public class ObjectInputStream extends InputStream implements ObjectInput, ObjectStreamConstants
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 specific class.
For example to read from a stream as written by the example in
ObjectOutputStream:
FileInputStream fis = new FileInputStream("t.tmp");
ObjectInputStream ois = new ObjectInputStream(fis);
int i = ois.readInt();
String today = (String) ois.readObject();
Date date = (Date) ois.readObject();
ois.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 the following methods:
private void writeObject(java.io.ObjectOutputStream stream)
throws IOException;
private void readObject(java.io.ObjectInputStream stream)
throws IOException, ClassNotFoundException;
private void readObjectNoData()
throws ObjectStreamException;
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.
Any attempt to read object data which exceeds the boundaries of the custom data written by the corresponding writeObject method will cause an OptionalDataException to be thrown with an eof field value of true. Non-object reads which exceed the end of the allotted data will reflect the end of data in the same way that they would indicate the end of the stream: bytewise reads will return -1 as the byte read or number of bytes read, and primitive reads will throw EOFExceptions. If there is no corresponding writeObject method, then the end of default serialized data marks the end of the allotted data.
Primitive and object read calls issued from within a readExternal method
behave in the same manner--if the stream is already positioned at the end of
data written by the corresponding writeExternal method, object reads will
throw OptionalDataExceptions with eof set to true, bytewise reads will
return -1, and primitive reads will throw EOFExceptions. Note that this
behavior does not hold for streams written with the old
ObjectStreamConstants.PROTOCOL_VERSION_1 protocol, in which the
end of data written by writeExternal methods is not demarcated, and hence
cannot be detected.
The readObjectNoData method is responsible for initializing the state of the object for its particular class in the event that the serialization stream does not list the given class as a superclass of the object being deserialized. This may occur in cases where the receiving party uses a different version of the deserialized instance's class than the sending party, and the receiver's version extends classes that are not extended by the sender's version. This may also occur if the serialization stream has been tampered; hence, readObjectNoData is useful for initializing deserialized objects properly despite a "hostile" or incomplete source stream.
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.
Enum constants are deserialized 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
deserialize an enum constant, ObjectInputStream reads the constant name from
the stream; the deserialized constant is then obtained by calling the static
method Enum.valueOf(Class, String) with the enum constant's
base type and the received constant name as arguments. 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 deserialized cannot be
customized: any class-specific readObject, readObjectNoData, and readResolve
methods defined by enum types are ignored during deserialization.
Similarly, any serialPersistentFields or serialVersionUID field declarations
are also ignored--all enum types have a fixed serialVersionUID of 0L.
DataInput,
ObjectOutputStream,
Serializable,
Object Serialization Specification, Section 3, Object Input Classes| Modifier and Type | Class and Description |
|---|---|
static class |
ObjectInputStream.GetField
Provide access to the persistent fields read from the input stream.
|
baseWireHandle, PROTOCOL_VERSION_1, PROTOCOL_VERSION_2, SC_BLOCK_DATA, SC_ENUM, 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_ENUM, TC_EXCEPTION, TC_LONGSTRING, TC_MAX, TC_NULL, TC_OBJECT, TC_PROXYCLASSDESC, TC_REFERENCE, TC_RESET, TC_STRING| Modifier | Constructor and Description |
|---|---|
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)
Creates an ObjectInputStream that reads from the specified InputStream.
|
| Modifier and Type | Method and Description |
|---|---|
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[] buf,
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[] buf)
Reads bytes, blocking until all bytes are read.
|
void |
readFully(byte[] buf,
int off,
int len)
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.
|
Object |
readUnshared()
Reads an "unshared" object from the ObjectInputStream.
|
int |
readUnsignedByte()
Reads an unsigned 8 bit byte.
|
int |
readUnsignedShort()
Reads an unsigned 16 bit short.
|
String |
readUTF()
Reads a String in
modified UTF-8
format.
|
void |
registerValidation(ObjectInputValidation obj,
int prio)
Register an object to be validated before the graph is returned.
|
protected Class<?> |
resolveClass(ObjectStreamClass desc)
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.
|
mark, markSupported, read, reset, skipclone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, waitread, skippublic ObjectInputStream(InputStream in) 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 ObjectInputStream.readFields or ObjectInputStream.readUnshared methods.
in - input stream to read fromStreamCorruptedException - if the stream header is incorrectIOException - if an I/O error occurs while reading stream headerSecurityException - if untrusted subclass illegally overrides
security-sensitive methodsNullPointerException - if in is nullObjectInputStream(),
readFields(),
ObjectOutputStream.ObjectOutputStream(OutputStream)protected ObjectInputStream()
throws IOException,
SecurityException
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.
SecurityException - if a security manager exists and its
checkPermission method denies enabling
subclassing.IOExceptionSecurityManager.checkPermission(java.security.Permission),
SerializablePermissionpublic final Object readObject() throws IOException, ClassNotFoundException
The root object is completely 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.
readObject in interface ObjectInputClassNotFoundException - 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.protected Object readObjectOverride() throws IOException, ClassNotFoundException
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 streamObjectInputStream(),
readObject()public Object readUnshared() throws IOException, ClassNotFoundException
ObjectInputStream 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.
ClassNotFoundException - if class of an object to deserialize
cannot be foundStreamCorruptedException - if control information in the stream
is inconsistentObjectStreamException - if object to deserialize has already
appeared in streamOptionalDataException - if primitive data is next in streamIOException - if an I/O error occurs during deserializationpublic void defaultReadObject()
throws IOException,
ClassNotFoundException
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.public ObjectInputStream.GetField readFields() throws IOException, ClassNotFoundException
GetField object representing the persistent
fields of the object being deserializedClassNotFoundException - 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.public void registerValidation(ObjectInputValidation obj, int prio) throws NotActiveException, InvalidObjectException
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.NotActiveException - The stream is not currently reading objects
so it is invalid to register a callback.InvalidObjectException - The validation object is null.protected Class<?> resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException
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, if the class is not an array
class, its serialVersionUID is compared to the serialVersionUID of the
serialized class, and if there is a mismatch, the deserialization fails
and an InvalidClassException is thrown.
The default implementation of this method in
ObjectInputStream returns the result of calling
Class.forName(desc.getName(), false, loader)
where loader is determined as follows: if there is a
method on the current thread's stack whose declaring class was
defined by a user-defined class loader (and was not a generated to
implement reflective invocations), then loader is class
loader corresponding to the closest such method to the currently
executing frame; otherwise, loader is
null. If this call results in a
ClassNotFoundException and the name of the passed
ObjectStreamClass instance is the Java language keyword
for a primitive type or void, then the Class object
representing that primitive type or void will be returned
(e.g., an ObjectStreamClass with the name
"int" will be resolved to Integer.TYPE).
Otherwise, the ClassNotFoundException will be thrown to
the caller of this method.desc - an instance of class ObjectStreamClassClass object corresponding to descIOException - any of the usual Input/Output exceptions.ClassNotFoundException - if class of a serialized object cannot
be found.protected Class<?> resolveProxyClass(String[] interfaces) throws IOException, ClassNotFoundException
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). Unless any
of the resolved interfaces are non-public, this same value of
loader is also the class loader passed to
Proxy.getProxyClass; if non-public interfaces are present,
their class loader is passed instead (if more than one non-public
interface class loader is encountered, an
IllegalAccessError is thrown).
If Proxy.getProxyClass throws an
IllegalArgumentException, resolveProxyClass
will throw a ClassNotFoundException containing the
IllegalArgumentException.interfaces - the list of interface names that were
deserialized in the proxy class descriptorIOException - any exception thrown by the underlying
InputStreamClassNotFoundException - if the proxy class or any of the
named interfaces could not be foundObjectOutputStream.annotateProxyClass(Class)protected Object resolveObject(Object obj) throws IOException
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.
obj - object to be substitutedIOException - Any of the usual Input/Output exceptions.protected boolean enableResolveObject(boolean enable)
throws SecurityException
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.
enable - true for enabling use of resolveObject for
every object being deserializedSecurityException - if a security manager exists and its
checkPermission method denies enabling the stream
to allow objects read from the stream to be replaced.SecurityManager.checkPermission(java.security.Permission),
SerializablePermissionprotected void readStreamHeader()
throws IOException,
StreamCorruptedException
IOException - if there are I/O errors while reading from the
underlying InputStreamStreamCorruptedException - if control information in the stream
is inconsistentprotected ObjectStreamClass readClassDescriptor() throws IOException, ClassNotFoundException
writeClassDescriptor method). By default,
this method reads class descriptors according to the format defined in
the Object Serialization specification.IOException - If an I/O error has occurred.ClassNotFoundException - If the Class of a serialized object used
in the class descriptor representation cannot be foundObjectOutputStream.writeClassDescriptor(java.io.ObjectStreamClass)public int read()
throws IOException
read in interface ObjectInputread in class InputStreamIOException - If an I/O error has occurred.public int read(byte[] buf,
int off,
int len)
throws IOException
read in interface ObjectInputread in class InputStreambuf - the buffer into which the data is readoff - the start offset of the datalen - the maximum number of bytes readIOException - If an I/O error has occurred.DataInputStream.readFully(byte[],int,int)public int available()
throws IOException
available in interface ObjectInputavailable in class InputStreamIOException - if there are I/O errors while reading from the
underlying InputStreampublic void close()
throws IOException
close in interface Closeableclose in interface ObjectInputclose in interface AutoCloseableclose in class InputStreamIOException - If an I/O error has occurred.public boolean readBoolean()
throws IOException
readBoolean in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public byte readByte()
throws IOException
readByte in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public int readUnsignedByte()
throws IOException
readUnsignedByte in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public char readChar()
throws IOException
readChar in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public short readShort()
throws IOException
readShort in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public int readUnsignedShort()
throws IOException
readUnsignedShort in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public int readInt()
throws IOException
readInt in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public long readLong()
throws IOException
readLong in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public float readFloat()
throws IOException
readFloat in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public double readDouble()
throws IOException
readDouble in interface DataInputEOFException - If end of file is reached.IOException - If other I/O error has occurred.public void readFully(byte[] buf)
throws IOException
readFully in interface DataInputbuf - the buffer into which the data is readEOFException - If end of file is reached.IOException - If other I/O error has occurred.public void readFully(byte[] buf,
int off,
int len)
throws IOException
readFully in interface DataInputbuf - the buffer into which the data is readoff - the start offset of the datalen - the maximum number of bytes to readEOFException - If end of file is reached.IOException - If other I/O error has occurred.public int skipBytes(int len)
throws IOException
skipBytes in interface DataInputlen - the number of bytes to be skippedIOException - If an I/O error has occurred.@Deprecated public String readLine() throws IOException
readLine in interface DataInputIOException - if there are I/O errors while reading from the
underlying InputStreampublic String readUTF() throws IOException
readUTF in interface DataInputIOException - if there are I/O errors while reading from the
underlying InputStreamUTFDataFormatException - if read bytes do not represent a valid
modified UTF-8 encoding of a string