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 methods (neither private nor final) in C itself." [[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.
canonicalization : Reducing the input to its equivalent simplest known form.
class variable : A class variable is a static field associated with the containing class.
conflicting accesses :"Two accesses to (reads of or writes to) the same variable are said to be conflicting if at least one of the accesses is a write." [[JLS 05]].
data race : "When a program contains two conflicting accesses (§17.4.1) that are not ordered by a happens-before relationship, it is said to contain a data race." [[JLS 05]]. "A data race occurs in an execution of a program if there are conflicting actions in that execution that are not ordered by synchronization." [[JSR-133 04]]. "
"Data races cause nonatomic execution of critical sections and are failures in (nondeterministic) programs that access and update shared data in critical sections." [[Netzer 92]].
deadlock : Two or more threads are said to have deadlocked when both block waiting for each others' locks. Neither thread can make any progress.
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 05]].
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 05]].
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;[12] and
- It is properly constructed (the
thisreference does not escape during construction).
[12] 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 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 sub-objects, provided the state of the sub-objects 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 05]].
instance variable : An instance variable is a non-static field that is a part of every instance of the class
liveness : Every operation or method invocation executes to completion without interruptions, even if it goes against safety.
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 06]]. "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 05]].
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" [[Unicode 08]].
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 06]].
obsolete reference : "An obsolete reference is simply a reference
that will never be dereferenced again." [[Bloch 08]].
open call : "An alien method invoked outside of a synchronized region is known as an open call [Lea00 2.4.1.3]". [[Bloch 08]] and [[Lea 00]].
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 04]]
program order is the order that inter-thread 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 06]].
race condition: "General races cause nondeterministic execution and are failures in programs intended to be deterministic." [[Netzer 92]].
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 06]].
safety : Its main goal is to ensure that all objects maintain consistent states in a multi-threaded environment [[Lea 00]].
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.
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, then all executions of the program will appear to be sequentially consistent (§17.4.3)." [[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 05]].
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 05]].
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 which performs some time consuming operation, starves other threads.
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 06]]. 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 06]]
trusted code : Code that is loaded by the primordial class loader, irrespective of whether it constitutes the Java API or not. 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 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 99]]. 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 allows an attacker to violate an explicit or implicit security policy" [[Seacord 05]].