找好工作，快人一步ObjectOutputStream (Java Platform SE 7 )
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Class ObjectOutputStream
java.io.ObjectOutputStream
All Implemented Interfaces:
public class ObjectOutputStream
implements ,
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.
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
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, ClassNotFoundE
private void writeObject(java.io.ObjectOutputStream stream)
throws IOException
private void readObjectNoData()
throws ObjectStreamE
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.
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
The methods of the Externalizable interface, writeExternal and
readExternal, are called to save and restore the objects state.
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
field values of the constant are not transmitted.
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
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.
See Also:,
Nested Class Summary
Nested Classes&
Modifier and Type
Class and Description
static class&
Provide programmatic access to the persistent fields to be written
to ObjectOutput.
Field Summary
Fields inherited from interface&java.io.
, , , , , , , , , , , , , , , , , , , , , , , , , , , ,
Constructor Summary
Constructors&
Constructor and Description
Provide a way for subclasses that are completely reimplementing
ObjectOutputStream to not have to allocate private data just used by
this implementation of ObjectOutputStream.
Creates an ObjectOutputStream that writes to the specified OutputStream.
Method Summary
Modifier and Type
Method and Description
protected void
Subclasses may implement this method to allow class data to be stored in
the stream.
protected void
Subclasses may implement this method to store custom data in the stream
along with descriptors for dynamic proxy classes.
Closes the stream.
Write the non-static and non-transient fields of the current class to
this stream.
protected void
Drain any buffered data in ObjectOutputStream.
protected boolean
(boolean&enable)
Enable the stream to do replacement of objects in the stream.
Flushes the stream.
Retrieve the object used to buffer persistent fields to be written to
the stream.
This method will allow trusted subclasses of ObjectOutputStream to
substitute one object for another during serialization.
Reset will disregard the state of any objects already written to the
(int&version)
Specify stream protocol version to use when writing the stream.
(byte[]&buf)
Writes an array of bytes.
(byte[]&buf,
Writes a sub array of bytes.
Writes a byte.
(boolean&val)
Writes a boolean.
Writes an 8 bit byte.
Writes a String as a sequence of bytes.
Writes a 16 bit char.
Writes a String as a sequence of chars.
protected void
Write the specified class descriptor to the ObjectOutputStream.
(double&val)
Writes a 64 bit double.
Write the buffered fields to the stream.
(float&val)
Writes a 32 bit float.
Writes a 32 bit int.
(long&val)
Writes a 64 bit long.
Write the specified object to the ObjectOutputStream.
protected void
Method used by subclasses to override the default writeObject method.
Writes a 16 bit short.
protected void
The writeStreamHeader method is provided so subclasses can append or
prepend their own header to the stream.
Writes an "unshared" object to the ObjectOutputStream.
Primitive data write of this String in
Methods inherited from class&java.lang.
, , , , , , , , , ,
Constructor Detail
ObjectOutputStream
public&ObjectOutputStream(&out)
Creates an ObjectOutputStream that writes to the specified OutputStream.
This constructor writes the serialization stream header to the
callers may wish to flush the stream immediately to
ensure that constructors for receiving ObjectInputStreams will not block
when reading the header.
If a security manager is installed, this constructor will check for
the "enableSubclassImplementation" SerializablePermission when invoked
directly or indirectly by the constructor of a subclass which overrides
the ObjectOutputStream.putFields or ObjectOutputStream.writeUnshared
Parameters:out - output stream to write to
- if an I/O error occurs while writing stream header
- if untrusted subclass illegally overrides
security-sensitive methods
- if out is nullSince:
See Also:,
ObjectOutputStream
protected&ObjectOutputStream()
Provide a way for subclasses that are completely reimplementing
ObjectOutputStream to not have to allocate private data just used by
this implementation of ObjectOutputStream.
If there is a security manager installed, this method first calls the
security manager's checkPermission method with a
SerializablePermission("enableSubclassImplementation")
permission to ensure it's ok to enable subclassing.
- if a security manager exists and its
checkPermission method denies enabling
subclassing.
See Also:,
Method Detail
useProtocolVersion
public&void&useProtocolVersion(int&version)
Specify stream protocol version to use when writing the stream.
This routine provides a hook to enable the current version of
Serialization to write in a format that is backwards compatible to a
previous version of the stream format.
Every effort will be made to avoid introducing additional
backwar however, sometimes there is no
other alternative.
Parameters:version - use ProtocolVersion from java.io.ObjectStreamConstants.
- if called after any objects
have been serialized.
- if invalid version is passed in.
- if I/O errors occurSince:
See Also:,
writeObject
public final&void&writeObject(&obj)
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
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.
Specified by:
&in interface&
Parameters:obj - the object to be written
- Something is wrong with a class used by
serialization.
- Some object to be serialized does not
implement the java.io.Serializable interface.
- Any exception thrown by the underlying
OutputStream.
writeObjectOverride
protected&void&writeObjectOverride(&obj)
Method used by subclasses to override the default writeObject method.
This method is called by trusted subclasses of ObjectInputStream that
constructed ObjectInputStream using the protected no-arg constructor.
The subclass is expected to provide an override method with the modifier
Parameters:obj - object to be written to the underlying stream
- if there are I/O errors while writing to the
underlying streamSince:
See Also:,
writeUnshared
public&void&writeUnshared(&obj)
Writes an "unshared" object to the ObjectOutputStream.
This method is
identical to writeObject, except that it always writes the given object
as a new, unique object in the stream (as opposed to a back-reference
pointing to a previously serialized instance).
Specifically:
An object written via writeUnshared is always serialized in the
same manner as a newly appearing object (an object that has not
been written to the stream yet), regardless of whether or not the
object has been written previously.
If writeObject is used to write an object that has been previously
written with writeUnshared, the previous writeUnshared operation
is treated as if it were a write of a separate object.
words, ObjectOutputStream will never generate back-references to
object data written by calls to writeUnshared.
While writing an object via writeUnshared does not in itself guarantee a
unique reference to the object when it is deserialized, it allows a
single object to be defined multiple times in a stream, so that multiple
calls to readUnshared by the receiver will not conflict.
Note that the
rules described above only apply to the base-level object written with
writeUnshared, and not to any transitively referenced sub-objects in the
object graph to be serialized.
ObjectOutputStream subclasses which override this method can only be
constructed in security contexts possessing the
"enableSubclassImplementation" SerializableP any attempt to
instantiate such a subclass without this permission will cause a
SecurityException to be thrown.
Parameters:obj - object to write to stream
- if an object in the graph to be
serialized does not implement the Serializable interface
- if a problem exists with the class of an
object to be serialized
- if an I/O error occurs during serializationSince:
defaultWriteObject
public&void&defaultWriteObject()
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.
- if I/O errors occur while writing to the underlying
OutputStream
public&&putFields()
Retrieve the object used to buffer persistent fields to be written to
the stream.
The fields will be written to the stream when writeFields
method is called.
Returns:an instance of the class Putfield that holds the serializable
- if I/O errors occurSince:
writeFields
public&void&writeFields()
Write the buffered fields to the stream.
- if I/O errors occur while writing to the underlying
- Called when a classes writeObject method was
not called to write the state of the object.Since:
public&void&reset()
Reset will disregard the state of any objects already written to the
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
They will be written to the stream again.
- if reset() is invoked while serializing an object.
annotateClass
protected&void&annotateClass(&?&&cl)
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.
Parameters:cl - the class to annotate custom data for
- Any exception thrown by the underlying
OutputStream.
annotateProxyClass
protected&void&annotateProxyClass(&?&&cl)
Subclasses may implement this method to store custom data in the stream
along with descriptors for dynamic proxy classes.
This method is called exactly once for each unique proxy class
descriptor in the stream.
The default implementation of this method in
ObjectOutputStream does nothing.
The corresponding method in ObjectInputStream is
resolveProxyClass.
For a given subclass of
ObjectOutputStream that overrides this method, the
resolveProxyClass method in the corresponding subclass of
ObjectInputStream must read any data or objects written by
annotateProxyClass.
Parameters:cl - the proxy class to annotate custom data for
- any exception thrown by the underlying
OutputStreamSince:
replaceObject
protected&&replaceObject(&obj)
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
replacement can be trusted.
The first occurrence of each object written
into the serialization stream is passed to replaceObject.
Subsequent
references to the object are replaced by the object returned by the
original call to replaceObject.
To ensure that the private state of
objects is not unintentionally exposed, only trusted streams may use
replaceObject.
The ObjectOutputStream.writeObject method takes a parameter of type
Object (as opposed to type Serializable) to allow for cases where
non-serializable objects are replaced by serializable ones.
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
Parameters:obj - the object to be replaced
Returns:the alternate object that replaced the specified one
- Any exception thrown by the underlying
OutputStream.
enableReplaceObject
protected&boolean&enableReplaceObject(boolean&enable)
Enable the stream to do replacement of objects in the stream.
enabled, the replaceObject method is called for every object being
serialized.
If enable is true, and there is a security manager
installed, this method first calls the security manager's
checkPermission method with a
SerializablePermission("enableSubstitution") permission to
ensure it's ok to enable the stream to do replacement of objects in the
Parameters:enable - boolean parameter to enable replacement of objects
Returns:the previous setting before this method was invoked
- if a security manager exists and its
checkPermission method denies enabling the stream
to do replacement of objects in the stream.See Also:,
writeStreamHeader
protected&void&writeStreamHeader()
The writeStreamHeader method is provided so subclasses can append or
prepend their own header to the stream.
It writes the magic number and
version to the stream.
- if I/O errors occur while writing to the underlying
writeClassDescriptor
protected&void&writeClassDescriptor(&desc)
Write the specified class descriptor to the ObjectOutputStream.
descriptors are used to identify the classes of objects written to the
Subclasses of ObjectOutputStream may override this method to
customize the way in which class descriptors are written to the
serialization stream.
The corresponding method in ObjectInputStream,
readClassDescriptor, should then be overridden to
reconstitute the class descriptor from its custom stream representation.
By default, this method writes class descriptors according to the format
defined in the Object Serialization specification.
Note that this method will only be called if the ObjectOutputStream
is not using the old serialization stream format (set by calling
ObjectOutputStream's useProtocolVersion method).
serialization stream is using the old format
(PROTOCOL_VERSION_1), the class descriptor will be written
internally in a manner that cannot be overridden or customized.
Parameters:desc - class descriptor to write to the stream
- If an I/O error has occurred.Since:
See Also:,
public&void&write(int&val)
Writes a byte. This method will block until the byte is actually
Specified by:
&in interface&
Specified by:
&in interface&
Specified by:
&in class&
Parameters:val - the byte to be written to the stream
- If an I/O error has occurred.
public&void&write(byte[]&buf)
Writes an array of bytes. This method will block until the bytes are
actually written.
Specified by:
&in interface&
Specified by:
&in interface&
Overrides:
&in class&
Parameters:buf - the data to be written
- If an I/O error has occurred.See Also:
public&void&write(byte[]&buf,
Writes a sub array of bytes.
Specified by:
&in interface&
Specified by:
&in interface&
Overrides:
&in class&
Parameters:buf - the data to be writtenoff - the start offset in the datalen - the number of bytes that are written
- If an I/O error has occurred.
public&void&flush()
Flushes the stream. This will write any buffered output bytes and flush
through to the underlying stream.
Specified by:
&in interface&
Specified by:
&in interface&
Overrides:
&in class&
- If an I/O error has occurred.
protected&void&drain()
Drain any buffered data in ObjectOutputStream.
Similar to flush but
does not propagate the flush to the underlying stream.
- if I/O errors occur while writing to the underlying
public&void&close()
Closes the stream. This method must be called to release any resources
associated with the stream.
Specified by:
&in interface&
Specified by:
&in interface&
Specified by:
&in interface&
Overrides:
&in class&
- If an I/O error has occurred.
writeBoolean
public&void&writeBoolean(boolean&val)
Writes a boolean.
Specified by:
&in interface&
Parameters:val - the boolean to be written
- if I/O errors occur while writing to the underlying
public&void&writeByte(int&val)
Writes an 8 bit byte.
Specified by:
&in interface&
Parameters:val - the byte value to be written
- if I/O errors occur while writing to the underlying
writeShort
public&void&writeShort(int&val)
Writes a 16 bit short.
Specified by:
&in interface&
Parameters:val - the short value to be written
- if I/O errors occur while writing to the underlying
public&void&writeChar(int&val)
Writes a 16 bit char.
Specified by:
&in interface&
Parameters:val - the char value to be written
- if I/O errors occur while writing to the underlying
public&void&writeInt(int&val)
Writes a 32 bit int.
Specified by:
&in interface&
Parameters:val - the integer value to be written
- if I/O errors occur while writing to the underlying
public&void&writeLong(long&val)
Writes a 64 bit long.
Specified by:
&in interface&
Parameters:val - the long value to be written
- if I/O errors occur while writing to the underlying
writeFloat
public&void&writeFloat(float&val)
Writes a 32 bit float.
Specified by:
&in interface&
Parameters:val - the float value to be written
- if I/O errors occur while writing to the underlying
writeDouble
public&void&writeDouble(double&val)
Writes a 64 bit double.
Specified by:
&in interface&
Parameters:val - the double value to be written
- if I/O errors occur while writing to the underlying
writeBytes
public&void&writeBytes(&str)
Writes a String as a sequence of bytes.
Specified by:
&in interface&
Parameters:str - the String of bytes to be written
- if I/O errors occur while writing to the underlying
writeChars
public&void&writeChars(&str)
Writes a String as a sequence of chars.
Specified by:
&in interface&
Parameters:str - the String of chars to be written
- if I/O errors occur while writing to the underlying
public&void&writeUTF(&str)
Primitive data write of this String in
Note that there is a
significant difference between writing a String into the stream as
primitive data or as an Object. A String instance written by writeObject
is written into the stream as a String initially. Future writeObject()
calls write references to the string into the stream.
Specified by:
&in interface&
Parameters:str - the String to be written
- if I/O errors occur while writing to the underlying
For further API reference and developer documentation, see . That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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