| 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 the stream. If the stream is a network socket stream, the
objects can be reconstituted 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 fos = new FileOutputStream("t.tmp");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeInt(12345);
oos.writeObject("Today");
oos.writeObject(new Date());
oos.close();
Classes that require special handling during the serialization and
deserialization process must implement special methods with these exact
signatures:
private void readObject(java.io.ObjectInputStream stream)
throws IOException, ClassNotFoundException;
private void writeObject(java.io.ObjectOutputStream stream)
throws IOException
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 state.
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
serialization 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 serialized 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
serialize an enum constant, ObjectOutputStream writes the string returned by
the constant's name method. 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 serialized cannot be customized; any class-specific
writeObject and writeReplace methods defined by enum types are ignored
during serialization. Similarly, any serialPersistentFields or
serialVersionUID field declarations are also ignored--all enum types have a
fixed serialVersionUID of 0L.
Primitive data, excluding serializable fields and externalizable data, is
written to the ObjectOutputStream in block-data records. A block data record
is composed of a header and data. The block data header consists of a marker
and the number of bytes to follow the header. Consecutive primitive data
writes are merged into one block-data record. The blocking factor used for
a block-data record will be 1024 bytes. Each block-data record will be
filled up to 1024 bytes, or be written whenever there is a termination of
block-data mode. Calls to the ObjectOutputStream methods writeObject,
defaultWriteObject and writeFields initially terminate any existing
block-data record.
author:
Mike Warres
author:
Roger Riggs
version:
1.145, 04/05/28
See Also: java.io.DataOutput
See Also: java.io.ObjectInputStream
See Also: java.io.Serializable
See Also: java.io.Externalizable
See Also: Object Serialization Specification, Section 2, Object Output Classes
since:
JDK1.1 | 