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## java.io
Class ObjectOutputStream

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

All Implemented Interfaces:
  DataOutput, 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 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.

Fields inherited from interface java.io.ObjectStreamConstants
`baseWireHandle, PROTOCOL_VERSION_1, PROTOCOL_VERSION_2, SC_BLOCK_DATA, 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_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[] 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.
`protected void` | `writeClassDescriptor(ObjectStreamClass classdesc)` Write the specified class descriptor to the ObjectOutputStream.
`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)` Method used by subclasses to override the default writeObject method.
`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 s)` 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.
  Parameters:
    `out` - `OutputStream` to read from
  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)`, `SerializablePermission`

Method Detail

### 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)`

### 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.
    `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`
  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.
  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

    public void writeFields()
                     throws IOException

  Write the buffered fields to the stream.
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream
    `NotActiveException` - Called when a classes writeObject method was not called to write the state of the object.
  Since:
    1.2

### 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.
  Throws:
    `IOException` - if reset() is invoked while serializing an object.

### 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.
  Parameters:
    `cl` - the class to annotate custom data for
  Throws:
    `IOException` - Any exception thrown by the underlying OutputStream.

### annotateProxyClass

    protected void annotateProxyClass(Class cl)
                               throws IOException

  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 writtem by `annotateProxyClass`.

  Parameters:
    `cl` - the proxy class to annotate custom data for
  Throws:
    `IOException` - any exception thrown by the underlying `OutputStream`
  Since:
    1.3
  See Also:
    `ObjectInputStream.resolveProxyClass(String[])`

### 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. 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.

  Parameters:
    `obj` - the object to be replaced
  Returns:
    the alternate object that replaced the specified one
  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.

  Parameters:
    `enable` - boolean parameter to enable replacement of objects
  Returns:
    the previous setting before this method was invoked
  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)`, `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.
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeClassDescriptor

    protected void writeClassDescriptor(ObjectStreamClass classdesc)
                                 throws IOException

  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.

  Parameters:
    `classdesc` - class descriptor to write to the stream
  Throws:
    `IOException` - If an I/O error has occurred.
  Since:
    1.3
  See Also:
    `ObjectInputStream.readClassDescriptor()`, `useProtocolVersion(int)`, `ObjectStreamConstants.PROTOCOL_VERSION_1`

### 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`
  Overrides:
    `write` in class `OutputStream`
  Parameters:
    `data` - the byte to be written to the stream
  Throws:
    `IOException` - If an I/O error has occurred.

### 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`
  Overrides:
    `write` in class `OutputStream`
  Parameters:
    `b` - the data to be written
  Throws:
    `IOException` - If an I/O error has occurred.

### write

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

  Writes a sub array of bytes.
  Specified by:
    `write` in interface `ObjectOutput`
  Overrides:
    `write` in class `OutputStream`
  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.

### 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`
  Overrides:
    `flush` in class `OutputStream`
  Throws:
    `IOException` - If an I/O error has occurred.

### 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.
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying 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`
  Overrides:
    `close` in class `OutputStream`
  Throws:
    `IOException` - If an I/O error has occurred.

### writeBoolean

    public void writeBoolean(boolean data)
                      throws IOException

  Writes a boolean.
  Specified by:
    `writeBoolean` in interface `DataOutput`
  Parameters:
    `data` - the boolean to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeByte

    public void writeByte(int data)
                   throws IOException

  Writes an 8 bit byte.
  Specified by:
    `writeByte` in interface `DataOutput`
  Parameters:
    `data` - the byte value to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeShort

    public void writeShort(int data)
                    throws IOException

  Writes a 16 bit short.
  Specified by:
    `writeShort` in interface `DataOutput`
  Parameters:
    `data` - the short value to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeChar

    public void writeChar(int data)
                   throws IOException

  Writes a 16 bit char.
  Specified by:
    `writeChar` in interface `DataOutput`
  Parameters:
    `data` - the char value to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeInt

    public void writeInt(int data)
                  throws IOException

  Writes a 32 bit int.
  Specified by:
    `writeInt` in interface `DataOutput`
  Parameters:
    `data` - the integer value to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeLong

    public void writeLong(long data)
                   throws IOException

  Writes a 64 bit long.
  Specified by:
    `writeLong` in interface `DataOutput`
  Parameters:
    `data` - the long value to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeFloat

    public void writeFloat(float data)
                    throws IOException

  Writes a 32 bit float.
  Specified by:
    `writeFloat` in interface `DataOutput`
  Parameters:
    `data` - the float value to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeDouble

    public void writeDouble(double data)
                     throws IOException

  Writes a 64 bit double.
  Specified by:
    `writeDouble` in interface `DataOutput`
  Parameters:
    `data` - the double value to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeBytes

    public void writeBytes(String data)
                    throws IOException

  Writes a String as a sequence of bytes.
  Specified by:
    `writeBytes` in interface `DataOutput`
  Parameters:
    `data` - the String of bytes to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeChars

    public void writeChars(String data)
                    throws IOException

  Writes a String as a sequence of chars.
  Specified by:
    `writeChars` in interface `DataOutput`
  Parameters:
    `data` - the String of chars to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeUTF

    public void writeUTF(String s)
                  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.
  Specified by:
    `writeUTF` in interface `DataOutput`
  Parameters:
    `s` - the String in UTF format
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

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