/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package java.util.concurrent.atomic;
import sun.misc.Unsafe;
import java.lang.reflect.*;
/**
* A reflection-based utility that enables atomic updates to
* designated <tt>volatile</tt> 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
*
* <pre>
* 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
* }
* </pre>
*
* <p> Note that the guarantees of the <tt>compareAndSet</tt>
* 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
* <tt>compareAndSet</tt> and <tt>set</tt>.
* @since 1.5
* @author Doug Lea
* @param <T> The type of the object holding the updatable field
* @param <V> The type of the field
*/
public abstract class AtomicReferenceFieldUpdater<T, V> {
/**
* 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.
*/
public static <U, W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass, Class<W> vclass, String fieldName) {
// Currently rely on standard intrinsics implementation
return new AtomicReferenceFieldUpdaterImpl<U,W>(tclass,
vclass,
fieldName);
}
/**
* Protected do-nothing constructor for use by subclasses.
*/
protected AtomicReferenceFieldUpdater() {
}
/**
* Atomically set the value of the field of the given object managed
* by this Updater to the given updated value if the current value
* <tt>==</tt> the expected value. This method is guaranteed to be
* atomic with respect to other calls to <tt>compareAndSet</tt> and
* <tt>set</tt>, 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 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
* <tt>==</tt> the expected value. This method is guaranteed to be
* atomic with respect to other calls to <tt>compareAndSet</tt> and
* <tt>set</tt>, 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.
*/
public abstract boolean weakCompareAndSet(T obj, V expect, V update);
/**
* 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 <tt>compareAndSet</tt>.
* @param obj An object whose field to set
* @param newValue the new value
*/
public abstract void set(T obj, V newValue);
/**
* 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 abstract V get(T obj);
/**
* 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
*/
public V getAndSet(T obj, V newValue) {
for (;;) {
V current = get(obj);
if (compareAndSet(obj, current, newValue))
return current;
}
}
/**
* Standard hotspot implementation using intrinsics
*/
private static class AtomicReferenceFieldUpdaterImpl<T,V> extends AtomicReferenceFieldUpdater<T,V> {
// BEGIN android-changed
private static final Unsafe unsafe = UnsafeAccess.THE_ONE;
// END android-changed
private final long offset;
private final Class<T> tclass;
private final Class<V> vclass;
AtomicReferenceFieldUpdaterImpl(Class<T> tclass, Class<V> vclass, String fieldName) {
Field field = null;
Class fieldClass = null;
try {
field = tclass.getDeclaredField(fieldName);
fieldClass = field.getType();
} catch(Exception ex) {
throw new RuntimeException(ex);
}
if (vclass != fieldClass)
throw new ClassCastException();
if (!Modifier.isVolatile(field.getModifiers()))
throw new IllegalArgumentException("Must be volatile type");
this.tclass = tclass;
this.vclass = vclass;
offset = unsafe.objectFieldOffset(field);
}
public boolean compareAndSet(T obj, V expect, V update) {
if (!tclass.isInstance(obj) ||
(update != null && !vclass.isInstance(update)))
throw new ClassCastException();
return unsafe.compareAndSwapObject(obj, offset, expect, update);
}
public boolean weakCompareAndSet(T obj, V expect, V update) {
// same implementation as strong form for now
if (!tclass.isInstance(obj) ||
(update != null && !vclass.isInstance(update)))
throw new ClassCastException();
return unsafe.compareAndSwapObject(obj, offset, expect, update);
}
public void set(T obj, V newValue) {
if (!tclass.isInstance(obj) ||
(newValue != null && !vclass.isInstance(newValue)))
throw new ClassCastException();
unsafe.putObjectVolatile(obj, offset, newValue);
}
public V get(T obj) {
if (!tclass.isInstance(obj))
throw new ClassCastException();
return (V)unsafe.getObjectVolatile(obj, offset);
}
}
}
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