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java.lang.Objectjava.io.OutputStream
java.io.ObjectOutputStream
public class ObjectOutputStream
An ObjectOutputStream writes primitive data types and graphs of Java objects to an OutputStream. The objects can be read (reconstituted) using an ObjectInputStream. Persistent storage of objects can be accomplished by using a file for the stream. If the stream is a network socket stream, the objects can be reconsituted on another host or in another process.
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
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.
DataOutput
,
ObjectInputStream
,
Serializable
,
Externalizable
,
Object Serialization Specification, Section 2, Object Output ClassesNested Class Summary | |
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static class |
ObjectOutputStream.PutField
Provide programatic access to the persistent fields to be written to ObjectOutput. |
Field Summary |
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Constructor Summary | |
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ObjectOutputStream(OutputStream out)
Creates an ObjectOutputStream that writes to the specified OutputStream. |
Method Summary | |
---|---|
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 UTF format. |
Methods inherited from class java.lang.Object |
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equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Constructor Detail |
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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 to
IOException
- if an I/O error occurs while writing stream header
SecurityException
- if untrusted subclass illegally overrides
security-sensitive methods
NullPointerException
- if out
is null
ObjectOutputStream()
,
putFields()
,
ObjectInputStream.ObjectInputStream(InputStream)
Method Detail |
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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_2
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.
writeObject
in interface ObjectOutput
obj
- the object to be written
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.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 stream
NotSerializableException
- if an object in the graph to be
serialized does not implement the Serializable interface
InvalidClassException
- if a problem exists with the class of an
object to be serialized
IOException
- if an I/O error occurs during serializationpublic void defaultWriteObject() throws IOException
IOException
- if I/O errors occur while writing to the underlying
OutputStream
public 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
stream
NotActiveException
- 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 DataOutput
write
in interface ObjectOutput
write
in class OutputStream
val
- the byte to be written to the stream
IOException
- If an I/O error has occurred.public void write(byte[] buf) throws IOException
write
in interface DataOutput
write
in interface ObjectOutput
write
in class OutputStream
buf
- the data to be written
IOException
- 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 DataOutput
write
in interface ObjectOutput
write
in class OutputStream
buf
- the data to be writtenoff
- the start offset in the datalen
- the number of bytes that are writtencopyOnWrite
- do not expose b to overrides of ObjectStream.write,
copy the contents of b to a buffer before writing.
IOException
- If an I/O error has occurred.public void flush() throws IOException
flush
in interface ObjectOutput
flush
in class OutputStream
IOException
- If an I/O error has occurred.public void close() throws IOException
close
in interface ObjectOutput
close
in class OutputStream
IOException
- If an I/O error has occurred.public void writeBoolean(boolean val) throws IOException
writeBoolean
in interface DataOutput
val
- the boolean to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeByte(int val) throws IOException
writeByte
in interface DataOutput
val
- the byte value to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeShort(int val) throws IOException
writeShort
in interface DataOutput
val
- the short value to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeChar(int val) throws IOException
writeChar
in interface DataOutput
val
- the char value to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeInt(int val) throws IOException
writeInt
in interface DataOutput
val
- the integer value to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeLong(long val) throws IOException
writeLong
in interface DataOutput
val
- the long value to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeFloat(float val) throws IOException
writeFloat
in interface DataOutput
val
- the float value to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeDouble(double val) throws IOException
writeDouble
in interface DataOutput
val
- the double value to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeBytes(String str) throws IOException
writeBytes
in interface DataOutput
str
- the String of bytes to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeChars(String str) throws IOException
writeChars
in interface DataOutput
str
- the String of chars to be written
IOException
- if I/O errors occur while writing to the underlying
streampublic void writeUTF(String str) throws IOException
writeUTF
in interface DataOutput
str
- the String in UTF format
IOException
- if I/O errors occur while writing to the underlying
stream
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