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2.1 The ObjectOutputStream Class

Class ObjectOutputStream implements object serialization. It maintains the state of the stream including the set of objects already serialized. Its methods control the traversal of objects to be serialized to save the specified objects and the objects to which they refer.

package java.io;

public class ObjectOutputStream
    extends OutputStream
    implements ObjectOutput, ObjectStreamConstants
{
    public ObjectOutputStream(OutputStream out)
        throws IOException;

    public final void writeObject(Object obj)
        throws IOException;

    public void defaultWriteObject();
        throws IOException, NotActiveException;

    public PutField putFields()
        throws IOException;

    public writeFields()
        throws IOException;

    public void reset() throws IOException;

    protected void annotateClass(Class cl) throws IOException;

    protected void writeClassDescriptor(ObjectStreamClass desc)
        throws IOException;

    protected Object replaceObject(Object obj) throws IOException;

    protected boolean enableReplaceObject(boolean enable)
        throws SecurityException;

    protected void writeStreamHeader() throws IOException;

    public void write(int data) throws IOException;

    public void write(byte b[]) throws IOException;

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

    public void flush() throws IOException;

    protected void drain() throws IOException;

    public void close() throws IOException;

    public void writeBoolean(boolean data) throws IOException;

    public void writeByte(int data) throws IOException;

    public void writeShort(int data) throws IOException;

    public void writeChar(int data) throws IOException;

    public void writeInt(int data) throws IOException;

    public void writeLong(long data) throws IOException;

    public void writeFloat(float data) throws IOException;

    public void writeDouble(double data) throws IOException;

    public void writeBytes(String data) throws IOException;

    public void writeChars(String data) throws IOException;

    public void writeUTF(String data) throws IOException;

    // Inner class to provide access to serializable fields.
    abstract static public class PutField
    {
        public void put(String name, boolean value)
            throws IOException, IllegalArgumentException;

        public void put(String name, char data)
            throws IOException, IllegalArgumentException;

        public void put(String name, byte data)
            throws IOException, IllegalArgumentException;

        public void put(String name, short data)
            throws IOException, IllegalArgumentException;

        public void put(String name, int data)
            throws IOException, IllegalArgumentException;

        public void put(String name, long data)
            throws IOException, IllegalArgumentException;

        public void put(String name, float data)
            throws IOException, IllegalArgumentException;

        public void put(String name, double data)
            throws IOException, IllegalArgumentException;

        public void put(String name, Object data)
            throws IOException, IllegalArgumentException;
    }

    public void useProtocolVersion(int version) throws IOException;

    protected ObjectOutputStream()
        throws IOException;

     protected writeObjectOverride()
        throws NotActiveException, IOException;
}

The ObjectOutputStream constructor requires an OutputStream. The constructor calls writeStreamHeader to write a magic number and version to the stream, that will be read and verified by the corresponding readStreamHeader in the ObjectInputStream constructor.

The writeObject method is used to serialize an object to the stream. An object is serialized as follows:

1. If a subclass is overriding the implementation, call the writeObjectOverride method and return. Overriding the implementation is described at the end of this section.
2. If there is data in the block-data buffer, the data is written to the stream and the buffer is reset.
3. If the object is null, null is put in the stream and writeObject returns.
4. If the object has been previously replaced, as described in Step 8, write the handle of the replacement to the stream and writeObject returns.
5. If the object has already been written to the stream, its handle is written to the stream and writeObject returns.
6. If the object is a Class, the corresponding ObjectStreamClass is written to the stream, a handle is assigned for the class, and writeObject returns.
7. If the object is an ObjectStreamClass, a descriptor for the class is written to the stream including its name, serialVersionUID, and the list of fields by name and type. A handle is assigned for the descriptor. The annotateClass subclass method is called before writeObject returns.
8. Process potential substitutions by the class of the object and/or by a subclass of ObjectInputStream.
1. If the class of an object defines the appropriate writeReplace method, the method is called. Optionally, it can return a substitute object to be serialized.
2. Then, if enabled by calling the enableReplaceObject method, the replaceObject method is called to allow subclasses of ObjectOutputStream to substitute for the object being serialized. If the original object was replaced in the previous step, the replaceObject method is called with the replacement object.

If the original object was replaced by either one or both steps above, the mapping from the original object to the replacement is recorded for later use in Step 4. Then, Steps 3 through 7 are repeated on the new object.

If the replacement object is not one of the types covered by Steps 3 through 7, processing resumes using the replacement object at Step 10.

1. If the object is a java.lang.String, the string is written in Universal Transfer Format (UTF) format, or a variant of UTF for long strings (for details, refer to Section 6.2, "Stream Elements"). A handle is assigned to the string, and writeObject returns.
2. If the object is an array, writeObject is called recursively to write the ObjectStreamClass of the array. The handle for the array is assigned. It is followed by the length of the array. Each element of the array is then written to the stream, after which writeObject returns.
3. For regular objects, the ObjectStreamClass for the class of the object is written by recursively calling writeObject. It will appear in the stream only the first time it is referenced. A handle is assigned for this object. Starting in Java^TM 2 SDK, Standard Edition, v1.3, writeObject calls writeClassDescriptor to output the ObjectStreamClass object.
4. The contents of the object are written to the stream.
1. If the object is serializable, the highest serializable class is located. For that class, and each derived class, that class's fields are written. If the class does not have a writeObject method, the defaultWriteObject method is called to write the serializable fields to the stream. If the class does have a writeObject method, it is called. It may call defaultWriteObject or putFields and writeFields to save the state of the object, and then it can write other information to the stream.
2. If the object is externalizable, the writeExternal method of the object is called.
3. If the object is neither serializable or externalizable, the NotSerializableException is thrown.

Exceptions may occur during the traversal or may occur in the underlying stream. For any subclass of IOException, the exception is written to the stream using the exception protocol and the stream state is discarded. If a second IOException is thrown while attempting to write the first exception into the stream, the stream is left in an unknown state and StreamCorruptedException is thrown from writeObject. For other exceptions, the stream is aborted and left in an unknown and unusable state.

The defaultWriteObject method implements the default serialization mechanism for the current class. This method may be called only from a class's writeObject method. The method writes all of the serializable fields of the current class to the stream. If called from outside the writeObject method, the NotActiveException is thrown.

The putFields method returns a PutField object the caller uses to set the values of the serializable fields in the stream. The fields may be set in any order. After all of the fields have been set, writeFields must be called to write the field values in the canonical order to the stream. If a field is not set, the default value appropriate for its type will be written to the stream. This method may only be called from within the writeObject method of a serializable class. It may not be called more than once or if defaultWriteObject has been called. Only after writeFields has been called can other data be written to the stream.

The reset method resets the stream state to be the same as if it had just been constructed. Reset will discard the state of any objects already written to the stream. The current point in the stream is marked as reset, so the corresponding ObjectInputStream will reset at the same point. Objects previously written to the stream will not be remembered as already having been written to the stream. They will be written to the stream again. This is useful when the contents of an object or objects must be sent again. Reset may not be called while objects are being serialized. If called inappropriately, an IOException is thrown.

Starting with the Java^TM 2 SDK, Standard Edition, v1.3, the writeClassDescriptor method is called when an ObjectStreamClass needs to be serialized. writeClassDescriptor is responsible for writing a representation of the ObjectStreamClass to the serialization stream. Subclasses may override this method to customize the way in which class descriptors are written to the serialization stream. If this method is overridden, then the corresponding readClassDescriptor method in ObjectInputStream should also be overridden to reconstitute the class descriptor from its custom stream representation. By default, writeClassDescriptor writes class descriptors according to the format specified in Section 6.4, "Grammar for the Stream Format". Note that this method will only be called if the ObjectOutputStream is not using the old serialization stream format (see Section 6.3, "Stream Protocol Versions"). If the serialization stream is using the old format (ObjectStreamConstants.PROTOCOL_VERSION_1), the class descriptor will be written internally in a manner that cannot be overridden or customized.

The annotateClass method is called while a Class is being serialized, and after the class descriptor has been written to the stream. Subclasses may extend this method and write other information to the stream about the class. This information must be read by the resolveClass method in a corresponding ObjectInputStream subclass.

An ObjectOutputStream subclass can implement the replaceObject method to monitor or replace objects during serialization. Replacing objects must be enabled explicitly by calling enableReplaceObject before calling writeObject with the first object to be replaced. Once enabled, replaceObject is called for each object just prior to serializing the object for the first time. Note that the replaceObject method is not called for objects of the specially handled classes, Class and ObjectStreamClass. An implementation of a subclass may return a substitute object that will be serialized instead of the original. The substitute object must be serializable. All references in the stream to the original object will be replaced by the substitute object.

When objects are being replaced, the subclass must ensure that the substituted object is compatible with every field where the reference will be stored, or that a complementary substitution will be made during deserialization. Objects, whose type is not a subclass of the type of the field or array element, will later abort the deserialization by raising a ClassCastException and the reference will not be stored.

The enableReplaceObject method can be called by trusted subclasses of ObjectOutputStream to enable the substitution of one object for another during serialization. Replacing objects is disabled until enableReplaceObject is called with a true value. It may thereafter be disabled by setting it to false. The previous setting is returned. The enableReplaceObject method checks that the stream requesting the replacement can be trusted. To ensure that the private state of objects is not unintentionally exposed, only trusted stream subclasses may use replaceObject. Trusted classes are those classes that belong to a security protection domain with permission to enable Serializable substitution.

If the subclass of ObjectOutputStream is not considered part of the system domain, SerializablePermission "enableSubstitution" must be added to the security policy file. AccessControlException is thrown if the protection domain of the subclass of ObjectInputStream does not have permission to "enableSubstitution" by calling enableReplaceObject. See the document Java^TM Security Architecture (JDK1.2) for additional information about the security model.

The writeStreamHeader method writes the magic number and version to the stream. This information must be read by the readStreamHeader method of ObjectInputStream. Subclasses may need to implement this method to identify the stream's unique format.

The flush method is used to empty any buffers being held by the stream and to forward the flush to the underlying stream. The drain method may be used by subclassers to empty only the ObjectOutputStream's buffers without forcing the underlying stream to be flushed.

All of the write methods for primitive types encode their values using a DataOutputStream to put them in the standard stream format. The bytes are buffered into block data records so they can be distinguished from the encoding of objects. This buffering allows primitive data to be skipped if necessary for class versioning. It also allows the stream to be parsed without invoking class-specific methods.

To override the implementation of serialization, the subclass of ObjectOutputStream should call the protected no-arg ObjectOutputStream, constructor. There is a security check within the no-arg constructor for SerializablePermission "enableSubclassImplementation" to ensure that only trusted classes are allowed to override the default implementation. This constructor does not allocate any private data for ObjectOutputStream and sets a flag that indicates that the final writeObject method should invoke the writeObjectOverride method and return. All other ObjectOutputStream methods are not final and can be directly overridden by the subclass.