Finalizer
Encyclopedia
In object-oriented
programming language
s that use garbage collection
, a finalizer is a special method
that is executed when an object
is garbage collected. It is similar in function to a destructor
. In less technical terms, a finalizer is a piece of code that ensures that certain necessary actions are taken when an acquired resource (such as a file
or access to a hardware device) is no longer being used. This could be closing the file or signalling to the operating system
that the hardware device is no longer needed. However, as noted below, finalizers are not the preferred way to accomplish this and for the most part are fail-safe
.
Unlike destructors, finalizers are usually not deterministic. A destructor is run when the program explicitly frees an object. In contrast, a finalizer is executed when the internal garbage collection system frees the object. Depending on the garbage collection method used, this may happen at an arbitrary moment after the object is freed, possibly never.
Programming languages that use finalizers include C# and Java
. In C#, and a few others that support finalizers, the syntax
for declaring a finalizer mimics that of destructors in C++
.
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. For example, destructors are often used to free expensive resources such as files or network handles. If placed in a finalizer, the resources may remain in use for long periods of time after the program is finished with them. Instead, most languages encourage the dispose pattern whereby the object has a method to clean up the object's resources, leaving the finalizer fail-safe in the case where the dispose method doesn't get called. (The C# language supports the dispose pattern explicitly, via the
.)
will put resurrected objects into a "zombie" state, where they log all messages sent to them, but do nothing else http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/GarbageCollection/Articles/gcFinalize.html#//apple_ref/doc/uid/TP40002453-SW2.
Object resurrection is useful to handle a pool of commonly used objects, but it obscures code and makes it more confusing.. It should be used only for objects that may be frequently used and where the construction/destruction of it is time-consuming. An example could be an array of random numbers, where a large number of them is created and destroyed in a short time, but where actually only a small number is in use at the same time. With object resurrection, a pooling technique would reduce the unnecessary overhead of creation and destruction. Here, a pool manager would get onto its object stack information in the form of a reference to the object, if it is currently to be destructed. The pool manager will keep the object for reuse later.
Object-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s that use garbage collection
Garbage collection (computer science)
In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program...
, a finalizer is a special method
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
that is executed when an object
Object (computer science)
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
is garbage collected. It is similar in function to a destructor
Destructor (computer science)
In object-oriented programming, a destructor is a method which is automatically invoked when the object is destroyed...
. In less technical terms, a finalizer is a piece of code that ensures that certain necessary actions are taken when an acquired resource (such as a file
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
or access to a hardware device) is no longer being used. This could be closing the file or signalling to the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
that the hardware device is no longer needed. However, as noted below, finalizers are not the preferred way to accomplish this and for the most part are fail-safe
Fail-safe
A fail-safe or fail-secure device is one that, in the event of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or danger to personnel....
.
Unlike destructors, finalizers are usually not deterministic. A destructor is run when the program explicitly frees an object. In contrast, a finalizer is executed when the internal garbage collection system frees the object. Depending on the garbage collection method used, this may happen at an arbitrary moment after the object is freed, possibly never.
Programming languages that use finalizers include C# and Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
. In C#, and a few others that support finalizers, the syntax
Syntax
In linguistics, syntax is the study of the principles and rules for constructing phrases and sentences in natural languages....
for declaring a finalizer mimics that of destructors in C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
.
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. For example, destructors are often used to free expensive resources such as files or network handles. If placed in a finalizer, the resources may remain in use for long periods of time after the program is finished with them. Instead, most languages encourage the dispose pattern whereby the object has a method to clean up the object's resources, leaving the finalizer fail-safe in the case where the dispose method doesn't get called. (The C# language supports the dispose pattern explicitly, via the
IDisposable interface and the using keywordKeyword (computer programming)
In computer programming, a keyword is a word or identifier that has a particular meaning to the programming language. The meaning of keywords — and, indeed, the meaning of the notion of keyword — differs widely from language to language....
.)
Object resurrection
Languages with finalizers must also address the problem of object resurrection. Resurrection occurs when an object's finalizer causes the object to become reachable (that is, not garbage). The garbage collector must determine if the object has been resurrected by the finalizer or risk creating a dangling reference. Languages have adopted several different solutions. For example, Java will not free the object until it has proven that the object is once again unreachable, but will not run the finalizer more than once. Objective-C 2.0Objective-C
Objective-C is a reflective, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.Today, it is used primarily on Apple's Mac OS X and iOS: two environments derived from the OpenStep standard, though not compliant with it...
will put resurrected objects into a "zombie" state, where they log all messages sent to them, but do nothing else http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/GarbageCollection/Articles/gcFinalize.html#//apple_ref/doc/uid/TP40002453-SW2.
Object resurrection is useful to handle a pool of commonly used objects, but it obscures code and makes it more confusing.. It should be used only for objects that may be frequently used and where the construction/destruction of it is time-consuming. An example could be an array of random numbers, where a large number of them is created and destroyed in a short time, but where actually only a small number is in use at the same time. With object resurrection, a pooling technique would reduce the unnecessary overhead of creation and destruction. Here, a pool manager would get onto its object stack information in the form of a reference to the object, if it is currently to be destructed. The pool manager will keep the object for reuse later.
See also
- Garbage collectionGarbage collection (computer science)In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program...
, specifically the section on Determinism - Object lifetimeObject lifetimeIn computer science, the object lifetime of an object in object-oriented programming is the time between an object's creation till the object is no longer used, and is destructed or freed.In object-oriented programming , the meaning of creating objects is far more subtle than simple...