java.util.concurrent.atomic
class AtomicReferenceFieldUpdater

java.lang.Object java.util.concurrent.atomic.AtomicReferenceFieldUpdater

public abstract class AtomicReferenceFieldUpdater
extends Object

A reflection-based utility that enables atomic updates to designated {@code volatile} reference fields of designated classes. This class is designed for use in atomic data structures in which several reference fields of the same node are independently subject to atomic updates. For example, a tree node might be declared as

 class Node {
   private volatile Node left, right;

   private static final AtomicReferenceFieldUpdater<Node, Node> leftUpdater =
     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left");
   private static AtomicReferenceFieldUpdater<Node, Node> rightUpdater =
     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right");

   Node getLeft() { return left;  }
   boolean compareAndSetLeft(Node expect, Node update) {
     return leftUpdater.compareAndSet(this, expect, update);
   }
   // ... and so on
 }
 

Note that the guarantees of the {@code compareAndSet} method in this class are weaker than in other atomic classes. Because this class cannot ensure that all uses of the field are appropriate for purposes of atomic access, it can guarantee atomicity only with respect to other invocations of {@code compareAndSet} and {@code set} on the same updater.

AtomicReferenceFieldUpdater

protected AtomicReferenceFieldUpdater()

Protected do-nothing constructor for use by subclasses.

compareAndSet

public abstract boolean compareAndSet(Object obj,
                                      Object expect,
                                      Object update)

Atomically sets the field of the given object managed by this updater to the given updated value if the current value {@code ==} the expected value. This method is guaranteed to be atomic with respect to other calls to {@code compareAndSet} and {@code set}, but not necessarily with respect to other changes in the field.

Parameters:

obj - An object whose field to conditionally set

expect - the expected value

update - the new value

Returns:

true if successful.

get

public abstract Object get(Object obj)

Gets the current value held in the field of the given object managed by this updater.

Parameters:

obj - An object whose field to get

Returns:

the current value

getAndSet

public Object getAndSet(Object obj,
                        Object newValue)

Atomically sets the field of the given object managed by this updater to the given value and returns the old value.

Parameters:

obj - An object whose field to get and set

newValue - the new value

Returns:

the previous value

lazySet

public abstract void lazySet(Object obj,
                             Object newValue)

Eventually sets the field of the given object managed by this updater to the given updated value.

Parameters:

obj - An object whose field to set

newValue - the new value

newUpdater

public static AtomicReferenceFieldUpdater newUpdater(Class tclass,
                                                     Class vclass,
                                                     String fieldName)

Creates and returns an updater for objects with the given field. The Class arguments are needed to check that reflective types and generic types match.

Parameters:

tclass - the class of the objects holding the field.

vclass - the class of the field

fieldName - the name of the field to be updated.

Returns:

the updater

set

public abstract void set(Object obj,
                         Object newValue)

Sets the field of the given object managed by this updater to the given updated value. This operation is guaranteed to act as a volatile store with respect to subsequent invocations of {@code compareAndSet}.

Parameters:

obj - An object whose field to set

newValue - the new value

weakCompareAndSet

public abstract boolean weakCompareAndSet(Object obj,
                                          Object expect,
                                          Object update)

Atomically sets the field of the given object managed by this updater to the given updated value if the current value {@code ==} the expected value. This method is guaranteed to be atomic with respect to other calls to {@code compareAndSet} and {@code set}, but not necessarily with respect to other changes in the field.

May fail spuriously and does not provide ordering guarantees, so is only rarely an appropriate alternative to {@code compareAndSet}.

Parameters:

obj - An object whose field to conditionally set

expect - the expected value

update - the new value

Returns:

true if successful.