Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
In computer science, a reference is a value that enables a program to indirectly access a particular data item, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the data item, and accessing those data is called...
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...
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...
In computer programming, a strong reference is a normal reference that protects the referred object from collection by a garbage collector. The term is used to distinguish the reference from weak references.a Category:Programming constructs...
). An object referenced only by weak references is considered unreachable
Unreachable memory
In computer science, unreachable memory is a block of memory allocated dynamically where the program that allocated the memory no longer has any reachable pointer that refers to it. Similarly, an unreachable object is a dynamically allocated object that has no reachable reference to it...
(or "weakly reachable") and so may be collected at any time. Some garbage-collected languages feature or support various levels of weak references, such as 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...
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
, Perl, and Lisp.
Garbage collection
Garbage collection is used to clean up unused objects and so reduce the potential for memory leak
Memory leak
A memory leak, in computer science , occurs when a computer program consumes memory but is unable to release it back to the operating system. In object-oriented programming, a memory leak happens when an object is stored in memory but cannot be accessed by the running code...
s and data corruption. There are two main types of garbage collection: tracing and reference counting
Reference counting
In computer science, reference counting is a technique of storing the number of references, pointers, or handles to a resource such as an object, block of memory, disk space or other resource...
. Reference counting schemes record the number of references to a given object and collect the object when the reference count becomes zero. Reference-counting cannot collect cyclic (or circular) references because only one object may be collected at a time. Groups of mutually referencing objects which are not directly referenced by other objects and are unreachable can thus become permanently resident; if an application continually generates such unreachable groups of unreachable objects this will have the effect of a memory leak
Memory leak
A memory leak, in computer science , occurs when a computer program consumes memory but is unable to release it back to the operating system. In object-oriented programming, a memory leak happens when an object is stored in memory but cannot be accessed by the running code...
. Weak references (references which are not counted in reference counting) may be used to solve the problem of circular references if the reference cycles are avoided by using weak references for some of the references within the group. For example, Apple's Cocoa
Cocoa (API)
Cocoa is Apple's native object-oriented application programming interface for the Mac OS X operating system and—along with the Cocoa Touch extension for gesture recognition and animation—for applications for the iOS operating system, used on Apple devices such as the iPhone, the iPod Touch, and...
Weak references are also used to minimize the number of unnecessary objects in memory by allowing the program to indicate which objects are not critical by only weakly referencing them.
Variations
Some languages have multiple levels of weak reference strength. For example, 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...
A soft reference is one of the strengths or levels of 'non strong' reference defined in the Java programming language, the others being weak and phantom....
A phantom reference is one of the strengths or levels of 'non strong' reference defined in the Java programming language; the others being weak and soft....
A Java package is a mechanism for organizing Java classes into namespaces similar to the modules of Modula. Java packages can be stored in compressed files called JAR files, allowing classes to download faster as a group rather than one at a time...
java.lang.ref. Each reference type has an associated notion of reachability. The garbage collector (GC) uses an object's type of reachability to determine when to free the object. It is safe for the GC to free an object that is softly reachable, but the GC may decide not to do so if it believes the JVM can spare the memory (e.g. the JVM has lots of unused heap space). The GC will free a weakly reachable object as soon as the GC notices the object. Unlike the other reference types, a phantom reference cannot be followed. On the other hand, phantom references provide a mechanism to notify the program when an object has been freed (notification is implemented using ReferenceQueues).
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...
, provide weak/strong reference functionality as part of supporting garbage collection libraries. In the case of C++, normal pointers are "weak" and smart pointer
Smart pointer
In computer science, a smart pointer is an abstract data type that simulates a pointer while providing additional features, such as automatic garbage collection or bounds checking. These additional features are intended to reduce bugs caused by the misuse of pointers while retaining efficiency...
s are "strong" (although pointers are not true weak references, as weak references are supposed to know when the object becomes unreachable).
Examples
Weak references can be useful in keeping track of the current variables being referenced in the application. This list must have weak links to the objects. Otherwise, once objects are added to the list, they will be referenced by it and will persist forever (or until the program stops).
Java Example
If you create a weak reference and then elsewhere in the code you can use get to get the actual object, the weak reference isn't strong enough to prevent garbage collection, so you may find (if there are no strong references to the object) that get suddenly starts returning null.
import java.lang.ref.WeakReference;
public class ReferenceTest {
public static void main(String[] args) throws InterruptedException {
Student s1 = new Student(1);
System.out.println(s1);
WeakReference ws = new WeakReference(s1);
System.out.println(ws.get);
s1 = null;
System.gc;
Thread.sleep(1000);
System.out.println(ws.get);
}
}
class Student {
public Student(int id) {
this.id = id;
}
int id;
public String toString {
return "[id=" + id + "]";
}
}
Another use of weak references is in writing a cache
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
. Using, for example, a weak hash map, one can store in the cache the various referred objects via a weak reference. When the garbage collector runs — when for example the application's memory usage gets sufficiently high — those cached objects which are no longer directly referenced by other objects are removed from the cache.
