Conversion (Oracle - Berkeley DB Java API)
Date: 2014-01-24

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## com.sleepycat.persist.evolve
Interface Conversion

All Superinterfaces:
  Serializable

    public interface Conversion

extends Serializable

Converts an old version of an object value to conform to the current class or field definition.

The `Conversion` interface is implemented by the user. A `Conversion` instance is passed to the `Converter.Converter(java.lang.String, int, java.lang.String, com.sleepycat.persist.evolve.Conversion)` constructor.

The `Conversion` interface extends `Serializable` and the `Conversion` instance is serialized for storage using standard Java serialization. Normally, the `Conversion` class should only have transient fields that are initialized in the `initialize(com.sleepycat.persist.model.EntityModel)` method. While non-transient fields are allowed, care must be taken to only include fields that are serializable and will not pull in large amounts of data.

When a class conversion is specified, two special considerations apply:

1. A class conversion is only applied when to instances of that class. The conversion will not be applied when the class when it appears as a superclass of the instance's class. In this case, a conversion for the instance's class must also be specified.
2. Although field renaming (as well as all other changes) is handled by the conversion method, a field Renamer is still needed when a secondary key field is renamed and field Deleter is still needed when a secondary key field is deleted. This is necessary for evolution of the metadata; specifically, if the key name changes the database must be renamed and if the key field is deleted the secondary database must be deleted.

The `Conversion` class must implement the standard equals method. See `equals(java.lang.Object)` for more information.

Conversions of simple types are generally simple. For example, a `String` field that contains only integer values can be easily converted to an `int` field:

      // The old class.  Version 0 is implied.
      //
      @Persistent
      class Address {
          String zipCode;
          ...
      }

      // The new class.  A new version number must be assigned.
      //
      @Persistent(version=1)
      class Address {
          int zipCode;
          ...
      }

      // The conversion class.
      //
      class MyConversion1 implements Conversion {

          public void initialize(EntityModel model) {
              // No initialization needed.
          }

          public Object convert(Object fromValue) {
              return Integer.valueOf((String) fromValue);
          }

          @Override
          public boolean equals(Object o) {
              return o instanceof MyConversion1;
          }
      }

      // Create a field converter mutation.
      //
      Converter converter = new Converter(Address.class.getName(), 0,
                                          "zipCode", new MyConversion1());

      // Configure the converter as described here.

A conversion may perform arbitrary transformations on an object. For example, a conversion may transform a single String address field into an Address object containing four fields for street, city, state and zip code.

      // The old class.  Version 0 is implied.
      //
      @Entity
      class Person {
          String address;
          ...
      }

      // The new class.  A new version number must be assigned.
      //
      @Entity(version=1)
      class Person {
          Address address;
          ...
      }

      // The new address class.
      //
      @Persistent
      class Address {
          String street;
          String city;
          String state;
          int zipCode;
          ...
      }

      class MyConversion2 implements Conversion {
          private transient RawType addressType;

          public void initialize(EntityModel model) {
              addressType = model.getRawType(Address.class.getName());
          }

          public Object convert(Object fromValue) {

              // Parse the old address and populate the new address fields
              //
              String oldAddress = (String) fromValue;
              Map<String, Object> addressValues = new HashMap<String, Object>();
              addressValues.put("street", parseStreet(oldAddress));
              addressValues.put("city", parseCity(oldAddress));
              addressValues.put("state", parseState(oldAddress));
              addressValues.put("zipCode", parseZipCode(oldAddress));

              // Return new raw Address object
              //
              return new RawObject(addressType, addressValues, null);
          }

          @Override
          public boolean equals(Object o) {
              return o instanceof MyConversion2;
          }

          private String parseStreet(String oldAddress) { ... }
          private String parseCity(String oldAddress) { ... }
          private String parseState(String oldAddress) { ... }
          private Integer parseZipCode(String oldAddress) { ... }
      }

      // Create a field converter mutation.
      //
      Converter converter = new Converter(Person.class.getName(), 0,
                                          "address", new MyConversion2());

      // Configure the converter as described here.

Note that when a conversion returns a `RawObject`, it must return it with a `RawType` that is current as defined by the current class definitions. The proper types can be obtained from the `EntityModel` in the conversion's `initialize` method.

A variation on the example above is where several fields in a class (street, city, state and zipCode) are converted to a single field (address). In this case a class converter rather than a field converter is used.

      // The old class.  Version 0 is implied.
      //
      @Entity
      class Person {
          String street;
          String city;
          String state;
          int zipCode;
          ...
      }

      // The new class.  A new version number must be assigned.
      //
      @Entity(version=1)
      class Person {
          Address address;
          ...
      }

      // The new address class.
      //
      @Persistent
      class Address {
          String street;
          String city;
          String state;
          int zipCode;
          ...
      }

      class MyConversion3 implements Conversion {
          private transient RawType newPersonType;
          private transient RawType addressType;

          public void initialize(EntityModel model) {
              newPersonType = model.getRawType(Person.class.getName());
              addressType = model.getRawType(Address.class.getName());
          }

          public Object convert(Object fromValue) {

              // Get field value maps for old and new objects.
              //
              RawObject person = (RawObject) fromValue;
              Map<String, Object> personValues = person.getValues();
              Map<String, Object> addressValues = new HashMap<String, Object>();
              RawObject address = new RawObject(addressType, addressValues, null);

              // Remove the old address fields and insert the new one.
              //
              addressValues.put("street", personValues.remove("street"));
              addressValues.put("city", personValues.remove("city"));
              addressValues.put("state", personValues.remove("state"));
              addressValues.put("zipCode", personValues.remove("zipCode"));
              personValues.put("address", address);

              return new RawObject(newPersonType, personValues, person.getSuper());
          }

          @Override
          public boolean equals(Object o) {
              return o instanceof MyConversion3;
          }
      }

      // Create a class converter mutation.
      //
      Converter converter = new Converter(Person.class.getName(), 0,
                                          new MyConversion3());

      // Configure the converter as described here.

A conversion can also handle changes to class hierarchies. For example, if a "name" field originally declared in class A is moved to its superclass B, a conversion can move the field value accordingly:

      // The old classes.  Version 0 is implied.
      //
      @Persistent
      class A extends B {
          String name;
          ...
      }
      @Persistent
      abstract class B {
          ...
      }

      // The new classes.  A new version number must be assigned.
      //
      @Persistent(version=1)
      class A extends B {
          ...
      }
      @Persistent(version=1)
      abstract class B {
          String name;
          ...
      }

      class MyConversion4 implements Conversion {
          private transient RawType newAType;
          private transient RawType newBType;

          public void initialize(EntityModel model) {
              newAType = model.getRawType(A.class.getName());
              newBType = model.getRawType(B.class.getName());
          }

          public Object convert(Object fromValue) {
              RawObject oldA = (RawObject) fromValue;
              RawObject oldB = oldA.getSuper();
              Map<String, Object> aValues = oldA.getValues();
              Map<String, Object> bValues = oldB.getValues();
              bValues.put("name", aValues.remove("name"));
              RawObject newB = new RawObject(newBType, bValues, oldB.getSuper());
              RawObject newA = new RawObject(newAType, aValues, newB);
              return newA;
          }

          @Override
          public boolean equals(Object o) {
              return o instanceof MyConversion4;
          }
      }

      // Create a class converter mutation.
      //
      Converter converter = new Converter(A.class.getName(), 0,
                                          new MyConversion4());

      // Configure the converter as described here.

A conversion may return an instance of a different class entirely, as long as it conforms to current class definitions and is the type expected in the given context (a subtype of the old type, or a type compatible with the new field type). For example, a field that is used to discriminate between two types of objects could be removed and replaced by two new subclasses:

      // The old class.  Version 0 is implied.
      //
      @Persistent
      class Pet {
          boolean isCatNotDog;
          ...
      }

      // The new classes.  A new version number must be assigned to the Pet class.
      //
      @Persistent(version=1)
      class Pet {
          ...
      }
      @Persistent
      class Cat extends Pet {
          ...
      }
      @Persistent
      class Dog extends Pet {
          ...
      }

      class MyConversion5 implements Conversion {
          private transient RawType newPetType;
          private transient RawType dogType;
          private transient RawType catType;

          public void initialize(EntityModel model) {
              newPetType = model.getRawType(Pet.class.getName());
              dogType = model.getRawType(Dog.class.getName());
              catType = model.getRawType(Cat.class.getName());
          }

          public Object convert(Object fromValue) {
              RawObject pet = (RawObject) fromValue;
              Map<String, Object> petValues = pet.getValues();
              Boolean isCat = (Boolean) petValues.remove("isCatNotDog");
              RawObject newPet = new RawObject(newPetType, petValues,
                                               pet.getSuper());
              RawType newSubType = isCat ? catType : dogType;
              return new RawObject(newSubType, Collections.emptyMap(), newPet);
          }

          @Override
          public boolean equals(Object o) {
              return o instanceof MyConversion5;
          }
      }

      // Create a class converter mutation.
      //
      Converter converter = new Converter(Pet.class.getName(), 0,
                                          new MyConversion5());

      // Configure the converter as described here.

The primary limitation of a conversion is that it may access at most a single entity instance at one time. Conversions involving multiple entities at once may be made by performing a store conversion.

See Also:
  `Class Evolution`

Method Summary
`Object` | `convert(Object fromValue)` Converts an old version of an object value to conform to the current class or field definition.
`boolean` | `equals(Object other)` The standard `equals` method that must be implemented by conversion class.
`void` | `initialize(EntityModel model)` Initializes the conversion, allowing it to obtain raw type information from the entity model.

Method Detail

### initialize

    void initialize(EntityModel model)

  Initializes the conversion, allowing it to obtain raw type information from the entity model.

### convert

    Object convert(Object fromValue)

  Converts an old version of an object value to conform to the current class or field definition.

  If a `RuntimeException` is thrown by this method, it will be thrown to the original caller. Similarly, a `IllegalArgumentException` will be thrown to the original caller if the object returned by this method does not conform to current class definitions.

  The class of the input and output object may be one of the simple types or `RawObject`. For primitive types, the primitive wrapper class is used.

  Parameters:
    `fromValue` - the object value being converted. The type of this value is defined by the old class version that is being converted.
  Returns:
    the converted object. The type of this value must conform to a current class definition. If this is a class conversion, it must be the current version of the class. If this is a field conversion, it must be of a type compatible with the current declared type of the field.

### equals

    boolean equals(Object other)

  The standard `equals` method that must be implemented by conversion class.

  When mutations are specified when opening a store, the specified and previously stored mutations are compared for equality. If they are equal, there is no need to replace the existing mutations in the stored catalog. To accurately determine equality, the conversion class must implement the `equals` method.

  If the `equals` method is not explicitly implemented by the conversion class or a superclass other than `Object`, `IllegalArgumentException` will be thrown when the store is opened.

  Normally whenever `equals` is implemented the `hashCode` method should also be implemented to support hash sets and maps. However, hash sets and maps containing `Conversion` objects are not used by the DPL and therefore the DPL does not require `hashCode` to be implemented.

  Overrides:
    `equals` in class `Object`

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