Class ObjectOutputStream
- All Implemented Interfaces:
Closeable
,DataOutput
,Flushable
,ObjectOutput
,ObjectStreamConstants
,AutoCloseable
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
:
try (FileOutputStream fos = new FileOutputStream("t.tmp");
ObjectOutputStream oos = new ObjectOutputStream(fos)) {
oos.writeObject("Today");
oos.writeObject(LocalDateTime.now());
} catch (Exception ex) {
// handle exception
}
Serializable classes that require special handling during the serialization and deserialization process should implement methods with the following 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 method name, modifiers, return type, and number and type of parameters must match exactly for the method to be used by serialization or deserialization. The methods should only be declared to throw checked exceptions consistent with these signatures.
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.
Records are serialized differently than ordinary serializable or externalizable objects, see record serialization.
- Since:
- 1.1
- External Specifications
- See Also:
-
Nested Class Summary
Modifier and TypeClassDescriptionstatic class
Provide programmatic access to the persistent fields to be written to ObjectOutput. -
Field Summary
Fields inherited from interface java.io.ObjectStreamConstants
baseWireHandle, PROTOCOL_VERSION_1, PROTOCOL_VERSION_2, SC_BLOCK_DATA, SC_ENUM, SC_EXTERNALIZABLE, SC_SERIALIZABLE, SC_WRITE_METHOD, SERIAL_FILTER_PERMISSION, 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 Summary
ModifierConstructorDescriptionprotected
Provide a way for subclasses that are completely reimplementing ObjectOutputStream to not have to allocate private data just used by this implementation of ObjectOutputStream.Creates an ObjectOutputStream that writes to the specified OutputStream. -
Method Summary
Modifier and TypeMethodDescriptionprotected void
annotateClass
(Class<?> cl) Subclasses may implement this method to allow class data to be stored in the stream.protected void
annotateProxyClass
(Class<?> cl) Subclasses may implement this method to store custom data in the stream along with descriptors for dynamic proxy classes.void
close()
Closes the stream.void
Write the non-static and non-transient fields of the current class to this stream.protected void
drain()
Drain any buffered data in ObjectOutputStream.protected boolean
enableReplaceObject
(boolean enable) Enables the stream to do replacement of objects written to the stream.void
flush()
Flushes the stream.Retrieve the object used to buffer persistent fields to be written to the stream.protected Object
replaceObject
(Object obj) This method will allow trusted subclasses of ObjectOutputStream to substitute one object for another during serialization.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.protected void
Write the specified class descriptor to the ObjectOutputStream.void
writeDouble
(double val) Writes a 64-bit double.void
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.final void
writeObject
(Object obj) Write the specified object to the ObjectOutputStream.protected void
Method used by subclasses to override the default writeObject method.void
writeShort
(int val) Writes a 16-bit short.protected void
The writeStreamHeader method is provided so subclasses can append or prepend their own header to the stream.void
writeUnshared
(Object obj) Writes an "unshared" object to the ObjectOutputStream.void
Primitive data write of this String in modified UTF-8 format.Methods inherited from class java.io.OutputStream
nullOutputStream
-
Constructor Details
-
ObjectOutputStream
Creates an ObjectOutputStream that writes to the specified OutputStream. This constructor writes the serialization stream header to the underlying stream; callers may wish to flush the stream immediately to ensure that constructors for receiving ObjectInputStreams will not block when reading the header.- Parameters:
out
- output stream to write to- Throws:
IOException
- if an I/O error occurs while writing stream headerNullPointerException
- ifout
isnull
- Since:
- 1.4
- See Also:
-
ObjectOutputStream
Provide a way for subclasses that are completely reimplementing ObjectOutputStream to not have to allocate private data just used by this implementation of ObjectOutputStream.- Throws:
IOException
- if an I/O error occurs while creating this stream
-
-
Method Details
-
useProtocolVersion
Specify stream protocol version to use when writing the stream.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.
- Parameters:
version
- use ProtocolVersion from java.io.ObjectStreamConstants.- Throws:
IllegalStateException
- if called after any objects have been serialized.IllegalArgumentException
- if invalid version is passed in.IOException
- if I/O errors occur- Since:
- 1.2
- See Also:
-
writeObject
Write the specified object to the ObjectOutputStream. The class of the object, the signature of the class, and the values of the non-transient and non-static fields of the class and all of its supertypes are written. Default serialization for a class can be overridden using the writeObject and the readObject methods. Objects referenced by this object are written transitively so that a complete equivalent graph of objects can be reconstructed by an ObjectInputStream.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.
- Specified by:
writeObject
in interfaceObjectOutput
- Parameters:
obj
- the object to be written- Throws:
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.
-
writeObjectOverride
Method used by subclasses to override the default writeObject method. This method is called by trusted subclasses of ObjectOutputStream that constructed ObjectOutputStream using the protected no-arg constructor. The subclass is expected to provide an override method with the modifier "final".- Parameters:
obj
- object to be written to the underlying stream- Throws:
IOException
- if there are I/O errors while writing to the underlying stream- Since:
- 1.2
- See Also:
-
defaultWriteObject
Write the non-static and non-transient fields of the current class to this stream. This may only be called from the writeObject method of the class being serialized. It will throw the NotActiveException if it is called otherwise.- Throws:
IOException
- if I/O errors occur while writing to the underlyingOutputStream
-
putFields
Retrieve the object used to buffer persistent fields to be written to the stream. The fields will be written to the stream when writeFields method is called.- Returns:
- an instance of the class Putfield that holds the serializable fields
- Throws:
IOException
- if I/O errors occur- Since:
- 1.2
-
writeFields
Write the buffered fields to the stream.- Throws:
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.- Since:
- 1.2
-
reset
Reset will disregard the state of any objects already written to the stream. The state is reset to be the same as a new ObjectOutputStream. The current point in the stream is marked as reset so the corresponding ObjectInputStream will be reset at the same point. Objects previously written to the stream will not be referred to as already being in the stream. They will be written to the stream again.- Throws:
IOException
- if reset() is invoked while serializing an object.
-
annotateClass
Subclasses may implement this method to allow class data to be stored in the stream. By default this method does nothing. The corresponding method in ObjectInputStream is resolveClass. This method is called exactly once for each unique class in the stream. The class name and signature will have already been written to the stream. This method may make free use of the ObjectOutputStream to save any representation of the class it deems suitable (for example, the bytes of the class file). The resolveClass method in the corresponding subclass of ObjectInputStream must read and use any data or objects written by annotateClass.- Parameters:
cl
- the class to annotate custom data for- Throws:
IOException
- Any exception thrown by the underlying OutputStream.
-
annotateProxyClass
Subclasses may implement this method to store custom data in the stream along with descriptors for dynamic proxy classes.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
isresolveProxyClass
. For a given subclass ofObjectOutputStream
that overrides this method, theresolveProxyClass
method in the corresponding subclass ofObjectInputStream
must read any data or objects written byannotateProxyClass
.- Parameters:
cl
- the proxy class to annotate custom data for- Throws:
IOException
- any exception thrown by the underlyingOutputStream
- Since:
- 1.3
- See Also:
-
replaceObject
This method will allow trusted subclasses of ObjectOutputStream to substitute one object for another during serialization. Replacing objects is disabled until enableReplaceObject is called. The enableReplaceObject method checks that the stream requesting to do replacement can be trusted. The first occurrence of each object written into the serialization stream is passed to replaceObject. Subsequent references to the object are replaced by the object returned by the original call to replaceObject. To ensure that the private state of objects is not unintentionally exposed, only trusted streams may use replaceObject.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 ensure 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
NullPointerException
in classes that contain references to the original object since they may be expecting an object instead of null.- Parameters:
obj
- the object to be replaced- Returns:
- the alternate object that replaced the specified one
- Throws:
IOException
- Any exception thrown by the underlying OutputStream.
-
enableReplaceObject
protected boolean enableReplaceObject(boolean enable) Enables the stream to do replacement of objects written to the stream. When enabled, thereplaceObject(java.lang.Object)
method is called for every object being serialized.- Parameters:
enable
- true for enabling use ofreplaceObject
for every object being serialized- Returns:
- the previous setting before this method was invoked
-
writeStreamHeader
The writeStreamHeader method is provided so subclasses can append or prepend their own header to the stream. It writes the magic number and version to the stream.- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeClassDescriptor
Write the specified class descriptor to the ObjectOutputStream. Class descriptors are used to identify the classes of objects written to the stream. Subclasses of ObjectOutputStream may override this method to customize the way in which class descriptors are written to the serialization stream. The corresponding method in ObjectInputStream,readClassDescriptor
, 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 Java 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.- Parameters:
desc
- class descriptor to write to the stream- Throws:
IOException
- If an I/O error has occurred.- Since:
- 1.3
- External Specifications
- See Also:
-
write
Writes a byte. This method will block until the byte is actually written.- Specified by:
write
in interfaceDataOutput
- Specified by:
write
in interfaceObjectOutput
- Specified by:
write
in classOutputStream
- Parameters:
val
- the byte to be written to the stream- Throws:
IOException
- If an I/O error has occurred.
-
write
Writes an array of bytes. This method will block until the bytes are actually written.- Specified by:
write
in interfaceDataOutput
- Specified by:
write
in interfaceObjectOutput
- Overrides:
write
in classOutputStream
- Parameters:
buf
- the data to be written- Throws:
IOException
- If an I/O error has occurred.- See Also:
-
write
Writes a sub array of bytes.- Specified by:
write
in interfaceDataOutput
- Specified by:
write
in interfaceObjectOutput
- Overrides:
write
in classOutputStream
- Parameters:
buf
- the data to be writtenoff
- the start offset in the datalen
- the number of bytes that are written- Throws:
IOException
- if an I/O error occurs. In particular, anIOException
is thrown if the output stream is closed.IndexOutOfBoundsException
- Ifoff
is negative,len
is negative, orlen
is greater thanb.length - off
-
flush
Flushes the stream. This will write any buffered output bytes and flush through to the underlying stream.- Specified by:
flush
in interfaceFlushable
- Specified by:
flush
in interfaceObjectOutput
- Overrides:
flush
in classOutputStream
- Throws:
IOException
- if an I/O error occurs.
-
drain
Drain any buffered data in ObjectOutputStream. Similar to flush but does not propagate the flush to the underlying stream.- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
close
Closes the stream. This method must be called to release any resources associated with the stream.- Specified by:
close
in interfaceAutoCloseable
- Specified by:
close
in interfaceCloseable
- Specified by:
close
in interfaceObjectOutput
- Overrides:
close
in classOutputStream
- Throws:
IOException
- If an I/O error has occurred.
-
writeBoolean
Writes a boolean.- Specified by:
writeBoolean
in interfaceDataOutput
- Parameters:
val
- the boolean to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeByte
Writes an 8-bit byte.- Specified by:
writeByte
in interfaceDataOutput
- Parameters:
val
- the byte value to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeShort
Writes a 16-bit short.- Specified by:
writeShort
in interfaceDataOutput
- Parameters:
val
- the short value to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeChar
Writes a 16-bit char.- Specified by:
writeChar
in interfaceDataOutput
- Parameters:
val
- the char value to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeInt
Writes a 32-bit int.- Specified by:
writeInt
in interfaceDataOutput
- Parameters:
val
- the integer value to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeLong
Writes a 64-bit long.- Specified by:
writeLong
in interfaceDataOutput
- Parameters:
val
- the long value to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeFloat
Writes a 32-bit float.- Specified by:
writeFloat
in interfaceDataOutput
- Parameters:
val
- the float value to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeDouble
Writes a 64-bit double.- Specified by:
writeDouble
in interfaceDataOutput
- Parameters:
val
- the double value to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeBytes
Writes a String as a sequence of bytes.- Specified by:
writeBytes
in interfaceDataOutput
- Parameters:
str
- the String of bytes to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeChars
Writes a String as a sequence of chars.- Specified by:
writeChars
in interfaceDataOutput
- Parameters:
str
- the String of chars to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-
writeUTF
Primitive data write of this String in modified UTF-8 format. Note that there is a significant difference between writing a String into the stream as primitive data or as an Object. A String instance written by writeObject is written into the stream as a String initially. Future writeObject() calls write references to the string into the stream.- Specified by:
writeUTF
in interfaceDataOutput
- Parameters:
str
- the String to be written- Throws:
IOException
- if I/O errors occur while writing to the underlying stream
-