java.lang.Object java.io.OutputStream java.io.ObjectOutputStream
Direct Known Subclasses:
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
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
private void readObject(java.io.ObjectInputStream stream) throws IOException, ClassNotFoundException; private void writeObject(java.io.ObjectOutputStream stream) throws IOException private void readObjectNoData() throws ObjectStreamException;
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.
Enum constants are serialized differently than ordinary serializable or externalizable objects. The serialized form of an enum constant consists solely of its name; field values of the constant are not transmitted. To serialize an enum constant, ObjectOutputStream writes the string returned by the constant's name method. Like other serializable or externalizable objects, enum constants can function as the targets of back references appearing subsequently in the serialization stream. The process by which enum constants are serialized cannot be customized; any class-specific writeObject and writeReplace methods defined by enum types are ignored during serialization. Similarly, any serialPersistentFields or serialVersionUID field declarations are also ignored--all enum types have a fixed serialVersionUID of 0L.
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.
|Nested Class Summary:|
|abstract public static class||ObjectOutputStream.PutField||Provide programmatic access to the persistent fields to be written to ObjectOutput.|
If there is a security manager installed, this method first calls the
public ObjectOutputStream(OutputStream out) throws IOException
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.
|Method from java.io.ObjectOutputStream Summary:|
|annotateClass, annotateProxyClass, close, defaultWriteObject, drain, enableReplaceObject, flush, getProtocolVersion, putFields, replaceObject, reset, useProtocolVersion, write, write, write, writeBoolean, writeByte, writeBytes, writeChar, writeChars, writeClassDescriptor, writeDouble, writeFields, writeFloat, writeInt, writeLong, writeObject, writeObjectOverride, writeShort, writeStreamHeader, writeTypeString, writeUTF, writeUnshared|
|Methods from java.io.OutputStream:|
|close, flush, write, write, write|
|Methods from java.lang.Object:|
|clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait|
|Method from java.io.ObjectOutputStream Detail:|
protected void annotateClass(Class<?> cl) throws IOException
protected void annotateProxyClass(Class<?> cl) throws IOException
This method is called exactly once for each unique proxy class
descriptor in the stream. The default implementation of this method in
The corresponding method in
public void close() throws IOException
public void defaultWriteObject() throws IOException
protected void drain() throws IOException
protected boolean enableReplaceObject(boolean enable) throws SecurityException
public void flush() throws IOException
public PutField putFields() throws IOException
protected Object replaceObject(Object obj) throws IOException
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.
public void reset() throws IOException
public void useProtocolVersion(int version) throws IOException
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.
public void write(int val) throws IOException
public void write(byte buf) throws IOException
public void write(byte buf, int off, int len) throws IOException
public void writeBoolean(boolean val) throws IOException
public void writeByte(int val) throws IOException
public void writeBytes(String str) throws IOException
public void writeChar(int val) throws IOException
public void writeChars(String str) throws IOException
protected void writeClassDescriptor(ObjectStreamClass desc) throws IOException
Note that this method will only be called if the ObjectOutputStream
is not using the old serialization stream format (set by calling
public void writeDouble(double val) throws IOException
public void writeFields() throws IOException
public void writeFloat(float val) throws IOException
public void writeInt(int val) throws IOException
public void writeLong(long val) throws IOException
public final void writeObject(Object obj) throws IOException
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.
protected void writeObjectOverride(Object obj) throws IOException
public void writeShort(int val) throws IOException
protected void writeStreamHeader() throws IOException
void writeTypeString(String str) throws IOException
public void writeUTF(String str) throws IOException
public void writeUnshared(Object obj) throws IOException
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.