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 overridable methods (neither private nor final) in C itself [[Goetz 2006]].
anti-pattern An anti-pattern is a pattern that may be commonly used but is ineffective and/or counterproductive in practice Laplante 2005.
availability The degree to which a system or component is operational and accessible when required for use. Often expressed as a probability [[IEEE Std 610.12 1990]].
big-endian "Multibyte data items are always stored in big-endian order, where the high bytes come first." [[JVMSpec 1999]] 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]].
canonicalization Reducing the input to its equivalent simplest known form.
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]].
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]].
conflicting accesses Two accesses to (reads of or writes to) the same variable provided that at least one of the accesses is a write [[JLS 2005]].
data race Conflicting accesses of the same variable that are not ordered by a happens-before relationship [[JLS 2005]].
deadlock Two or more threads are said to have deadlocked when both block waiting for each other's locks. Neither thread can make any progress.
error tolerance The ability of a system or component to continue normal operation despite the presence of erroneous inputs [[IEEE Std 610.12 1990]].
exploit A piece of software or a technique that takes advantage of a security vulnerability to violate an explicit or implicit [security policy] [[Seacord 2005a]].
fail safe Pertaining to a system or component that automatically places itself in a safe operating mode in the event of a failure – for example, a traffic light that reverts to blinking red in all directions when normal operation fails [[IEEE Std 610.12 1990]].
fail soft Pertaining to a system or component that continues to provide partial operational capability in the event of certain failures – for example, a traffic light that continues to alternate between red and green if the yellow light fails [[IEEE Std 610.12 1990]].
fault tolerance The ability of a system or component to continue normal operation despite the presence of hardware or software faults [[IEEE Std 610.12 1990]].
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]].
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]].
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]] §8.4.8.2 for the formal definition. Contrast with [override].
immutable When applied to an object, this 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
thisreference 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]].
Immutable objects are inherently thread-safe; they may be shared among 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.
initialization safety An object is considered to be completely initialized when its constructor finishes. A thread that can only see a reference to an object after that object has been completely initialized is guaranteed to see the correctly initialized values for that object's final fields [[JLS 2005]].
instance variable An instance variable is 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]].
interruption policy An interruption policy determines how a thread interprets an interruption 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]].
invariant A property that is assumed to be true at certain points during program execution, but not formally specified. They may be used in assert statements, or informally specified in comments. Invariants are often used to prove program correctness.
liveness A property that every operation or method invocation executes to completion without interruptions, even if it goes against safety.
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 2003.
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]]. "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]].
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]].
normalization (URI) Normalization is 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 upon opaque URIs [[API 2006]].
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] the obscured identifier. This can happen when the obscuring identifier is a variable while the obscured identifier is a type, for example. See [[JLS 2005]] §6.3.2 for more information.
obsolete reference An obsolete reference is simply a reference
that will never be dereferenced again [[Bloch 2008]].
open call An alien method invoked outside of a synchronized region is known as an open call [Lea00 2.4.1.3] [[Bloch 2008]], [[Lea 2000]].
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]] §8.4.8.1 for the formal definition. Contrast with [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 is a "subset of" is a partial order on sets [[Black 2004]]
program order The order that interthread actions are performed by a thread according to the intra-thread 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).
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]].
race condition "General races cause nondeterministic execution and are failures in programs intended to be deterministic" [[Netzer 1992]]. "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]].
relativization (URI) [Relativization] is 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]].
safe publication To publish an object safely, both the reference to the object and the object's state 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.
- Storing a reference to it into a final field.
Storing a reference to it into a field that is properly guarded by a (synchronized) lock.
[[Goetz 2006, Section 3.5 "Safe Publication"]]
safety Its main goal is to ensure that all objects maintain consistent states in a multi-threaded environment [[Lea 2000]].
sanitization Sanitization is a term used for validating input and transforming it to a representation that conforms to the input requirements of a complex subsystem. For example, a database may require all invalid characters to be escaped or eliminated prior to their storage. Input sanitization refers to the elimination of unwanted characters from the input by means of removal, replacement, encoding or escaping the characters.
security flaw A software defect that poses a potential security risk [[Seacord 2005]].
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.
sensitive data Any data that must be kept secure. Consequences of this security requirement include:
- Untrusted code is forbidden to access sensitive data.
- Trusted code is forbidden to leak sensitive data to untrusted code.
Examples of sensitive data include passwords and personally identifiable information.
sequential consistency "Sequential consistency is 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, all executions of the program will appear to be sequentially consistent (§17.4.3)" [[JLS 2005]]. 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]].
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]] §6.3.1 for more information. Contrast with [obscure].
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]].
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 some time-consuming operation starves other threads.
tainted data Data that either originate from an untrusted source or result 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]].
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]]. Immutable classes are thread-safe by definition. Mutable classes may also be thread-safe if they are properly synchronized.
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]].
trusted code Code that is loaded by the primordial class loader, irrespective of whether or not it constitutes the Java API. In this text, this meaning is extended to include code that is obtained from a known entity and given permissions that untrusted code lacks. By this definition, untrusted and trusted code can coexist in the namespace of a single class loader (not necessarily the primordial class loader). In such cases, the security policy must make this distinction clear by assigning appropriate privileges to trusted code while denying the same from untrusted code.
untrusted code Code of unknown origin that can potentially cause some harm when executed. Untrusted code may not always be malicious, but this is usually hard to determine automatically. Consequently, untrusted code should be run in a sandboxed environment.
volatile "A write to a volatile field (§8.3.1.4) happens-before every subsequent read of that field" [[JLS 2005]]. "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; however, it does not guarantee the atomicity of composite operations such as variable incrementation (read-modify-write sequence).
vulnerability A set of conditions that allow an attacker to violate an explicit or implicit security policy [[Seacord 2005]].