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

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## 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 be specified as a persistent proxy class in the entity model. When 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 proxied class may not be used as a superclass for a persistent class or entity class. For example, the following is not allowed.

      @Persistent
      class LocalizedText extends Locale { // NOT ALLOWED
          String text;
      }

A proxy for proxied class P does not handle instances of subclasses of P. To proxy a subclass 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 `RawStore` or `Converter`, only the raw data of the proxy instance will be visible. Raw data for the proxied instance never exists.

Currently a proxied object may not contain a reference to itself. For simple proxied objects such as the Locale class shown above, this naturally won't occur. But for proxied objects that are containers -- the built-in Collection and Map classes for example -- this can occur if the container is added as an element of itself. This should be avoided. If an attempt to store such an object is made, an `IllegalArgumentException` will be thrown.

Note that a proxy class may not be a subclass of an entity class.

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.

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