Page History

The Java language annotation facility is useful for documenting design intent. Source code annotation is a mechanism for associating metadata with a program element and making it available to the compiler, analyzers, debuggers, or Java Virtual Machine (JVM) for examination. Several annotations are available for documenting thread-safety or the lack thereof.

Obtaining Concurrency Annotations

Two sets of concurrency annotations are freely available and licensed for use in any code. The first set consists of four annotations described in Java Concurrency in Practice (JCIP) [Goetz 2006], which can be downloaded from jcip.net (jar, javadoc, source). The JCIP annotations are released under the Creative Commons Attribution License.

...

To use the annotations, download and add one or both of the aforementioned JAR files to the code's build path. The use of these annotations to document thread-safety is described in the following sections.

Documenting Intended Thread-Safety

JCIP provides three class-level annotations to describe the programmer's design intent with respect to thread-safety.

...

According to Joshua Bloch [Bloch 2008],

It is not necessary to document the immutability of enum types. Unless it is obvious from the return type, static factories must document the thread safety of the returned object, as demonstrated by Collections.synchronizedMap.

...

 package java.util.ArrayList;

 @NotThreadSafe
 public class ArrayList<E> extends ... {
   // ...
 }

Documenting Locking Policies

It is important to document all the locks that are being used to protect shared state. According to Brian Goetz and colleagues [Goetz 2006],

For each mutable state variable that may be accessed by more than one thread, all accesses to that variable must be performed with the same lock held. In this case, we say that the variable is guarded by that lock. (p. 28)

...

In this example, a locking policy named StateLock is used to indicate that locking on stateLock protects the named AircraftPosition region, which includes the mutable state used to represent the position of the aircraft.

Construction of Mutable Objects

Typically, object construction is considered an exception to the locking policy because objects are thread-confined when they are created. An object is confined to the thread that uses the new operator to create its instance. After creation, the object can be published to other threads safely. However, the object is not shared until the thread that created the instance allows it to be shared. Safe publication approaches discussed in TSM01-J. Do not let the this reference escape during object construction can be expressed succinctly with the @Unique("return") annotation.

...

 @RegionLock("Lock is this protects Instance")
 public final class Example {
   private int x = 1;
   private int y;

   @Unique("return")
   public Example(int y) {
     this.y = y;
   }
   // ...
 }

Documenting Thread-Confinement Policies

Dean Sutherland and William Scherlis propose annotations that can document thread-confinement policies. Their approach allows verification of the annotations against as-written code [Sutherland 2010].

For example, the following annotations express the design intent that a program has, at most, one Abstract Window Toolkit (AWT) event dispatch thread and several compute threads, and that the compute threads are forbidden to handle AWT data structures or events:

@ThreadRole AWT, Compute
@IncompatibleThreadRoles AWT, Compute
@MaxRoleCount AWT 1

Documenting Wait-Notify Protocols

According to Goetz and colleagues [Goetz 2006],

A state-dependent class should either fully expose (and document) its waiting and notification protocols to subclasses, or prevent subclasses from participating in them at all. (This is an extension of "design and document for inheritance, or else prohibit it" [EJ Item 15].) At the very least, designing a state-dependent class for inheritance requires exposing the condition queues and locks and documenting the condition predicates and synchronization policy; it may also require exposing the underlying state variables. (The worst thing a state-dependent class can do is expose its state to subclasses but not document its protocols for waiting and notification; this is like a class exposing its state variables but not documenting its invariants.) (p. 395)

Wait-notify protocols should be documented adequately. Currently, we are not aware of any annotations for this purpose.

Applicability

Annotating concurrent code helps document the design intent and can be used to automate the detection and prevention of race conditions and data races.

Bibliography

...