remain in the Convert annotations users may consider the "attribute" path is required for every MappedSuperclass or Embeddable.


@Target({METHOD, FIELD, TYPE})
@Retention(RUNTIME)
public @interface Convert {

  /**
   * Specifies the converter to be applied.  A value for this
   * element must be specified if multiple converters would
   * otherwise apply.
   */
  Class converter() default void.class;

  /**
   * The attributeName must be specified unless the Convert annotation
* is on an attribute of basic type or on an element collection of basic
   * type.  In these cases, attributeName must not be specified.
   */
  String attributeName() default "";
If we use the AttributeOverride this attribute could be renamed "key". This would eliminate the empty string and the requirement not to specify the attribute name.

  /**
   * Used to disable an auto-apply or inherited converter.
   * If disableConversion is true, the converter element should
   * not be specified.
   */
  boolean disableConversion() default false;
}

/**
 * Used to group Convert annotations
 */
@Target({METHOD, FIELD, TYPE})
@Retention(RUNTIME)
public @interface Converts {
  Convert[] value();
}
This annotation should no longer be needed if we place convert in AttributeOverrides



The Convert annotation is used to specify the conversion of a Basic
(whether explicit or default) field or property.  The Convert annotation
should not be used to specify conversion of the following:
Id attributes, version attributes, relationship attributes, and attributes
explicitly annotated (or designated via XML) as Enumerated or Temporal.
Applications that specify such conversions will not be portable.


The Convert annotation may be applied on a basic attribute or
an element collection of basic type (in which case the converter
is applied to the elements of the collection).  In these cases, the
attributeName element must not be specified.

Examples:

@Converter
public class BooleanToIntegerConverter implements AttributeConverter<Boolean, Integer> { ... }

@Converter(autoApply=true)
public class EmployeeDateConverter implements AttributeConverter<com.acme.EmployeeDate, java.sql.Date> { ... }

@Entity
public class Employee {
   @Id long id;

   @Convert(BooleanToIntegerConverter.class)
   boolean fullTime;
   ...
    // EmployeeDateConverter is applied automatically
   EmployeeDate startDate;
}


// Apply a converter to an element collection of basic type
@ElementCollection
@Convert(NameConverter.class) // applies to each element in the collection
List<String>  names;


// Apply a converter to an element collection that is a map of basic values
// The converter is applied to the map *value*
@ElementCollection
@Convert(EmployeeNameConverter.class)
Map<String, String>  responsibilities;

When the Convert annotation is applied to a map to specify conversion
of a map key of basic type, "key" must be used to specify that it
is the map key that is to be converted.

// Apply a converter to a Map key of basic type (relationship)
@OneToMany
@Convert(converter=ResponsibilityCodeConverter.class, attributeName="key")
Alternatively : @Convert(converter=ResponsibilityCodeConverter.class, key=true)
Map<String, Employee>  responsibilities;

// Apply a converter to a Map key of basic type (element collection)
@ElementCollection
@Convert(converter=ResponsibilityCodeConverter.class, attributeName="key")
Map<String, String>  responsibilities;


// Disable conversion in the presence of an autoApply converter
@Convert(disableConversion=true)
String myString;


The Convert annotation may be applied on an embedded attribute or on a
map collection whose key or value is of embeddable type (in which case
the converter is applied to the specified attribute of the embeddables
contained in the collection).  In these cases the attributeName
element must be specified.

To override conversion mappings at multiple levels of embedding, a dot
(".")  notation form must be used in the attributeName element to
indicate an attribute within an embedded attribute.  The value of each
identifier used with the dot notation is the name of the respective
embedded field or property.

When the Convert annotation is applied to a map containing
embeddables, the attributeName element must be specified, and "key."
or "value." must be used to prefix the name of the attribute that is
to be converted in order to specify it as part of the map key or map
value.

// Apply a converter to an embeddable attribute
@Embedded
@Convert(converter=CountryConverter.class, attributeName="country")
Or: @AttributeOverride(name="country", convert=@Convert(converter=CountryConverter.class))
This would require changing "column" to be no longer required but I see not issue with that.
Address address;


// Apply a converter to a nested embeddable attribute:
@Embedded
@Convert(converter=CityConverter.class, attributeName="region.city")
Address address;

@Entity public class PropertyRecord {
  ...
  // Apply a converter to a nested attribute of an embeddable that is
  // a map key of an element collection
  @Convert(name="key.region.city", converter=CityConverter.class)
  @ElementCollection
  Map<Address, PropertyInfo> parcels;
}

@OneToMany
// Apply to an embeddable that is a map key for a relationship
@Convert(attributeName="key.type", converter=ResponsibilityTypeConverter.class)
Map<Responsibility, Employee>  responsibilities;


The Convert annotation may be applied to an entity class that extends
a mapped superclass to specify or override a conversion mapping
for an inherited basic or embedded attribute.

// Override conversion mappings for attributes inherited from a mapped superclass
@Entity
@Converts({
  @Convert(attributeName="startDate", converter=DateConverter.class),
  @Convert(attributeName="endDate", converter=DateConverter.class)})
public class FullTimeEmployee extends GenericEmployee { ... }



Open Issue:

Do we need an EntityManagerFactory method to dynamically add converters?
This is where a customization event during EMF initialization would be very helpful. Adding converters dynamically at runtime would have unpredictable behaviour.








[jsr338-experts] Re: converters

Gordon Yorke 03/05/2012
 
 
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