Burkhard Burkhard - 1 month ago 9
Java Question

Java Serialization with non serializable parts

I have:

class MyClass extends MyClass2 implements Serializable {
//...
}


In MyClass2 is a property that is not serializable. How can I serialize (and de-serialize) this object?

Correction: MyClass2 is, of course, not an interface but a class.

Answer

As someone else noted, chapter 11 of Josh Bloch's Effective Java is an indispensible resource on Java Serialization.

A couple points from that chapter pertinent to your question:

  • assuming you want to serialize the state of the non-serializable field in MyClass2, that field must be accessible to MyClass, either directly or through getters and setters. MyClass will have to implement custom serialization by providing readObject and writeObject methods.
  • the non-serializable field's Class must have an API to allow getting it's state (for writing to the object stream) and then instantiating a new instance with that state (when later reading from the object stream.)
  • per Item 74 of Effective Java, MyClass2 must have a no-arg constructor accessible to MyClass, otherwise it is impossible for MyClass to extend MyClass2 and implement Serializable.

I've written a quick example below illustrating this.


class MyClass extends MyClass2 implements Serializable{

  public MyClass(int quantity) {
    setNonSerializableProperty(new NonSerializableClass(quantity));
  }

  private void writeObject(java.io.ObjectOutputStream out)
  throws IOException{
    // note, here we don't need out.defaultWriteObject(); because
    // MyClass has no other state to serialize
    out.writeInt(super.getNonSerializableProperty().getQuantity());
  }

  private void readObject(java.io.ObjectInputStream in)
  throws IOException {
    // note, here we don't need in.defaultReadObject();
    // because MyClass has no other state to deserialize
    super.setNonSerializableProperty(new NonSerializableClass(in.readInt()));
  }
}

/* this class must have no-arg constructor accessible to MyClass */
class MyClass2 {

  /* this property must be gettable/settable by MyClass.  It cannot be final, therefore. */
  private NonSerializableClass nonSerializableProperty;

  public void setNonSerializableProperty(NonSerializableClass nonSerializableProperty) {
    this.nonSerializableProperty = nonSerializableProperty;
  }

  public NonSerializableClass getNonSerializableProperty() {
    return nonSerializableProperty;
  }
}

class NonSerializableClass{

  private final int quantity;

  public NonSerializableClass(int quantity){
    this.quantity = quantity;
  }

  public int getQuantity() {
    return quantity;
  }
}
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