ObjectOutputStream

See also:

• DataOutput
• ObjectInputStream
• Serializable
• Externalizable
• Object Serialization Specification, Section 2, Object Output Classes

ObjectOutputStream

public ObjectOutputStream (OutputStream out)

Creates an ObjectOutputStream that writes to the specified OutputStream. This constructor writes the serialization stream header to the underlying stream; 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 methods.

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.

close

public void close ()

Closes the stream. This method must be called to release any resources associated with the stream.

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.

flush

public void flush ()

Flushes the stream. This will write any buffered output bytes and flush through to the underlying stream.

putFields

public ObjectOutputStream.PutField 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.

reset

public void reset ()

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 referred to as already being in the stream. They will be written to the stream again.

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 backwards incompatibilities; however, sometimes there is no other alternative.

write

public void write (byte[] buf)

Writes an array of bytes. This method will block until the bytes are actually written.

write

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

Writes a sub array of bytes.

write

public void write (int val)

Writes a byte. This method will block until the byte is actually written.

writeBoolean

public void writeBoolean (boolean val)

Writes a boolean.

writeByte

public void writeByte (int val)

Writes an 8 bit byte.

writeBytes

public void writeBytes (String str)

Writes a String as a sequence of bytes.

writeChar

public void writeChar (int val)

Writes a 16 bit char.

writeChars

public void writeChars (String str)

Writes a String as a sequence of chars.

writeDouble

public void writeDouble (double val)

Writes a 64 bit double.

writeFields

public void writeFields ()

Write the buffered fields to the stream.

writeFloat

public void writeFloat (float val)

Writes a 32 bit float.

writeInt

public void writeInt (int val)

Writes a 32 bit int.

writeLong

public void writeLong (long val)

Writes a 64 bit long.

writeObject

public final void writeObject (Object 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 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.

writeShort

public void writeShort (int val)

Writes a 16 bit short.

writeUTF

public void writeUTF (String str)

Primitive data write of this String in modified UTF-8 format. 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.

writeUnshared

public void writeUnshared (Object 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. In other words, ObjectOutputStream will never generate back-references to object data written by calls to writeUnshared.

ObjectOutputStream subclasses which override this method can only be constructed in security contexts possessing the "enableSubclassImplementation" SerializablePermission; any attempt to instantiate such a subclass without this permission will cause a SecurityException to be thrown.

annotateClass

protected void annotateClass (Class<?> 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.

annotateProxyClass

protected void annotateProxyClass (Class<?> 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.

drain

protected void drain ()

Drain any buffered data in ObjectOutputStream. Similar to flush but does not propagate the flush to the underlying stream.

enableReplaceObject

protected boolean enableReplaceObject (boolean enable)

Enable the stream to do replacement of objects in the stream. When 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 stream.

replaceObject

protected Object replaceObject (Object 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 null.

writeClassDescriptor

protected void writeClassDescriptor (ObjectStreamClass desc)

Write the specified class descriptor to the ObjectOutputStream. Class descriptors are used to identify the classes of objects written to the stream. 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). If this 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.

writeObjectOverride

protected void writeObjectOverride (Object 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 "final".

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.