Berkeley DB Java Edition
version 3.0.12

com.sleepycat.persist.model
Interface PersistentProxy<T>


public interface PersistentProxy<T>

Implemented by a proxy class to represent the persistent state of a (non-persistent) proxied class. Normally classes that are outside the scope of the developer's control must be proxied since they cannot be annotated, and because it is desirable to insulate the stored format from changes to the instance fields of the proxied class. This is useful for classes in the standard Java libraries, for example.

PersistentProxy objects are not required to be thread-safe. A single thread will create and call the methods of a given PersistentProxy object.

There are three requirements for a proxy class:

  1. It must implement the PersistentProxy interface.
  2. It must specified as a persistent proxy class in the entity model. Using the AnnotationModel, a proxy class is indicated by the Persistent annotation with the Persistent.proxyFor() property.
  3. It must be explicitly registered by calling EntityModel.registerClass(java.lang.Class) before opening the store.

In order to serialize an instance of the proxied class before it is stored, an instance of the proxy class is created. The proxied instance is then passed to the proxy's initializeProxy method. When this method returns, the proxy instance contains the state of the proxied instance. The proxy instance is then serialized and stored in the same way as for any persistent object.

When an instance of the proxy object is deserialized after it is retrieved from storage, its convertProxy() method is called. The instance of the proxied class returned by this method is then returned as a field in the persistent instance.

For example:

  import java.util.Locale;

  @Persistent(proxyFor=Locale.class)
  class LocaleProxy implements PersistentProxy<Locale> {

      String language;
      String country;
      String variant;

      private LocaleProxy() {}

      public void initializeProxy(Locale object) {
          language = object.getLanguage();
          country = object.getCountry();
          variant = object.getVariant();
      }

      public Locale convertProxy() {
          return new Locale(language, country, variant);
      }
  }

The above definition allows the Locale class to be used in any persistent class, for example:

  @Persistent
  class LocalizedText {
      String text;
      Locale locale;
  }

A proxy for proxied class P does not handle instances of subclasses of P. To proxy subclasses of P, a separate proxy class is needed.

Several built in proxy types are used implicitly. An application defined proxy will be used instead of a built-in proxy, if both exist for the same proxied class.

With respect to class evolution, a proxy instance is no different than any other persistent instance. When using a ConversionStore, RawStore or Converter, only the raw data of the proxy instance will be visible. Raw data for the proxied instance never exists.

Generic Types

Special consideration is needed when the proxied class has generic type parameters. The proxy class should have the same number of type parameters as the proxied class. For example, a proxy class for HashMap<K,V> might declare arrays of type K and V as shown below:

  @Persistent(proxyFor=HashSet.class)
  class HashMapProxy<K,V> implements PersistentProxy<HashSet<K,V>> {
      private K[] keys;
      private V[] values;
      ...
  }

The type parameters of the proxy class and proxied class are linked by the Direct Persistence Layer to determine actual types. Knowing actual types is important for efficient class evolution, as described under Generic Types and Class Evolution.

Using the example of the proxy class above, imagine that a field in a persistent class is declared with type HashMap<Integer,Color>. The Direct Persistence Layer can determine that the actual type arguments in the field declaration, Integer and Color, are the types of the keys and values arrays in the proxy class. The type parameters K and V are the links between types in the proxy class and types in the proxied class.

Author:
Mark Hayes

Method Summary
 T convertProxy()
          Returns a new proxied class instance to which the state of this proxy instance has been copied.
 void initializeProxy(T object)
          Copies the state of a given proxied class instance to this proxy instance.
 

Method Detail

initializeProxy

void initializeProxy(T object)
Copies the state of a given proxied class instance to this proxy instance.


convertProxy

T convertProxy()
Returns a new proxied class instance to which the state of this proxy instance has been copied.


Berkeley DB Java Edition
version 3.0.12

Copyright (c) 1996-2006 Sleepycat Software, Inc. - All rights reserved.