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3.1 The ObjectInputStream Class

Class ObjectInputStream implements object deserialization. It maintains the state of the stream including the set of objects already deserialized. Its methods allow primitive types and objects to be read from a stream written by ObjectOutputStream. It manages restoration of the object and the objects that it refers to from the stream.

package java.io;

public class ObjectInputStream
    extends InputStream
    implements ObjectInput, ObjectStreamConstants
{
    public ObjectInputStream(InputStream in)
        throws StreamCorruptedException, IOException;

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

    public Object readUnshared()
        throws OptionalDataException, ClassNotFoundException, 
            IOException;

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

    public GetField readFields()
        throws IOException;

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

    protected ObjectStreamClass readClassDescriptor()
        throws IOException, ClassNotFoundException;

    protected Class resolveClass(ObjectStreamClass v)
        throws IOException, ClassNotFoundException;

    protected Object resolveObject(Object obj)
        throws IOException;

    protected boolean enableResolveObject(boolean enable)
        throws SecurityException;

    protected void readStreamHeader()
        throws IOException, StreamCorruptedException;

    public int read() throws IOException;

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

    public int available() throws IOException;

    public void close() throws IOException;

    public boolean readBoolean() throws IOException;

    public byte readByte() throws IOException;

    public int readUnsignedByte() throws IOException;

    public short readShort() throws IOException;

    public int readUnsignedShort() throws IOException;

    public char readChar() throws IOException;

    public int readInt() throws IOException;

    public long readLong() throws IOException;

    public float readFloat() throws IOException;

    public double readDouble() throws IOException;

    public void readFully(byte[] data) throws IOException;

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

    public int skipBytes(int len) throws IOException;

    public String readLine() throws IOException;

    public String readUTF() throws IOException;

    // Class to provide access to serializable fields.
    static abstract public class GetField
    {
        public ObjectStreamClass getObjectStreamClass();

        public boolean defaulted(String name)
            throws IOException, IllegalArgumentException;

        public char get(String name, char default)
            throws IOException, IllegalArgumentException;

        public boolean get(String name, boolean default)
            throws IOException, IllegalArgumentException;

        public byte get(String name, byte default)
            throws IOException, IllegalArgumentException;

        public short get(String name, short default)
            throws IOException, IllegalArgumentException;

        public int get(String name, int default)
            throws IOException, IllegalArgumentException;

        public long get(String name, long default)
            throws IOException, IllegalArgumentException;

        public float get(String name, float default)
            throws IOException, IllegalArgumentException;

        public double get(String name, double default)
            throws IOException, IllegalArgumentException;

        public Object get(String name, Object default)
            throws IOException, IllegalArgumentException;
    }

    protected ObjectInputStream()
        throws StreamCorruptedException, IOException;

    protected readObjectOverride()
        throws OptionalDataException, ClassNotFoundException,
            IOException;
}

The single-argument ObjectInputStream constructor requires an InputStream. The constructor calls readStreamHeader to read and verifies the header and version written by the corresponding ObjectOutputStream.writeStreamHeader method. If a security manager is installed, this constructor checks for the "enableSubclassImplementation" SerializablePermission when invoked directly or indirectly by the constructor of a subclass which overrides the readFields and/or readUnshared methods.

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Note - The ObjectInputStream constructor blocks until it completes reading the serialization stream header. Code which waits for an ObjectInputStream to be constructed before creating the corresponding ObjectOutputStream for that stream will deadlock, since the ObjectInputStream constructor will block until a header is written to the stream, and the header will not be written to the stream until the ObjectOutputStream constructor executes. This problem can be resolved by creating the ObjectOutputStream before the ObjectInputStream, or otherwise removing the timing dependency between completion of ObjectInputStream construction and the creation of the ObjectOutputStream.

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The readObject method is used to deserialize an object from the stream. It reads from the stream to reconstruct an object.

1. If the ObjectInputStream subclass is overriding the implementation, call the readObjectOverride method and return. Reimplementation is described at the end of this section.
2. If a block data record occurs in the stream, throw a BlockDataException with the number of available bytes.
3. If the object in the stream is null, return null.
4. If the object in the stream is a handle to a previous object, return the object.
5. If the object in the stream is a Class, read its ObjectStreamClass descriptor, add it and its handle to the set of known objects, and return the corresponding Class object.
6. If the object in the stream is an ObjectStreamClass, read its name, serialVersionUID, and fields. Add it and its handle to the set of known objects. Call the resolveClass method on the stream to get the local class for this descriptor, and throw an exception if the class cannot be found. Return the ObjectStreamClass object.
7. If the object in the stream is a String, read its UTF encoding, add it and its handle to the set of known objects, and proceed to Step 11.
8. If the object in the stream is an array, read its ObjectStreamClass and the length of the array. Allocate the array, and add it and its handle in the set of known objects. Read each element using the appropriate method for its type and assign it to the array. Proceed to Step 11.
9. For all other objects, the ObjectStreamClass of the object is read from the stream. The local class for that ObjectStreamClass is retrieved. The class must be serializable or externalizable.
10. An instance of the class is allocated. The instance and its handle are added to the set of known objects. The contents restored appropriately:
    1. For serializable objects, the no-arg constructor for the first non-serializable supertype is run. For serializable classes, the fields are initialized to the default value appropriate for its type. Then the fields of each class are restored by calling class-specific readObject methods, or if these are not defined, by calling the defaultReadObject method. Note that field initializers and constructors are not executed for serializable classes during deserialization. In the normal case, the version of the class that wrote the stream will be the same as the class reading the stream. In this case, all of the supertypes of the object in the stream will match the supertypes in the currently-loaded class. If the version of the class that wrote the stream had different supertypes than the loaded class, the ObjectInputStream must be more careful about restoring or initializing the state of the differing classes. It must step through the classes, matching the available data in the stream with the classes of the object being restored. Data for classes that occur in the stream, but do not occur in the object, is discarded. For classes that occur in the object, but not in the stream, the class fields are set to default values by default serialization.
    2. For externalizable objects, the no-arg constructor for the class is run and then the readExternal method is called to restore the contents of the object.
11. Process potential substitutions by the class of the object and/or by a subclass of ObjectInputStream:
    1. If the class of the object defines the appropriate readResolve method, the method is called to allow the object to replace itself.
    2. Then if previously enabled by enableResolveObject, the resolveObject method is called to allow subclasses of the stream to examine and replace the object. If the previous step did replace the original object, the resolveObject method is called with the replacement object.

If a replacement took place, the table of known objects is updated so the replacement object is associated with the handle. The replacement object is then returned from readObject.

All of the methods for reading primitives types only consume bytes from the block data records in the stream. If a read for primitive data occurs when the next item in the stream is an object, the read methods return -1 or the EOFException as appropriate. The value of a primitive type is read by a DataInputStream from the block data record.

The exceptions thrown reflect errors during the traversal or exceptions that occur on the underlying stream. If any exception is thrown, the underlying stream is left in an unknown and unusable state.

When the reset token occurs in the stream, all of the state of the stream is discarded. The set of known objects is cleared.

When the exception token occurs in the stream, the exception is read and a new WriteAbortedException is thrown with the terminating exception as an argument. The stream context is reset as described earlier.

The readUnshared method is used to read "unshared" objects from the stream. This method is identical to readObject, except that it prevents subsequent calls to readObject and readUnshared from returning additional references to the deserialized instance returned by the original call to readUnshared. Specifically:

• If readUnshared is called to deserialize a back-reference (the stream representation of an object which has been written previously to the stream), an ObjectStreamException will be thrown.
• If readUnshared returns successfully, then any subsequent attempts to deserialize back-references to the stream handle deserialized by readUnshared will cause an ObjectStreamException to be thrown.

Deserializing an object via readUnshared invalidates the stream handle associated with the returned object. Note that this in itself does not always guarantee that the reference returned by readUnshared is unique; the deserialized object may define a readResolve method which returns an object visible to other parties, or readUnshared may return a Class object obtainable elsewhere in the stream or through external means. However, for objects which are not instances of java.lang.Class and do not define readResolve methods, readUnshared guarantees that the returned object reference is unique and cannot be obtained a second time from the ObjectInputStream that created it, even if the underlying data stream has been manipulated. This guarantee applies only to the base-level object returned by readUnshared, and not to any transitively referenced sub-objects in the returned object graph.

The defaultReadObject method is used to read the fields and object from the stream. It uses the class descriptor in the stream to read the fields in the canonical order by name and type from the stream. The values are assigned to the matching fields by name in the current class. Details of the versioning mechanism can be found in Section 5.5, "Compatible Java Type Evolution." Any field of the object that does not appear in the stream is set to its default value. Values that appear in the stream, but not in the object, are discarded. This occurs primarily when a later version of a class has written additional fields that do not occur in the earlier version. This method may only be called from the readObject method while restoring the fields of a class. When called at any other time, the NotActiveException is thrown.

The readFields method reads the values of the serializable fields from the stream and makes them available via the GetField class. The readFields method is only callable from within the readObject method of a serializable class. It cannot be called more than once or if defaultReadObject has been called. The GetFields object uses the current object's ObjectStreamClass to verify the fields that can be retrieved for this class. The GetFields object returned by readFields is only valid during this call to the classes readObject method. The fields may be retrieved in any order. Additional data may only be read directly from stream after readFields has been called.

The registerValidation method can be called to request a callback when the entire graph has been restored but before the object is returned to the original caller of readObject. The order of validate callbacks can be controlled using the priority. Callbacks registered with higher values are called before those with lower values. The object to be validated must support the ObjectInputValidation interface and implement the validateObject method. It is only correct to register validations during a call to a class's readObject method. Otherwise, a NotActiveException is thrown. If the callback object supplied to registerValidation is null, an InvalidObjectException is thrown.

Starting with the Java SDK, Standard Edition, v1.3, the readClassDescriptor method is used to read in all ObjectStreamClass objects. readClassDescriptor 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 described in Section 6.4, "Grammar for the Stream Format".

The resolveClass method is called while a class is being deserialized, and after the class descriptor has been read. Subclasses may extend this method to read other information about the class written by the corresponding subclass of ObjectOutputStream. The method must find and return the class with the given name and serialVersionUID. The default implementation locates the class by calling the class loader of the closest caller of readObject that has a class loader. If the class cannot be found ClassNotFoundException should be thrown. Prior to JDK 1.1.6, the resolveClass method was required to return the same fully qualified class name as the class name in the stream. In order to accommodate package renaming across releases, method resolveClass only needs to return a class with the same base class name and SerialVersionUID in JDK 1.1.6 and later versions.

The resolveObject method is used by trusted subclasses to monitor or substitute one object for another during deserialization. Resolving objects must be enabled explicitly by calling enableResolveObject before calling readObject for the first object to be resolved. Once enabled, resolveObject is called once for each serializable object just prior to the first time it is being returned from readObject. Note that the resolveObject method is not called for objects of the specially handled classes, Class, ObjectStreamClass, String, and arrays. A subclass's implementation of resolveObject may return a substitute object that will be assigned or returned instead of the original. The object returned must be of a type that is consistent and assignable to every reference of the original object or else a ClassCastException will be thrown. All assignments are type-checked. All references in the stream to the original object will be replaced by references to the substitute object.

The enableResolveObject method is called by trusted subclasses of ObjectOutputStream to enable the monitoring or substitution of one object for another during deserialization. Replacing objects is disabled until enableResolveObject is called with a true value. It may thereafter be disabled by setting it to false. The previous setting is returned. The enableResolveObject method checks if the stream has permission to request substitution during serialization. To ensure that the private state of objects is not unintentionally exposed, only trusted streams may use resolveObject. Trusted classes are those classes with a class loader equal to null or belong to a security protection domain that provides permission to enable substitution.

If the subclass of ObjectInputStream is not considered part of the system domain, a line has to be added to the security policy file to provide to a subclass of ObjectInputStream permission to call enableResolveObject. The SerializablePermission to add is "enableSubstitution". AccessControlException is thrown if the protection domain of the subclass of ObjectStreamClass does not have permission to "enableSubstitution" by calling enableResolveObject. See the document Java Security Architecture (JDK 1.2) for additional information about the security model.

The readStreamHeader method reads and verifies the magic number and version of the stream. If they do not match, the StreamCorruptedMismatch is thrown.

To override the implementation of deserialization, a subclass of ObjectInputStream should call the protected no-arg ObjectInputStream, constructor. There is a security check within the no-arg constructor for SerializablePermission "enableSubclassImplementation" to ensure that only trusted classes are allowed to override the default implementation. This constructor does not allocate any private data for ObjectInputStream and sets a flag that indicates that the final readObject method should invoke the readObjectOverride method and return. All other ObjectInputStream methods are not final and can be directly overridden by the subclass.

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