Java tutorial
/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * 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/publicdomain/zero/1.0/ */ package java.util.concurrent.atomic; import java.lang.reflect.Field; import java.lang.reflect.Modifier; import java.security.AccessController; import java.security.PrivilegedActionException; import java.security.PrivilegedExceptionAction; import java.util.Objects; import java.util.function.BinaryOperator; import java.util.function.UnaryOperator; import jdk.internal.misc.Unsafe; import jdk.internal.reflect.CallerSensitive; import jdk.internal.reflect.Reflection; import java.lang.invoke.VarHandle; /** * 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> {@code * 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. * * <p>Object arguments for parameters of type {@code T} that are not * instances of the class passed to {@link #newUpdater} will result in * a {@link ClassCastException} being thrown. * * @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 * @param <U> the type of instances of tclass * @param <W> the type of instances of vclass * @return the updater * @throws ClassCastException if the field is of the wrong type * @throws IllegalArgumentException if the field is not volatile * @throws RuntimeException with a nested reflection-based * exception if the class does not hold field or is the wrong type, * or the field is inaccessible to the caller according to Java language * access control */ @CallerSensitive public static <U, W> AtomicReferenceFieldUpdater<U, W> newUpdater(Class<U> tclass, Class<W> vclass, String fieldName) { return new AtomicReferenceFieldUpdaterImpl<U, W>(tclass, vclass, fieldName, Reflection.getCallerClass()); } /** * 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 {@code 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>This operation may fail spuriously 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 {@code 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); /** * Returns 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) { V prev; do { prev = get(obj); } while (!compareAndSet(obj, prev, newValue)); return prev; } /** * Atomically updates (with memory effects as specified by {@link * VarHandle#compareAndSet}) the field of the given object managed * by this updater with the results of applying the given * function, returning the previous value. The function should be * side-effect-free, since it may be re-applied when attempted * updates fail due to contention among threads. * * @param obj An object whose field to get and set * @param updateFunction a side-effect-free function * @return the previous value * @since 1.8 */ public final V getAndUpdate(T obj, UnaryOperator<V> updateFunction) { V prev, next; do { prev = get(obj); next = updateFunction.apply(prev); } while (!compareAndSet(obj, prev, next)); return prev; } /** * Atomically updates (with memory effects as specified by {@link * VarHandle#compareAndSet}) the field of the given object managed * by this updater with the results of applying the given * function, returning the updated value. The function should be * side-effect-free, since it may be re-applied when attempted * updates fail due to contention among threads. * * @param obj An object whose field to get and set * @param updateFunction a side-effect-free function * @return the updated value * @since 1.8 */ public final V updateAndGet(T obj, UnaryOperator<V> updateFunction) { V prev, next; do { prev = get(obj); next = updateFunction.apply(prev); } while (!compareAndSet(obj, prev, next)); return next; } /** * Atomically updates (with memory effects as specified by {@link * VarHandle#compareAndSet}) the field of the given object managed * by this updater with the results of applying the given function * to the current and given values, returning the previous value. * The function should be side-effect-free, since it may be * re-applied when attempted updates fail due to contention among * threads. The function is applied with the current value as its * first argument, and the given update as the second argument. * * @param obj An object whose field to get and set * @param x the update value * @param accumulatorFunction a side-effect-free function of two arguments * @return the previous value * @since 1.8 */ public final V getAndAccumulate(T obj, V x, BinaryOperator<V> accumulatorFunction) { V prev, next; do { prev = get(obj); next = accumulatorFunction.apply(prev, x); } while (!compareAndSet(obj, prev, next)); return prev; } /** * Atomically updates (with memory effects as specified by {@link * VarHandle#compareAndSet}) the field of the given object managed * by this updater with the results of applying the given function * to the current and given values, returning the updated value. * The function should be side-effect-free, since it may be * re-applied when attempted updates fail due to contention among * threads. The function is applied with the current value as its * first argument, and the given update as the second argument. * * @param obj An object whose field to get and set * @param x the update value * @param accumulatorFunction a side-effect-free function of two arguments * @return the updated value * @since 1.8 */ public final V accumulateAndGet(T obj, V x, BinaryOperator<V> accumulatorFunction) { V prev, next; do { prev = get(obj); next = accumulatorFunction.apply(prev, x); } while (!compareAndSet(obj, prev, next)); return next; } private static final class AtomicReferenceFieldUpdaterImpl<T, V> extends AtomicReferenceFieldUpdater<T, V> { private static final Unsafe U = Unsafe.getUnsafe(); private final long offset; /** * if field is protected, the subclass constructing updater, else * the same as tclass */ private final Class<?> cclass; /** class holding the field */ private final Class<T> tclass; /** field value type */ private final Class<V> vclass; /* * 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(final Class<T> tclass, final Class<V> vclass, final String fieldName, final Class<?> caller) { final Field field; final Class<?> fieldClass; final int modifiers; try { field = AccessController.doPrivileged(new PrivilegedExceptionAction<Field>() { public Field run() throws NoSuchFieldException { return tclass.getDeclaredField(fieldName); } }); modifiers = field.getModifiers(); sun.reflect.misc.ReflectUtil.ensureMemberAccess(caller, tclass, null, modifiers); ClassLoader cl = tclass.getClassLoader(); ClassLoader ccl = caller.getClassLoader(); if ((ccl != null) && (ccl != cl) && ((cl == null) || !isAncestor(cl, ccl))) { sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass); } fieldClass = field.getType(); } catch (PrivilegedActionException pae) { throw new RuntimeException(pae.getException()); } catch (Exception ex) { throw new RuntimeException(ex); } if (vclass != fieldClass) throw new ClassCastException(); if (vclass.isPrimitive()) throw new IllegalArgumentException("Must be reference type"); if (!Modifier.isVolatile(modifiers)) throw new IllegalArgumentException("Must be volatile type"); // Access to protected field members is restricted to receivers only // of the accessing class, or one of its subclasses, and the // accessing class must in turn be a subclass (or package sibling) // of the protected member's defining class. // If the updater refers to a protected field of a declaring class // outside the current package, the receiver argument will be // narrowed to the type of the accessing class. this.cclass = (Modifier.isProtected(modifiers) && tclass.isAssignableFrom(caller) && !isSamePackage(tclass, caller)) ? caller : tclass; this.tclass = tclass; this.vclass = vclass; this.offset = U.objectFieldOffset(field); } /** * Returns true if the second classloader can be found in the first * classloader's delegation chain. * Equivalent to the inaccessible: first.isAncestor(second). */ private static boolean isAncestor(ClassLoader first, ClassLoader second) { ClassLoader acl = first; do { acl = acl.getParent(); if (second == acl) { return true; } } while (acl != null); return false; } /** * Returns true if the two classes have the same class loader and * package qualifier */ private static boolean isSamePackage(Class<?> class1, Class<?> class2) { return class1.getClassLoader() == class2.getClassLoader() && Objects.equals(class1.getPackageName(), class2.getPackageName()); } /** * Checks that target argument is instance of cclass. On * failure, throws cause. */ private final void accessCheck(T obj) { if (!cclass.isInstance(obj)) throwAccessCheckException(obj); } /** * Throws access exception if accessCheck failed due to * protected access, else ClassCastException. */ private final void throwAccessCheckException(T obj) { if (cclass == tclass) throw new ClassCastException(); else 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())); } private final void valueCheck(V v) { if (v != null && !(vclass.isInstance(v))) throwCCE(); } static void throwCCE() { throw new ClassCastException(); } public final boolean compareAndSet(T obj, V expect, V update) { accessCheck(obj); valueCheck(update); return U.compareAndSetReference(obj, offset, expect, update); } public final boolean weakCompareAndSet(T obj, V expect, V update) { // same implementation as strong form for now accessCheck(obj); valueCheck(update); return U.compareAndSetReference(obj, offset, expect, update); } public final void set(T obj, V newValue) { accessCheck(obj); valueCheck(newValue); U.putReferenceVolatile(obj, offset, newValue); } public final void lazySet(T obj, V newValue) { accessCheck(obj); valueCheck(newValue); U.putReferenceRelease(obj, offset, newValue); } @SuppressWarnings("unchecked") public final V get(T obj) { accessCheck(obj); return (V) U.getReferenceVolatile(obj, offset); } @SuppressWarnings("unchecked") public final V getAndSet(T obj, V newValue) { accessCheck(obj); valueCheck(newValue); return (V) U.getAndSetReference(obj, offset, newValue); } } }