Atomics

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:

      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`. The weak version may be more efficient in the normal case, but differs in that any given invocation of `weakCompareAndSet` method may fail, even spuriously (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.

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 {@link edu.emory.mathcs.backport.java.util.concurrent.atomic.AtomicBoolean}, {@link edu.emory.mathcs.backport.java.util.concurrent.atomic.AtomicInteger}, {@link edu.emory.mathcs.backport.java.util.concurrent.atomic.AtomicLong}, and {@link edu.emory.mathcs.backport.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:

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

The memory effects for accesses and updates of atomics generally follow the rules for volatiles, as stated in The Java Language Specification, Third Edition (17.4 Memory Model):

- `get` has the memory effects of reading a `volatile` variable.
- `set` has the memory effects of writing (assigning) a `volatile` variable.
- `lazySet` has the memory effects of writing (assigning) a `volatile` variable except that it permits reorderings with subsequent (but not previous) memory actions that do not themselves impose reordering constraints with ordinary non-`volatile` writes. Among other usage contexts, `lazySet` may apply when nulling out, for the sake of garbage collection, a reference that is never accessed again.
- `weakCompareAndSet` atomically reads and conditionally writes a variable, is ordered with respect to other memory operations on that variable, but otherwise acts as an ordinary non-volatile memory operation.
- `compareAndSet` and all other read-and-update operations such as `getAndIncrement` have the memory effects of both reading and writing `volatile` variables.

The {@link edu.emory.mathcs.backport.java.util.concurrent.atomic.AtomicIntegerArray}, {@link edu.emory.mathcs.backport.java.util.concurrent.atomic.AtomicLongArray}, and {@link edu.emory.mathcs.backport.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 {@link edu.emory.mathcs.backport.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 {@link edu.emory.mathcs.backport.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 `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 `Float.floatToIntBits` and `Float.intBitstoFloat` conversions, and doubles using `Double.doubleToLongBits` and `Double.longBitsToDouble` conversions. @since 1.5