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
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.
DataOutputObjectInputStreamSerializableExternalizableIf 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.
out output stream to write toIOException if an I/O error occurs while writing stream headerjava.lang.SecurityException if untrusted subclass illegally overrides
security-sensitive methodsjava.lang.NullPointerException if out is nullObjectOutputStream() putFields()ObjectInputStream.(java.io.InputStream) 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.
java.lang.SecurityException if a security manager exists and its
checkPermission method denies enabling
subclassing.java.lang.SecurityManager.checkPermission(java.security.Permission)SerializablePermissionThis 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.
version use ProtocolVersion from java.io.ObjectStreamConstants.java.lang.IllegalStateException if called after any objects
have been serialized.java.lang.IllegalArgumentException if invalid version is passed in.IOException if I/O errors occurObjectStreamConstants.PROTOCOL_VERSION_1ObjectStreamConstants.PROTOCOL_VERSION_2Exceptions 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.
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.obj object to be written to the underlying streamIOException if there are I/O errors while writing to the
underlying streamObjectOutputStream() writeObject(java.lang.Object)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.
obj object to write to streamNotSerializableException if an object in the graph to be
serialized does not implement the Serializable interfaceInvalidClassException if a problem exists with the class of an
object to be serializedIOException if an I/O error occurs during serializationIOException if I/O errors occur while writing to the underlying
OutputStreamIOException if I/O errors occurIOException if I/O errors occur while writing to the underlying
streamNotActiveException Called when a classes writeObject method was
not called to write the state of the object.IOException if reset() is invoked while serializing an object.cl the class to annotate custom data forIOException Any exception thrown by the underlying
OutputStream.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 written by
annotateProxyClass.
cl the proxy class to annotate custom data forIOException any exception thrown by the underlying
OutputStreamObjectInputStream.resolveProxyClass(java.lang.String[])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.
obj the object to be replacedIOException Any exception thrown by the underlying
OutputStream.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.
enable boolean parameter to enable replacement of objectsjava.lang.SecurityException if a security manager exists and its
checkPermission method denies enabling the stream
to do replacement of objects in the stream.java.lang.SecurityManager.checkPermission(java.security.Permission)SerializablePermissionIOException if I/O errors occur while writing to the underlying
streamreadClassDescriptor, 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.
desc class descriptor to write to the streamIOException If an I/O error has occurred.ObjectInputStream.readClassDescriptor()useProtocolVersion(int)ObjectStreamConstants.PROTOCOL_VERSION_1val the byte to be written to the streamIOException If an I/O error has occurred.buf the data to be writtenIOException If an I/O error has occurred.buf the data to be writtenoff the start offset in the datalen the number of bytes that are writtenIOException If an I/O error has occurred.IOException If an I/O error has occurred.IOException if I/O errors occur while writing to the underlying
streamIOException If an I/O error has occurred.val the boolean to be writtenIOException if I/O errors occur while writing to the underlying
streamval the byte value to be writtenIOException if I/O errors occur while writing to the underlying
streamval the short value to be writtenIOException if I/O errors occur while writing to the underlying
streamval the char value to be writtenIOException if I/O errors occur while writing to the underlying
streamval the integer value to be writtenIOException if I/O errors occur while writing to the underlying
streamval the long value to be writtenIOException if I/O errors occur while writing to the underlying
streamval the float value to be writtenIOException if I/O errors occur while writing to the underlying
streamval the double value to be writtenIOException if I/O errors occur while writing to the underlying
streamstr the String of bytes to be writtenIOException if I/O errors occur while writing to the underlying
streamstr the String of chars to be writtenIOException if I/O errors occur while writing to the underlying
streamstr the String to be writtenIOException if I/O errors occur while writing to the underlying
streamname the name of the serializable fieldval the value to assign to the fieldjava.lang.IllegalArgumentException 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 is not
booleanname the name of the serializable fieldval the value to assign to the fieldjava.lang.IllegalArgumentException 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 is not
bytename the name of the serializable fieldval the value to assign to the fieldjava.lang.IllegalArgumentException 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 is not
charname the name of the serializable fieldval the value to assign to the fieldjava.lang.IllegalArgumentException 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 is not
shortname the name of the serializable fieldval the value to assign to the fieldjava.lang.IllegalArgumentException 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 is not
intname the name of the serializable fieldval the value to assign to the fieldjava.lang.IllegalArgumentException 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 is not
longname the name of the serializable fieldval the value to assign to the fieldjava.lang.IllegalArgumentException 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 is not
floatname the name of the serializable fieldval the value to assign to the fieldjava.lang.IllegalArgumentException 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 is not
doublename the name of the serializable fieldval the value to assign to the field
(which may be null)java.lang.IllegalArgumentException 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 is not a
reference typePutField object.
PutField object in a proper format, and may
result in corruption of the serialization stream. The
correct way to write PutField data is by
calling the ObjectOutputStream.writeFields()out the stream to write the data and fields toIOException if I/O errors occur while writing to the
underlying streamjava.lang.IllegalArgumentException if the specified stream is not
the same stream that produced this PutField
object