Phantom reference

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A phantom reference is a kind of reference in Java, where the memory can be reclaimed. The 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. [1] Phantom reference are the weakest level of reference in Java; in order from strongest to weakest, they are: strong, soft, weak, phantom.

Java (programming language) Object-oriented programming language

Java is a general-purpose programming language that is class-based, object-oriented, and designed to have as few implementation dependencies as possible. It is intended to let application developers "write once, run anywhere" (WORA), meaning that compiled Java code can run on all platforms that support Java without the need for recompilation. Java applications are typically compiled to "bytecode" that can run on any Java virtual machine (JVM) regardless of the underlying computer architecture. The syntax of Java is similar to C and C++, but it has fewer low-level facilities than either of them. As of 2018, Java was one of the most popular programming languages in use according to GitHub, particularly for client-server web applications, with a reported 9 million developers.

In computer programming, a weak reference is a reference that does not protect the referenced object from collection by a garbage collector, unlike a strong reference. An object referenced only by weak references – meaning "every chain of references that reaches the object includes at least one weak reference as a link" – is considered weakly reachable, and can be treated as unreachable and so may be collected at any time. Some garbage-collected languages feature or support various levels of weak references, such as C#, Java, Lisp, OCaml, Perl, and Python.

A soft reference is a reference that is garbage-collected less aggressively. The 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. In order from strongest to weakest, they are: strong, soft, weak, phantom.

Contents

An object is phantomly referenced after it has been finalized.

In object-oriented programming, a finalizer or finalize method is a special method that performs finalization, generally some form of cleanup. A finalizer is executed during object destruction, prior to the object being deallocated, and is complementary to an initializer, which is executed during object creation, following allocation. Finalizers are strongly discouraged by some, due to difficulty in proper use and the complexity they add, and alternatives are suggested instead, primarily the dispose pattern – see problems with finalizers.

In Java 8 and earlier versions, the reference needs to be cleared before the memory for a finalized referent can be reclaimed. A change in Java 9 [2] will allow memory from a finalized referent to be reclaimable immediately.

Use

Phantom references are of limited use, primarily narrow technical uses. [3] First, it can be used instead of a finalize method, guaranteeing that the object is not resurrected during finalization. This allows the object to be garbage collected in a single cycle, rather than needing to wait for a second GC cycle to ensure that it has not been resurrected. A second use is to detect exactly when an object has been removed from memory (by using in combination with a ReferenceQueue object), ensuring that its memory is available, for example deferring allocation of a large amount of memory (e.g., a large image) until previous memory is freed.

See also

An ephemeron is a data structure that solves two related problems in garbage collected systems. On the one hand, an ephemeron provides a notification when some object is about to be collected. On the other hand, an ephemeron allows data to be associated with some object without creating a reference to that object that will prevent the object from being collected. An ephemeron is a key-value pair, where the key is the object that the ephemeron guards, notifying the system when that object is collectable, and the value can be any data associated with the object such as a property list, and which may be empty. Since the elements of the property list may refer back to the key, they may prevent collection of that key. But the ephemeron is treated specially by the garbage collector. The value field is not traced until the key is found to be reachable from the system roots other than through ephemeron keys. The set of ephemerons whose keys are only reachable from ephemeron keys are then holding onto keys that are ready to be collected; these objects are not reachable from the roots except through ephemerons. When the garbage collector detects such a set, the ephemerons are queued for notification and their keys and values are traced. Hence ephemerons both detect objects that are ready for collection and break the cycles that can prevent objects from being collected.

Circular reference chain of logical connections that form a loop

A circular reference is a series of references where the last object references the first, resulting in a closed loop.

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In computer science, garbage collection (GC) 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. Garbage collection was invented by John McCarthy around 1959 to simplify manual memory management in Lisp.

Java virtual machine runtime environment that can execute Java bytecode as a result of compiling computer programs written in the Java programming language

A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describes what is required of a JVM implementation. Having a specification ensures interoperability of Java programs across different implementations so that program authors using the Java Development Kit (JDK) need not worry about idiosyncrasies of the underlying hardware platform.

Java Platform, Standard Edition is a computing platform for development and deployment of portable code for desktop and server environments. Java SE was formerly known as Java 2 Platform, Standard Edition (J2SE).

This is a comparison of Java and C++, two prominent object-oriented programming languages.

In computer programming, tracing garbage collection is a form of automatic memory management that consists of determining which objects should be deallocated by tracing which objects are reachable by a chain of references from certain "root" objects, and considering the rest as "garbage" and collecting them. Tracing garbage collection is the most common type of garbage collection – so much so that "garbage collection" often refers to tracing garbage collection, rather than other methods such as reference counting – and there are a large number of algorithms used in implementation.

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Java Card refers to a software technology that allows Java-based applications (applets) to be run securely on smart cards and similar small memory footprint devices. Java Card is the tiniest of Java platforms targeted for embedded devices. Java Card gives the user the ability to program the devices and make them application specific. It is widely used in SIM cards and ATM cards. The first Java Card was introduced in 1996 by Schlumberger's card division which later merged with Gemplus to form Gemalto. Java Card products are based on the Java Card Platform specifications developed by Sun Microsystems. Many Java card products also rely on the GlobalPlatform specifications for the secure management of applications on the card.

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. Informally, unreachable memory is dynamic memory that the program can not reach directly, nor get to by starting at an object it can reach directly, and then following a chain of pointer references.

Java (software platform) set of several computer software products and specifications

Java is a set of computer software and specifications developed by James Gosling at Sun Microsystems, which was later acquired by the Oracle Corporation, that provides a system for developing application software and deploying it in a cross-platform computing environment. Java is used in a wide variety of computing platforms from embedded devices and mobile phones to enterprise servers and supercomputers. Java applets, which are less common than standalone Java applications, were commonly run in secure, sandboxed environments to provide many features of native applications through being embedded in HTML pages. It's still possible to run Java in web browsers after most of them having dropped support for Java's VM.

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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.

References

  1. ↑ "java.lang.ref (Java Platform SE 8 )". Java™ Platform, Standard Edition 8 API Specification. Oracle. Retrieved 6 August 2016.
  2. ↑ oracle.com, kim barrett at (28 December 2015). "hg: jdk9/hs-rt/jdk: 8071507: (ref) Clear phantom reference as soft and weak references do".
  3. ↑ Nicholas, Ethan (May 4, 2006). "Understanding Weak References". www.java.net. Archived from the original on August 19, 2010. Retrieved October 1, 2010.


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