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Java 2 Platform SE v1.3.1: Class ObjectInputStream
Overview  Package   Class  Use  Tree  Deprecated  Index  Help 
JavaTM 2 Platform
Std. Ed. v1.3.1
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java.io
Class ObjectInputStream

java.lang.Object
  |
  +--java.io.InputStream
        |
        +--java.io.ObjectInputStream
All Implemented Interfaces:
DataInput, ObjectInput, ObjectStreamConstants
public class ObjectInputStream
extends InputStream
implements ObjectInput, ObjectStreamConstants

An ObjectInputStream deserializes primitive data and objects previously written using an ObjectOutputStream. ObjectOutputStream and ObjectInputStream can provide an application with persistent storage for graphs of objects when used with a FileOutputStream and FileInputStream respectively. ObjectInputStream is used to recover those objects previously serialized. Other uses include passing objects between hosts using a socket stream or for marshaling and unmarshaling arguments and parameters in a remote communication system.

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();
Classes control how they are serialized by implementing either the java.io.Serializable or java.io.Externalizable interfaces.

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.

Since:
JDK1.1
See Also:
DataInput, ObjectOutputStream, Serializable, Object Serialization Specification, Section 3, Object Input Classes

Inner Class Summary
static class ObjectInputStream.GetField
          Provide access to the persistent fields read from the input stream.
 
Fields inherited from interface java.io.ObjectStreamConstants
baseWireHandle, PROTOCOL_VERSION_1, PROTOCOL_VERSION_2, SC_BLOCK_DATA, 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_EXCEPTION, TC_LONGSTRING, TC_MAX, TC_NULL, TC_OBJECT, TC_PROXYCLASSDESC, TC_REFERENCE, TC_RESET, TC_STRING
 
Constructor Summary
protected ObjectInputStream()
          Provide a way for subclasses that are completely reimplementing ObjectInputStream to not have to allocate private data just used by this implementation of ObjectInputStream.
  ObjectInputStream(InputStream in)
          Create an ObjectInputStream that reads from the specified InputStream.
 
Method Summary
 int available()
          Returns the number of bytes that can be read without blocking.
 void close()
          Closes the input stream.
 void defaultReadObject()
          Read the non-static and non-transient fields of the current class from this stream.
protected  boolean enableResolveObject(boolean enable)
          Enable the stream to allow objects read from the stream to be replaced.
 int read()
          Reads a byte of data.
 int read(byte[] b, int off, int len)
          Reads into an array of bytes.
 boolean readBoolean()
          Reads in a boolean.
 byte readByte()
          Reads an 8 bit byte.
 char readChar()
          Reads a 16 bit char.
protected  ObjectStreamClass readClassDescriptor()
          Read a class descriptor from the serialization stream.
 double readDouble()
          Reads a 64 bit double.
 ObjectInputStream.GetField readFields()
          Reads the persistent fields from the stream and makes them available by name.
 float readFloat()
          Reads a 32 bit float.
 void readFully(byte[] data)
          Reads bytes, blocking until all bytes are read.
 void readFully(byte[] data, int offset, int size)
          Reads bytes, blocking until all bytes are read.
 int readInt()
          Reads a 32 bit int.
 String readLine()
          Deprecated. This method does not properly convert bytes to characters. see DataInputStream for the details and alternatives.
 long readLong()
          Reads a 64 bit long.
 Object readObject()
          Read an object from the ObjectInputStream.
protected  Object readObjectOverride()
          This method is called by trusted subclasses of ObjectOutputStream that constructed ObjectOutputStream using the protected no-arg constructor.
 short readShort()
          Reads a 16 bit short.
protected  void readStreamHeader()
          The readStreamHeader method is provided to allow subclasses to read and verify their own stream headers.
 int readUnsignedByte()
          Reads an unsigned 8 bit byte.
 int readUnsignedShort()
          Reads an unsigned 16 bit short.
 String readUTF()
          Reads a UTF format String.
 void registerValidation(ObjectInputValidation obj, int prio)
          Register an object to be validated before the graph is returned.
protected  Class resolveClass(ObjectStreamClass v)
          Load the local class equivalent of the specified stream class description.
protected  Object resolveObject(Object obj)
          This method will allow trusted subclasses of ObjectInputStream to substitute one object for another during deserialization.
protected  Class resolveProxyClass(String[] interfaces)
          Returns a proxy class that implements the interfaces named in a proxy class descriptor; subclasses may implement this method to read custom data from the stream along with the descriptors for dynamic proxy classes, allowing them to use an alternate loading mechanism for the interfaces and the proxy class.
 int skipBytes(int len)
          Skips bytes, block until all bytes are skipped.
 
Methods inherited from class java.io.InputStream
mark, markSupported, read, reset, skip
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 
Methods inherited from interface java.io.ObjectInput
read, skip
 

Constructor Detail

ObjectInputStream

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.
Parameters:
in - the underlying InputStream from which to read
Throws:
StreamCorruptedException - The version or magic number are incorrect.
IOException - An exception occurred in the underlying stream.

ObjectInputStream

protected ObjectInputStream()
                     throws IOException,
                            SecurityException
Provide a way for subclasses that are completely reimplementing ObjectInputStream to not have to allocate private data just used by this implementation of ObjectInputStream.

If there is a security manager installed, this method first calls the security manager's checkPermission method with the SerializablePermission("enableSubclassImplementation") permission to ensure it's ok to enable subclassing.

Throws:
IOException - Thrown if not called by a subclass.
SecurityException - if a security manager exists and its checkPermission method denies enabling subclassing.
See Also:
SecurityManager.checkPermission(java.security.Permission), SerializablePermission
Method Detail

readObject

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.

Specified by:
readObject in interface ObjectInput
Throws:
ClassNotFoundException - Class of a serialized object cannot be found.
InvalidClassException - Something is wrong with a class used by serialization.
StreamCorruptedException - Control information in the stream is inconsistent.
OptionalDataException - Primitive data was found in the stream instead of objects.
IOException - Any of the usual Input/Output related exceptions.

readObjectOverride

protected Object readObjectOverride()
                             throws OptionalDataException,
                                    ClassNotFoundException,
                                    IOException
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".
Returns:
the Object read from the stream.
Throws:
ClassNotFoundException - Class definition of a serialized object cannot be found.
OptionalDataException - Primitive data was found in the stream instead of objects.
IOException - if I/O errors occurred while reading from the underlying stream
Since:
1.2
See Also:
ObjectInputStream(), readObject()

defaultReadObject

public 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.
IOException - if an I/O error occurs.
NotActiveException - if the stream is not currently reading objects.

readFields

public ObjectInputStream.GetField readFields()
                                      throws IOException,
                                             ClassNotFoundException,
                                             NotActiveException
Reads the persistent fields from the stream and makes them available by name.
Returns:
the GetField object representing the persistent fields of the object being deserialized
Throws:
ClassNotFoundException - if the class of a serialized object could not be found.
IOException - if an I/O error occurs.
NotActiveException - if the stream is not currently reading objects.
Since:
1.2

registerValidation

public 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.
InvalidObjectException - The validation object is null.

resolveClass

protected Class resolveClass(ObjectStreamClass v)
                      throws IOException,
                             ClassNotFoundException
Load the local class equivalent of the specified stream class description. 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.

Parameters:
v - an instance of class ObjectStreamClass
Returns:
a Class object corresponding to v
Throws:
IOException - Any of the usual Input/Output exceptions.
ClassNotFoundException - If class of a serialized object cannot be found.

resolveProxyClass

protected Class resolveProxyClass(String[] interfaces)
                           throws IOException,
                                  ClassNotFoundException
Returns a proxy class that implements the interfaces named in a proxy class descriptor; subclasses may implement this method to read custom data from the stream along with the descriptors for dynamic proxy classes, allowing them to use an alternate loading mechanism for the interfaces and the proxy class.

This method is called exactly once for each unique proxy class descriptor in the stream.

The corresponding method in ObjectOutputStream is annotateProxyClass. For a given subclass of ObjectInputStream that overrides this method, the annotateProxyClass method in the corresponding subclass of ObjectOutputStream must write any data or objects read by this method.

The default implementation of this method in ObjectInputStream returns the result of calling Proxy.getProxyClass with the list of Class objects for the interfaces that are named in the interfaces parameter. The Class object for each interface name i is the value returned by calling 

     Class.forName(i, false, loader)
where loader is that of the first non-null class loader up the execution stack, or null if no non-null class loaders are on the stack (the same class loader choice used by the resolveClass method). This same value of loader is also the class loader passed to Proxy.getProxyClass. If Proxy.getProxyClass throws an IllegalArgumentException, resolveProxyClass will throw a ClassNotFoundException containing the IllegalArgumentException.
Parameters:
interfaces - the list of interface names that were deserialized in the proxy class descriptor
Returns:
a proxy class for the specified interfaces
Throws:
IOException - any exception thrown by the underlying InputStream
ClassNotFoundException - if the proxy class or any of the named interfaces could not be found
Since:
1.3
See Also:
ObjectOutputStream.annotateProxyClass(Class)

resolveObject

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 same 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.

Parameters:
obj - object to be substituted
Returns:
the substituted object
Throws:
IOException - Any of the usual Input/Output exceptions.

enableResolveObject

protected boolean enableResolveObject(boolean enable)
                               throws SecurityException
Enable the stream to allow objects read from the stream to be replaced. When enabled, the resolveObject method is called for every object being deserialized. If enable is true, and there is a security manager installed, this method first calls the security manager's checkPermission method with the SerializablePermission("enableSubstitution") permission to ensure it's ok to enable the stream to allow objects read from the stream to be replaced.
Parameters:
enable - true for enabling use of resolveObject for every object being deserialized
Returns:
the previous setting before this method was invoked
Throws:
SecurityException - if a security manager exists and its checkPermission method denies enabling the stream to allow objects read from the stream to be replaced.
See Also:
SecurityManager.checkPermission(java.security.Permission), SerializablePermission

readStreamHeader

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.
Throws:
IOException - if there are I/O errors while reading from the underlying InputStream
StreamCorruptedException - if control information in the stream is inconsistent

readClassDescriptor

protected ObjectStreamClass readClassDescriptor()
                                         throws IOException,
                                                ClassNotFoundException
Read a class descriptor from the serialization stream. This method is called when the ObjectInputStream expects a class descriptor as the next item in the serialization stream. Subclasses of ObjectInputStream may override this method to read in class descriptors that have been written in non-standard formats (by subclasses of ObjectOutputStream which have overridden the writeClassDescriptor method). By default, this method reads class descriptors according to the format defined in the Object Serialization specification.

Returns:
the class descriptor read
Throws:
IOException - If an I/O error has occurred.
ClassNotFoundException - If the Class of a serialized object used in the class descriptor representation cannot be found
Since:
1.3
See Also:
ObjectOutputStream.writeClassDescriptor(java.io.ObjectStreamClass)

read

public int read()
         throws IOException
Reads a byte of data. This method will block if no input is available.
Specified by:
read in interface ObjectInput
Overrides:
read in class InputStream
Returns:
the byte read, or -1 if the end of the stream is reached.
Throws:
IOException - If an I/O error has occurred.

read

public int read(byte[] b,
                int off,
                int len)
         throws IOException
Reads into an array of bytes. This method will block until some input is available. Consider using java.io.DataInputStream.readFully to read exactly 'length' bytes.
Specified by:
read in interface ObjectInput
Overrides:
read in class InputStream
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.
See Also:
DataInputStream.readFully(byte[],int,int)

available

public int available()
              throws IOException
Returns the number of bytes that can be read without blocking.
Specified by:
available in interface ObjectInput
Overrides:
available in class InputStream
Returns:
the number of available bytes.
Throws:
IOException - if there are I/O errors while reading from the underlying InputStream

close

public void close()
           throws IOException
Closes the input stream. Must be called to release any resources associated with the stream.
Specified by:
close in interface ObjectInput
Overrides:
close in class InputStream
Throws:
IOException - If an I/O error has occurred.

readBoolean

public boolean readBoolean()
                    throws IOException
Reads in a boolean.
Specified by:
readBoolean in interface DataInput
Returns:
the boolean read.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readByte

public byte readByte()
              throws IOException
Reads an 8 bit byte.
Specified by:
readByte in interface DataInput
Returns:
the 8 bit byte read.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readUnsignedByte

public int readUnsignedByte()
                     throws IOException
Reads an unsigned 8 bit byte.
Specified by:
readUnsignedByte in interface DataInput
Returns:
the 8 bit byte read.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readShort

public short readShort()
                throws IOException
Reads a 16 bit short.
Specified by:
readShort in interface DataInput
Returns:
the 16 bit short read.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readUnsignedShort

public int readUnsignedShort()
                      throws IOException
Reads an unsigned 16 bit short.
Specified by:
readUnsignedShort in interface DataInput
Returns:
the 16 bit short read.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readChar

public char readChar()
              throws IOException
Reads a 16 bit char.
Specified by:
readChar in interface DataInput
Returns:
the 16 bit char read.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readInt

public int readInt()
            throws IOException
Reads a 32 bit int.
Specified by:
readInt in interface DataInput
Returns:
the 32 bit integer read.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readLong

public long readLong()
              throws IOException
Reads a 64 bit long.
Specified by:
readLong in interface DataInput
Returns:
the read 64 bit long.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readFloat

public float readFloat()
                throws IOException
Reads a 32 bit float.
Specified by:
readFloat in interface DataInput
Returns:
the 32 bit float read.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readDouble

public double readDouble()
                  throws IOException
Reads a 64 bit double.
Specified by:
readDouble in interface DataInput
Returns:
the 64 bit double read.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readFully

public void readFully(byte[] data)
               throws IOException
Reads bytes, blocking until all bytes are read.
Specified by:
readFully in interface DataInput
Parameters:
data - the buffer into which the data is read
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readFully

public void readFully(byte[] data,
                      int offset,
                      int size)
               throws IOException
Reads bytes, blocking until all bytes are read.
Specified by:
readFully in interface DataInput
Parameters:
data - the buffer into which the data is read
offset - the start offset of the data
size - the maximum number of bytes to read
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

skipBytes

public int skipBytes(int len)
              throws IOException
Skips bytes, block until all bytes are skipped.
Specified by:
skipBytes in interface DataInput
Parameters:
len - the number of bytes to be skipped
Returns:
the actual number of bytes skipped.
Throws:
EOFException - If end of file is reached.
IOException - If other I/O error has occurred.

readLine

public String readLine()
                throws IOException
Deprecated. This method does not properly convert bytes to characters. see DataInputStream for the details and alternatives.

Reads in a line that has been terminated by a \n, \r, \r\n or EOF.
Specified by:
readLine in interface DataInput
Returns:
a String copy of the line.
Throws:
IOException - if there are I/O errors while reading from the underlying InputStream

readUTF

public String readUTF()
               throws IOException
Reads a UTF format String.
Specified by:
readUTF in interface DataInput
Returns:
the String.
Throws:
IOException - if there are I/O errors while reading from the underlying InputStream
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