An ObjectOutputStream writes primitive data types and graphs of
Java objects to an OutputStream. The objects can be read
(reconstituted) using an ObjectInputStream.
Persistent storage of objects can be accomplished by using a file for
If the stream is a network socket stream, the objects can be reconsituted
on another host or in another process.
Only objects that support the java.io.Serializable interface can be
written to streams.
The class of each serializable object is encoded including the class
name and signature of the class, the values of the
object's fields and arrays, and the closure of any other objects
referenced from the initial objects.
The method writeObject is used to write an object
to the stream. Any object, including Strings and arrays, is
written with writeObject. Multiple objects or primitives can be
written to the stream. The objects must be read back from the
corresponding ObjectInputstream with the same types and in the same
order as they were written.
Primitive data types can also be written to the stream using the
appropriate methods from DataOutput. Strings can also be written
using the writeUTF method.
The default serialization mechanism for an object writes the class
of the object, the class signature, and the values of all
non-transient and non-static fields. References to other objects
(except in transient or static fields) cause those objects to be
written also. Multiple references to a single object are encoded
using a reference sharing mechanism so that graphs of objects can
be restored to the same shape as when the original was written.
For example to write an object that can be read by the example in ObjectInputStream:
FileOutputStream ostream = new FileOutputStream("t.tmp");
ObjectOutputStream p = new ObjectOutputStream(ostream);
Classes that require special handling during the serialization and deserialization
process must implement special methods with these exact signatures:
The writeObject method is responsible for writing the state of
the object for its particular class so that the corresponding
readObject method can restore it.
The method does not need to concern itself with the
state belonging to the object's superclasses or subclasses.
State is saved by writing the individual fields to the ObjectOutputStream
using the writeObject method or by using the methods for
primitive data types supported by DataOutput.
Serialization does not write out the fields of any object that does
not implement the java.io.Serializable interface. Subclasses of
Objects that are not serializable can be serializable. In this case
the non-serializable class must have a no-arg constructor to allow
its fields to be initialized. In this case it is the
responsibility of the subclass to save and restore the state of the
non-serializable class. It is frequently the case that the fields
of that class are accessible (public, package, or protected) or
that there are get and set methods that can be used to restore the
Serialization of an object can be prevented by implementing writeObject
and readObject methods that throw the NotSerializableException.
The exception will be caught by the ObjectOutputStream and abort the
Implementing the Externalizable interface allows the object to
assume complete control over the contents and format of the object's
serialized form. The methods of the Externalizable interface,
writeExternal and readExternal, are called to save and restore the
objects state. When implemented by a class they can write and read
their own state using all of the methods of ObjectOutput and
ObjectInput. It is the responsibility of the objects to handle any
versioning that occurs.
Creates an ObjectOutputStream that writes to the specified OutputStream.
The stream header is written to the stream. The caller may want to call
flush immediately so that the corresponding ObjectInputStream can read
the header immediately.
Write the specified object to the ObjectOutputStream.
The class of the object, the signature of the class, and the values
of the non-transient and non-static fields of the class and all
of its supertypes are written. Default serialization for a class can be
overridden using the writeObject and the readObject methods.
Objects referenced by this object are written transitively so
that a complete equivalent graph of objects can be
reconstructed by an ObjectInputStream.
Exceptions are thrown for
problems with the OutputStream and for classes that should not be
serialized. All exceptions are fatal to the OutputStream, which
is left in an indeterminate state, and it is up to the caller
to ignore or recover the stream state.
Write the non-static and non-transient fields of the current class
to this stream. This may only be called from the writeObject method
of the class being serialized. It will throw the NotActiveException
if it is called otherwise.
Reset will disregard the state of any objects already written
to the stream. The state is reset to be the same as a new
ObjectOutputStream. The current point in the stream is marked
as reset so the corresponding ObjectInputStream will be reset
at the same point. Objects previously written to the stream
will not be refered to as already being in the stream. They
will be written to the stream again.
Subclasses may implement this method to allow class data to be stored
in the stream. By default this method does nothing.
The corresponding method in ObjectInputStream is resolveClass.
This method is called exactly once for each unique class in the stream.
The class name and signature will have already been written to the stream.
This method may make free use of the ObjectOutputStream to save
any representation of the class it deems suitable (for example,
the bytes of the class file). The resolveClass method in the corresponding
subclass of ObjectInputStream must read and use any data or objects
written by annotateClass.
annotateClass is called only for normal classes. Arrays are not normal classes.
This method will allow trusted subclasses of ObjectOutputStream
to substitute one object for another during
serialization. Replacing objects is disabled until
enableReplaceObject is called. The enableReplaceObject method
checks that the stream requesting to do replacment can be
trusted. Every reference to serializable objects is passed to
replaceObject. To insure that the private state of objects is
not unintentionally exposed only trusted streams may use
When a subclass is replacing objects it must insure that either
a complementary substitution must be made during
deserialization or that the substituted object is compatible
with every field where the reference will be stored. Objects
whose type is not a subclass of the type of the field or array
element abort the serialization by raising an exception and the
object is not be stored.
This method is called only once when each object is first encountered.
All subsequent references to the object will be redirected to the
new object. This method should return the object to be substituted or
the original object.
Null can be returned as the object to be substituted, but may
cause NullReferenceException in classes that contain references
to the original object since they may be expecting an object
instead of null.