Serialization Changes and Enhancements in J2SE 1.4

Documentation Contents

The following are enhancements to serialization in version 1.4 of the Java 2 Platform. For a summary of enhancements in previous releases, see Enhancements in Previous Releases.

Support for deserialization of unshared objects

Serialization now provides extra support for deserialization of objects which are known to be unshared in the data-serialization stream. The new support is provided by the following API additions in package java.io:

ObjectInputStream.readUnshared()

java.lang.Object)">ObjectOutputStream.writeUnshared(Object obj)

ObjectStreamField(String name, Class type, boolean unshared)

Previously, security-conscious programmers had to clone private internal objects after deserializing them to guard against the possibility that outside parties with access to the serialization stream could append spurious back handles to the sensitive objects, resulting in extra references to them during deserialization. However, this solution slows performance and wastes memory -- two objects must be created and a copy operation invoked in order to ensure a unique reference to a single usable object. The new API additions provide a more efficient solution by allowing receivers to invalidate "unshared" objects as they are read in, making it impossible for third parties to dereference back-handles to those objects occurring later in the stream.

Security permissions now required to override putFields, readFields

Beginning with J2SE 1.4.0, ObjectOutputStream's java.io.OutputStream)">public one-argument constructor requires the "enableSubclassImplementation" SerializablePermission when invoked (either directly or indirectly) by a subclass which overrides ObjectOutputStream.putFields or java.lang.Object)">ObjectOutputStream.writeUnshared.

Also beginning with J2SE 1.4.0, ObjectInputStream's java.io.InputStream)">public one-argument constructor requires the "enableSubclassImplementation" SerializablePermission when invoked (either directly or indirectly) by a subclass which overrides ObjectInputStream.readFields or ObjectInputStream.readUnshared.

These changes will not affect the great majority of applications. However, it will affect any ObjectInputStream/ObjectOutputStream" subclasses which override the putFields or readFields methods without also overriding the rest of the serialization infrastructure.

Support for class-defined readObjectNoData method

In addition to supporting class-defined writeObject() and readObject() methods, serialization now includes support for class-defined readObjectNoData() methods. Each class-defined readObjectNoData() method is required to have the following signature:

private void readObjectNoData() throws ObjectStreamException;

The readObjectNoData() method is analogous to the class-defined readObject() method, except that (if defined) it is called in cases where the class descriptor for a superclass of the object being deserialized (and hence the object data described by that class descriptor) is not present in the serialization stream. More formally:

If object O of class C is being deserialized, and S is a superclass of C in the VM which is deserializing O, then S.readObjectNoData() is invoked by ObjectInputStream during the deserialization of O if and only if the following conditions are true:

1. S implements java.io.Serializable (directly or indirectly).
2. S defines an readObjectNoData() method with the signature listed above.
3. The serialization stream containing O does not include a class descriptor for S among its list of superclass descriptors for C.

Note that readObjectNoData() is never invoked in cases where a class-defined readObject() method could be called, though serializable class implementors can call readObjectNoData() from within readObject() as a means of consolidating initialization code.

See the class description in the API specification of ObjectInputStream for more information.

Bug fix: Deserialization fails for Class object of primitive type

In previous releases, bug 4171142 caused attempts to deserialize Class objects of primitive types to fail with a ClassNotFoundException. The problem was that ObjectInputStream.resolveClass() did not work for ObjectStreamClass descriptors for primitive types. This bug is fixed in J2SE 1.4.0.

Bug fix: ObjectInputStream.resolveProxyClass can fail for non-public interface cases

In previous releases, ObjectInputStream.resolveProxyClass would not always select the proper class loader to define the proxy class in if one or more of the proxy interfaces were non-public. In this release, if ObjectInputStream.resolveProxyClass detects a non-public interface, it attempts to define the implementing proxy class in the same class loader as the interface (barring conflicts, in which case an exception is thrown), which is necessary in order for the proxy to implement the interface.

Bug fix: Invalid serialPersistentFields field name causes NullPointerException

In previous releases, bug 4387368 caused NullPointerExceptions to be thrown when serializing objects which used default serialization but also declared serialPersistentField entries which did not map to actual class fields. Serialization will now throw InvalidClassExceptions in such cases (since it is never necessary to define such "unbacked" serialPersistentFields when using default serialization).

Bug fix: ClassNotFoundException in skipped objects causes serialization to fail

In previous releases, ClassNotFoundExceptions triggered by "skipped" objects--objects associated with fields not present in the classes loaded by the deserializing party--would cause deserialization of the entire object graph to fail, even though the skipped values would not be included in the graph. This release of serialization addresses this problem by ignoring ClassNotFoundExceptions associated with such skipped objects, thus eliminating a class of unnecessary deserialization errors. Other miscellaneous changes have also been made to improve the overall robustness of serialization with regards to ClassNotFoundExceptions encountered during deserialization.