The Java language annotation facility is useful for documenting the design intent behind specific concurrency properties of code. Source code annotation is a mechanism for associating metadata with a program element and making it available to the compiler, tools such as debuggers, or the VM for examination. Several annotations are available for documenting thread-safety, or the lack thereof.
Obtaining Concurrency Annotations
There are two sets of concurrency annotations that 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 06]] which can be downloaded from jcip.net
(jar, javadoc, source). The JCIP annotations are released under the Creative Commons Attribution License.
The second, larger, set of concurrency annotations is available from and supported by SureLogic. The SureLogic annotations are released under the The Apache Software License, Version 2.0 and can be downloaded from surelogic.com (jar, javadoc, source). These annotations can be verified by the SureLogic JSure tool and are useful for documenting code even if the tool is unavailable. The SureLogic annotations also include the JCIP annotations because they are supported by the JSure tool (JSure also supports use of the JCIP Jar file).
To use the annotations, download and add one or both of the above Jar files to the code's build path. The use of these annotations to document thread-safety is discussed below.
Documenting Intended Thread-safety
JCIP provides three class-level annotations to describe the programmer's design intent with respect to thread-safety.
The @ThreadSafe annotation is applied to a class to indicate that it is thread-safe. This means that no sequences of accesses (reads and writes to public fields, calls to public methods) may leave the object in an inconsistent state, regardless of the interleaving of these accesses by the runtime or any synchronization or coordination on part of the caller.
For example, the Aircraft class shown below specifies that it is thread-safe as part of its locking policy documentation. This class protects the x and y fields using a ReentrantLock.
@ThreadSafe
@Region("private AircraftState")
@RegionLock("StateLock is stateLock protects AircraftState")
public final class Aircraft {
private final Lock stateLock = new ReentrantLock();
// ...
@InRegion("AircraftState")
private long x, y;
// ...
public void setPosition(long x, long y) {
stateLock.lock();
try {
this.x = x;
this.y = y;
} finally {
stateLock.unlock();
}
}
// ...
}
The @Region and @RegionLock annotations document the precise locking policy that the promise of thread-safety is predicated upon.
Even if one or more @RegionLock or @GuardedBy annotations have been used to document the locking policy of a class, the @ThreadSafe annotation provides an intuitive way for reviewers to learn that the class is thread-safe.
@Immutable: This annotation is applied to immutable classes. Immutable objects are inherently thread-safe; they may be safely shared amongst multiple threads after they are fully constructed, and published by declaring the corresponding reference as volatile.
The following example shows an immutable Point class:
@Immutable
public final class Point {
private final int f_x;
private final int f_y;
public Point(int x, int y) {
f_x = x;
f_y = y;
}
public int getX() {
return f_x;
}
public int getY() {
return f_y;
}
}
@NotThreadSafe This annotation is applied to classes that are not thread-safe. Several classes do not document whether they are safe for multithreaded use or not. Consequently, a programmer has no easy way to determine whether the class is thread-safe. This annotation provides clear indication of the class's lack of thread-safety.
For example, most of the collection implementations provided in java.util are not thread-safe. The class java.util.ArrayList which is not thread-safe could document this as shown below.
package java.util;
@NotThreadSafe
public class ArrayList extends ... {
...
}
Documenting Locking Policies
According to Goetz et al. [[Goetz 06, pg 28]]:
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.
Consequently, it is important to document all the locks that are being used to protect shared state. JCIP provides the @GuardedBy annotation for this purpose while SureLogic provides the @RegionLock annotation.
@GuardedBy: The field or method to which this annotation is applied can only be accessed when holding a particular lock, which may be an intrinsic lock, or a dynamic lock such as java.util.concurrent.Lock.
For example, the following MovablePoint class implements a movable point which has the capability of remembering its past locations using an array list, memo.
@ThreadSafe
public final class MovablePoint {
@GuardedBy("this") double xPos = 1.0;
@GuardedBy("this") double yPos = 1.0;
@GuardedBy("itself") static final List<MovablePoint> memo = new ArrayList<MovablePoint>();
public void move(double slope, double distance) {
rememberPoint(this);
synchronized (this) {
xPos += (1 / slope) * distance;
yPos += slope * distance;
}
}
public static void rememberPoint(MovablePoint value) {
synchronized (memo) {
memo.add(value);
}
}
}
The @GuardedBy annotations on the xPos and yPos fields indicate that access to these fields is protected by holding a lock on this (as is done in the move() method which mutates these fields). The @GuardedBy annotation on the memo list indicates that a lock on the ArrayList object protects its contents (as is done in the rememberPoint() method).
One issue with the @GuardedBy annotation is that it does not clarify that there is a relationship between the two fields in the above example. This limitation can be overcome by using the SureLogic @RegionLock annotation.
@RegionLock: Declares a new region lock for the class to which this annotation is applied. This declaration creates a new named lock that associates a particular lock object with a region of the class. The region may only be accessed when the lock is held.
For example, the SimpleLock locking policy indicates that synchronizing on the instance protects all of the instance's state:
@RegionLock("SimpleLock is this protects Instance")
class Simple { ... }
Unlike @GuardedBy, the @RegionLock annotation allows the programmer to give an explicit, and hopefully meaningful, name to the locking policy.
In addition to naming the locking policy, the @Region annotation allows a name to be given to the region of the state that is being protected. This makes it clear that the state belongs together with respect to the locking policy. This is demonstrated in the following example:
@Region("private AircraftPosition")
@RegionLock("StateLock is stateLock protects AircraftPosition")
public final class Aircraft {
private final Lock stateLock = new ReentrantLock();
@InRegion("AircraftPosition")
private long x, y;
@InRegion("AircraftPosition")
private long altitude;
// ...
public void setPosition(long x, long y) {
stateLock.lock();
try {
this.x = x;
this.y = y;
} finally {
stateLock.unlock();
}
}
// ...
}
In this example, a locking policy named StateLock is used to indicate that locking on the java.util.concurrent.Lock stateLock protects the named region, AircraftPosition, 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 safely published to other threads. However, the object is not shared until the thread that created the instance allows. Safe publication approaches discussed in CON14-J. Do not let the "this" reference escape during object construction can be succinctly expressed with the @Unique("return") annotation.
For example, in the code shown below, the @Unique("return") annotation documents that the object returned from the constructor is a unique reference.
@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
Sutherland and Scherlis [[Sutherland 10]] propose annotations that can document thread-confinement policies. Their approach is designed to allow verification of the annotations against code, as it exists.
For example, to express the design intent that a program has at most one AWT event dispatch thread, several Compute threads, and the Compute threads are forbidden to handle AWT data structures or events, use the following annotations:
@ColorDeclare AWT, Compute @IncompatibleColors AWT, Compute @MaxColorCount AWT 1
Documenting wait-notify protocols
"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.)" [Goetz 06, pg 395]
Wait-notify protocols should be adequately documented. Currently, we are not aware of any annotations for this purpose.