ObjectOutputStream

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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 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.

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
Version:
  1.105, 08/09/01
Author:
  Mike Warres, Roger Riggs
See Also:
  `DataOutput`, `ObjectInputStream`, `Serializable`, `Externalizable`, Object Serialization Specification, Section 2, Object Output Classes

Nested Class Summary
`static class` | `ObjectOutputStream.PutField` Provide programatic 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_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
`ObjectOutputStream(OutputStream out)` Creates an ObjectOutputStream that writes to the specified OutputStream.

Method Summary
`void` | `close()` Closes the stream.
`void` | `defaultWriteObject()` Write the non-static and non-transient fields of the current class to this stream.
`void` | `flush()` Flushes the stream.
`ObjectOutputStream.PutField` | `putFields()` Retrieve the object used to buffer persistent fields to be written to the stream.
`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.
`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.
`void` | `writeShort(int val)` Writes a 16 bit short.
`void` | `writeUnshared(Object obj)` Writes an "unshared" object to the ObjectOutputStream.
`void` | `writeUTF(String str)` Primitive data write of this String in UTF format.

Methods inherited from class java.lang.Object
`equals, 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)`

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.

### 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

    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.

### write

    public void write(int val)
               throws IOException

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

  Specified by:
    `write` in interface `DataOutput`
  Specified by:
    `write` in interface `ObjectOutput`
  Specified by:
    `write` in class `OutputStream`
  Parameters:
    `val` - the byte to be written to the stream
  Throws:
    `IOException` - If an I/O error has occurred.

### write

    public void write(byte[] buf)
               throws IOException

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

  Specified by:
    `write` in interface `DataOutput`
  Specified by:
    `write` in interface `ObjectOutput`
  Overrides:
    `write` in class `OutputStream`
  Parameters:
    `buf` - the data to be written
  Throws:
    `IOException` - If an I/O error has occurred.
  See Also:
    `OutputStream.write(byte[], int, int)`

### write

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

  Writes a sub array of bytes.

  Specified by:
    `write` in interface `DataOutput`
  Specified by:
    `write` in interface `ObjectOutput`
  Overrides:
    `write` in class `OutputStream`
  Parameters:
    `buf` - the data to be written
    `off` - the start offset in the data
    `len` - the number of bytes that are written
    `copyOnWrite` - do not expose b to overrides of ObjectStream.write, copy the contents of b to a buffer before writing.
  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.

### 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 val)
                      throws IOException

  Writes a boolean.

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

### writeByte

    public void writeByte(int val)
                   throws IOException

  Writes an 8 bit byte.

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

### writeShort

    public void writeShort(int val)
                    throws IOException

  Writes a 16 bit short.

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

### writeChar

    public void writeChar(int val)
                   throws IOException

  Writes a 16 bit char.

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

### writeInt

    public void writeInt(int val)
                  throws IOException

  Writes a 32 bit int.

  Specified by:
    `writeInt` in interface `DataOutput`
  Parameters:
    `val` - the integer value to be written
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

### writeLong

    public void writeLong(long val)
                   throws IOException

  Writes a 64 bit long.

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

### writeFloat

    public void writeFloat(float val)
                    throws IOException

  Writes a 32 bit float.

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

### writeDouble

    public void writeDouble(double val)
                     throws IOException

  Writes a 64 bit double.

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

### writeBytes

    public void writeBytes(String str)
                    throws IOException

  Writes a String as a sequence of bytes.

  Specified by:
    `writeBytes` in interface `DataOutput`
  Parameters:
    `str` - 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 str)
                    throws IOException

  Writes a String as a sequence of chars.

  Specified by:
    `writeChars` in interface `DataOutput`
  Parameters:
    `str` - 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 str)
                  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:
    `str` - the String in UTF format
  Throws:
    `IOException` - if I/O errors occur while writing to the underlying stream

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