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.
(package private) static class ObjectOutputStream.PutFieldImpl
          Provide access to the persistent fields to be written to the output stream.
(package private) static class ObjectOutputStream.Stack
          Unsynchronized Stack.
 
Field Summary
private  IOException abortIOException
           
private  boolean blockDataMode
           
private  byte[] buf
           
private  ObjectOutputStream.Stack classDescStack
           
private  int count
           
private  ObjectStreamClass currentClassDesc
           
private  Object currentObject
           
private  ObjectOutputStream.PutField currentPutFields
           
private  DataOutputStream dos
           
(package private)  boolean enableReplace
           
private  boolean enableSubclassImplementation
           
private  int nextReplaceOffset
           
private  int nextWireOffset
           
private static boolean NOT_REPLACEABLE
           
private  OutputStream out
           
private  int recursionDepth
           
private static boolean REPLACEABLE
           
private  Object[] replaceObjects
           
(package private)  boolean useDeprecatedExternalizableFormat
           
private  ArrayList wireHandle2Object
           
private  int[] wireHash2Handle
           
private  int wireHashCapacity
           
private  int wireHashLoadFactor
           
private  int wireHashSizePower
           
private  int[] wireNextHandle
           
private  Object[] writeObjectArglist
           
 
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
private  void addReplacement(Object orig, Object replacement)
           
protected  void annotateClass(Class cl)
          Subclasses may implement this method to allow class data to be stored in the stream.
private  void assignWireOffset(Object obj)
           
private  void bufferedWrite(byte[] b, int off, int len)
           
private  boolean checkSpecialClasses(Object obj)
           
private  boolean checkSubstitutableSpecialClasses(Object obj, Class cl)
           
 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.
private  int findWireOffset(Object obj)
           
 void flush()
          Flushes the stream.
private  void growWireHash2Handle()
           
private  void hashInsert(Object obj, int offset)
           
private  void invokeObjectWriter(Object obj)
           
private  Object lookupReplace(Object obj)
           
private  void outputArray(Object obj)
          Write an array out.
private  void outputClass(Class aclass)
           
private  void outputClassDescriptor(ObjectStreamClass classdesc)
           
private  void outputClassFields(Object o, Class cl, ObjectStreamField[] fields)
           
private  void outputObject(Object obj)
           
private  void outputString(String s)
          Write a string to 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.
private  void resetStream()
           
private  boolean serializeNullAndRepeat(Object obj, boolean checkForReplace)
           
private  boolean setBlockData(boolean mode)
           
 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.
private  void writeBlockDataHeader(int len)
           
 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.
private  void writeCanonical(byte[] b, int off, int len)
           
 void writeChar(int data)
          Writes a 16 bit char.
 void writeChars(String data)
          Writes a String as a sequence of chars.
private  void writeCode(int tag)
           
 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.
private  void writeInternal(byte[] b, int off, int len, boolean copyOnWrite)
           
 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.
(package private)  void writeTypeString(String typeString)
           
 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, registerNatives, toString, wait, wait, wait
 

Field Detail

blockDataMode

private boolean blockDataMode

buf

private byte[] buf

count

private int count

out

private OutputStream out

dos

private DataOutputStream dos

abortIOException

private IOException abortIOException

wireHandle2Object

private ArrayList wireHandle2Object

nextWireOffset

private int nextWireOffset

wireHash2Handle

private int[] wireHash2Handle

wireNextHandle

private int[] wireNextHandle

wireHashSizePower

private int wireHashSizePower

wireHashLoadFactor

private int wireHashLoadFactor

wireHashCapacity

private int wireHashCapacity

currentObject

private Object currentObject

currentClassDesc

private ObjectStreamClass currentClassDesc

classDescStack

private ObjectOutputStream.Stack classDescStack

currentPutFields

private ObjectOutputStream.PutField currentPutFields

writeObjectArglist

private Object[] writeObjectArglist

enableReplace

boolean enableReplace

replaceObjects

private Object[] replaceObjects

nextReplaceOffset

private int nextReplaceOffset

REPLACEABLE

private static final boolean REPLACEABLE

NOT_REPLACEABLE

private static final boolean NOT_REPLACEABLE

recursionDepth

private int recursionDepth

useDeprecatedExternalizableFormat

boolean useDeprecatedExternalizableFormat

enableSubclassImplementation

private boolean enableSubclassImplementation
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.

checkSpecialClasses

private boolean checkSpecialClasses(Object obj)
                             throws IOException

checkSubstitutableSpecialClasses

private boolean checkSubstitutableSpecialClasses(Object obj,
                                                 Class cl)
                                          throws IOException

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.

resetStream

private void resetStream()
                  throws IOException

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.

outputString

private void outputString(String s)
                   throws IOException
Write a string to the stream. Note that since Strings are Objects, writeObject will behave identically.

outputClass

private void outputClass(Class aclass)
                  throws IOException

outputClassDescriptor

private void outputClassDescriptor(ObjectStreamClass classdesc)
                            throws IOException

outputArray

private void outputArray(Object obj)
                  throws IOException
Write an array out. Note that since Arrays are Objects, writeObject(obj) will behave identically.

Parameters:
o - can represent an array of any type/dimension.

writeTypeString

void writeTypeString(String typeString)
               throws IOException

outputObject

private void outputObject(Object obj)
                   throws IOException

lookupReplace

private Object lookupReplace(Object obj)

serializeNullAndRepeat

private boolean serializeNullAndRepeat(Object obj,
                                       boolean checkForReplace)
                                throws IOException

findWireOffset

private int findWireOffset(Object obj)

assignWireOffset

private void assignWireOffset(Object obj)
                       throws IOException

growWireHash2Handle

private void growWireHash2Handle()

hashInsert

private void hashInsert(Object obj,
                        int offset)

addReplacement

private void addReplacement(Object orig,
                            Object replacement)

writeCode

private void writeCode(int tag)
                throws IOException

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

writeInternal

private void writeInternal(byte[] b,
                           int off,
                           int len,
                           boolean copyOnWrite)
                    throws IOException

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

bufferedWrite

private void bufferedWrite(byte[] b,
                           int off,
                           int len)
                    throws IOException

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

setBlockData

private boolean setBlockData(boolean mode)
                      throws IOException

writeBlockDataHeader

private void writeBlockDataHeader(int len)
                           throws IOException

writeCanonical

private void writeCanonical(byte[] b,
                            int off,
                            int len)
                     throws IOException

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

outputClassFields

private void outputClassFields(Object o,
                               Class cl,
                               ObjectStreamField[] fields)
                        throws IOException,
                               InvalidClassException

invokeObjectWriter

private void invokeObjectWriter(Object obj)
                         throws IOException