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FlexDoc/Javadoc 2.0 Demo Java Doc |
Warning: Deserialization of untrusted data is inherently dangerous and should be avoided. Untrusted data should be carefully validated according to the "Serialization and Deserialization" section of the Secure Coding Guidelines for Java SE. Serialization Filtering describes best practices for defensive use of serial filters.
The key to disabling deserialization attacks is to prevent instances of arbitrary classes from being deserialized, thereby preventing the direct or indirect execution of their methods. ObjectInputFilter describes how to use filters and ObjectInputFilter.Config describes how to configure the filter and filter factory. Each stream has an optional deserialization filter to check the classes and resource limits during deserialization. The JVM-wide filter factory ensures that a filter can be set on every ObjectInputStream and every object read from the stream can be checked. The ObjectInputStream constructors invoke the filter factory to select the initial filter which may be updated or replaced by setObjectInputFilter(ObjectInputFilter).
If an ObjectInputStream has a filter, the ObjectInputFilter can check that the classes, array lengths, number of references in the stream, depth, and number of bytes consumed from the input stream are allowed and if not, can terminate deserialization.
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.
Records are serialized differently than ordinary serializable or externalizable objects. During deserialization the record's canonical constructor is invoked to construct the record object. Certain serialization-related methods, such as readObject and writeObject, are ignored for serializable records. See Java Object Serialization Specification, Section 1.13, "Serialization of Records" for additional information.
Nested Class Summary |
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abstract static class |
Provide access to the persistent fields read from the input stream.
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Field Summary |
Constructor Summary |
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protected |
Provide a way for subclasses that are completely reimplementing
ObjectInputStream to not have to allocate private data just used by this
implementation of ObjectInputStream.
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Creates an ObjectInputStream that reads from the specified InputStream.
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Method Summary |
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int |
Returns the number of bytes that can be read without blocking.
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void |
close()
Closes the input stream.
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void |
Read the non-static and non-transient fields of the current class from
this stream.
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protected boolean |
enableResolveObject(boolean enable)
Enables the stream to do replacement of objects read from the stream.
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final ObjectInputFilter |
Returns the deserialization filter for this stream.
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int |
read()
Reads a byte of data.
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int |
read(byte[] buf, int off, int len)
Reads into an array of bytes.
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boolean |
Reads in a boolean.
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byte |
readByte()
Reads an 8 bit byte.
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char |
readChar()
Reads a 16 bit char.
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protected ObjectStreamClass |
Read a class descriptor from the serialization stream.
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double |
Reads a 64 bit double.
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Reads the persistent fields from the stream and makes them available by
name.
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float |
Reads a 32 bit float.
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void |
readFully(byte[] buf)
Reads bytes, blocking until all bytes are read.
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void |
readFully(byte[] buf, int off, int len)
Reads bytes, blocking until all bytes are read.
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int |
readInt()
Reads a 32 bit int.
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readLine()
Deprecated. This method does not properly convert bytes to characters.
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long |
readLong()
Reads a 64 bit long.
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final Object |
Read an object from the ObjectInputStream.
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protected Object |
This method is called by trusted subclasses of ObjectInputStream that
constructed ObjectInputStream using the protected no-arg constructor.
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short |
Reads a 16 bit short.
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protected void |
The readStreamHeader method is provided to allow subclasses to read and
verify their own stream headers.
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Reads an "unshared" object from the ObjectInputStream.
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int |
Reads an unsigned 8 bit byte.
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int |
Reads an unsigned 16 bit short.
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readUTF()
Reads a String in
modified UTF-8
format.
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void |
registerValidation(ObjectInputValidation obj, int prio)
Register an object to be validated before the graph is returned.
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protected Class<?> |
Load the local class equivalent of the specified stream class
description.
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protected Object |
resolveObject(Object obj)
This method will allow trusted subclasses of ObjectInputStream to
substitute one object for another during deserialization.
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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.
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final void |
Set the deserialization filter for the stream.
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int |
skipBytes(int len)
Skips bytes.
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Methods inherited from class java.io.InputStream |
Methods inherited from class java.lang.Object |
public ObjectInputStream |
(InputStream in) |
throws |
The constructor initializes the deserialization filter to the filter returned by invoking the Config#getSerialFilterFactory() with null for the current filter and the static JVM-wide filter for the requested filter.
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.
protected ObjectInputStream |
() |
throws |
The constructor initializes the deserialization filter to the filter returned by invoking the Config#getSerialFilterFactory() with null for the current filter and the static JVM-wide filter for the requested filter.
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.
public final Object readObject |
() |
throws |
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.
The deserialization filter, when not null, is invoked for each object (regular or class) read to reconstruct the root object. See setObjectInputFilter for details.
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.
protected Object readObjectOverride |
() |
throws |
public Object readUnshared |
() |
throws |
The deserialization filter, when not null, is invoked for each object (regular or class) read to reconstruct the root object. See setObjectInputFilter for details.
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.
public void defaultReadObject |
() |
throws |
public ObjectInputStream.GetField readFields |
() |
throws |
public void registerValidation |
(ObjectInputValidation obj, int prio) |
throws |
protected Class<?> resolveClass |
(ObjectStreamClass desc) |
throws |
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 the first class loader on the current thread's stack (starting from the currently executing method) that is neither the platform class loader nor its ancestor; otherwise, loader is the platform class loader. 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.
protected Class<?> resolveProxyClass |
(String[] interfaces) |
throws |
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 the first class loader on the current thread's stack (starting from the currently executing method) that is neither the platform class loader nor its ancestor; otherwise, loader is the platform class loader. 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.
protected Object resolveObject |
(Object obj) |
throws |
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 deserialization 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.
protected boolean enableResolveObject |
(boolean enable) |
throws |
If object replacement is currently not enabled, and 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 that the caller is permitted to enable the stream to do replacement of objects read from the stream.
protected void readStreamHeader |
() |
throws |
protected ObjectStreamClass readClassDescriptor |
() |
throws |
public int read |
() |
throws |
public int read |
(byte[] buf, int off, int len) |
throws |
public int available |
() |
throws |
public void close |
() |
throws |
public boolean readBoolean |
() |
throws |
public byte readByte |
() |
throws |
public int readUnsignedByte |
() |
throws |
public char readChar |
() |
throws |
public short readShort |
() |
throws |
public int readUnsignedShort |
() |
throws |
public int readInt |
() |
throws |
public long readLong |
() |
throws |
public float readFloat |
() |
throws |
public double readDouble |
() |
throws |
public void readFully |
(byte[] buf) |
throws |
public void readFully |
(byte[] buf, int off, int len) |
throws |
public int skipBytes |
(int len) |
throws |
public String readLine |
() |
throws |
public String readUTF |
() |
throws |
public final ObjectInputFilter getObjectInputFilter |
() |
public final void setObjectInputFilter |
(ObjectInputFilter filter) |
It is not permitted to replace a non-null filter with a null filter. If the current filter is non-null, the value returned from the filter factory must be non-null.
The filter's checkInput method is called for each class and reference in the stream. The filter can check any or all of the class, the array length, the number of references, the depth of the graph, and the size of the input stream. The depth is the number of nested readObject calls starting with the reading of the root of the graph being deserialized and the current object being deserialized. The number of references is the cumulative number of objects and references to objects already read from the stream including the current object being read. The filter is invoked only when reading objects from the stream and not for primitives.
If the filter returns Status.REJECTED, null or throws a RuntimeException, the active readObject or readUnshared throws InvalidClassException, otherwise deserialization continues uninterrupted.
Each call to readObject or readUnshared increases the depth by 1 before reading an object and decreases by 1 before returning normally or exceptionally. The depth starts at 1 and increases for each nested object and decrements when each nested call returns. The count of references in the stream starts at 1 and is increased before reading an object.
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FlexDoc/Javadoc 2.0 Demo Java Doc |