AtomicReferenceFieldUpdater (Java 2 Platform SE 5.0) - JDK 5 Documentation v1.5.0, Java 2 SDK 英文文档

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Java^TM 2 Platform
Standard Ed. 5.0

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

java.lang.Object
  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
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

Constructor Summary

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

Method Summary

abstract boolean compareAndSet(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.

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

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

static <U,W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass, Class<W> vclass, String fieldName)
          Creates an updater for objects with the given field.

abstract void set(T obj, V newValue)
          Set the field of the given object managed by this updater.

abstract boolean weakCompareAndSet(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.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

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

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 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 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.

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)

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)

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.

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

get

public abstract V get(T obj)

Get the current value held in the field by the given object.

Parameters:: obj - An object whose field to get
Returns:: the current value

getAndSet

public V getAndSet(T obj,
                   V newValue)

Set to the given value and return the old value.

Parameters:: obj - An object whose field to get and set; newValue - the new value
Returns:: the previous value