Java.Util.Concurrent.Atomic Namespace

This method (which varies in argument types across different classes) atomically sets a variable to the updateValue if it currently holds the expectedValue, reporting true on success.


A small toolkit of classes that support lock-free thread-safe programming on single variables. In essence, the classes in this package extend the notion of volatile values, fields, and array elements to those that also provide an atomic conditional update operation of the form:

java Example

boolean compareAndSet(expectedValue, updateValue);

This method (which varies in argument types across different classes) atomically sets a variable to the updateValue if it currently holds the expectedValue, reporting true on success. The classes in this package also contain methods to get and unconditionally set values, as well as a weaker conditional atomic update operation weakCompareAndSet described below.

The specifications of these methods enable implementations to employ efficient machine-level atomic instructions that are available on contemporary processors. However on some platforms, support may entail some form of internal locking. Thus the methods are not strictly guaranteed to be non-blocking -- a thread may block transiently before performing the operation.

Instances of classes Java.Util.Concurrent.Atomic.AtomicBoolean, Java.Util.Concurrent.Atomic.AtomicInteger, Java.Util.Concurrent.Atomic.AtomicLong, and Java.Util.Concurrent.Atomic.AtomicReference each provide access and updates to a single variable of the corresponding type. Each class also provides appropriate utility methods for that type. For example, classes AtomicLong and AtomicInteger provide atomic increment methods. One application is to generate sequence numbers, as in:

java Example

class Sequencer {
   private final AtomicLong sequenceNumber
     = new AtomicLong(0);
   public long next() {
     return sequenceNumber.getAndIncrement();

It is straightforward to define new utility functions that, like getAndIncrement, apply a function to a value atomically. For example, given some transformation

java Example

long transform(long input)
write your utility method as follows:

java Example

long getAndTransform(AtomicLong var) {
   while (true) {
     long current = var.get();
     long next = transform(current);
     if (var.compareAndSet(current, next))
         return current;
         // return next; for transformAndGet

The memory effects for accesses and updates of atomics generally follow the rules for volatiles, as stated in :

In addition to classes representing single values, this package contains Updater classes that can be used to obtain compareAndSet operations on any selected volatile field of any selected class. Java.Util.Concurrent.Atomic.AtomicReferenceFieldUpdater, Java.Util.Concurrent.Atomic.AtomicIntegerFieldUpdater, and Java.Util.Concurrent.Atomic.AtomicLongFieldUpdater are reflection-based utilities that provide access to the associated field types. These are mainly of use in atomic data structures in which several volatile fields of the same node (for example, the links of a tree node) are independently subject to atomic updates. These classes enable greater flexibility in how and when to use atomic updates, at the expense of more awkward reflection-based setup, less convenient usage, and weaker guarantees.

The Java.Util.Concurrent.Atomic.AtomicIntegerArray, Java.Util.Concurrent.Atomic.AtomicLongArray, and Java.Util.Concurrent.Atomic.AtomicReferenceArray classes further extend atomic operation support to arrays of these types. These classes are also notable in providing volatile access semantics for their array elements, which is not supported for ordinary arrays.

The atomic classes also support method weakCompareAndSet, which has limited applicability. On some platforms, the weak version may be more efficient than compareAndSet in the normal case, but differs in that any given invocation of the weakCompareAndSet method may return falsespuriously (that is, for no apparent reason). A false return means only that the operation may be retried if desired, relying on the guarantee that repeated invocation when the variable holds expectedValue and no other thread is also attempting to set the variable will eventually succeed. (Such spurious failures may for example be due to memory contention effects that are unrelated to whether the expected and current values are equal.) Additionally weakCompareAndSet does not provide ordering guarantees that are usually needed for synchronization control. However, the method may be useful for updating counters and statistics when such updates are unrelated to the other happens-before orderings of a program. When a thread sees an update to an atomic variable caused by a weakCompareAndSet, it does not necessarily see updates to any other variables that occurred before the weakCompareAndSet. This may be acceptable when, for example, updating performance statistics, but rarely otherwise.

The Java.Util.Concurrent.Atomic.AtomicMarkableReference class associates a single boolean with a reference. For example, this bit might be used inside a data structure to mean that the object being referenced has logically been deleted. The Java.Util.Concurrent.Atomic.AtomicStampedReference class associates an integer value with a reference. This may be used for example, to represent version numbers corresponding to series of updates.

Atomic classes are designed primarily as building blocks for implementing non-blocking data structures and related infrastructure classes. The compareAndSet method is not a general replacement for locking. It applies only when critical updates for an object are confined to a single variable.

Atomic classes are not general purpose replacements for java.lang.Integer and related classes. They do not define methods such as equals, hashCode and compareTo. (Because atomic variables are expected to be mutated, they are poor choices for hash table keys.) Additionally, classes are provided only for those types that are commonly useful in intended applications. For example, there is no atomic class for representing byte. In those infrequent cases where you would like to do so, you can use an AtomicInteger to hold byte values, and cast appropriately. You can also hold floats using Java.Lang.Float.FloatToRawIntBits(float) and Java.Lang.Float.IntBitsToFloat(int) conversions, and doubles using Java.Lang.Double.DoubleToRawLongBits(double) and Java.Lang.Double.LongBitsToDouble(long) conversions.


AtomicBooleanA boolean value that may be updated atomically.
AtomicIntegerAn int value that may be updated atomically.
AtomicIntegerArrayAn int array in which elements may be updated atomically.
AtomicIntegerFieldUpdaterA reflection-based utility that enables atomic updates to designated volatile int fields of designated classes.
AtomicLongA long value that may be updated atomically.
AtomicLongArrayA long array in which elements may be updated atomically.
AtomicLongFieldUpdaterA reflection-based utility that enables atomic updates to designated volatile long fields of designated classes.
AtomicMarkableReferenceAn AtomicMarkableReference maintains an object reference along with a mark bit, that can be updated atomically.
AtomicReferenceAn object reference that may be updated atomically.
AtomicReferenceArrayAn array of object references in which elements may be updated atomically.
AtomicReferenceFieldUpdaterA reflection-based utility that enables atomic updates to designated volatile reference fields of designated classes.
AtomicStampedReferenceAn AtomicStampedReference maintains an object reference along with an integer "stamp", that can be updated atomically.