Java Object Serialization Specification: 2

The ObjectOutputStream Class
The ObjectOutputStream.PutField Class
The writeObject Method
The writeExternal Method
The writeReplace Method
The useProtocolVersion Method

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 writeUnshared(Object obj)
        throws IOException;

    public void defaultWriteObject()
        throws IOException, NotActiveException;

    public PutField putFields()
        throws IOException;

    public void writeFields()
        throws IOException;

    public void reset() throws IOException;

    protected void annotateClass(Class<?> cl) throws IOException;

    protected void annotateProxyClass(Class<?> cl) throws IOException;

    protected void writeClassDescriptor(ObjectStreamClass desc)
        throws IOException;

    protected Object replaceObject(Object obj) throws IOException;

    protected boolean enableReplaceObject(boolean enable);

    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.
    public abstract static class PutField
    {
        public void put(String name, boolean value)
            throws IllegalArgumentException;

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

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

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

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

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

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

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

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

    public void useProtocolVersion(int version) throws IOException;

    protected ObjectOutputStream()
        throws IOException;

    protected writeObjectOverride()
        throws NotActiveException, IOException;
}

The single-argument ObjectOutputStream constructor creates an ObjectOutputStream that serializes objects to the given OutputStream. The constructor calls writeStreamHeader to write a magic number and version to the stream that will be read and verified by a corresponding call to readStreamHeader in the single-argument ObjectInputStream constructor.

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

If a subclass is overriding the implementation, call the writeObjectOverride method and return. Overriding the implementation is described at the end of this section.
If there is data in the block-data buffer, the data is written to the stream and the buffer is reset.
If the object is null, null is put in the stream and writeObject returns.
If the object has been previously replaced, as described in Step 8, write the handle of the replacement to the stream and writeObject returns.
If the object has already been written to the stream, its handle is written to the stream and writeObject returns.
If the object is a Class, the corresponding ObjectStreamClass is written to the stream, a handle is assigned for the class, and writeObject returns.
If the object is an ObjectStreamClass, a handle is assigned to the object, after which it is written to the stream using one of the class descriptor formats described in Section 4.3, "Serialized Form". In versions 1.3 and later of the Java 2 SDK, Standard Edition, the writeClassDescriptor method is called to output the ObjectStreamClass if it represents a class that is not a dynamic proxy class, as determined by passing the associated Class object to the isProxyClass method of java.lang.reflect.Proxy. Afterwards, an annotation for the represented class is written: if the class is a dynamic proxy class, then the annotateProxyClass method is called; otherwise, the annotateClass method is called. The writeObject method then returns.
Process potential substitutions by the class of the object and/or by a subclass of ObjectInputStream.
1. If the class of an object is not an enum type and 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.
If the object is a java.lang.String, the string is written as length information followed by the contents of the string encoded in modified UTF-8. For details, refer to Section 6.2, "Stream Elements". A handle is assigned to the string, and writeObject returns.
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.
If the object is an enum constant, the ObjectStreamClass for the enum type of the constant is written by recursively calling writeObject. It will appear in the stream only the first time it is referenced. A handle is assigned for the enum constant. Next, the value returned by the name method of the enum constant is written as a String object, as described in step 9. Note that if the same name string has appeared previously in the stream, a back reference to it will be written. The writeObject method then returns.
If the object is a record object, the ObjectStreamClass for the class of the record 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 the record object.

The components of the record object are written to the stream.
```
 a.  If the record object is serializable or externalizable, the record
     components are written, as if by invoking the `defaultWriteObject`
     method.

 b.  If the object is neither serializable or externalizable, the
     `NotSerializableException` is thrown.
```
The writeObject method then returns.

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 the object.

The contents of the object are written to the stream.

a.  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.

b.  If the object is externalizable, the `writeExternal` method of the
    object is called.

c.  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 writeUnshared method 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 the ObjectInputStream.readUnshared method (see Section 3.1, "The ObjectInputStream Class") 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.

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 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 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 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. 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.

2.2 The ObjectOutputStream.PutField Class

Class PutField provides the API for setting values of the serializable fields for a class when the class does not use default serialization. Each method puts the specified named value into the stream. An IllegalArgumentException is thrown if name does not match the name of a serializable field for the class whose fields are being written, or if the type of the named field does not match the second parameter type of the specific put method invoked.

2.3 The writeObject Method

For serializable objects, the writeObject method allows a class to control the serialization of its own fields. Here is its signature:

private void writeObject(ObjectOutputStream stream)
    throws IOException;

Each subclass of a serializable object may define its own writeObject method. If a class does not implement the method, the default serialization provided by defaultWriteObject will be used. When implemented, the class is only responsible for writing its own fields, not those of its supertypes or subtypes.

The class's writeObject method, if implemented, is responsible for saving the state of the class. Either ObjectOutputStream's defaultWriteObject or writeFields method must be called once (and only once) before writing any optional data that will be needed by the corresponding readObject method to restore the state of the object; even if no optional data is written, defaultWriteObject or writeFields must still be invoked once. If defaultWriteObject or writeFields is not invoked once prior to the writing of optional data (if any), then the behavior of instance deserialization is undefined in cases where the ObjectInputStream cannot resolve the class which defined the writeObject method in question.

The responsibility for the format, structure, and versioning of the optional data lies completely with the class.

2.4 The writeExternal Method

Objects implementing java.io.Externalizable must implement the writeExternal method to save the entire state of the object. It must coordinate with its superclasses to save their state. All of the methods of ObjectOutput are available to save the object's primitive typed fields and object fields.

public void writeExternal(ObjectOutput stream)
    throws IOException;

A new default format for writing Externalizable data has been introduced in JDK 1.2. The new format specifies that primitive data will be written in block data mode by writeExternal methods. Additionally, a tag denoting the end of the External object is appended to the stream after the writeExternal method returns. The benefits of this format change are discussed in Section 3.6, "The readExternal Method". Compatibility issues caused by this change are discussed in Section 2.6, "The useProtocolVersion Method".

2.5 The writeReplace Method

For Serializable and Externalizable classes, the writeReplace method allows a class of an object to nominate its own replacement in the stream before the object is written. By implementing the writeReplace method, a class can directly control the types and instances of its own instances being serialized.

The method is defined as follows:

ANY-ACCESS-MODIFIER Object writeReplace()
             throws ObjectStreamException;

The writeReplace method is called when ObjectOutputStream is preparing to write the object to the stream. The ObjectOutputStream checks whether the class defines the writeReplace method. If the method is defined, the writeReplace method is called to allow the object to designate its replacement in the stream. The object returned should be either of the same type as the object passed in or an object that when read and resolved will result in an object of a type that is compatible with all references to the object. If it is not, a ClassCastException will occur when the type mismatch is discovered.

2.6 The useProtocolVersion Method

Due to a stream protocol change that was not backwards compatible, a mechanism has been added to enable the current Virtual Machine to write a serialization stream that is readable by a previous release. Of course, the problems that are corrected by the new stream format will exist when using the backwards compatible protocol.

Stream protocol versions are discussed in Section 6.3, "Stream Protocol Versions".