java.io
Class 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. 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

Inner Class Summary

static class

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

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.
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. 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.
void	write(byte[] b) Writes an array of bytes.
void	write(byte[] b, int off, int len) Writes a sub array of bytes.
void	write(int data) Writes a byte.
void	writeBoolean(boolean data) Writes a boolean.
void	writeByte(int data) Writes an 8 bit byte.
void	writeBytes(String data) Writes a String as a sequence of bytes.
void	writeChar(int data) Writes a 16 bit char.
void	writeChars(String data) Writes a String as a sequence of chars.
void	writeDouble(double data) Writes a 64 bit double.
void	writeFields() Write the buffered fields to the stream.
void	writeFloat(float data) Writes a 32 bit float.
void	writeInt(int data) Writes a 32 bit int.
void	writeLong(long data) Writes a 64 bit long.
void	writeObject(Object obj) Write the specified object to the ObjectOutputStream.
protected void	writeObjectOverride(Object obj) This method is called by trusted subclasses of ObjectInputStream that constructed ObjectInputStream using the protected no-arg constructor.
void	writeShort(int data) 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	writeUTF(String data) Primitive data write of this String in UTF 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. 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.

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:

IOException - Thrown if not called by a subclass.

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

See Also:

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

Method Detail

writeObjectOverride

protected void writeObjectOverride(Object obj)
                            throws IOException

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".

Since:

JDK 1.2

See Also:

ObjectOutputStream(), writeObject(Object)

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 - Thrown if called after any objects have been serialized.

IllegalArgument - if invalid version is passed in.

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

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.

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.

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.

Since:

JDK1.2

writeFields

public void writeFields()
                 throws IOException

Write the buffered fields to the stream.

Throws:

NotActiveException - Called when a classes writeObject method was not called to write the state of the object.

Since:

JDK1.2

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.

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.

Throws:

IOException - Any exception thrown by the underlying OutputStream.

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.

enableReplaceObject

protected boolean enableReplaceObject(boolean enable)
                               throws SecurityException

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.

Throws:

SecurityException - if a security manager exists and its checkPermission method denies enabling the stream to do replacement of objects in the stream.

See Also:

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

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.

write

public void write(int data)
           throws IOException

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

Specified by:

write in interface ObjectOutput

Parameters:

b - the byte

Throws:

IOException - If an I/O error has occurred.

Overrides:

write in class OutputStream

write

public void write(byte[] b)
           throws IOException

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

Specified by:

write in interface ObjectOutput

Parameters:

b - the data to be written

Throws:

IOException - If an I/O error has occurred.

Overrides:

write in class OutputStream

write

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

Writes a sub array of bytes.

Specified by:

write in interface ObjectOutput

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

flush

public void flush()
           throws IOException

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

Specified by:

flush in interface ObjectOutput

Throws:

IOException - If an I/O error has occurred.

Overrides:

flush in class OutputStream

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.

close

public void close()
           throws IOException

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

Specified by:

close in interface ObjectOutput

Throws:

IOException - If an I/O error has occurred.

Overrides:

close in class OutputStream

writeBoolean

public void writeBoolean(boolean data)
                  throws IOException

Writes a boolean.

Parameters:

data - the boolean to be written

writeByte

public void writeByte(int data)
               throws IOException

Writes an 8 bit byte.

Parameters:

data - the byte value to be written

writeShort

public void writeShort(int data)
                throws IOException

Writes a 16 bit short.

Parameters:

data - the short value to be written

writeChar

public void writeChar(int data)
               throws IOException

Writes a 16 bit char.

Parameters:

data - the char value to be written

writeInt

public void writeInt(int data)
              throws IOException

Writes a 32 bit int.

Parameters:

data - the integer value to be written

writeLong

public void writeLong(long data)
               throws IOException

Writes a 64 bit long.

Parameters:

data - the long value to be written

writeFloat

public void writeFloat(float data)
                throws IOException

Writes a 32 bit float.

Parameters:

data - the float value to be written

writeDouble

public void writeDouble(double data)
                 throws IOException

Writes a 64 bit double.

Parameters:

data - the double value to be written

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

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

writeUTF

public void writeUTF(String data)
              throws IOException

Primitive data write of this String in UTF 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.

Parameters:

str - the String in UTF format