| An ObjectInputStream deserializes primitive data and objects previously
written using an ObjectOutputStream.
ObjectOutputStream and ObjectInputStream can provide an application with
persistent storage for graphs of objects when used with a FileOutputStream
and FileInputStream respectively. ObjectInputStream is used to recover
those objects previously serialized. Other uses include passing objects
between hosts using a socket stream or for marshaling and unmarshaling
arguments and parameters in a remote communication system.
ObjectInputStream ensures that the types of all objects in the graph
created from the stream match the classes present in the Java Virtual
Machine. Classes are loaded as required using the standard mechanisms.
Only objects that support the java.io.Serializable or
java.io.Externalizable interface can be read from streams.
The method readObject is used to read an object from the
stream. Java's safe casting should be used to get the desired type. In
Java, strings and arrays are objects and are treated as objects during
serialization. When read they need to be cast to the expected type.
Primitive data types can be read from the stream using the appropriate
method on DataInput.
The default deserialization mechanism for objects restores the contents
of each field to the value and type it had when it was written. Fields
declared as transient or static are ignored by the deserialization process.
References to other objects cause those objects to be read from the stream
as necessary. Graphs of objects are restored correctly using a reference
sharing mechanism. New objects are always allocated when deserializing,
which prevents existing objects from being overwritten.
Reading an object is analogous to running the constructors of a new
object. Memory is allocated for the object and initialized to zero (NULL).
No-arg constructors are invoked for the non-serializable classes and then
the fields of the serializable classes are restored from the stream starting
with the serializable class closest to java.lang.object and finishing with
the object's most specific class.
For example to read from a stream as written by the example in
ObjectOutputStream:
FileInputStream fis = new FileInputStream("t.tmp");
ObjectInputStream ois = new ObjectInputStream(fis);
int i = ois.readInt();
String today = (String) ois.readObject();
Date date = (Date) ois.readObject();
ois.close();
Classes control how they are serialized by implementing either the
java.io.Serializable or java.io.Externalizable interfaces.
Implementing the Serializable interface allows object serialization to
save and restore the entire state of the object and it allows classes to
evolve between the time the stream is written and the time it is read. It
automatically traverses references between objects, saving and restoring
entire graphs.
Serializable classes that require special handling during the
serialization and deserialization process should implement the following
methods:
private void writeObject(java.io.ObjectOutputStream stream)
throws IOException;
private void readObject(java.io.ObjectInputStream stream)
throws IOException, ClassNotFoundException;
private void readObjectNoData()
throws ObjectStreamException;
The readObject method is responsible for reading and restoring the state
of the object for its particular class using data written to the stream by
the corresponding writeObject method. The method does not need to concern
itself with the state belonging to its superclasses or subclasses. State is
restored by reading data from the ObjectInputStream for the individual
fields and making assignments to the appropriate fields of the object.
Reading primitive data types is supported by DataInput.
Any attempt to read object data which exceeds the boundaries of the
custom data written by the corresponding writeObject method will cause an
OptionalDataException to be thrown with an eof field value of true.
Non-object reads which exceed the end of the allotted data will reflect the
end of data in the same way that they would indicate the end of the stream:
bytewise reads will return -1 as the byte read or number of bytes read, and
primitive reads will throw EOFExceptions. If there is no corresponding
writeObject method, then the end of default serialized data marks the end of
the allotted data.
Primitive and object read calls issued from within a readExternal method
behave in the same manner--if the stream is already positioned at the end of
data written by the corresponding writeExternal method, object reads will
throw OptionalDataExceptions with eof set to true, bytewise reads will
return -1, and primitive reads will throw EOFExceptions. Note that this
behavior does not hold for streams written with the old
ObjectStreamConstants.PROTOCOL_VERSION_1 protocol, in which the
end of data written by writeExternal methods is not demarcated, and hence
cannot be detected.
The readObjectNoData method is responsible for initializing the state of
the object for its particular class in the event that the serialization
stream does not list the given class as a superclass of the object being
deserialized. This may occur in cases where the receiving party uses a
different version of the deserialized instance's class than the sending
party, and the receiver's version extends classes that are not extended by
the sender's version. This may also occur if the serialization stream has
been tampered; hence, readObjectNoData is useful for initializing
deserialized objects properly despite a "hostile" or incomplete source
stream.
Serialization does not read or assign values to 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 state.
Any exception that occurs while deserializing an object will be caught by
the ObjectInputStream and abort the reading process.
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.
Enum constants are deserialized differently than ordinary serializable or
externalizable objects. The serialized form of an enum constant consists
solely of its name; field values of the constant are not transmitted. To
deserialize an enum constant, ObjectInputStream reads the constant name from
the stream; the deserialized constant is then obtained by calling the static
method Enum.valueOf(Class, String) with the enum constant's
base type and the received constant name as arguments. Like other
serializable or externalizable objects, enum constants can function as the
targets of back references appearing subsequently in the serialization
stream. The process by which enum constants are deserialized cannot be
customized: any class-specific readObject, readObjectNoData, and readResolve
methods defined by enum types are ignored during deserialization.
Similarly, any serialPersistentFields or serialVersionUID field declarations
are also ignored--all enum types have a fixed serialVersionUID of 0L.
author:
Mike Warres
author:
Roger Riggs
version:
1.155, 04/05/28
See Also: java.io.DataInput
See Also: java.io.ObjectOutputStream
See Also: java.io.Serializable
See Also: Object Serialization Specification, Section 3, Object Input Classes
since:
JDK1.1 | 