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AtomicReferenceFieldUpdater.javaAPI DocJava SE 5 API8587Fri Aug 26 14:57:26 BST 2005java.util.concurrent.atomic

AtomicReferenceFieldUpdater

public abstract class AtomicReferenceFieldUpdater 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 and volatile semantics only with respect to other invocations of compareAndSet and set.

since
1.5
author
Doug Lea
param
The type of the object holding the updatable field
param
The type of the field

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

    
Methods Summary
public abstract booleancompareAndSet(T obj, V expect, V update)
Atomically set the value of 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.

param
obj An object whose field to conditionally set
param
expect the expected value
param
update the new value
return
true if successful.

public abstract Vget(T obj)
Get the current value held in the field by the given object.

param
obj An object whose field to get
return
the current value

public VgetAndSet(T obj, V newValue)
Set to the given value and return the old value.

param
obj An object whose field to get and set
param
newValue the new value
return
the previous value

        for (;;) {
            V current = get(obj);
            if (compareAndSet(obj, current, newValue))
                return current;
        }
    
public static java.util.concurrent.atomic.AtomicReferenceFieldUpdaternewUpdater(java.lang.Class tclass, java.lang.Class vclass, java.lang.String fieldName)
Creates an updater for objects with the given field. The Class arguments are needed to check that reflective types and generic types match.

param
tclass the class of the objects holding the field.
param
vclass the class of the field
param
fieldName the name of the field to be updated.
return
the updater
throws
IllegalArgumentException if the field is not a volatile reference type.
throws
RuntimeException with a nested reflection-based exception if the class does not hold field or is the wrong type.

        // Currently rely on standard intrinsics implementation
        return new AtomicReferenceFieldUpdaterImpl<U,W>(tclass, 
                                                        vclass, 
                                                        fieldName);
    
public abstract voidset(T obj, V newValue)
Set the field of the given object managed by this updater. This operation is guaranteed to act as a volatile store with respect to subsequent invocations of compareAndSet.

param
obj An object whose field to set
param
newValue the new value

public abstract booleanweakCompareAndSet(T obj, V expect, V update)
Atomically set the value of 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, and may fail spuriously.

param
obj An object whose field to conditionally set
param
expect the expected value
param
update the new value
return
true if successful.