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

  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
  Author:
    Doug Lea

- - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `AtomicReferenceFieldUpdater()` Protected do-nothing constructor for use by subclasses.

  - ### Method Summary

    All Methods Static Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `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

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