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java.lang.Object | +----java.io.OutputStream | +----java.io.ObjectOutputStream
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 ostream = new FileOutputStream("t.tmp"); ObjectOutputStream p = new ObjectOutputStream(ostream); p.writeInt(12345); p.writeObject("Today"); p.writeObject(new Date()); p.flush(); ostream.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
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. 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.
Inner Class Summary | |
static | ObjectOutputStream.PutField
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Constructor Summary | |
ObjectOutputStream(OutputStream out)
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ObjectOutputStream()
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Method Summary | |
void | annotateClass(Class cl)
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void | close()
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void | defaultWriteObject()
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void | drain()
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boolean | enableReplaceObject(boolean enable)
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void | flush()
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ObjectOutputStream.PutField | putFields()
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Object | replaceObject(Object obj)
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void | reset()
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void | useProtocolVersion(int version)
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.
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void | write(int data)
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void | write(byte[] b)
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void | write(byte[] b,
int off,
int len)
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void | writeBoolean(boolean data)
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void | writeByte(int data)
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void | writeBytes(String data)
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void | writeChar(int data)
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void | writeChars(String data)
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void | writeDouble(double data)
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void | writeFields()
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void | writeFloat(float data)
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void | writeInt(int data)
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void | writeLong(long data)
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void | writeObject(Object obj)
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void | writeObjectOverride(Object obj)
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void | writeShort(int data)
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void | writeStreamHeader()
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void | writeUTF(String data)
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Methods inherited from class java.io.OutputStream |
close, flush, write, write, write |
Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Constructor Detail |
public ObjectOutputStream(OutputStream out) throws IOException
protected ObjectOutputStream() throws IOException, SecurityException
permission SerializablePermission "enableSubclassImplementation" ;
Method Detail |
protected void writeObjectOverride(Object obj) 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.
version
- use ProtocolVersion from java.io.ObjectStreamConstants.
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.
public void defaultWriteObject() throws IOException
public ObjectOutputStream.PutField putFields() throws IOException
public void writeFields() throws IOException
public void reset() throws IOException
protected void annotateClass(Class cl) throws IOException
protected Object replaceObject(Object obj) throws IOException
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.
protected boolean enableReplaceObject(boolean enable) throws SecurityException
protected void writeStreamHeader() throws IOException
public void write(int data) throws IOException
b
- the byte
public void write(byte[] b) throws IOException
b
- the data to be written
public void write(byte[] b, int off, int len) throws IOException
b
- the data to be written
off
- the start offset in the data
len
- the number of bytes that are written
public void flush() throws IOException
protected void drain() throws IOException
public void close() throws IOException
public void writeBoolean(boolean data) throws IOException
data
- the boolean to be written
public void writeByte(int data) throws IOException
data
- the byte value to be written
public void writeShort(int data) throws IOException
data
- the short value to be written
public void writeChar(int data) throws IOException
data
- the char value to be written
public void writeInt(int data) throws IOException
data
- the integer value to be written
public void writeLong(long data) throws IOException
data
- the long value to be written
public void writeFloat(float data) throws IOException
data
- the float value to be written
public void writeDouble(double data) throws IOException
data
- the double value to be written
public void writeBytes(String data) throws IOException
s
- the String of bytes to be written
public void writeChars(String data) throws IOException
s
- the String of chars to be written
public void writeUTF(String data) throws IOException
str
- the String in UTF format
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