public class ObjectOutputStream extends OutputStream implements ObjectOutput, ObjectStreamConstants
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.
DataOutput,
ObjectInputStream,
Serializable,
Externalizable,
Object Serialization Specification, Section 2, Object Output Classes| Modifier and Type | Class and Description |
|---|---|
static class |
ObjectOutputStream.PutField
Provide programmatic access to the persistent fields to be written
to ObjectOutput.
|
baseWireHandle, PROTOCOL_VERSION_1, PROTOCOL_VERSION_2, SC_BLOCK_DATA, SC_ENUM, 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_ENUM, TC_EXCEPTION, TC_LONGSTRING, TC_MAX, TC_NULL, TC_OBJECT, TC_PROXYCLASSDESC, TC_REFERENCE, TC_RESET, TC_STRING| Constructor and Description |
|---|
ObjectOutputStream(OutputStream out)
Creates an ObjectOutputStream that writes to the specified OutputStream.
|
| Modifier and Type | Method and Description |
|---|---|
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
modified UTF-8
format.
|
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.
out - output stream to write toIOException - if an I/O error occurs while writing stream headerSecurityException - if untrusted subclass illegally overrides
security-sensitive methodsNullPointerException - if out is nullObjectOutputStream(),
putFields(),
ObjectInputStream.ObjectInputStream(InputStream)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.
version - use ProtocolVersion from java.io.ObjectStreamConstants.IllegalStateException - if called after any objects
have been serialized.IllegalArgumentException - if invalid version is passed in.IOException - if I/O errors occurObjectStreamConstants.PROTOCOL_VERSION_1,
ObjectStreamConstants.PROTOCOL_VERSION_2public 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.
writeObject in interface ObjectOutputobj - the object to be writtenInvalidClassException - 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.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.
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 serializationpublic void defaultWriteObject()
throws IOException
IOException - if I/O errors occur while writing to the underlying
OutputStreampublic ObjectOutputStream.PutField putFields() throws IOException
IOException - if I/O errors occurpublic void writeFields()
throws IOException
IOException - 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.public void reset()
throws IOException
IOException - if reset() is invoked while serializing an object.public void write(int val)
throws IOException
write in interface DataOutputwrite in interface ObjectOutputwrite in class OutputStreamval - the byte to be written to the streamIOException - If an I/O error has occurred.public void write(byte[] buf)
throws IOException
write in interface DataOutputwrite in interface ObjectOutputwrite in class OutputStreambuf - the data to be writtenIOException - If an I/O error has occurred.OutputStream.write(byte[], int, int)public void write(byte[] buf,
int off,
int len)
throws IOException
write in interface DataOutputwrite in interface ObjectOutputwrite in class OutputStreambuf - 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.public void flush()
throws IOException
flush in interface Flushableflush in interface ObjectOutputflush in class OutputStreamIOException - If an I/O error has occurred.public void close()
throws IOException
close in interface Closeableclose in interface ObjectOutputclose in interface AutoCloseableclose in class OutputStreamIOException - If an I/O error has occurred.public void writeBoolean(boolean val)
throws IOException
writeBoolean in interface DataOutputval - the boolean to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeByte(int val)
throws IOException
writeByte in interface DataOutputval - the byte value to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeShort(int val)
throws IOException
writeShort in interface DataOutputval - the short value to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeChar(int val)
throws IOException
writeChar in interface DataOutputval - the char value to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeInt(int val)
throws IOException
writeInt in interface DataOutputval - the integer value to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeLong(long val)
throws IOException
writeLong in interface DataOutputval - the long value to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeFloat(float val)
throws IOException
writeFloat in interface DataOutputval - the float value to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeDouble(double val)
throws IOException
writeDouble in interface DataOutputval - the double value to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeBytes(String str) throws IOException
writeBytes in interface DataOutputstr - the String of bytes to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeChars(String str) throws IOException
writeChars in interface DataOutputstr - the String of chars to be writtenIOException - if I/O errors occur while writing to the underlying
streampublic void writeUTF(String str) throws IOException
writeUTF in interface DataOutputstr - the String to be writtenIOException - if I/O errors occur while writing to the underlying
stream