Effective Java 78 Consider serialization proxies instead of serialized instances

Serialization proxy pattern

1. private static nested class (has a single constructor whose parameter type is the enclosing class, the constructor just copies all the data of its argument) of the serializable class that concisely represents the logical state of an instance of the enclosing class.  

   // Serialization proxy for Period class

   private static class SerializationProxy implements Serializable {

   private final Date start;

   private final Date end;

   SerializationProxy(Period p) {

   this.start = p.start;

   this.end = p.end;

   }

   private static final long serialVersionUID = 234098243823485285L; // Any number will do (Item 75)

   }

2. Both the enclosing class and its serialization proxy class must be declared to implement serializable.
3. Add writeRepalce method in the enclosing class.

   // writeReplace method for the serialization proxy pattern

   private Object writeReplace() {

   return new SerializationProxy(this);

   }

4. To guarantee no generation of a serialized instance of the enclosing class fabrication readObject method should be added.

// readObject method for the serialization proxy pattern

private void readObject(ObjectInputStream stream)

throws InvalidObjectException {

throw new InvalidObjectException("Proxy required");

}

1. Finally, provide a readResolve method on the SerializationProxy class that returns a logically equivalent instance of the enclosing class.

   // readResolve method for Period.SerializationProxy

   private Object readResolve() {

   return new Period(start, end); // Uses public constructor

   }

Advantage of serialization proxy

The serialization proxy pattern allows the deserialized instance to have a different class from the originally serialized instance.

// EnumSet's serialization proxy

private static class SerializationProxy <E extends Enum<E>>

implements Serializable {

// The element type of this enum set.

private final Class<E> elementType;

// The elements contained in this enum set.

private final Enum[] elements;

SerializationProxy(EnumSet<E> set) {

elementType = set.elementType;

elements = set.toArray(EMPTY_ENUM_ARRAY); // (Item 43)

}

private Object readResolve() {

EnumSet<E> result = EnumSet.noneOf(elementType);

for (Enum e : elements)

result.add((E)e);

return result;

}

private static final long serialVersionUID = 362491234563181265L;

}

Limitations of serialization proxy

1. It is not compatible with classes that are extendable by their clients (Item 17).
2. It is not compatible with some classes whose object graphs contain circularities.
3. Performance of it is worse than defensive copying.

Summary

Consider the serialization proxy pattern whenever you find yourself having to write a readObject or writeObject method on a class that is not extendable by its clients. This pattern is perhaps the easiest way to robustly serialize objects with nontrivial invariants.