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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent.atomic; |
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import sun.misc.Unsafe; |
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import java.lang.reflect.Field; |
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import java.lang.reflect.Modifier; |
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import java.security.AccessController; |
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import java.security.PrivilegedExceptionAction; |
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import java.security.PrivilegedActionException; |
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|
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/** |
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* A reflection-based utility that enables atomic updates to |
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* designated {@code volatile} reference fields of designated |
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* classes. This class is designed for use in atomic data structures |
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* in which several reference fields of the same node are |
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* independently subject to atomic updates. For example, a tree node |
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* might be declared as |
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* |
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* <pre> {@code |
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* class Node { |
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* private volatile Node left, right; |
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* |
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* private static final AtomicReferenceFieldUpdater<Node, Node> leftUpdater = |
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* AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left"); |
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* private static AtomicReferenceFieldUpdater<Node, Node> rightUpdater = |
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* AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right"); |
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* |
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* Node getLeft() { return left; } |
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* boolean compareAndSetLeft(Node expect, Node update) { |
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* return leftUpdater.compareAndSet(this, expect, update); |
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* } |
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* // ... and so on |
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* }}</pre> |
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* |
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* <p>Note that the guarantees of the {@code compareAndSet} |
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* method in this class are weaker than in other atomic classes. |
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* Because this class cannot ensure that all uses of the field |
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* are appropriate for purposes of atomic access, it can |
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* guarantee atomicity only with respect to other invocations of |
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* {@code compareAndSet} and {@code set} on the same updater. |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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* @param <T> The type of the object holding the updatable field |
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* @param <V> The type of the field |
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*/ |
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public abstract class AtomicReferenceFieldUpdater<T,V> { |
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|
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/** |
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* Creates and returns an updater for objects with the given field. |
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* The Class arguments are needed to check that reflective types and |
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* generic types match. |
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* |
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* @param tclass the class of the objects holding the field |
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* @param vclass the class of the field |
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* @param fieldName the name of the field to be updated |
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* @param <U> the type of instances of tclass |
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* @param <W> the type of instances of vclass |
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* @return the updater |
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* @throws IllegalArgumentException if the field is not a volatile reference type |
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* @throws RuntimeException with a nested reflection-based |
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* exception if the class does not hold field or is the wrong type, |
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* or the field is inaccessible to the caller according to Java language |
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* access control |
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*/ |
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public static <U,W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass, |
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Class<W> vclass, |
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String fieldName) { |
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return new AtomicReferenceFieldUpdaterImpl<U,W> |
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(tclass, vclass, fieldName); |
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} |
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|
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/** |
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* Protected do-nothing constructor for use by subclasses. |
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*/ |
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protected AtomicReferenceFieldUpdater() { |
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} |
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|
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/** |
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* Atomically sets the field of the given object managed by this updater |
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* to the given updated value if the current value {@code ==} the |
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* expected value. This method is guaranteed to be atomic with respect to |
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* other calls to {@code compareAndSet} and {@code set}, but not |
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* necessarily with respect to other changes in the field. |
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* |
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* @param obj An object whose field to conditionally set |
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* @param expect the expected value |
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* @param update the new value |
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* @return true if successful |
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*/ |
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public abstract boolean compareAndSet(T obj, V expect, V update); |
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|
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/** |
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* Atomically sets the field of the given object managed by this updater |
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* to the given updated value if the current value {@code ==} the |
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* expected value. This method is guaranteed to be atomic with respect to |
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* other calls to {@code compareAndSet} and {@code set}, but not |
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* necessarily with respect to other changes in the field. |
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* |
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* <p><a href="package-summary.html#weakCompareAndSet">May fail |
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* spuriously and does not provide ordering guarantees</a>, so is |
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* only rarely an appropriate alternative to {@code compareAndSet}. |
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* |
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* @param obj An object whose field to conditionally set |
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* @param expect the expected value |
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* @param update the new value |
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* @return true if successful |
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*/ |
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public abstract boolean weakCompareAndSet(T obj, V expect, V update); |
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|
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/** |
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* Sets the field of the given object managed by this updater to the |
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* given updated value. This operation is guaranteed to act as a volatile |
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* store with respect to subsequent invocations of {@code compareAndSet}. |
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* |
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* @param obj An object whose field to set |
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* @param newValue the new value |
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*/ |
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public abstract void set(T obj, V newValue); |
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|
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/** |
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* Eventually sets the field of the given object managed by this |
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* updater to the given updated value. |
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* |
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* @param obj An object whose field to set |
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* @param newValue the new value |
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* @since 1.6 |
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*/ |
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public abstract void lazySet(T obj, V newValue); |
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|
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/** |
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* Gets the current value held in the field of the given object managed |
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* by this updater. |
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* |
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* @param obj An object whose field to get |
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* @return the current value |
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*/ |
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public abstract V get(T obj); |
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|
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/** |
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* Atomically sets the field of the given object managed by this updater |
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* to the given value and returns the old value. |
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* |
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* @param obj An object whose field to get and set |
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* @param newValue the new value |
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* @return the previous value |
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*/ |
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public V getAndSet(T obj, V newValue) { |
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for (;;) { |
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V current = get(obj); |
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if (compareAndSet(obj, current, newValue)) |
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return current; |
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} |
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} |
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|
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private static final class AtomicReferenceFieldUpdaterImpl<T,V> |
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extends AtomicReferenceFieldUpdater<T,V> { |
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private static final Unsafe unsafe = Unsafe.getUnsafe(); |
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private final long offset; |
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private final Class<T> tclass; |
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private final Class<V> vclass; |
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private final Class<?> cclass; |
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|
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/* |
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* Internal type checks within all update methods contain |
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* internal inlined optimizations checking for the common |
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* cases where the class is final (in which case a simple |
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* getClass comparison suffices) or is of type Object (in |
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* which case no check is needed because all objects are |
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* instances of Object). The Object case is handled simply by |
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* setting vclass to null in constructor. The targetCheck and |
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* updateCheck methods are invoked when these faster |
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* screenings fail. |
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*/ |
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|
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AtomicReferenceFieldUpdaterImpl(final Class<T> tclass, |
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Class<V> vclass, |
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final String fieldName) { |
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final Field field; |
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final Class<?> fieldClass; |
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final Class<?> caller; |
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final int modifiers; |
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try { |
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field = AccessController.doPrivileged( |
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new PrivilegedExceptionAction<Field>() { |
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public Field run() throws NoSuchFieldException { |
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return tclass.getDeclaredField(fieldName); |
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} |
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}); |
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caller = sun.reflect.Reflection.getCallerClass(3); |
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modifiers = field.getModifiers(); |
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sun.reflect.misc.ReflectUtil.ensureMemberAccess( |
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caller, tclass, null, modifiers); |
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ClassLoader cl = tclass.getClassLoader(); |
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ClassLoader ccl = caller.getClassLoader(); |
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if ((ccl != null) && (ccl != cl) && |
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((cl == null) || !isAncestor(cl, ccl))) { |
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sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass); |
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} |
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fieldClass = field.getType(); |
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} catch (PrivilegedActionException pae) { |
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throw new RuntimeException(pae.getException()); |
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} catch (Exception ex) { |
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throw new RuntimeException(ex); |
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} |
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|
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if (vclass != fieldClass) |
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throw new ClassCastException(); |
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|
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if (!Modifier.isVolatile(modifiers)) |
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throw new IllegalArgumentException("Must be volatile type"); |
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|
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this.cclass = (Modifier.isProtected(modifiers) && |
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caller != tclass) ? caller : null; |
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this.tclass = tclass; |
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if (vclass == Object.class) |
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this.vclass = null; |
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else |
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this.vclass = vclass; |
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offset = unsafe.objectFieldOffset(field); |
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} |
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|
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/** |
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* Returns true if the second classloader can be found in the first |
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* classloader's delegation chain. |
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* Equivalent to the inaccessible: first.isAncestor(second). |
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*/ |
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private static boolean isAncestor(ClassLoader first, ClassLoader second) { |
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ClassLoader acl = first; |
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do { |
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acl = acl.getParent(); |
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if (second == acl) { |
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return true; |
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} |
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} while (acl != null); |
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return false; |
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} |
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|
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void targetCheck(T obj) { |
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if (!tclass.isInstance(obj)) |
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throw new ClassCastException(); |
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if (cclass != null) |
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ensureProtectedAccess(obj); |
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} |
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|
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void updateCheck(T obj, V update) { |
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if (!tclass.isInstance(obj) || |
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(update != null && vclass != null && !vclass.isInstance(update))) |
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throw new ClassCastException(); |
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if (cclass != null) |
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ensureProtectedAccess(obj); |
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} |
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|
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public boolean compareAndSet(T obj, V expect, V update) { |
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if (obj == null || obj.getClass() != tclass || cclass != null || |
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(update != null && vclass != null && |
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vclass != update.getClass())) |
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updateCheck(obj, update); |
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return unsafe.compareAndSwapObject(obj, offset, expect, update); |
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} |
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|
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public boolean weakCompareAndSet(T obj, V expect, V update) { |
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// same implementation as strong form for now |
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if (obj == null || obj.getClass() != tclass || cclass != null || |
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(update != null && vclass != null && |
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vclass != update.getClass())) |
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updateCheck(obj, update); |
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return unsafe.compareAndSwapObject(obj, offset, expect, update); |
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} |
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|
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public void set(T obj, V newValue) { |
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if (obj == null || obj.getClass() != tclass || cclass != null || |
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(newValue != null && vclass != null && |
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vclass != newValue.getClass())) |
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updateCheck(obj, newValue); |
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unsafe.putObjectVolatile(obj, offset, newValue); |
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} |
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|
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public void lazySet(T obj, V newValue) { |
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if (obj == null || obj.getClass() != tclass || cclass != null || |
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(newValue != null && vclass != null && |
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vclass != newValue.getClass())) |
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updateCheck(obj, newValue); |
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unsafe.putOrderedObject(obj, offset, newValue); |
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} |
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|
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@SuppressWarnings("unchecked") |
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public V get(T obj) { |
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if (obj == null || obj.getClass() != tclass || cclass != null) |
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targetCheck(obj); |
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return (V)unsafe.getObjectVolatile(obj, offset); |
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} |
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|
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private void ensureProtectedAccess(T obj) { |
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if (cclass.isInstance(obj)) { |
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return; |
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} |
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throw new RuntimeException( |
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new IllegalAccessException("Class " + |
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cclass.getName() + |
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" can not access a protected member of class " + |
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tclass.getName() + |
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" using an instance of " + |
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obj.getClass().getName() |
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) |
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); |
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} |
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} |
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} |