Serialization

This article is about data structure encoding. For other uses, see Serialization (disambiguation).

In computer science, in the context of data storage and transmission, serialization is the process of converting a data structure or object state into a format that can be stored (for example, in a file or memory buffer, or transmitted across a network connection link) and "resurrected" later in the same or another computer environment. When the resulting series of bits is reread according to the serialization format, it can be used to create a semantically identical clone of the original object. For many complex objects, such as those that make extensive use of references, this process is not straightforward. Serialization of object oriented objects does not include any of their associated methods with which they were previously inextricably linked.

This process of serializing an object is also called deflating or marshalling an object. The opposite operation, extracting a data structure from a series of bytes, is deserialization (which is also called inflating or unmarshalling).

Uses

Serialization provides:

• a method of persisting objects which is more convenient than writing their properties to a text file on disk, and re-assembling them by reading this back in.
• a method of remote procedure calls, e.g., as in SOAP
• a method for distributing objects, especially in software componentry such as COM, CORBA, etc.
• a method for detecting changes in time-varying data.

For some of these features to be useful, architecture independence must be maintained. For example, for maximal use of distribution, a computer running on a different hardware architecture should be able to reliably reconstruct a serialized data stream, regardless of endianness. This means that the simpler and faster procedure of directly copying the memory layout of the data structure cannot work reliably for all architectures. Serializing the data structure in an architecture independent format means that we do not suffer from the problems of byte ordering, memory layout, or simply different ways of representing data structures in different programming languages.

Inherent to any serialization scheme is that, because the encoding of the data is by definition serial, extracting one part of the serialized data structure requires that the entire object be read from start to end, and reconstructed. In many applications this linearity is an asset, because it enables simple, common I/O interfaces to be utilized to hold and pass on the state of an object. In applications where higher performance is an issue, it can make sense to expend more effort to deal with a more complex, non-linear storage organization.

Even on a single machine, primitive pointer objects are too fragile to save, because the objects to which they point may be reloaded to a different location in memory. To deal with this, the serialization process includes a step called unswizzling or pointer unswizzling and the deserialization process includes a step called pointer swizzling.

Since both serializing and deserializing can be driven from common code, (for example, the Serialize function in Microsoft Foundation Classes) it is possible for the common code to do both at the same time, and thus 1) detect differences between the objects being serialized and their prior copies, and 2) provide the input for the next such detection. It is not necessary to actually build the prior copy, since differences can be detected "on the fly". This is a way to understand the technique called differential execution. It is useful in the programming of user interfaces whose contents are time-varying — graphical objects can be created, removed, altered, or made to handle input events without necessarily having to write separate code to do those things.

Consequences

Serialization, however, breaks the opacity of an abstract data type by potentially exposing private implementation details. To discourage competitors from making compatible products, publishers of proprietary software often keep the details of their programs' serialization formats a trade secret. Some deliberately obfuscate or even encrypt the serialized data.

Yet, interoperability requires that applications be able to understand each other's serialization formats. Therefore, remote method call architectures such as CORBA define their serialization formats in detail.

Serialization formats

The Xerox Network Systems Courier technology in the early 1980s influenced the first widely-adopted standard. Sun Microsystems published the External Data Representation (XDR) in 1987.

In the late 1990s, a push to provide an alternative to the standard serialization protocols started: XML was used to produce a human readable text-based encoding. Such an encoding can be useful for persistent objects that may be read and understood by humans, or communicated to other systems regardless of programming language. It has the disadvantage of losing the more compact, byte-stream-based encoding, but by this point larger storage and transmission capacities made file size less of a concern than in the early days of computing. Binary XML has been proposed as a compromise which is not readable by plain-text editors, but is more compact than regular XML. In the 2000s, XML is often used for asynchronous transfer of structured data between client and server in Ajax web applications.

JSON is a more lightweight plain-text alternative to XML which is also commonly used for client-server communication in web applications. JSON is based on JavaScript syntax, but is supported in other programming languages as well.

Another alternative, YAML, is effectively a superset of JSON and includes features that make it more powerful for serialization, more "human friendly," and potentially more compact. These features include a notion of tagging data types, support for non-hierarchical data structures, the option to data structure with indentation, and multiple forms of scalar data quoting.

Another human-readable serialization format is the property list format used in NeXTSTEP, GNUstep, and Mac OS X Cocoa.

For large volume scientific datasets, such as satellite data and output of numerical climate, weather, or ocean models, specific binary serialization standards have been developed, e.g. HDF, netCDF and the older GRIB.

Programming language support

Several object-oriented programming languages directly support object serialization (or object archival), either by syntactic sugar elements or providing a standard interface for doing so.

Some of these programming languages are Ruby, Smalltalk, Python, PHP, Objective-C, Java, and the .NET family of languages.

There are also libraries available that add serialization support to languages that lack native support for it.

.NET Framework

In the .NET languages, classes can be serialized and deserialized by adding the Serializable attribute to the class.

If new members are added to a serializable class, they can be tagged with the OptionalField attribute to allow previous versions of the object to be deserialized without error. This attribute affects only deserialization, and prevents the runtime from throwing an exception if a member is missing from the serialized stream. A member can also be marked with the NonSerialized attribute to indicate that it should not be serialized. This will allow the details of those members to be kept secret.

To modify the default deserialization (for example, to automatically initialize a member marked NonSerialized), the class must implement the IDeserializationCallback interface and define the IDeserializationCallback.OnDeserialization method.

Objects may be serialized in binary format for deserialization by other .NET applications. The framework also provides the SoapFormatter and XmlSerializer objects to support serialization in human-readable, cross-platform XML.

Objective-C

In the Objective-C programming language, serialization (more commonly known as archiving) is achieved by overriding the write: and read: methods in the Object root class. (NB This is in the GNU runtime variant of Objective-C. In the NeXT-style runtime, the implementation is very similar.)

Java

Java provides automatic serialization which requires that the object be marked by implementing the java.io.Serializable interface. Implementing the interface marks the class as "okay to serialize," and Java then handles serialization internally. There are no serialization methods defined on the Serializable interface, but a serializable class can optionally define methods with certain special names and signatures that if defined, will be called as part of the serialization/deserialization process. The language also allows the developer to override the serialization process more thoroughly by implementing another interface, the Externalizable interface, which includes two special methods that are used to save and restore the object's state.

There are three primary reasons why objects are not serializable by default and must implement the Serializable interface to access Java's serialization mechanism.

1. Not all objects capture useful semantics in a serialized state. For example, a Thread object is tied to the state of the current JVM. There is no context in which a deserialized Thread object would maintain useful semantics.
2. The serialized state of an object forms part of its class's compatibility contract. Maintaining compatibility between versions of serializable classes requires additional effort and consideration. Therefore, making a class serializable needs to be a deliberate design decision and not a default condition.
3. Serialization allows access to non-transient private members of a class that are not otherwise accessible. Classes containing sensitive information (for example, a password) should not be serializable nor externalizable.

The standard encoding method uses a simple translation of the fields into a byte stream. Primitives as well as non-transient, non-static referenced objects are encoded into the stream. Each object that is referenced by the serialized object and not marked as transient must also be serialized; and if any object in the complete graph of non-transient object references is not serializable, then serialization will fail. The developer can influence this behavior by marking objects as transient, or by redefining the serialization for an object so that some portion of the reference graph is truncated and not serialized.

It is possible to serialize Java objects through JDBC and store them into a database.

While Swing components do implement the Serializable interface, they are not portable between different versions of the Java Virtual Machine. As such, a Swing component, or any component which inherits it, may be serialized to an array of bytes, but it is not guaranteed that this storage will be readable on another machine.

ColdFusion

ColdFusion allows data structures to be serialized to WDDX with the <cfwddx> tag and to JSON with the SerializeJSON() function.

OCaml

OCaml's standard library provides marshalling through the Marshal module (its documentation) and the Pervasives functions output_value and input_value. While OCaml programming is statically type-checked, uses of the Marshal module may break type guarantees, as there is no way to check whether an unmarshalled stream represents objects of the expected type. In OCaml it is difficult to marshal a function or a data structure which contains a function (e.g. an object which contains a method), because executable code in functions cannot be transmitted across different programs. (There is a flag to marshal the code position of a function but it can only be unmarshalled in exactly the same program.)

Perl

Several Perl modules available from CPAN provide serialization mechanisms, including Storable and FreezeThaw.

Storable includes functions to serialize and deserialize Perl data structures to and from files or Perl scalars.

In addition to serializing directly to files, Storable includes the freeze function to return a serialized copy of the data packed into a scalar, and thaw to deserialize such a scalar. This is useful for sending a complex data structure over a network socket or storing it in a database.

When serializing structures with Storable, there are network safe functions that always store their data in a format that is readable on any computer at a small cost of speed. These functions are named nstore, nfreeze, etc. There are no "n" functions for deserializing these structures — the regular thaw and retrieve deserialize structures serialized with the "n" functions and their machine-specific equivalents.

C/C++

C and C++ do not provide direct support for serialization. However, compiler-based solutions, such as the ODB ORM system for C++, are capable of automatically producing serialization code with few or no modifications to class declarations. Another popular serialization framework is Boost.Serialization from the Boost Framework.

Python

Python implements serialization through the standard library module pickle, and to a lesser extent, the older marshal. marshal does offer the ability to serialize Python code objects, unlike pickle. In addition, Python offers the cPickle module, which (as the name suggests) is a C implementation of the pickle module. It can be up to 1000 times faster than the pure Python pickle module, but has a few limitations. The shelve module is based on the pickle module and can be regarded as a serialized Python dictionary.

As of version 2.6, Python's standard library also includes support for JSON and for XML-encoded property lists. (See json and plistlib, respectively.) However, these modules only handle basic Python types like strings, integers, and collections of basic types, whereas pickle is intended for arbitrary objects.

PHP

PHP implements serialization through the built-in serialize and unserialize functions. PHP can serialize any of its data types except resources (file pointers, sockets, etc.).

For objects (as of at least PHP 4) there are two "magic methods" that can be implemented within a class — __sleep() and __wakeup() — that are called from within serialize() and unserialize(), respectively, that can clean up and restore an object. For example, it may be desirable to close a database connection on serialization and restore the connection on deserialization; this functionality would be handled in these two magic methods. They also permit the object to pick which properties are serialized.

As of PHP 5.1.0 there's another method to hook into internal serialize/unserialize mechanism - Serializable interface.

R

R has the function dput which writes an ASCII text representation of an R object to a file or connection. A representation can be read from a file using dget.

REBOL

REBOL will serialize to file (save/all) or to a string! (mold/all). Strings and files can be deserialized using the polymorphic load function.

Ruby

Ruby includes the standard module Marshal with 2 methods dump and load, akin to the standard Unix utilities dump and restore. These methods serialize to the standard class String, that is, they effectively become a sequence of bytes.

Some objects cannot be serialized (doing so would raise a TypeError exception):

• bindings,
• procedure objects,
• instances of class IO,
• singleton objects

If a class requires custom serialization (for example, it requires certain cleanup actions done on dumping / restoring), it can be done by implementing 2 methods: _dump and _load. The instance method _dump should return a String object containing all the information necessary to reconstitute objects of this class and all referenced objects up to a maximum depth given as an integer parameter (a value of -1 implies that depth checking should be disabled). The class method _load should take a String and return an object of this class.

Smalltalk

Squeak Smalltalk

There are several ways in Squeak Smalltalk to serialize and store objects. The easiest and most used method will be shown below (where?). Other classes of interest in Squeak for serializing objects are SmartRefStream and ImageSegment.

Cincom Smalltalk and Smalltalk/X

Both provide a so called "binary-object storage framework", which support serialization into and retrieval from a compact binary form. Both handle cyclic, recursive and shared structures, storage/retrieval of class and metaclass info and include mechanisms for "on the fly" object migration (i.e. to convert instances which were written by an older version of a class with a different object layout). The APIs are similar (storeBinary/readBinary), but the encoding details are different, making these two formats incompatible. However, the Smalltalk/X code is open source and free and can be loaded into other Smalltalks to allow for cross-dialect object interchange.

Other Smalltalk dialects

Object serialization is not part of the ANSI Smalltalk specification. As a result, the code to serialize an object varies by Smalltalk implementation. The resulting binary data also varies. For instance, a serialized object created in Squeak Smalltalk cannot be restored in Ambrai Smalltalk. Consequently, various applications that do work on multiple Smalltalk implementations that rely on object serialization cannot share data between these different implementations. These applications include the MinneStore object database [2] and some RPC packages. A solution to this problem is SIXX [3], which is an package for multiple Smalltalks that uses an XML-based format for serialization.

Lisp

Generally a Lisp data structure can be serialized with the functions "read" and "print". A variable foo containing, for example, a list of arrays would be printed by (print foo). Similarly an object can be read from a stream named s by (read s). These two parts of the Lisp implementation are called the Printer and the Reader. The output of "print" is human readable; it uses lists demarked by parentheses, for example: (4 2.9 "x" y).

In many types of Lisp, including Common Lisp, the printer cannot represent every type of data because it is not clear how to do so. In Common Lisp for example the printer cannot print CLOS objects. Instead the programmer may write a method on the generic function print-object, this will be invoked when the object is printed. This is somewhat similar to the method used in Ruby.

Lisp code itself is written in the syntax of the reader, called read syntax. Most languages use separate and different parsers to deal with code and data, Lisp only uses one. A file containing lisp code may be read into memory as a data structure, transformed by another program, then possibly executed or written out. See REPL.

Notice that not all readers/writers support cyclic, recursive or shared structures.

Haskell

In Haskell, serialization is supported for types by inheritance of the Read and Show type classes. Every type that inherits the Read class defines a function that will extract the data from the string representation of the dumped data. The Show class, in turn, contains the show function from which a string representation of the object can be generated.

The programmer need not define the functions explicitly—merely declaring a type to be deriving Read or deriving Show, or both, can make the compiler generate the appropriate functions for many cases (but not all: function types, for example, cannot automatically derive Show or Read).

Windows PowerShell

Windows PowerShell implements serialization through the built-in cmdlet Export-CliXML. Export-CliXML serializes .NET objects and stores the resulting XML in a file.

To reconstitute the objects, use the Import-CliXML cmdlet, which generates a deserialized object from the XML in the exported file. Deserialized objects, often known as "property bags" are not live objects; they are snapshots that have properties, but no methods.

Two dimensional data structures can also be (de)serialized in CSV format using the built-in cmdlets Import-CSV and Export-CSV.

See also

• Comparison of data serialization formats
• Hibernate (Java)
• Persistor.NET
• XML Schema
• Basic Encoding Rules

External links

For ASP.NET:

• Serialize Any ASP.NET Object to a JSON String
• aspnet Callback technique using csharp

For Java:

• Java Object Serialization documentation
• Java 1.4 Object Serialization documentation.
• Java Object Serialization Specification
• Java Object Serialization
• Durable Java: Serialization
• XML Data Binding Resources
• JOAFIP serialization in file and more...
• Databoard Binary serialization with partial and random access, type system, RPC, type adaption, and text format.
• Kryo Fast and efficient object graph serialization framework for Java

For C:

• c11n
• tpl

For C++:

• Serialization and Unserialization
• Boost Serialization
• s11n
• Sweet Persist
• C++ Middleware Writer

For PHP:

• Object Serialization in PHP
• Online serialize & unserialize tool with recovering capacities. Other resources about PHP serialization
• Recovering Truncated PHP Serialized Arrays

Serialization systems that support multiple languages:

• Google Protocol Buffers Developer Guide (C++, Java, Python)
• Apache Avro