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Class java.io.ObjectOutputStream

java.lang.Object
   |
   +----java.io.OutputStream
           |
           +----java.io.ObjectOutputStream

public class ObjectOutputStream
extends OutputStream
implements ObjectOutput, ObjectStreamConstants
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 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);
	p.writeInt(12345);
	p.writeObject("Today");
	p.writeObject(new Date());
	p.flush();
	ostream.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.

See Also:
DataOutput, ObjectInputStream, Serializable, Externalizable

Constructor Index

 o ObjectOutputStream(OutputStream)
Creates an ObjectOutputStream that writes to the specified OutputStream.

Method Index

 o annotateClass(Class)
Subclasses may implement this method to allow class data to be stored in the stream.
 o close()
Closes the stream.
 o defaultWriteObject()
Write the non-static and non-transient fields of the current class to this stream.
 o drain()
Drain any buffered data in ObjectOutputStream.
 o enableReplaceObject(boolean)
Enable the stream to do replacement of objects in the stream.
 o flush()
Flushes the stream.
 o replaceObject(Object)
This method will allow trusted subclasses of ObjectOutputStream to substitute one object for another during serialization.
 o reset()
Reset will disregard the state of any objects already written to the stream.
 o write(byte[])
Writes an array of bytes.
 o write(byte[], int, int)
Writes a sub array of bytes.
 o write(int)
Writes a byte.
 o writeBoolean(boolean)
Writes a boolean.
 o writeByte(int)
Writes an 8 bit byte.
 o writeBytes(String)
Writes a String as a sequence of bytes.
 o writeChar(int)
Writes a 16 bit char.
 o writeChars(String)
Writes a String as a sequence of chars.
 o writeDouble(double)
Writes a 64 bit double.
 o writeFloat(float)
Writes a 32 bit float.
 o writeInt(int)
Writes a 32 bit int.
 o writeLong(long)
Writes a 64 bit long.
 o writeObject(Object)
Write the specified object to the ObjectOutputStream.
 o writeShort(int)
Writes a 16 bit short.
 o writeStreamHeader()
The writeStreamHeader method is provided so subclasses can append or prepend their own header to the stream.
 o writeUTF(String)
Writes a String in UTF format.

Constructors

 o ObjectOutputStream
 public ObjectOutputStream(OutputStream out) throws IOException
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.

Throws: IOException
Any exception thrown by the underlying OutputStream.

Methods

 o writeObject
 public final void writeObject(Object obj) throws IOException
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.

Throws: InvalidClassException
Something is wrong with a class used by serialization.
Throws: NotSerializableException
Some object to be serialized does not implement the java.io.Serializable interface.
Throws: IOException
Any exception thrown by the underlying OutputStream.
 o defaultWriteObject
 public final void defaultWriteObject() throws IOException
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.

 o reset
 public void reset() throws IOException
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.

 o annotateClass
 protected void annotateClass(Class cl) throws IOException
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.

Throws: IOException
Any exception thrown by the underlying OutputStream.
 o replaceObject
 protected Object replaceObject(Object obj) throws IOException
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 replaceObject.

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.

Throws: IOException
Any exception thrown by the underlying OutputStream.
 o enableReplaceObject
 protected final boolean enableReplaceObject(boolean enable) throws SecurityException
Enable the stream to do replacement of objects in the stream. If the stream is a trusted class it is allowed to enable replacement. Trusted classes are those classes with a classLoader equals null.

When enabled the replaceObject method is called for every object being serialized.

Throws: SecurityException
The classloader of this stream object is non-null.
 o writeStreamHeader
 protected void writeStreamHeader() throws IOException
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.

 o write
 public void write(int data) throws IOException
Writes a byte. This method will block until the byte is actually written.

Parameters:
b - the byte
Throws: IOException
If an I/O error has occurred.
Overrides:
write in class OutputStream
 o write
 public void write(byte b[]) throws IOException
Writes an array of bytes. This method will block until the bytes are actually written.

Parameters:
b - the data to be written
Throws: IOException
If an I/O error has occurred.
Overrides:
write in class OutputStream
 o flush
 public void flush() throws IOException
Flushes the stream. This will write any buffered output bytes and flush through to the underlying stream.

Throws: IOException
If an I/O error has occurred.
Overrides:
flush in class OutputStream
 o drain
 protected void drain() throws IOException
Drain any buffered data in ObjectOutputStream. Similar to flush but does not propagate the flush to the underlaying stream.

 o close
 public void close() throws IOException
Closes the stream. This method must be called to release any resources associated with the stream.

Throws: IOException
If an I/O error has occurred.
Overrides:
close in class OutputStream
 o writeBoolean
 public void writeBoolean(boolean data) throws IOException
Writes a boolean.

Parameters:
data - the boolean to be written
 o write
 public void write(byte b[],
                   int off,
                   int len) throws IOException
Writes a sub array of bytes.

Parameters:
b - the data to be written
off - the start offset in the data
len - the number of bytes that are written
Throws: IOException
If an I/O error has occurred.
Overrides:
write in class OutputStream
 o writeByte
 public void writeByte(int data) throws IOException
Writes an 8 bit byte.

Parameters:
data - the byte value to be written
 o writeShort
 public void writeShort(int data) throws IOException
Writes a 16 bit short.

Parameters:
data - the short value to be written
 o writeChar
 public void writeChar(int data) throws IOException
Writes a 16 bit char.

Parameters:
data - the char value to be written
 o writeInt
 public void writeInt(int data) throws IOException
Writes a 32 bit int.

Parameters:
data - the integer value to be written
 o writeLong
 public void writeLong(long data) throws IOException
Writes a 64 bit long.

Parameters:
data - the long value to be written
 o writeFloat
 public void writeFloat(float data) throws IOException
Writes a 32 bit float.

Parameters:
data - the float value to be written
 o writeDouble
 public void writeDouble(double data) throws IOException
Writes a 64 bit double.

Parameters:
data - the double value to be written
 o writeBytes
 public void writeBytes(String data) throws IOException
Writes a String as a sequence of bytes.

Parameters:
s - the String of bytes to be written
 o writeChars
 public void writeChars(String data) throws IOException
Writes a String as a sequence of chars.

Parameters:
s - the String of chars to be written
 o writeUTF
 public void writeUTF(String data) throws IOException
Writes a String in UTF format.

Parameters:
str - the String in UTF format

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