/*
* @(#)AtomicReferenceFieldUpdater.java 1.15 06/06/15
*
* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util.concurrent.atomic;
import sun.misc.Unsafe;
import java.lang.reflect.*;
/**
* A reflection-based utility that enables atomic updates to
* designated {@code 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
*
* <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 {@code 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
* {@code compareAndSet} and {@code set} on the same updater.
*
* @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 and returns 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) {
return new AtomicReferenceFieldUpdaterImpl<U,W>(tclass,
vclass,
fieldName);
}
/**
* Protected do-nothing constructor for use by subclasses.
*/
protected AtomicReferenceFieldUpdater() {
}
/**
* Atomically sets the field of the given object managed by this updater
* to the given updated value if the current value {@code ==} the
* expected value. This method is guaranteed to be atomic with respect to
* other calls to {@code compareAndSet} and {@code 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 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 {@code ==} the
* expected value. This method is guaranteed to be atomic with respect to
* other calls to {@code compareAndSet} and {@code set}, but not
* necessarily with respect to other changes in the field.
*
* <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
* and does not provide ordering guarantees, so is only rarely an
* appropriate alternative to {@code compareAndSet}.
*
* @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);
/**
* 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 {@code compareAndSet}.
*
* @param obj An object whose field to set
* @param newValue the new value
*/
public abstract void set(T obj, V newValue);
/**
* Eventually sets the field of the given object managed by this
* updater to the given updated value.
*
* @param obj An object whose field to set
* @param newValue the new value
* @since 1.6
*/
public abstract void lazySet(T obj, V newValue);
/**
* Gets the current value held in the field of the given object managed
* by this updater.
*
* @param obj An object whose field to get
* @return the current value
*/
public abstract V get(T obj);
/**
* Atomically sets the field of the given object managed by this updater
* to the given value and returns 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;
}
}
private static final class AtomicReferenceFieldUpdaterImpl<T,V>
extends AtomicReferenceFieldUpdater<T,V> {
private static final Unsafe unsafe = Unsafe.getUnsafe();
private final long offset;
private final Class<T> tclass;
private final Class<V> vclass;
private final Class cclass;
/*
* Internal type checks within all update methods contain
* internal inlined optimizations checking for the common
* cases where the class is final (in which case a simple
* getClass comparison suffices) or is of type Object (in
* which case no check is needed because all objects are
* instances of Object). The Object case is handled simply by
* setting vclass to null in constructor. The targetCheck and
* updateCheck methods are invoked when these faster
* screenings fail.
*/
AtomicReferenceFieldUpdaterImpl(Class<T> tclass,
Class<V> vclass,
String fieldName) {
Field field = null;
Class fieldClass = null;
Class caller = null;
int modifiers = 0;
try {
field = tclass.getDeclaredField(fieldName);
caller = sun.reflect.Reflection.getCallerClass(3);
modifiers = field.getModifiers();
sun.reflect.misc.ReflectUtil.ensureMemberAccess(
caller, tclass, null, modifiers);
sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
fieldClass = field.getType();
} catch (Exception ex) {
throw new RuntimeException(ex);
}
if (vclass != fieldClass)
throw new ClassCastException();
if (!Modifier.isVolatile(modifiers))
throw new IllegalArgumentException("Must be volatile type");
this.cclass = (Modifier.isProtected(modifiers) &&
caller != tclass) ? caller : null;
this.tclass = tclass;
if (vclass == Object.class)
this.vclass = null;
else
this.vclass = vclass;
offset = unsafe.objectFieldOffset(field);
}
void targetCheck(T obj) {
if (!tclass.isInstance(obj))
throw new ClassCastException();
if (cclass != null)
ensureProtectedAccess(obj);
}
void updateCheck(T obj, V update) {
if (!tclass.isInstance(obj) ||
(update != null && vclass != null && !vclass.isInstance(update)))
throw new ClassCastException();
if (cclass != null)
ensureProtectedAccess(obj);
}
public boolean compareAndSet(T obj, V expect, V update) {
if (obj == null || obj.getClass() != tclass || cclass != null ||
(update != null && vclass != null &&
vclass != update.getClass()))
updateCheck(obj, update);
return unsafe.compareAndSwapObject(obj, offset, expect, update);
}
public boolean weakCompareAndSet(T obj, V expect, V update) {
// same implementation as strong form for now
if (obj == null || obj.getClass() != tclass || cclass != null ||
(update != null && vclass != null &&
vclass != update.getClass()))
updateCheck(obj, update);
return unsafe.compareAndSwapObject(obj, offset, expect, update);
}
public void set(T obj, V newValue) {
if (obj == null || obj.getClass() != tclass || cclass != null ||
(newValue != null && vclass != null &&
vclass != newValue.getClass()))
updateCheck(obj, newValue);
unsafe.putObjectVolatile(obj, offset, newValue);
}
public void lazySet(T obj, V newValue) {
if (obj == null || obj.getClass() != tclass || cclass != null ||
(newValue != null && vclass != null &&
vclass != newValue.getClass()))
updateCheck(obj, newValue);
unsafe.putOrderedObject(obj, offset, newValue);
}
public V get(T obj) {
if (obj == null || obj.getClass() != tclass || cclass != null)
targetCheck(obj);
return (V)unsafe.getObjectVolatile(obj, offset);
}
private void ensureProtectedAccess(T obj) {
if (cclass.isInstance(obj)) {
return;
}
throw new RuntimeException (
new IllegalAccessException("Class " +
cclass.getName() +
" can not access a protected member of class " +
tclass.getName() +
" using an instance of " +
obj.getClass().getName()
)
);
}
}
}
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