java.io
Class ObjectOutputStream

java.lang.Object   
  java.io.OutputStream   
      java.io.ObjectOutputStream

All Implemented Interfaces:

Closeable , DataOutput , Flushable , ObjectOutput , ObjectStreamConstants

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 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.

Since:

JDK1.1

See Also:

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

Nested Class Summary

static class

ObjectOutputStream.PutField Provide programmatic access to the persistent fields to be written to ObjectOutput.

Field Summary

Fields inherited from interface java.io.ObjectStreamConstants

baseWireHandle , PROTOCOL_VERSION_1 , PROTOCOL_VERSION_2 , SC_BLOCK_DATA , SC_ENUM , SC_EXTERNALIZABLE , SC_SERIALIZABLE , SC_WRITE_METHOD , STREAM_MAGIC , STREAM_VERSION , SUBCLASS_IMPLEMENTATION_PERMISSION , SUBSTITUTION_PERMISSION , TC_ARRAY , TC_BASE , TC_BLOCKDATA , TC_BLOCKDATALONG , TC_CLASS , TC_CLASSDESC , TC_ENDBLOCKDATA , TC_ENUM , TC_EXCEPTION , TC_LONGSTRING , TC_MAX , TC_NULL , TC_OBJECT , TC_PROXYCLASSDESC , TC_REFERENCE , TC_RESET , TC_STRING

Constructor Summary

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

Method Summary

protected void	annotateClass (Class <?> cl) Subclasses may implement this method to allow class data to be stored in the stream.
protected void	annotateProxyClass (Class <?> cl) Subclasses may implement this method to store custom data in the stream along with descriptors for dynamic proxy classes.
void	close () Closes the stream.
void	defaultWriteObject () Write the non-static and non-transient fields of the current class to this stream.
protected void	drain () Drain any buffered data in ObjectOutputStream.
protected boolean	enableReplaceObject (boolean enable) Enable the stream to do replacement of objects in the stream.
void	flush () Flushes the stream.
ObjectOutputStream.PutField	putFields () Retrieve the object used to buffer persistent fields to be written to the stream.
protected Object	replaceObject (Object obj) This method will allow trusted subclasses of ObjectOutputStream to substitute one object for another during serialization.
void	reset () Reset will disregard the state of any objects already written to the stream.
void	useProtocolVersion (int version) Specify stream protocol version to use when writing the stream.
void	write (byte[] buf) Writes an array of bytes.
void	write (byte[] buf, int off, int len) Writes a sub array of bytes.
void	write (int val) Writes a byte.
void	writeBoolean (boolean val) Writes a boolean.
void	writeByte (int val) Writes an 8 bit byte.
void	writeBytes (String str) Writes a String as a sequence of bytes.
void	writeChar (int val) Writes a 16 bit char.
void	writeChars (String str) Writes a String as a sequence of chars.
protected void	writeClassDescriptor (ObjectStreamClass desc) Write the specified class descriptor to the ObjectOutputStream.
void	writeDouble (double val) Writes a 64 bit double.
void	writeFields () Write the buffered fields to the stream.
void	writeFloat (float val) Writes a 32 bit float.
void	writeInt (int val) Writes a 32 bit int.
void	writeLong (long val) Writes a 64 bit long.
void	writeObject (Object obj) Write the specified object to the ObjectOutputStream.
protected void	writeObjectOverride (Object obj) Method used by subclasses to override the default writeObject method.
void	writeShort (int val) Writes a 16 bit short.
protected void	writeStreamHeader () The writeStreamHeader method is provided so subclasses can append or prepend their own header to the stream.
void	writeUnshared (Object obj) Writes an "unshared" object to the ObjectOutputStream.
void	writeUTF (String str) Primitive data write of this String in modified UTF-8 format.

Methods inherited from class java.lang.Object

clone , equals , finalize , getClass , hashCode , notify , notifyAll , toString , wait , wait , wait

Constructor Detail

ObjectOutputStream

public ObjectOutputStream(OutputStream    out)
                   throws IOException   

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.

Parameters:

out - output stream to write to

Throws:

IOException - if an I/O error occurs while writing stream header

SecurityException - if untrusted subclass illegally overrides security-sensitive methods

NullPointerException - if out is null

See Also:

ObjectOutputStream() , putFields() , ObjectInputStream.ObjectInputStream(InputStream)

ObjectOutputStream

protected ObjectOutputStream()
                      throws IOException   ,
                             SecurityException   

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.

Throws:

SecurityException - if a security manager exists and its checkPermission method denies enabling subclassing.

IOException

See Also:

SecurityManager.checkPermission(java.security.Permission) , SerializablePermission

Method Detail

useProtocolVersion

public void useProtocolVersion(int version)
                        throws IOException   

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.

Parameters:

version - use ProtocolVersion from java.io.ObjectStreamConstants.

Throws:

IllegalStateException - if called after any objects have been serialized.

IllegalArgumentException - if invalid version is passed in.

IOException - if I/O errors occur

Since:

1.2

See Also:

ObjectStreamConstants.PROTOCOL_VERSION_1 , ObjectStreamConstants.PROTOCOL_VERSION_2

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.

Specified by:

writeObject in interface ObjectOutput

Parameters:

obj - the object to be written

Throws:

InvalidClassException - Something is wrong with a class used by serialization.

NotSerializableException - Some object to be serialized does not implement the java.io.Serializable interface.

IOException - Any exception thrown by the underlying OutputStream.

writeObjectOverride

protected void writeObjectOverride(Object    obj)
                            throws IOException   

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".

Parameters:

obj - object to be written to the underlying stream

Throws:

IOException - if there are I/O errors while writing to the underlying stream

Since:

1.2

See Also:

ObjectOutputStream() , writeObject(Object)

writeUnshared

public void writeUnshared(Object    obj)
                   throws IOException   

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.

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" SerializablePermission; any attempt to instantiate such a subclass without this permission will cause a SecurityException to be thrown.

Parameters:

obj - object to write to stream

Throws:

NotSerializableException - if an object in the graph to be serialized does not implement the Serializable interface

InvalidClassException - if a problem exists with the class of an object to be serialized

IOException - if an I/O error occurs during serialization

defaultWriteObject

public 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.

Throws:

IOException - if I/O errors occur while writing to the underlying OutputStream

putFields

public ObjectOutputStream.PutField    putFields()
                                      throws IOException   

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 fields

Throws:

IOException - if I/O errors occur

Since:

1.2

writeFields