Java™ Platform
Standard Ed. 6

java.util.concurrent.atomic
Class AtomicReferenceFieldUpdater<T,V>

java.lang.Object
  extended by java.util.concurrent.atomic.AtomicReferenceFieldUpdater<T,V>
Type Parameters:
T - The type of the object holding the updatable field
V - The type of the field

public abstract class AtomicReferenceFieldUpdater<T,V>
extends Object

A reflection-based utility that enables atomic updates to designated 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 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 compareAndSet and set on the same updater.

Since:
1.5

Constructor Summary
protected AtomicReferenceFieldUpdater()
          Protected do-nothing constructor for use by subclasses.
 
Method Summary
abstract  boolean compareAndSet(T obj, V expect, V update)
          Atomically sets the field of the given object managed by this updater to the given updated value if the current value == the expected value.
abstract  V get(T obj)
          Gets the current value held in the field of the given object managed by this updater.
 V getAndSet(T obj, V newValue)
          Atomically sets the field of the given object managed by this updater to the given value and returns the old value.
abstract  void lazySet(T obj, V newValue)
          Eventually sets the field of the given object managed by this updater to the given updated value.
static
<U,W> AtomicReferenceFieldUpdater<U,W>
newUpdater(Class<U> tclass, Class<W> vclass, String fieldName)
          Creates and returns an updater for objects with the given field.
abstract  void set(T obj, V newValue)
          Sets the field of the given object managed by this updater to the given updated value.
abstract  boolean weakCompareAndSet(T obj, V expect, V update)
          Atomically sets the field of the given object managed by this updater to the given updated value if the current value == the expected value.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

AtomicReferenceFieldUpdater

protected AtomicReferenceFieldUpdater()
Protected do-nothing constructor for use by subclasses.

Method Detail

newUpdater

public static <U,W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass,
                                                                Class<W> 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
Throws:
IllegalArgumentException - if the field is not a volatile reference type.
RuntimeException - with a nested reflection-based exception if the class does not hold field or is the wrong type.

compareAndSet

public abstract boolean compareAndSet(T obj,
                                      V expect,
                                      V update)
Atomically sets the field of the given object managed by this updater to the given updated value if the current value == the expected value. This method is guaranteed to be atomic with respect to other calls to compareAndSet and 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.

weakCompareAndSet

public abstract boolean weakCompareAndSet(T obj,
                                          V expect,
                                          V update)
Atomically sets the field of the given object managed by this updater to the given updated value if the current value == the expected value. This method is guaranteed to be atomic with respect to other calls to compareAndSet and 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 compareAndSet.

Parameters:
obj - An object whose field to conditionally set
expect - the expected value
update - the new value
Returns:
true if successful.

set

public abstract void set(T obj,
                         V 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 compareAndSet.

Parameters:
obj - An object whose field to set
newValue - the new value

lazySet

public abstract void lazySet(T obj,
                             V 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
Since:
1.6

get

public abstract V get(T 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 V getAndSet(T obj,
                   V 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

Java™ Platform
Standard Ed. 6

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