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java.io

Class ObjectOutputStream

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

All Implemented Interfaces:

ObjectOutput, ObjectStreamConstants


public class ObjectOutputStream

extends OutputStream

implements ObjectOutput, ObjectStreamConstants

An ObjectOutputStream can be used to write objects as well as primitive data in a platform-independent manner to an OutputStream. The data produced by an ObjectOutputStream can be read and reconstituted by an ObjectInputStream. writeObject (Object) is used to write Objects, the write<type> methods are used to write primitive data (as in DataOutputStream). Strings can be written as objects or as primitive data. Not all objects can be written out using an ObjectOutputStream. Only those objects that are an instance of java.io.Serializable can be written. Using default serialization, information about the class of an object is written, all of the non-transient, non-static fields of the object are written, if any of these fields are objects, they are written out in the same manner. An object is only written out the first time it is encountered. If the object is encountered later, a reference to it is written to the underlying stream. Thus writing circular object graphs does not present a problem, nor are relationships between objects in a graph lost. Example usage:
Hashtable map = new Hashtable ();
map.put ("one", new Integer (1));
map.put ("two", new Integer (2));
ObjectOutputStream oos =
new ObjectOutputStream (new FileOutputStream ("numbers"));
oos.writeObject (map);
oos.close ();
ObjectInputStream ois =
new ObjectInputStream (new FileInputStream ("numbers"));
Hashtable newmap = (Hashtable)ois.readObject ();
System.out.println (newmap);
The default serialization can be overriden in two ways. By defining a method private void writeObject (ObjectOutputStream), a class can dictate exactly how information about itself is written. defaultWriteObject () may be called from this method to carry out default serialization. This method is not responsible for dealing with fields of super-classes or subclasses. By implementing java.io.Externalizable. This gives the class complete control over the way it is written to the stream. If this approach is used the burden of writing superclass and subclass data is transfered to the class implementing java.io.Externalizable.

See Also:

Constructor Summary

ObjectOutputStream(java.io.OutputStream out)

Creates a new ObjectOutputStream that will do all of its writing onto out.
ObjectOutputStream()

Protected constructor that allows subclasses to override serialization.

Method Summary

voidannotateClass(Class cl)

An empty hook that allows subclasses to write extra information about classes to the stream.
voidannotateProxyClass(Class cl)

voidclose()

voiddefaultWriteObject()

Writes the current objects non-transient, non-static fields from the current class to the underlying output stream.
voiddrain()

Causes the block-data buffer to be written to the underlying stream, but does not flush underlying stream.
booleanenableReplaceObject(boolean enable)

If enable is true and this object is trusted, then replaceObject (Object) will be called in subsequent calls to writeObject (Object).
voidflush()

java.io.ObjectOutputStream.PutFieldputFields()

java.lang.ObjectreplaceObject(java.lang.Object obj)

Allows subclasses to replace objects that are written to the stream with other objects to be written in their place.
voidreset()

Resets stream to state equivalent to the state just after it was constructed.
static voidsetDefaultProtocolVersion(int version)

GNU $classpath specific Changes the default stream protocol used by all ObjectOutputStreams.
voiduseProtocolVersion(int version)

Informs this ObjectOutputStream to write data according to the specified protocol.
voidwrite(int data)

voidwrite(byte[] b)

voidwrite(byte[] b, int off, int len)

voidwriteBoolean(boolean data)

voidwriteByte(int data)

voidwriteBytes(java.lang.String data)

voidwriteChar(int data)

voidwriteChars(java.lang.String data)

voidwriteDouble(double data)

voidwriteFields()

voidwriteFloat(float data)

voidwriteInt(int data)

voidwriteLong(long data)

voidwriteObject(java.lang.Object obj)

Writes a representation of obj to the underlying output stream by writing out information about its class, then writing out each of the objects non-transient, non-static fields.
voidwriteObjectOverride(java.lang.Object obj)

This method allows subclasses to override the default serialization mechanism provided by ObjectOutputStream.
voidwriteShort(int data)

voidwriteStreamHeader()

Writes stream magic and stream version information to the underlying stream.
voidwriteUTF(java.lang.String data)

Constructor Details

ObjectOutputStream

protected ObjectOutputStream()

Protected constructor that allows subclasses to override serialization. This constructor should be called by subclasses that wish to override writeObject (Object). This method does a security check NOTE: currently not implemented, then sets a flag that informs writeObject (Object) to call the subclasses writeObjectOverride (Object) method.

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ObjectOutputStream

public ObjectOutputStream(java.io.OutputStream out)

Creates a new ObjectOutputStream that will do all of its writing onto out. This method also initializes the stream by writing the header information (stream magic number and stream version).

Parameters:

Throws:

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Method Details

annotateClass

protected void annotateClass(Class cl)

An empty hook that allows subclasses to write extra information about classes to the stream. This method is called the first time each class is seen, and after all of the standard information about the class has been written.

Parameters:

Throws:

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annotateProxyClass

protected void annotateProxyClass(Class cl)

Parameters:


close

public void close()

See Also:


defaultWriteObject

public void defaultWriteObject()

Writes the current objects non-transient, non-static fields from the current class to the underlying output stream. This method is intended to be called from within a object's private void writeObject (ObjectOutputStream) method.

Throws:


drain

protected void drain()

Causes the block-data buffer to be written to the underlying stream, but does not flush underlying stream.

Throws:


enableReplaceObject

protected boolean enableReplaceObject(boolean enable)

If enable is true and this object is trusted, then replaceObject (Object) will be called in subsequent calls to writeObject (Object). Otherwise, replaceObject (Object) will not be called.

Parameters:

Throws:


flush

public void flush()

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putFields

public ObjectOutputStream.PutField putFields()


replaceObject

protected Object replaceObject(java.lang.Object obj)

Allows subclasses to replace objects that are written to the stream with other objects to be written in their place. This method is called the first time each object is encountered (modulo reseting of the stream). This method must be enabled before it will be called in the serialization process.

Parameters:

Throws:

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reset

public void reset()

Resets stream to state equivalent to the state just after it was constructed. Causes all objects previously written to the stream to be forgotten. A notification of this reset is also written to the underlying stream.

Throws:


setDefaultProtocolVersion

public static void setDefaultProtocolVersion(int version)

GNU $classpath specific Changes the default stream protocol used by all ObjectOutputStreams. There are currently two different protocols, specified by PROTOCOL_VERSION_1 and PROTOCOL_VERSION_2. The default default is PROTOCOL_VERSION_1.

Parameters:

Throws:

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useProtocolVersion

public void useProtocolVersion(int version)

Informs this ObjectOutputStream to write data according to the specified protocol. There are currently two different protocols, specified by PROTOCOL_VERSION_1 and PROTOCOL_VERSION_2. This implementation writes data using PROTOCOL_VERSION_1 by default, as is done by the JDK 1.1. A non-portable method, setDefaultProtocolVersion (int version) is provided to change the default protocol version. For an explination of the differences beween the two protocols see XXX: the Java ObjectSerialization Specification.

Parameters:

Throws:

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write

public void write(byte[] b)

Parameters:

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write

public void write(byte[] b, int off, int len)

Parameters:

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write

public void write(int data)

Parameters:

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writeBoolean

public void writeBoolean(boolean data)

Parameters:

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writeByte

public void writeByte(int data)

Parameters:

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writeBytes

public void writeBytes(java.lang.String data)

Parameters:

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writeChar

public void writeChar(int data)

Parameters:

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writeChars

public void writeChars(java.lang.String data)

Parameters:

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writeDouble

public void writeDouble(double data)

Parameters:

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writeFields

public void writeFields()


writeFloat

public void writeFloat(float data)

Parameters:

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writeInt

public void writeInt(int data)

Parameters:

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writeLong

public void writeLong(long data)

Parameters:

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writeObject

public final void writeObject(java.lang.Object obj)

Writes a representation of obj to the underlying output stream by writing out information about its class, then writing out each of the objects non-transient, non-static fields. If any of these fields are other objects, they are written out in the same manner. This method can be overriden by a class by implementing private void writeObject (ObjectOutputStream). If an exception is thrown from this method, the stream is left in an undefined state.

Parameters:

Throws:


writeObjectOverride

protected void writeObjectOverride(java.lang.Object obj)

This method allows subclasses to override the default serialization mechanism provided by ObjectOutputStream. To make this method be used for writing objects, subclasses must invoke the 0-argument constructor on this class from there constructor.

Parameters:

Throws:

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writeShort

public void writeShort(int data)

Parameters:

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writeStreamHeader

protected void writeStreamHeader()

Writes stream magic and stream version information to the underlying stream.

Throws:


writeUTF

public void writeUTF(java.lang.String data)

Parameters:

See Also: