com.sleepycat.persist.evolve.Conversion Maven / Gradle / Ivy
/*-
* Copyright (C) 2002, 2018, Oracle and/or its affiliates. All rights reserved.
*
* This file was distributed by Oracle as part of a version of Oracle Berkeley
* DB Java Edition made available at:
*
* http://www.oracle.com/technetwork/database/database-technologies/berkeleydb/downloads/index.html
*
* Please see the LICENSE file included in the top-level directory of the
* appropriate version of Oracle Berkeley DB Java Edition for a copy of the
* license and additional information.
*/
package com.sleepycat.persist.evolve;
import java.io.Serializable;
import com.sleepycat.persist.model.EntityModel;
import com.sleepycat.persist.raw.RawObject;
import com.sleepycat.persist.raw.RawType;
/**
* Converts an old version of an object value to conform to the current class
* or field definition.
*
* The {@code Conversion} interface is implemented by the user. A
* {@code Conversion} instance is passed to the {@link Converter#Converter}
* constructor.
*
* The {@code Conversion} interface extends {@link Serializable} and the
* {@code Conversion} instance is serialized for storage using standard Java
* serialization. Normally, the {@code Conversion} class should only have
* transient fields that are initialized in the {@link #initialize} 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:
*
* - 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.
* - 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 {@code Conversion} class must implement the standard equals method.
* See {@link #equals} for more information.
*
* Conversions of simple types are generally simple. For example, a {@code
* String} field that contains only integer values can be easily converted to
* an {@code int} field:
*
* // The old class. Version 0 is implied.
* //
* {@literal @Persistent}
* class Address {
* String zipCode;
* ...
* }
*
* // The new class. A new version number must be assigned.
* //
* {@literal @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);
* }
*
* {@code @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 {@link Mutations 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.
* //
* {@literal @Entity}
* class Person {
* String address;
* ...
* }
*
* // The new class. A new version number must be assigned.
* //
* {@literal @Entity(version=1)}
* class Person {
* Address address;
* ...
* }
*
* // The new address class.
* //
* {@literal @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;
* {@literal Map addressValues = new HashMap();}
* 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);
* }
*
* {@code @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 {@link Mutations here}.
*
* Note that when a conversion returns a {@link RawObject}, it must return
* it with a {@link RawType} that is current as defined by the current class
* definitions. The proper types can be obtained from the {@link EntityModel}
* in the conversion's {@link #initialize 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.
* //
* {@literal @Entity}
* class Person {
* String street;
* String city;
* String state;
* int zipCode;
* ...
* }
*
* // The new class. A new version number must be assigned.
* //
* {@literal @Entity(version=1)}
* class Person {
* Address address;
* ...
* }
*
* // The new address class.
* //
* {@literal @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;
* {@literal Map personValues = person.getValues();}
* {@literal Map addressValues = new HashMap();}
* 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());
* }
*
* {@code @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 {@link Mutations 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.
* //
* {@literal @Persistent}
* class A extends B {
* String name;
* ...
* }
* {@literal @Persistent}
* abstract class B {
* ...
* }
*
* // The new classes. A new version number must be assigned.
* //
* {@literal @Persistent(version=1)}
* class A extends B {
* ...
* }
* {@literal @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();
* {@literal Map aValues = oldA.getValues();}
* {@literal Map 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;
* }
*
* {@code @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 {@link Mutations 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.
* //
* {@literal @Persistent}
* class Pet {
* boolean isCatNotDog;
* ...
* }
*
* // The new classes. A new version number must be assigned to the Pet class.
* //
* {@literal @Persistent(version=1)}
* class Pet {
* ...
* }
* {@literal @Persistent}
* class Cat extends Pet {
* ...
* }
* {@literal @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;
* {@literal Map 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);
* }
*
* {@code @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 {@link Mutations 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 com.sleepycat.persist.evolve Class Evolution
* @author Mark Hayes
*/
public interface Conversion extends Serializable {
/**
* Initializes the conversion, allowing it to obtain raw type information
* from the entity model.
*
* @param model the EntityModel.
*/
void initialize(EntityModel model);
/**
* Converts an old version of an object value to conform to the current
* class or field definition.
*
* If a {@link RuntimeException} is thrown by this method, it will be
* thrown to the original caller. Similarly, a {@link
* 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 {@link RawObject}. For primitive types, the primitive wrapper
* class is used.
*
* @param fromValue the object value being converted. The type of this
* value is defined by the old class version that is being converted.
*
* @return 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.
*/
Object convert(Object fromValue);
/**
* The standard {@code 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 {@code equals} method.
*
* If the {@code equals} method is not explicitly implemented by the
* conversion class or a superclass other than {@code Object}, {@code
* IllegalArgumentException} will be thrown when the store is opened.
*
* Normally whenever {@code equals} is implemented the {@code 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
* {@code hashCode} to be implemented.
*/
boolean equals(Object other);
}