SER00-J. Enable serialization compatibility during class evolution

Once an object of a particular class has been serialized, future refactoring of the class's code often becomes problematic. Specifically, existing serialized forms (encoded representations) become part of the object's published API and must be supported for an indefinite period. This can be troublesome from a security perspective; not only does it promote dead code, it also forces the provider to maintain a compatible codebase for the lifetime of their products.

Classes that implement Serializable without overriding its functionality are said to be using the default serialized form. In the event the class changes, byte streams produced by users of old versions of the class become incompatible with the new implementation. Consequently, serializable classes that rely on the default serialized form cannot be evolved without compromising compatibility.

To enable compatible evolution of a serializable class, developers must use a custom serialized form, which is more flexible than a default form. Specifically,

• Use of a custom form severs the dependence of the stream format on the code of the implementing class.
• The code generated for deserializing a custom form can handle compatible deviations from the serialized form, like extra fields.

As a result, developers need neither maintain the earlier version of the code nor explicitly support the original serialized form.

Note that compliance with this rule, while necessary, is not sufficient to guarantee compatible evolution of serializable classes. For a full discussion of compatible evolution of serializable classes, see the Java Object Serialization Specification (version 6), Chapter 5, "Versioning of Serializable Objects" [Sun 2006].

Noncompliant Code Example

This noncompliant code example implements a GameWeapon class with a serializable field called numOfWeapons and uses the default serialized form. Any changes to the internal representation of the class can break the existing serialized form.

class GameWeapon implements Serializable {
  int numOfWeapons = 10;
	    
  public String toString() {
    return String.valueOf(numOfWeapons);
  }
}

Because this class does not provide a serialVersionUID, the Java Virtual Machine (JVM) assigns it one using implementation-defined methods. If the class definition changes, the serialVersionUID is also likely to change. Consequently, the JVM will refuse to associate the serialized form of an object with the class definition when the version IDs are different.

Compliant Solution (serialVersionUID)

In this solution, the class has an explicit serialVersionUID that contains a number unique to this version of the class. The JVM will make a good-faith effort to deserialize any serialized object with the same class name and version ID.

class GameWeapon implements Serializable {
  private static final long serialVersionUID = 24L;

  int numOfWeapons = 10;
	    
  public String toString() {
    return String.valueOf(numOfWeapons);
  }
}

Compliant Solution (serialPersistentFields)

Ideally, Serializable should be implemented only for stable classes. One way to maintain the original serialized form and allow the class to evolve is to use custom serialization with the help of serialPersistentFields. The static and transient qualifiers specify which fields should not be serialized, whereas the serialPersistentFields field specifies which fields should be serialized. It also relieves the class from defining the serializable field within the class implementation, decoupling the current implementation from the overall logic. New fields can easily be added without breaking compatibility across releases.

class WeaponStore implements Serializable {
  int numOfWeapons = 10; // Total number of weapons
}

public class GameWeapon implements Serializable {
  WeaponStore ws = new WeaponStore();
  private static final ObjectStreamField[] serialPersistentFields
      = {new ObjectStreamField("ws", WeaponStore.class)};

  private void readObject(ObjectInputStream ois)
      throws IOException, ClassNotFoundException {
    ObjectInputStream.GetField gf = ois.readFields();
    this.ws = (WeaponStore) gf.get("ws", ws);
  }
	 
  private void writeObject(ObjectOutputStream oos) throws IOException {
    ObjectOutputStream.PutField pf = oos.putFields();
    pf.put("ws", ws);
    oos.writeFields();
  }
	 
  public String toString() {
    return String.valueOf(ws);
  }
}

Risk Assessment

Failure to provide a consistent serialization mechanism across releases can limit the extensibility of classes. If classes are extended, compatibility issues may result.

Automated Detection

Automated detection of classes that use the default serialized form is straightforward.

Related Guidelines

Bibliography