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Wiki Markup*alien method:* "From the perspective of a class C, an alien method is one whose behavior is not fully specified by C. This includes methods in other classes as well as overrideable overridable methods (neither private nor final) in C itself" \[ [Goetz 2006|AA. References#Goetz 06]\].

atomicity: When applied to an operation on primitive data, indicates that other threads that might access the data might see the data as it exists before the operation occurs or after the operation has completed but may never see an intermediate value of the data.

Wiki Markup*big-endian:* " "Multibyte data items are always stored in big-endian order, where the high bytes come first" [The Java Virtual Machine Specification|AA. References#JVMSpec 99], [Chapter 4, "The {{class}} File Format"|http://java.sun.com/docs/books/jvms/second_edition/html/ClassFile.doc.html]. This term refers to the tension between Lilliput and Blefuscu (regarding whether to open soft-boiled eggs from the large or the small end) in Jonathan Swift's satirical novel _Gulliver's Travels_; it was first applied to the question of byte-ordering by Danny Cohen \[[Cohen 1981|AA. References#Cohen 81]\[JVMSpec 2013, Chapter 4, "The class File Format"]. This term refers to the tension between Lilliput and Blefuscu (regarding whether to open soft-boiled eggs from the large or the small end) in Jonathan Swift's satirical novel Gulliver's Travels; it was first applied to the question of byte-ordering by Danny Cohen [Cohen 1981].

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Wiki Markup*class variable:* "A class variable is a field declared using the keyword {{static}} within a class declaration, or with or without the keyword static within an interface declaration. A class variable is created when its class or interface is prepared and is initialized to a default value. The class variable effectively ceases to exist when its class or interface is unloaded" \[ [JLS 2005|AA. References#JLS 05]\]2013, §4.12.3, "Kinds of Variables"].

*condition predicate:* An expression constructed from the state variables of a class that must be true for a thread to continue execution. The thread pauses execution, via {{Object.wait()}}, {{Thread.sleep()}}, or some other mechanism, and is resumed later, presumably when the requirement is true and when it is notified \[ [Goetz 2006|AA. References#Goetz 06]\].

Wiki Markup*conflicting accesses:* Two  Two accesses to (reads of or writes to) the same variable provided that at least one of the accesses is a write \ [[JLS 2005|AA. References#JLS 05]\]JLS 2013, §17.4.1, "Shared Variables"].

controlling expression: The top-level expression in the conditional expression of an if, while, do...while, or switch statement.

Wiki Markup*data data race:* "Conflicting accesses of the same variable that are not ordered by a "When a program contains two conflicting accesses that are not ordered by a happens-before relationship" \[[JLS 2005|AA. References#JLS 05]\]. relationship, it is said to contain a data race" [JLS 2013, §17.4.5, "Happens-before Order"].

deadlock: Two or more threads are said to have deadlocked when both block waiting for each others' locks. Neither thread can make any progress.

Wiki Markup*happens-before order:* "Two actions can be ordered by a happens-before relationship. If one action happens-before another, then the first is visible to and ordered before the second. . . . It should be noted that the presence of a happens-before relationship between two actions does not necessarily imply that they have to take place in that order in an implementation. If the reordering produces results consistent with a legal execution, it is not illegal. . . . More specifically, if two actions share a happens-before relationship, they do not necessarily have to appear to have happened in that order to any code with which they do not share a happens-before relationship. Writes in one thread that are in a data race with reads in another thread may, for example, appear to occur out of order to those reads" \[[JLS 2005|AA. References#JLS 05]\]. [JLS 2013, §17.4.5, "Happens-before Order"].

*heap memory:* "Memory that can be shared between threads is called shared memory or heap memory. All instance fields, static fields and array elements are stored in heap memory....Local variables (§14.4), formal method parameters (§8.4.1) or exception handler parameters are never shared between threads and are unaffected by the memory model" \[[JLS 2005|AA. References#JLS 05]\]. variables, formal method parameters or exception handler parameters are never shared between threads and are unaffected by the memory model" [JLS 2013, §17.4.1, "Shared Variables"].

Wiki Markup*hide:* One class field hides a field in a superclass if they have the same identifier. The hidden field is not accessible from the class. Likewise, a class method hides a method in a superclass if they have the same identifier but incompatible signatures. The hidden method is not accessible from the class. See \[[JLS 2005|AA. References#JLS 05]\] section [8.4.8.2|http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.4.8.2] for the formal definition. Contrast with [override|BB. Definitions#override].See Java Language Specification, §8.4.8.2, "Hiding (by Class Methods)" [JLS 2013] for the formal definition. Contrast with override.

immutable: When applied to an object, immutable means that its state cannot be changed after being initialized. " An object is immutable if:

• Its state cannot be modified after construction;
• All its fields are final; and
• It is properly constructed (the this reference does not escape during construction).

It is technically possible to have an immutable object without all fields being {{final}}. {{String}} is such a class but this relies on delicate reasoning about benign data races that requires a deep understanding of the Java Memory Model. (For the curious: {{String}} lazily computes the hash code the first time {{hashCode}} is called and caches it in a nonfinal field, but this works only because that field can take on only one nondefault value that is the same every time it is computed because it is derived deterministically from immutable state" \[[Goetz 2006|AA. References#Goetz 06]\].

Immutable objects are inherently thread-safe; they may be shared between multiple threads or published without synchronization, though it is usually required to declare the fields containing their references volatile to ensure visibility. An immutable object may contain mutable subobjects, provided the state of the subobjects cannot be modified after construction of the immutable object has concluded.

• [Goetz 2006]

It is technically possible to have an immutable object without all fields being final. String is such a class but this relies on delicate reasoning about benign data races that requires a deep understanding of the Java Memory Model.

initialization safety: "An object is considered to be completely initialized when its constructor finishes. A thread that can see a reference to an object only after that object has been completely initialized is guaranteed to see the correctly initialized values for that object's final fields" [JLS 2013, §17.5, "final Field Semantics"] Anchor initialization safety initialization safety Wiki Markup*initialization safety:* "An object is considered to be completely initialized when its constructor finishes. A thread that can see a reference to an object only after that object has been completely initialized is guaranteed to see the correctly initialized values for that object's final fields" \[[JLS 2005|AA. References#JLS 05]\].

Wiki Markup*interruption policy:* "Determines how a thread interprets an interruption request—what request—what it does (if anything) when one is detected, what units of work are considered atomic with respect to interruption, and how quickly it reacts to interruption" \ [[Goetz 2006|AA. References#Goetz 06]\].

Wiki Markup*instance variable:* "A field declared within a class declaration without using the keyword {{static}}. If a class {{T}} has a field _a_ that is an instance variable, then a new instance variable _a_ is created and initialized to a default value as part of each newly created object of class {{T}} or of any class that is a subclass of {{T}}. The instance variable effectively ceases to exist when the object of which it is a field is no longer referenced, after any necessary finalization of the object has been completed" \[ [JLS 2005|AA. References#JLS 05]\]2013, §4.12.3, "Kinds of Variables"].

liveness: Every operation or method invocation executes to completion without interruptions, even if it goes against safety.

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memoization: An optimization technique used primarily to speed up computer programs by having function calls avoid repeating the calculation of results for previously processed inputs White, Tom, Memoization in Java Using Dynamic Proxy Classes, 08/20/2003inputs [White 2003].

Wiki Markup*memory model:* "The rules that determine how memory accesses are ordered and when they are guaranteed to be visible are known as the memory model of the Java programming language" \ [[JPL 2006|AA. References#JPL 06]\Arnold 2006]. "A memory model describes, given a program and an execution trace of that program, whether the execution trace is a legal execution of the program" \[ [JLS 2005|AA. References#JLS 05]\]. 2013, §17.4, "Memory Model"].

*normalization:* Lossy conversion of the data to its simplest known (and anticipated) form. "When implementations keep strings in a normalized form, they can be assured that equivalent strings have a unique binary representation" \ [[Davis 2008|AA. References#Davis 08]\].

Wiki Markup*normalization (URI):* The process of removing unnecessary "." and ".." segments from the path component of a hierarchical URI. Each "." segment is simply removed. A ".." segment is removed only if it is preceded by a non-".." segment. Normalization has no effect on opaque URIs \[[API 2006|AA. References#API 06]\][API 2013, Class URI].

Wiki Markup*obscure:* One scoped identifier obscures another identifier in a containing scope if the two identifiers are the same, but the obscuring identifier does not [shadow|BB. Definitions#shadow] the obscured identifier. This can happen if the obscuring identifier is a variable and the obscured identifier is a type, for example. See \[[JLS 2005|AA. References#JLS 05]\], section [6.3.2|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.3.2], for more example. See Java Language Specification, §6.4.2, "Obscuring" [JLS 2013], for more information.

*obsolete reference:* "An obsolete reference is simply a reference that will never be dereferenced again" \[ [Bloch 2008|AA. References#Bloch 08]\].

*open call:* "An alien method invoked outside of a synchronized region is known as an open call" \[Lea00 2[Lea 2000, §2.4.1.3\]. See \[See also Effective Java, 2nd ed. [Bloch 2008|AA. References#Bloch 08]\] and \[[Lea 2000|AA. References#Lea 00]\].

Wiki Markup*override:* One class method overrides a method in a superclass if they have compatible signatures. The overridden method is still accessible from the class via the {{super}} keyword. See \[[JLS 2005|AA. References#JLS 05]\], section [8.4.8.1|http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8. See Java Language Specification, §8.4.8.1, "Overriding (by Instance Methods)" [JLS 2013], for the formal definition. Contrast with [hide|BB. Definitions#hide].

*partial order:* An order defined for some, but not necessarily all, pairs of items. For instance, the sets \ {a, b\} and \ {a, c, d\} are subsets of \ {a, b, c, d\}, but neither is a subset of the other. So "subset of" is a partial order on sets \ [[Black 2004|AA. References#Black 04]\].

*program order:* The order that interthread actions are performed by a thread according to the intrathread semantics of the thread. "Program order \ [can be described\] as the order of bytecodes present in the .class file, as they would execute based on control flow values" (David Holmes, [JMM Mailing List|https://mailman.cs.umd.edu/mailman/private/javamemorymodel-discussion/2007-September/000086.html]).

Wiki Markup*publishing objects:* "Publishing an object means making it available to code outside of its current scope, such as by storing a reference to it where other code can find it, returning it from a nonprivate method, or passing it to a method in another class" \ [[Goetz 2006|AA. References#Goetz 06]\].

Wiki Markup*race condition:* "General races cause nondeterministic execution and are failures in programs intended to be deterministic" \[ [Netzer 1992|AA. References#Netzer 92]\]. "A race condition occurs when the correctness of a computation depends on the relative timing or interleaving of multiple threads by the runtime" \[ [Goetz 2006|AA. References#Goetz 06]\].

Wiki Markup*relativization (URI):* "The inverse of resolution. For example, relativizing the URI {{http://java.sun.com/j2se/1.3/docs/guide/index.html}} against the base URI {{http://java.sun.com/j2se/1.3}} yields the relative URI {{docs/guide/index.html}}" \ [[API 2006|AA. References#API 06]\]. Anchor safe publication safe publication Wiki Markup*safe publication:* Definition TBD \[[Goetz 2006, Section 3.5 "Safe Publication"|AA. References#Goetz 06]\]API 2013, Class URI].

safe publication: "To publish an object safely, both the reference to the object and the [state of the object] must be made visible to other threads at the same time. A properly constructed object can be safely published by:

• Initializing an object reference from a static initializer;
• Storing a reference to it into a volatile field or AtomicReference;
• Storing a reference to it into a final field of a properly constructed object; or
• Storing a reference to it into a field that is properly guarded by a lock" [Goetz 2006, §3.5 "Safe Publication"].

Wiki Markup*safety:* Its main goal is to ensure that all objects maintain consistent states in a multithreaded environment \[ [Lea 2000|AA. References#Lea 00]\].

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Wiki Markup*security flaw:* A software defect that poses a potential security risk \ [[Seacord 2005|AA. References#Seacord 05]\]Seacord 2013].

sensitive code: Any code that performs operations that would be forbidden to untrusted code. Also, any code that accesses sensitive data (q. v.). For example, code whose correct operation requires enhanced privileges is typically considered to be sensitive.

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Wiki Markup*sequential consistency:* "A very strong guarantee that is made about visibility and ordering in an execution of a program. Within a sequentially consistent execution, there is a total order over all individual actions (such as reads and writes) which is consistent with the order of the program, and each individual action is atomic and is immediately visible to every thread. . . . If a program is correctly synchronized, then all executions of the program will appear to be sequentially consistent (§17consistent" [JLS 2013, §17.4.3)." \[[JLS 2005|AA. References#JLS 05]\]. Sequential consistency implies there will be no compiler optimizations in the statements of the action. Adopting sequential consistency as the memory model and disallowing other primitives can be overly restrictive because, under this condition, the compiler is not allowed to make optimizations and reorder code \[[JLS 2005|AA. References#JLS 05]\]., "Programs and Program Order"]. Sequential consistency implies there will be no compiler optimizations in the statements of the action. Adopting sequential consistency as the memory model and disallowing other primitives can be overly restrictive because, under this condition, the compiler is not allowed to make optimizations and reorder code.

Wiki Markup*shadow:* One scoped identifier shadows another identifier in a containing scope if the two identifiers are the same and they both reference variables. They may also both reference methods or types. The shadowed identifier is not accessible in the scope of the shadowing identifier. See \[[JLS 2005|AA. References#JLS 05]\], section [6.3.1|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.3.1], for more information. Contrast with [obscure|BB. Definitions#obscure].See Java Language Specification, §6.4.1, "Shadowing" [JLS 2013], for more information. Contrast with obscure.

Wiki Markup*synchronization:* "The Java programming language provides multiple mechanisms for communicating between threads. The most basic of these methods is _synchronization_, which is implemented using monitors. Each object in Java is associated with a monitor, which a thread can lock or unlock. Only one thread at a time may hold a lock on a monitor. Any other threads attempting to lock that monitor are blocked until they can obtain a lock on that monitor." \[ [JLS 2005|AA. References#JLS 05]\]2013, §17.1, "Synchronization"].

starvation: A condition wherein one or more threads prevent other threads from accessing a shared resource over extended periods of time. For instance, a thread that invokes a synchronized method that performs somea time-time consuming operation starves other threads.

Wiki Markup*tainted data:* Data that either originate from an untrusted source or resulted from an operation whose inputs included tainted data. Tainted data can be _sanitized_ (also _untainted_) through suitable data validation. Note that all outputs from untrusted code must be considered to be tainted \ [[Jovanovich 2006|AA. References#Jovanovich 06]\Jovanovic 2006].

Wiki Markup*thread-safe:* An object is thread-safe if it can be shared by multiple threads without the possibility of any data races. "A thread-safe object performs synchronization internally, so multiple threads can freely access it through its public interface without further synchronization" \[ [Goetz 2006|AA. References#Goetz 06]\]. Immutable classes are thread-safe by definition. Mutable classes may also be thread-safe if they are properly synchronized.

Wiki Markup*total order:* An order defined for all pairs of items of a set. For instance, {{<=}} (less than or equal to) is a total order on integers; that is, for any two integers, one of them is less than or equal to the other \ [[Black 2006|AA. References#Black 06]\].

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untrusted code: Code of unknown origin that can potentially cause some harm when executed. Untrusted code may not always be malicious, but it is usually hard to determine automatically. Consequently, untrusted code should be run in a sandboxed environment.

Wiki Markup*volatile:* "A write to a volatile field (§8.3.1.4) happens-before every subsequent read of that field" \ [[JLS 2005|AA. References#JLS 05]\]. "Operations on the master copies of volatile variables on behalf of a thread are performed by the main memory in exactly the order that the thread requested" \[[JVMSpec 1999|AA. References#JVMSpec 99]\]. Accesses to a {{volatile}} variable are [sequentially consistent|BB. Definitions#sequential consistency], which also means that the operations are exempt from compiler optimizations. Declaring a variable {{volatile}} ensures that all threads see the most up-to-date value of the variable if any thread modifies it. Volatile guarantees atomic reads and writes of primitive values, but it does not guarantee the atomicity of composite operations such as variable incrementation JLS 2013, §17.4.5. "Happens-before Order"]. "Operations on the master copies of volatile variables on behalf of a thread are performed by the main memory in exactly the order that the thread requested" [JVMSpec 1999]. Accesses to a volatile variable are sequentially consistent, which also means that the operations are exempt from compiler optimizations. Declaring a variable volatile ensures that all threads see the most up-to-date value of the variable if any thread modifies it. Volatile guarantees atomic reads and writes of primitive values, but it does not guarantee the atomicity of composite operations such as variable incrementation (read-modify-write sequence).

*vulnerability:* "A set of conditions that allows an attacker to violate an explicit or implicit security policy" \ [[Seacord 2005|AA. References#Seacord 05]\Seacord 2013].