In
computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
, a finalizer or finalize method is a special
method
Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to:
*Scien ...
that performs finalization, generally some form of cleanup. A finalizer is executed during
object destruction
In object-oriented programming (OOP), the object lifetime (or life cycle) of an object is the time between an object's creation and its destruction. Rules for object lifetime vary significantly between languages, in some cases between implement ...
, prior to the object being
deallocated, and is complementary to an
initializer In computer programming, initialization (or initialisation) is the assignment of an initial value for a data object or variable. The manner in which initialization is performed depends on the programming language, as well as the type, storage class ...
, which is executed during
object creation
In object-oriented programming (OOP), the object lifetime (or life cycle) of an object is the time between an object's creation and its destruction. Rules for object lifetime vary significantly between languages, in some cases between implement ...
, following
allocation
Allocation may refer to:
Computing
* Block allocation map
* C++ allocators
* Delayed allocation
* File allocation table
* IP address allocation
* Memory allocation
* No-write allocation (cache)
* Register allocation
Economics
* Asset alloca ...
. Finalizers are strongly discouraged by some, due to difficulty in proper use and the complexity they add, and alternatives are suggested instead, mainly the
dispose pattern
In object-oriented programming, the dispose pattern is a design pattern for resource management. In this pattern, a resource is held by an object, and released by calling a conventional method – usually called close, dispose, free, release dep ...
(see
problems with finalizers).
The term ''finalizer'' is mostly used in
object-oriented
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pro ...
and
functional
Functional may refer to:
* Movements in architecture:
** Functionalism (architecture)
** Form follows function
* Functional group, combination of atoms within molecules
* Medical conditions without currently visible organic basis:
** Functional sy ...
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
s that use
garbage collection
Waste collection is a part of the process of waste management. It is the transfer of solid waste from the point of use and disposal to the point of treatment or landfill. Waste collection also includes the curbside collection of recyclable m ...
, of which the archetype is
Smalltalk
Smalltalk is an object-oriented, dynamically typed reflective programming language. It was designed and created in part for educational use, specifically for constructionist learning, at the Learning Research Group (LRG) of Xerox PARC by Alan Ka ...
. This is contrasted with a ''
destructor'', which is a method called for finalization in languages with deterministic object lifetimes, archetypically
C++
C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
. These are generally exclusive: a language will have either finalizers (if automatically garbage collected) or destructors (if manually memory managed), but in rare cases a language may have both, as in
C++/CLI
C++/CLI is a variant of the C++ programming language, modified for Common Language Infrastructure. It has been part of Visual Studio 2005 and later, and provides interoperability with other .NET languages such as C#. Microsoft created C++/CLI ...
and
D, and in case of
reference counting
In computer science, reference counting is a programming technique of storing the number of references, pointers, or handles to a resource, such as an object, a block of memory, disk space, and others.
In garbage collection algorithms, referenc ...
(instead of tracing garbage collection), terminology varies. In technical use, ''finalizer'' may also be used to refer to destructors, as these also perform finalization, and some subtler distinctions are drawn – see
terminology
Terminology is a group of specialized words and respective meanings in a particular field, and also the study of such terms and their use; the latter meaning is also known as terminology science. A ''term'' is a word, compound word, or multi-wor ...
. The term ''final'' is also used to indicate a class that cannot be
inherited; this is unrelated.
Terminology
The terminology of ''finalizer'' and ''finalization'' versus ''destructor'' and ''destruction'' varies between authors and is sometimes unclear.
In common use, a ''destructor'' is a method called deterministically on object destruction, and the archetype is C++ destructors; while a finalizer is called non-deterministically by the garbage collector, and the archetype is
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
finalize
methods.
For languages that implement garbage collection via
reference counting
In computer science, reference counting is a programming technique of storing the number of references, pointers, or handles to a resource, such as an object, a block of memory, disk space, and others.
In garbage collection algorithms, referenc ...
, terminology varies, with some languages such as
Objective-C
Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXTS ...
and
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offici ...
using ''destructor'', and other languages such as Python using ''finalizer'' (per spec, Python is garbage collected, but the reference
CPython
CPython is the reference implementation of the Python (programming language), Python programming language. Written in C (programming language), C and Python, CPython is the default and most widely used implementation of the Python language.
CP ...
implementation since its version 2.0 uses a combination of reference counting and garbage collection). This reflects that reference counting results in semi-deterministic object lifetime: for objects that are not part of a cycle, objects are destroyed deterministically when the reference count drops to zero, but objects that are part of a cycle are destroyed non-deterministically, as part of a separate form of garbage collection.
In certain narrow technical use, ''constructor'' and ''destructor'' are language-level terms, meaning ''methods defined in a class'', while ''initializer'' and ''finalizer'' are implementation-level terms, meaning ''methods called during object creation or destruction''. Thus for example the original specification for the
C# language referred to "destructors", even though C# is garbage-collected, but the specification for the
Common Language Infrastructure
The Common Language Infrastructure (CLI) is an open specification and technical standard originally developed by Microsoft and standardized by ISO/IEC (ISO/IEC 23271) and Ecma International (ECMA 335) that describes executable code and a runt ...
(CLI), and the implementation of its runtime environment as the
Common Language Runtime
The Common Language Runtime (CLR), the virtual machine component of Microsoft .NET Framework, manages the execution of .NET programs. Just-in-time compilation converts the managed code (compiled intermediate language code) into machine instructio ...
(CLR), referred to "finalizers". This is reflected in the C# language committee's notes, which read in part: "The C# compiler compiles destructors to ...
robablyinstance finalizer
. This terminology is confusing, and thus more recent versions of the C# spec refer to the language-level method as "finalizers".
[
Another language that does not make this terminology distinction is D. Although D classes are garbage collected, their cleanup functions are called destructors.
]
Use
Finalization is mostly used for cleanup, to release memory or other resources: to deallocate memory allocated via manual memory management
In computer science, manual memory management refers to the usage of manual instructions by the programmer to identify and deallocate unused objects, or garbage. Up until the mid-1990s, the majority of programming languages used in industry supp ...
; to clear references if reference counting
In computer science, reference counting is a programming technique of storing the number of references, pointers, or handles to a resource, such as an object, a block of memory, disk space, and others.
In garbage collection algorithms, referenc ...
is used (decrement reference counts); to release resources, particularly in the Resource Acquisition Is Initialization (RAII) idiom; or to unregister an object. The amount of finalization varies significantly between languages, from extensive finalization in C++, which has manual memory management, reference counting, and deterministic object lifetimes; to often no finalization in Java, which has non-deterministic object lifetimes and is often implemented with a tracing garbage collector. It is also possible for there to be little or no explicit (user-specified) finalization, but significant implicit finalization, performed by the compiler, interpreter, or runtime; this is common in case of automatic reference counting, as in the CPython
CPython is the reference implementation of the Python (programming language), Python programming language. Written in C (programming language), C and Python, CPython is the default and most widely used implementation of the Python language.
CP ...
reference implementation of Python, or in Automatic Reference Counting
Automatic Reference Counting (ARC) is a memory management feature of the Clang compiler providing automatic reference counting for the Objective-C and Swift programming languages. At compile time, it inserts into the object code messages retain an ...
in Apple's implementation of Objective-C
Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXTS ...
, which both automatically break references during finalization. A finalizer can include arbitrary code; a particularly complex use is to automatically return the object to an object pool
The object pool pattern is a software creational design pattern that uses a set of initialized objects kept ready to use – a "pool" – rather than allocating and destroying them on demand. A client of the pool will request an object from the po ...
.
Memory deallocation during finalization is common in languages like C++ where manual memory management is standard, but also occurs in managed languages when memory has been allocated outside of the managed heap (externally to the language); in Java this occurs with Java Native Interface
In software design, the Java Native Interface (JNI) is a foreign function interface programming framework that enables Java code running in a Java virtual machine (JVM) to call and be called by
native applications (programs specific to a hardwa ...
(JNI) and ByteBuffer
objects in New I/O java.nio (NIO stands for New Input/Output) is a collection of Java programming language APIs that offer features for intensive I/O operations. It was introduced with the J2SE 1.4 release of Java by Sun Microsystems to complement an existing stand ...
(NIO). This latter can cause problems due to the garbage collector being unable to track these external resources, so they will not be collected aggressively enough, and can cause out-of-memory errors due to exhausting unmanaged memory – this can be avoided by treating native memory as a resource and using the dispose pattern
In object-oriented programming, the dispose pattern is a design pattern for resource management. In this pattern, a resource is held by an object, and released by calling a conventional method – usually called close, dispose, free, release dep ...
, as discussed below.
Finalizers are generally both much less necessary and much less used than destructors. They are much less necessary because garbage collection automates memory management
Memory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when ...
, and much less used because they are not generally executed deterministically – they may not be called in a timely manner, or even at all, and the execution environment cannot be predicted – and thus any cleanup that must be done in a deterministic way must instead be done by some other method, most frequently manually via the dispose pattern
In object-oriented programming, the dispose pattern is a design pattern for resource management. In this pattern, a resource is held by an object, and released by calling a conventional method – usually called close, dispose, free, release dep ...
. Notably, both Java and Python do not guarantee that finalizers will ever be called, and thus they cannot be relied on for cleanup.
Due to the lack of programmer control over their execution, it is usually recommended to avoid finalizers for any but the most trivial operations. In particular, operations often performed in destructors are not usually appropriate for finalizers. A common anti-pattern
An anti-pattern in software engineering, project management, and business processes is a common response to a recurring problem that is usually ineffective and risks being highly counterproductive. The term, coined in 1995 by computer programmer An ...
is to write finalizers as if they were destructors, which is both unnecessary and ineffectual, due to differences between finalizers and destructors. This is particularly common among C++
C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
programmers, as destructors are heavily used in idiomatic C++, following the Resource Acquisition Is Initialization (RAII) idiom.
Syntax
Programming languages that use finalizers include C++/CLI
C++/CLI is a variant of the C++ programming language, modified for Common Language Infrastructure. It has been part of Visual Studio 2005 and later, and provides interoperability with other .NET languages such as C#. Microsoft created C++/CLI ...
, C#, Clean, Go, Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
, JavaScript
JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of Website, websites use JavaScript on the Client (computing), client side ...
and Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
. Syntax varies significantly by language.
In Java, a finalizer is a method called finalize
, which overrides the Object.finalize
method.[java.lang, Class Object]
finalize
/ref>
In JavaScript
FinalizationRegistry
allows you to request a callback when an object is garbage-collected.
In Python, a finalizer is a method called __del__
.
In Perl, a finalizer is a method called DESTROY
.
In C#, a finalizer (called "destructor" in earlier versions of the standard) is a method whose name is the class name with ~
prefixed, as in ~Foo
– this is the same syntax as a C++ ''destructor'', and these methods were originally called "destructors", by analogy with C++, despite having different behavior, but were renamed to "finalizers" due to the confusion this caused.
In C++/CLI, which has both destructors and finalizers, a destructor is a method whose name is the class name with ~
prefixed, as in ~Foo
(as in C#), and a finalizer is a method whose name is the class name with !
prefixed, as in !Foo
.
In Go finalizers are applied to a single pointer by calling the runtime.SetFinalizer
function in the standard library.
Implementation
A finalizer is called when an object
Object may refer to:
General meanings
* Object (philosophy), a thing, being, or concept
** Object (abstract), an object which does not exist at any particular time or place
** Physical object, an identifiable collection of matter
* Goal, an ...
is garbage collected – after an object has become garbage (unreachable), but before its memory is deallocated. Finalization occurs non-deterministically, at the discretion of the garbage collector, and might never occur. This contrasts with destructors, which are called deterministically as soon as an object is no longer in use, and are always called, except in case of uncontrolled program termination. Finalizers are most frequently instance method
A method in object-oriented programming (OOP) is a procedure associated with a message and an object. An object consists of ''state data'' and ''behavior''; these compose an ''interface'', which specifies how the object may be utilized by any ...
s, due to needing to do object-specific operations.
The garbage collector must also account for the possibility of object resurrection. Most commonly this is done by first executing finalizers, then checking whether any objects have been resurrected, and if so, aborting their destruction. This additional check is potentially expensive – a simple implementation re-checks all garbage if even a single object has a finalizer – and thus both slows down and complicates garbage collection. For this reason, objects with finalizers may be collected less frequently than objects without finalizers (only on certain cycles), exacerbating problems caused by relying on prompt finalization, such as resource leaks.
If an object is resurrected, there is the further question of whether its finalizer is called again, when it is next destroyed – unlike destructors, finalizers are potentially called multiple times. If finalizers are called for resurrected objects, objects may repeatedly resurrect themselves and be indestructible; this occurs in the CPython implementation of Python prior to Python 3.4, and in CLR languages such as C#. To avoid this, in many languages, including Java, Objective-C (at least in recent Apple implementations), and Python from Python 3.4, objects are finalized at most once, which requires tracking if the object has been finalized yet.
In other cases, notably CLR languages like C#, finalization is tracked separately from the objects themselves, and objects can be repeatedly registered or deregistered for finalization.
Problems
Depending on the implementation, finalizers can cause a significant number of problems, and are thus strongly discouraged by a number of authorities.[MET12-J. Do not use finalizers]
, Dhruv Mohindra
The CERT Oracle Secure Coding Standard for Java
05. Methods (MET)
/ref> These problems include:[
* Finalizers may not be called in a timely manner, or at all, so they cannot be relied upon to persist state, release scarce resources, or do anything else of importance.
* Finalizers may result in ]object resurrection
In object-oriented programming languages with garbage collection, object resurrection is when an object comes back to life during the process of object destruction, as a side effect of a finalizer being executed.
Object resurrection causes a num ...
, which is often a programming error and whose very possibility significantly slows down and complicates garbage collection.
* Finalizers are run based on garbage collection, which is generally based on managed memory pressure – they are not run in case of other resource scarcity, and thus are not suited for managing other scarce resources.
* Finalizers do not run in a specified order, and cannot rely on class invariant
In computer programming, specifically object-oriented programming, a class invariant (or type invariant) is an invariant used for constraining objects of a class. Methods of the class should preserve the invariant. The class invariant constrain ...
s (as they may refer to other objects that have already been finalized).
* Slow finalizers may delay other finalizers.
* Exceptions within finalizers generally cannot be handled, because the finalizer is run in an unspecified environment, and may be either ignored or cause uncontrolled program termination.
* Finalizers may reference and accidentally finalize live objects, violating program invariants.
* Finalizers may cause synchronization issues, even in otherwise sequential (single-threaded) programs, when finalization is done in a separate threads.
* Finalizers may cause deadlocks if synchronization mechanisms such as locks are used, due to not being run in a specified order and possibly being run concurrently.
* Finalizers that are run during program termination cannot rely on the usual runtime environment, and thus may fail due to incorrect assumptions – for this reason finalizers are often not run during termination.
Further, finalizers may fail to run due to objects remaining reachable beyond when they are expected to be garbage, either due to programming errors or due to unexpected reachability. For example, when Python catches an exception (or an exception is not caught in interactive mode), it keeps a reference to the stack frame where the exception was raised, which keeps objects referenced from that stack frame alive.
In Java, finalizers in a superclass can also slow down garbage collection in a subclass, as the finalizer can potentially refer to fields in the subclass, and thus the field cannot be garbage collected until the following cycle, once the finalizer has run.[ This can be avoided by using ]composition over inheritance
Composition over inheritance (or composite reuse principle) in object-oriented programming (OOP) is the principle that classes should achieve Polymorphism (computer science), polymorphic behavior and code reuse by their object composition, compo ...
.
Resource management
A common anti-pattern
An anti-pattern in software engineering, project management, and business processes is a common response to a recurring problem that is usually ineffective and risks being highly counterproductive. The term, coined in 1995 by computer programmer An ...
is to use finalizers to release resources, by analogy with the Resource Acquisition Is Initialization (RAII) idiom of C++: acquire a resource in the initializer (constructor), and release it in the finalizer (destructor). This does not work, for a number of reasons. Most basically, finalizers may never be called, and even if called, may not be called in a timely manner – thus using finalizers to release resources will generally cause resource leak
In computer science, a resource leak is a particular type of resource consumption by a computer program where the program does not release resources it has acquired. This condition is normally the result of a bug in a program. Typical resource lea ...
s. Further, finalizers are not called in a prescribed order, while resources often need to be released in a specific order, frequently the opposite order in which they were acquired. Also, as finalizers are called at the discretion of the garbage collector, they will often only be called under managed memory pressure (when there is little managed memory available), regardless of resource pressure – if scarce resources are held by garbage but there is plenty of managed memory available, garbage collection may not occur, thus not reclaiming these resources.
Thus instead of using finalizers for automatic resource management, in garbage-collected languages one instead must manually manage resources, generally by using the dispose pattern
In object-oriented programming, the dispose pattern is a design pattern for resource management. In this pattern, a resource is held by an object, and released by calling a conventional method – usually called close, dispose, free, release dep ...
. In this case resources may still be acquired in the initializer, which is called explicitly on object instantiation, but are released in the dispose method. The dispose method may be called explicitly, or implicitly by language constructs such as C#'s using
, Java's try
-with-resources, or Python's with
.
However, in certain cases both the dispose pattern and finalizers are used for releasing resources. This is mostly found in CLR languages such as C#, where finalization is used as a backup for disposal: when a resource is acquired, the acquiring object is queued for finalization so that the resource is released on object destruction, even if the resource is not released by manual disposal.
Deterministic and non-deterministic object lifetimes
In languages with deterministic object lifetimes, notably C++, resource management is frequently done by tying resource possession lifetime to object lifetime, acquiring resources during initialization and releasing them during finalization; this is known as Resource Acquisition Is Initialization (RAII). This ensures that resource possession is a class invariant
In computer programming, specifically object-oriented programming, a class invariant (or type invariant) is an invariant used for constraining objects of a class. Methods of the class should preserve the invariant. The class invariant constrain ...
, and that resources are released promptly when the object is destroyed.
However, in languages with non-deterministic object lifetimes – which include all major languages with garbage collection, such as C#, Java, and Python – this does not work, because finalization may not be timely or may not happen at all, and thus resources may not be released for a long time or even at all, causing resource leak
In computer science, a resource leak is a particular type of resource consumption by a computer program where the program does not release resources it has acquired. This condition is normally the result of a bug in a program. Typical resource lea ...
s. In these languages resources are instead generally managed manually via the dispose pattern
In object-oriented programming, the dispose pattern is a design pattern for resource management. In this pattern, a resource is held by an object, and released by calling a conventional method – usually called close, dispose, free, release dep ...
: resources may still be acquired during initialization, but are released by calling a dispose
method. Nevertheless, using finalization for releasing resources in these languages is a common anti-pattern
An anti-pattern in software engineering, project management, and business processes is a common response to a recurring problem that is usually ineffective and risks being highly counterproductive. The term, coined in 1995 by computer programmer An ...
, and forgetting to call dispose
will still cause a resource leak.
In some cases both techniques are combined, using an explicit dispose method, but also releasing any still-held resources during finalization as a backup. This is commonly found in C#, and is implemented by registering an object for finalization whenever a resource is acquired, and suppressing finalization whenever a resource is released.
Object resurrection
If user-specified finalizers are allowed, it is possible for finalization to cause object resurrection
In object-oriented programming languages with garbage collection, object resurrection is when an object comes back to life during the process of object destruction, as a side effect of a finalizer being executed.
Object resurrection causes a num ...
, as the finalizers can run arbitrary code, which may create references from live objects to objects being destroyed. For languages without garbage collection, this is a severe bug, and causes dangling reference
Dangling pointers and wild pointers in computer programming are pointers that do not point to a valid object of the appropriate type. These are special cases of memory safety violations. More generally, dangling references and wild references are ...
s and memory safety
Memory safety is the state of being protected from various software bugs and Vulnerability (computing), security vulnerabilities when dealing with random-access memory, memory access, such as buffer overflows and dangling pointers. For example, J ...
violations; for languages with garbage collection, this is prevented by the garbage collector, most commonly by adding another step to garbage collection (after running all user-specified finalizers, check for resurrection), which complicates and slows down garbage collection.
Further, object resurrection means that an object may not be destroyed, and in pathological cases an object can always resurrect itself during finalization, making itself indestructible. To prevent this, some languages, like Java and Python (from Python 3.4) only finalize objects once, and do not finalize resurrected objects. Concretely this is done by tracking if an object has been finalized on an object-by-object basis. Objective-C also tracks finalization (at least in recent Apple versions) for similar reasons, treating resurrection as a bug.
A different approach is used in the .NET Framework
The .NET Framework (pronounced as "''dot net"'') is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows. It was the predominant implementation of the Common Language Infrastructure (CLI) until bein ...
, notably C# and Visual Basic .NET
Visual Basic, originally called Visual Basic .NET (VB.NET), is a multi-paradigm, object-oriented programming language, implemented on .NET, Mono, and the .NET Framework. Microsoft launched VB.NET in 2002 as the successor to its original Visua ...
, where finalization is tracked by a "queue", rather than by object. In this case, if a user-specified finalizer is provided, by default the object is only finalized once (it is queued for finalization on creation, and dequeued once it is finalized), but this can be changed via calling the GC
module. Finalization can be prevented by calling GC.SuppressFinalize
, which dequeues the object, or reactivated by calling GC.ReRegisterForFinalize
, which enqueues the object. These are particularly used when using finalization for resource management as a supplement to the dispose pattern, or when implementing an object pool
The object pool pattern is a software creational design pattern that uses a set of initialized objects kept ready to use – a "pool" – rather than allocating and destroying them on demand. A client of the pool will request an object from the po ...
.
Contrast with initialization
Finalization is formally complementary to initialization – initialization occurs at the start of lifetime, finalization at the end – but differs significantly in practice. Both variables and objects are initialized, mostly to assign values, but in general only objects are finalized, and in general there is no need to clear values – the memory can simply be deallocated and reclaimed by the operating system.
Beyond assigning initial values, initialization is mostly used to acquire resources or to register an object with some service (like an event handler
In programming and software design, an event is an action or occurrence recognized by software, often originating asynchronously from the external environment, that may be handled by the software. Computer events can be generated or triggered ...
). These actions have symmetric release or unregister actions, and these can symmetrically be handled in a finalizer, which is done in RAII. However, in many languages, notably those with garbage collection, object lifetime is asymmetric: object creation happens deterministically at some explicit point in the code, but object destruction happens non-deterministically, in some unspecified environment, at the discretion of the garbage collector. This asymmetry means that finalization cannot be effectively used as the complement of initialization, because it does not happen in a timely manner, in a specified order, or in a specified environment. The symmetry is partially restored by also disposing of the object at an explicit point, but in this case disposal and destruction do not happen at the same point, and an object may be in a "disposed but still alive" state, which weakens the class invariant
In computer programming, specifically object-oriented programming, a class invariant (or type invariant) is an invariant used for constraining objects of a class. Methods of the class should preserve the invariant. The class invariant constrain ...
s and complicates use.
Variables are generally initialized at the start of their lifetime, but not finalized at the end of their lifetime – though if a variable has an object as its value, the ''object'' may be finalized. In some cases variables are also finalized: GCC extensions allow finalization of variables.
Connection with finally
As reflected in the naming, "finalization" and the finally
construct both fulfill similar purposes: performing some final action, generally cleaning up, after something else has finished. They differ in when they occur – a finally
clause is executed when program execution leaves the body of the associated try
clause – this occurs during stack unwind, and there is thus a stack of pending finally
clauses, in order – while finalization occurs when an object is destroyed, which happens depending on the memory management method, and in general there is simply a set of objects awaiting finalization – often on the heap – which need not happen in any specific order.
However, in some cases these coincide. In C++, object destruction is deterministic, and the behavior of a finally
clause can be produced by having a local variable with an object as its value, whose scope is a block corresponds to the body of a try
clause – the object is finalized (destructed) when execution exits this scope, exactly as if there were a finally
clause. For this reason, C++ does not have a finally
construct – the difference being that finalization is defined in the class definition as the destructor method, rather than at the call site in a finally
clause.
Conversely, in the case of a finally
clause in a coroutine
Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing execution to be suspended and resumed. Coroutines are well-suited for implementing familiar program components such as cooperative ...
, like in a Python generator, the coroutine may never terminate – only ever yielding – and thus in ordinary execution the finally
clause is never executed. If one interprets instances of a coroutine as objects, then the finally
clause can be considered a finalizer of the object, and thus can be executed when the instance is garbage collected. In Python terminology, the definition of a coroutine is a generator ''function,'' while an instance of it is a generator ''iterator,'' and thus a finally
clause in a generator function becomes a finalizer in generator iterators instantiated from this function.
History
The notion of finalization as a separate step in object destruction dates to , by analogy with the earlier distinction of initialization in object construction in . Literature prior to this point used "destruction" for this process, not distinguishing finalization and deallocation, and programming languages dating to this period, like C++ and Perl, use the term "destruction". The terms "finalize" and "finalization" are also used in the influential book ''Design Patterns
''Design Patterns: Elements of Reusable Object-Oriented Software'' (1994) is a software engineering book describing software design patterns. The book was written by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, with a foreword ...
'' (1994).["Every new class has a fixed implementation overhead (initialization, finalization, etc.).", "''destructor'' In C++, an operation that is automatically invoked to finalize an object that is about to be deleted."] The introduction of Java in 1995 contained finalize
methods, which popularized the term and associated it with garbage collection, and languages from this point generally make this distinction and use the term "finalization", particularly in the context of garbage collection.
See also
* Garbage collection
Waste collection is a part of the process of waste management. It is the transfer of solid waste from the point of use and disposal to the point of treatment or landfill. Waste collection also includes the curbside collection of recyclable m ...
, specifically the section on Determinism
Determinism is a philosophical view, where all events are determined completely by previously existing causes. Deterministic theories throughout the history of philosophy have developed from diverse and sometimes overlapping motives and consi ...
* Object lifetime
In object-oriented programming (OOP), the object lifetime (or life cycle) of an object is the time between an object's creation and its destruction. Rules for object lifetime vary significantly between languages, in some cases between implementa ...
* Initialization Process & related Initializer Pattern
Notes
References
Further reading
*
*
* {{Cite book , last=Montgomery , first=Stephen , date=January 24, 1994 , url=http://www.sciencedirect.com/science/book/9780125050401 , title=Object-Oriented Information Engineering: Analysis, Design, and Implementation , publisher=Academic Press , isbn=978-0-12-505040-1
External links
*
Finalize Instead Of Proper Destructor
, ''WikiWikiWeb
The WikiWikiWeb is the first wiki, or user-editable website. It was launched on 25 March 1995 by programmer Ward Cunningham to accompany the Portland Pattern Repository website discussing software design patterns. The name ''WikiWikiWeb'' origi ...
'' – comparison of Java finalizers with C++ destructors
* Paul Krill,
Oracle recommends axing Java object finalizer
, ''JavaWorld
''InfoWorld'' (abbreviated IW) is an information technology media business. Founded in 1978, it began as a monthly magazine. In 2007, it transitioned to a web-only publication. Its parent company today is International Data Group, and its siste ...
'' Mar 29, 2017.
Memory management
Method (computer programming)
Object-oriented programming
de:Garbage Collection#Finalisierung