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Class java.io.ObjectInputStream

java.lang.Object
   |
   +----java.io.InputStream
           |
           +----java.io.ObjectInputStream

public class ObjectInputStream

extends InputStream

implements ObjectInput, ObjectStreamConstants

ObjectInputStream ensures that the types of all objects in the graph created from the stream match the classes present in the Java Virtual Machine. Classes are loaded as required using the standard mechanisms.

Only objects that support the java.io.Serializable or java.io.Externalizable interface can be read from streams. The method readObject is used to read an object from the stream. Java's safe casting should be used to get the desired type. In Java, strings and arrays are objects and are treated as objects during serialization. When read they need to be cast to the expected type.

Primitive data types can be read from the stream using the appropriate method on DataInput.

The default deserialization mechanism for objects restores the contents of each field to the value and type it had when it was written. Fields declared as transient or static are ignored by the deserialization process. References to other objects cause those objects to be read from the stream as necessary. Graphs of objects are restored correctly using a reference sharing mechanism. New objects are always allocated when deserializing, which prevents existing objects from being overwritten.

Reading an object is analogous to running the constructors of a new object. Memory is allocated for the object and initialized to zero (NULL). No-arg constructors are invoked for the non-serializable classes and then the fields of the serializable classes are restored from the stream starting with the serializable class closest to java.lang.object and finishing with the object's most specifiec class.

For example to read from a stream as written by the example in ObjectOutputStream:

	FileInputStream istream = new FileInputStream("t.tmp");
	ObjectInputStream p = new ObjectInputStream(istream);
	int i = p.readInt();
	String today = (String)p.readObject();
	Date date = (Date)p.readObject();
	istream.close();
 

Implementing the Serializable interface allows object serialization to save and restore the entire state of the object and it allows classes to evolve between the time the stream is written and the time it is read. It automatically traverses references between objects, saving and restoring entire graphs. Serializable classes that require special handling during the serialization and deserialization process should implement both of these methods:

 private void writeObject(java.io.ObjectOutputStream stream)
     throws IOException;
 private void readObject(java.io.ObjectInputStream stream)
     throws IOException, ClassNotFoundException; 
 

The readObject method is responsible for reading and restoring the state of the object for its particular class using data written to the stream by the corresponding writeObject method. The method does not need to concern itself with the state belonging to its superclasses or subclasses. State is restored by reading data from the ObjectInputStream for the individual fields and making assignments to the appropriate fields of the object. Reading primitive data types is supported by DataInput.

Serialization does not read or assign values to 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.

Any exception that occurs while deserializing an object will be caught by the ObjectInputStream and abort the reading 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.

See Also:

DataInput, ObjectOutputStream, Serializable

ObjectInputStream(InputStream)

Create an ObjectInputStream that reads from the specified InputStream.

available()

Returns the number of bytes that can be read without blocking.

close()

Closes the input stream.

defaultReadObject()

Read the non-static and non-transient fields of the current class from this stream.

enableResolveObject(boolean)

Enable the stream to allow objects read from the stream to be replaced.

read()

Reads a byte of data.

read(byte[], int, int)

Reads into an array of bytes.

readBoolean()

Reads in a boolean.

readByte()

Reads an 8 bit byte.

readChar()

Reads a 16 bit char.

readDouble()

Reads a 64 bit double.

readFloat()

Reads a 32 bit float.

readFully(byte[])

Reads bytes, blocking until all bytes are read.

readFully(byte[], int, int)

Reads bytes, blocking until all bytes are read.

readInt()

Reads a 32 bit int.

readLine()

Reads in a line that has been terminated by a \n, \r, \r\n or EOF.

readLong()

Reads a 64 bit long.

readObject()

Read an object from the ObjectInputStream.

readShort()

Reads a 16 bit short.

readStreamHeader()

The readStreamHeader method is provided to allow subclasses to read and verify their own stream headers.

readUnsignedByte()

Reads an unsigned 8 bit byte.

readUnsignedShort()

Reads an unsigned 16 bit short.

readUTF()

Reads a UTF format String.

registerValidation(ObjectInputValidation, int)

Register an object to be validated before the graph is returned.

resolveClass(ObjectStreamClass)

Subclasses may implement this method to allow classes to be fetched from an alternate source.

resolveObject(Object)

This method will allow trusted subclasses of ObjectInputStream to substitute one object for another during deserialization.

skipBytes(int)

Skips bytes, block until all bytes are skipped.

 public ObjectInputStream(InputStream in) throws IOException, StreamCorruptedException

Create an ObjectInputStream that reads from the specified InputStream. The stream header containing the magic number and version number are read from the stream and verified. This method will block until the corresponding ObjectOutputStream has written and flushed the header.

Throws: StreamCorruptedException

The version or magic number are incorrect.

Throws: IOException

An exception occurred in the underlying stream.

 public final Object readObject() throws OptionalDataException, ClassNotFoundException, IOException

Read an object from the ObjectInputStream. 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 read. Default deserializing for a class can be overriden using the writeObject and readObject methods. Objects referenced by this object are read transitively so that a complete equivalent graph of objects is reconstructed by readObject.

The root object is completly restored when all of its fields and the objects it references are completely restored. At this point the object validation callbacks are executed in order based on their registered priorities. The callbacks are registered by objects (in the readObject special methods) as they are individually restored. Exceptions are thrown for problems with the InputStream and for classes that should not be deserialized. All exceptions are fatal to the InputStream and leave it in an indeterminate state; it is up to the caller to ignore or recover the stream state.

Throws: ClassNotFoundException

Class of a serialized object cannot be found.

Throws: InvalidClassException

Something is wrong with a class used by serialization.

Throws: StreamCorruptedException

Control information in the stream is inconsistent.

Throws: OptionalDataException

Primitive data was found in the stream instead of objects.

Throws: IOException

Any of the usual Input/Output related exceptions.

 public final void defaultReadObject() throws IOException, ClassNotFoundException, NotActiveException

Read the non-static and non-transient fields of the current class from this stream. This may only be called from the readObject method of the class being deserialized. It will throw the NotActiveException if it is called otherwise.

Throws: ClassNotFoundException

if the class of a serialized object could not be found.

Throws: IOException

if an I/O error occurs.

Throws: NotActiveException

if the stream is not currently reading objects.

 public synchronized void registerValidation(ObjectInputValidation obj,
                                             int prio) throws NotActiveException, InvalidObjectException

Register an object to be validated before the graph is returned. While similar to resolveObject these validations are called after the entire graph has been reconstituted. Typically, a readObject method will register the object with the stream so that when all of the objects are restored a final set of validations can be performed.

Parameters:

obj - the object to receive the validation callback.

prio - controls the order of callbacks;zero is a good default. Use higher numbers to be called back earlier, lower numbers for later callbacks. Within a priority, callbacks are processed in no particular order.

Throws: NotActiveException

The stream is not currently reading objects so it is invalid to register a callback.

Throws: InvalidObjectException

The validation object is null.

 protected Class resolveClass(ObjectStreamClass v) throws IOException, ClassNotFoundException

Subclasses may implement this method to allow classes to be fetched from an alternate source. The corresponding method in ObjectOutputStream is annotateClass. This method will be invoked only once for each unique class in the stream. This method can be implemented by subclasses to use an alternate loading mechanism but must return a Class object. Once returned, the serialVersionUID of the class is compared to the serialVersionUID of the serialized class. If there is a mismatch, the deserialization fails and an exception is raised.

By default the class name is resolved relative to the class that called readObject.

Throws: ClassNotFoundException

If class of a serialized object cannot be found.

 protected Object resolveObject(Object obj) throws IOException

This method will allow trusted subclasses of ObjectInputStream to substitute one object for another during deserialization. Replacing objects is disabled until enableResolveObject is called. The enableResolveObject method checks that the stream requesting to resolve object can be trusted. Every reference to serializable objects is passed to resolveObject. To insure that the private state of objects is not unintentionally exposed only trusted streams may use resolveObject.

This method is called after an object has been read but before it is returned from readObject. The default resolveObject method just returns the new object.

When a subclass is replacing objects it must insure 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.

Throws: IOException

Any of the usual Input/Output exceptions.

 protected final boolean enableResolveObject(boolean enable) throws SecurityException

Enable the stream to allow objects read from the stream to be replaced. If the stream is a trusted class it is allowed to enable replacment. Trusted classes are those classes with a classLoader equals null.

When enabled the resolveObject method is called for every object being deserialized.

Throws: SecurityException

The classloader of this stream object is non-null.

 protected void readStreamHeader() throws IOException, StreamCorruptedException

The readStreamHeader method is provided to allow subclasses to read and verify their own stream headers. It reads and verifies the magic number and version number.

 public int read() throws IOException

Reads a byte of data. This method will block if no input is available.

Returns:

the byte read, or -1 if the end of the stream is reached.

Throws: IOException

If an I/O error has occurred.

Overrides:

read in class InputStream

 public int read(byte data[],
                 int offset,
                 int length) throws IOException

Reads into an array of bytes. This method will block until some input is available.

Parameters:

b - the buffer into which the data is read

off - the start offset of the data

len - the maximum number of bytes read

Returns:

the actual number of bytes read, -1 is returned when the end of the stream is reached.

Throws: IOException

If an I/O error has occurred.

Overrides:

read in class InputStream

 public int available() throws IOException

Returns the number of bytes that can be read without blocking.

Returns:

the number of available bytes.

Overrides:

available in class InputStream

 public void close() throws IOException

Closes the input stream. Must be called to release any resources associated with the stream.

Throws: IOException

If an I/O error has occurred.

Overrides:

close in class InputStream

 public boolean readBoolean() throws IOException

Reads in a boolean.

Returns:

the boolean read.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public byte readByte() throws IOException

Reads an 8 bit byte.

Returns:

the 8 bit byte read.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public int readUnsignedByte() throws IOException

Reads an unsigned 8 bit byte.

Returns:

the 8 bit byte read.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public short readShort() throws IOException

Reads a 16 bit short.

Returns:

the 16 bit short read.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public int readUnsignedShort() throws IOException

Reads an unsigned 16 bit short.

Returns:

the 16 bit short read.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public char readChar() throws IOException

Reads a 16 bit char.

Returns:

the 16 bit char read.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public int readInt() throws IOException

Reads a 32 bit int.

Returns:

the 32 bit integer read.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public long readLong() throws IOException

Reads a 64 bit long.

Returns:

the read 64 bit long.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public float readFloat() throws IOException

Reads a 32 bit float.

Returns:

the 32 bit float read.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public double readDouble() throws IOException

Reads a 64 bit double.

Returns:

the 64 bit double read.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public void readFully(byte data[]) throws IOException

Reads bytes, blocking until all bytes are read.

Parameters:

b - the buffer into which the data is read

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public void readFully(byte data[],
                       int offset,
                       int size) throws IOException

Reads bytes, blocking until all bytes are read.

Parameters:

b - the buffer into which the data is read

off - the start offset of the data

len - the maximum number of bytes to read

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public int skipBytes(int len) throws IOException

Skips bytes, block until all bytes are skipped.

Parameters:

n - the number of bytes to be skipped

Returns:

the actual number of bytes skipped.

Throws: EOFException

If end of file is reached.

Throws: IOException

If other I/O error has occurred.

 public String readLine() throws IOException

Reads in a line that has been terminated by a \n, \r, \r\n or EOF.

Returns:

a String copy of the line.

 public String readUTF() throws IOException

Reads a UTF format String.

Returns:

the String.

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