AtomicLongFieldUpdater.java

/*
 * 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.invoke.VarHandle;
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.LongBinaryOperator;
import java.util.function.LongUnaryOperator;
import jdk.internal.misc.Unsafe;
import jdk.internal.reflect.CallerSensitive;
import jdk.internal.reflect.Reflection;
import sun.reflect.misc.ReflectUtil;

/**
 * A reflection-based utility that enables atomic updates to
 * designated {@code volatile long} fields of designated classes.
 * This class is designed for use in atomic data structures in which
 * several fields of the same node are independently subject to atomic
 * updates.
 *
 * <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.
 *
 * @param <T> The type of the object holding the updatable field
 *
 * @author Doug Lea
 * @since 1.5
 */
// 对目标类中volatile修饰的名为fieldName的long字段进行原子操作
public abstract class AtomicLongFieldUpdater<T> {
    
    /*▼ 构造器/工厂方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Protected do-nothing constructor for use by subclasses.
     */
    protected AtomicLongFieldUpdater() {
    }
    
    /**
     * Creates and returns an updater for objects with the given field.
     * The Class argument is needed to check that reflective types and
     * generic types match.
     *
     * @param tclass    the class of the objects holding the field
     * @param fieldName the name of the field to be updated
     * @param <U>       the type of instances of tclass
     *
     * @return the updater
     *
     * @throws IllegalArgumentException if the field is not a
     *                                  volatile long type
     * @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> AtomicLongFieldUpdater<U> newUpdater(Class<U> tclass, String fieldName) {
        Class<?> caller = Reflection.getCallerClass();
        
        // 如果JVM支持long的无锁compareAndSet
        if(AtomicLong.VM_SUPPORTS_LONG_CAS) {
            return new CASUpdater<U>(tclass, fieldName, caller);
        }
        
        // 如果JVM不支持long的无锁compareAndSet
        return new LockedUpdater<U>(tclass, fieldName, caller);
    }
    
    /*▲ 构造器/工厂方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 获取值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    // 获取对象obj中的目标字段值
    public abstract long get(T obj);
    
    /*▲ 获取值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 设置值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    // 设置对象obj中的目标字段的值为newValue
    public abstract void set(T obj, long 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
     */
    // 设置对象obj中的目标字段的值为newValue
    public abstract void lazySet(T obj, long newValue);
    
    /**
     * 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
     */
    // 设置对象obj中的目标字段的值为newValue，并返回旧值
    public long getAndSet(T obj, long newValue) {
        long prev;
        do {
            prev = get(obj);
        } while(!compareAndSet(obj, prev, newValue));
        return prev;
    }
    
    /*▲ 设置值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 比较并更新 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    // 如果obj中目标字段的值为expect，则将其更新为update
    public abstract boolean compareAndSet(T obj, long expect, long 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><a href="package-summary.html#weakCompareAndSet">May fail
     * spuriously and does not provide ordering guarantees</a>, 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
     */
    // 如果obj中目标字段的值为expect，则将其更新为update
    public abstract boolean weakCompareAndSet(T obj, long expect, long update);
    
    /*▲ 比较并更新 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 增/减 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Atomically adds the given value to the current value of the field of
     * the given object managed by this updater.
     *
     * @param obj   An object whose field to get and set
     * @param delta the value to add
     *
     * @return the previous value
     */
    // 将对象obj内的目标字段原子地增加delta，并返回旧值
    public long getAndAdd(T obj, long delta) {
        long prev, next;
        do {
            prev = get(obj);
            next = prev + delta;
        } while(!compareAndSet(obj, prev, next));
        return prev;
    }
    
    /**
     * Atomically adds the given value to the current value of the field of
     * the given object managed by this updater.
     *
     * @param obj   An object whose field to get and set
     * @param delta the value to add
     *
     * @return the updated value
     */
    // 将对象obj内的目标字段原子地增加delta，并返回新值
    public long addAndGet(T obj, long delta) {
        long prev, next;
        do {
            prev = get(obj);
            next = prev + delta;
        } while(!compareAndSet(obj, prev, next));
        return next;
    }
    
    /**
     * Atomically increments by one the current value of the field of the
     * given object managed by this updater.
     *
     * @param obj An object whose field to get and set
     *
     * @return the previous value
     */
    // 将对象obj内的目标字段原子地递增，并返回旧值
    public long getAndIncrement(T obj) {
        long prev, next;
        do {
            prev = get(obj);
            next = prev + 1;
        } while(!compareAndSet(obj, prev, next));
        return prev;
    }
    
    /**
     * Atomically increments by one the current value of the field of the
     * given object managed by this updater.
     *
     * @param obj An object whose field to get and set
     *
     * @return the updated value
     */
    // 将对象obj内的目标字段原子地递增，并返回新值
    public long incrementAndGet(T obj) {
        long prev, next;
        do {
            prev = get(obj);
            next = prev + 1;
        } while(!compareAndSet(obj, prev, next));
        return next;
    }
    
    /**
     * Atomically decrements by one the current value of the field of the
     * given object managed by this updater.
     *
     * @param obj An object whose field to get and set
     *
     * @return the previous value
     */
    // 将对象obj内的目标字段原子地递减，并返回旧值
    public long getAndDecrement(T obj) {
        long prev, next;
        do {
            prev = get(obj);
            next = prev - 1;
        } while(!compareAndSet(obj, prev, next));
        return prev;
    }
    
    /**
     * Atomically decrements by one the current value of the field of the
     * given object managed by this updater.
     *
     * @param obj An object whose field to get and set
     *
     * @return the updated value
     */
    // 将对象obj内的目标字段原子地递减，并返回新值
    public long decrementAndGet(T obj) {
        long prev, next;
        do {
            prev = get(obj);
            next = prev - 1;
        } while(!compareAndSet(obj, prev, next));
        return next;
    }
    
    /*▲ 增/减 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ lambda操作 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    // 对对象obj内的目标字段执行给定的操作。如果操作成功，则返回旧值（旧值会动态变化）
    public final long getAndUpdate(T obj, LongUnaryOperator updateFunction) {
        long prev, next;
        do {
            prev = get(obj);
            next = updateFunction.applyAsLong(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
     */
    // 对对象obj内的目标字段执行给定的操作。如果操作成功，则返回新值（旧值会动态变化）
    public final long updateAndGet(T obj, LongUnaryOperator updateFunction) {
        long prev, next;
        do {
            prev = get(obj);
            next = updateFunction.applyAsLong(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
     */
    // 对对象obj内的目标字段执行给定的操作。如果操作成功，则返回旧值（旧值会动态变化）
    public final long getAndAccumulate(T obj, long x, LongBinaryOperator accumulatorFunction) {
        long prev, next;
        do {
            prev = get(obj);
            next = accumulatorFunction.applyAsLong(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
     */
    // 对对象obj内的目标字段执行给定的操作。如果操作成功，则返回新值（旧值会动态变化）
    public final long accumulateAndGet(T obj, long x, LongBinaryOperator accumulatorFunction) {
        long prev, next;
        do {
            prev = get(obj);
            next = accumulatorFunction.applyAsLong(prev, x);
        } while(!compareAndSet(obj, prev, next));
        return next;
    }
    
    /*▲ lambda操作 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Returns true if the second classloader can be found in the first
     * classloader's delegation chain.
     * Equivalent to the inaccessible: first.isAncestor(second).
     */
    // 判断类加载器second是否为first的祖先（second在first的委托链上）
    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
     */
    // 判断class1与class2是否拥有相同的类加载器且位于同一个包
    static boolean isSamePackage(Class<?> class1, Class<?> class2) {
        return class1.getClassLoader() == class2.getClassLoader()
            && Objects.equals(class1.getPackageName(), class2.getPackageName());
    }
    
    
    
    
    
    
    // 如果JVM【支持】long的无锁compareAndSet
    private static final class CASUpdater<T> extends AtomicLongFieldUpdater<T> {
        private static final Unsafe U = Unsafe.getUnsafe();
        
        // 待访问的非静态字段的JVM偏移地址
        private final long offset;
        
        /** class holding the field */
        // 待访问字段所在的类
        private final Class<T> tclass;
        
        /**
         * if field is protected, the subclass constructing updater, else
         * the same as tclass
         */
        // 如果从不在同一个包中的子类caller中访问tclass，则cclass==caller，否则，cclass==tclass
        private final Class<?> cclass;
        
        
        
        /*▼ 构造器 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        CASUpdater(final Class<T> tclass, final String fieldName, final Class<?> caller) {
            final Field field;
            final int modifiers;
            try {
                // 返回tclass中指定名称的字段，但不包括父类/父接口中的字段
                field = AccessController.doPrivileged(new PrivilegedExceptionAction<Field>() {
                    public Field run() throws NoSuchFieldException {
                        return tclass.getDeclaredField(fieldName);
                    }
                });
                
                // 获取字段修饰符
                modifiers = field.getModifiers();
                
                // 确保caller可以访问tclass中具有modifiers修饰符的成员，否则会抛异常
                ReflectUtil.ensureMemberAccess(caller, tclass, null, modifiers);
                
                ClassLoader ccl = caller.getClassLoader();
                ClassLoader cl = tclass.getClassLoader();
                
                if((ccl != null) && (ccl != cl) && ((cl == null) || !isAncestor(cl, ccl))) {
                    // 使用系统安全管理器检查当前类对tclass所在的包的访问权限
                    ReflectUtil.checkPackageAccess(tclass);
                }
            } catch(PrivilegedActionException pae) {
                throw new RuntimeException(pae.getException());
            } catch(Exception ex) {
                throw new RuntimeException(ex);
            }
            
            // 字段类型必须为long
            if(field.getType() != long.class) {
                throw new IllegalArgumentException("Must be long type");
            }
            
            // 修饰符必须带有volatile
            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.offset = U.objectFieldOffset(field);
        }
        
        /*▲ 构造器 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /*▼ 获取值 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        // 获取对象obj中的目标字段值
        public final long get(T obj) {
            accessCheck(obj);
            return U.getLongVolatile(obj, offset);
        }
        
        /*▲ 获取值 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /*▼ 设置值 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        // 设置对象obj中的目标字段的值为newValue
        public final void set(T obj, long newValue) {
            accessCheck(obj);
            U.putLongVolatile(obj, offset, newValue);
        }
        
        // 设置对象obj中的目标字段的值为newValue
        public final void lazySet(T obj, long newValue) {
            accessCheck(obj);
            U.putLongRelease(obj, offset, newValue);
        }
        
        // 设置对象obj中的目标字段的值为newValue，并返回旧值
        public final long getAndSet(T obj, long newValue) {
            accessCheck(obj);
            return U.getAndSetLong(obj, offset, newValue);
        }
        
        /*▲ 设置值 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /*▼ 比较并更新 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        // 如果obj中目标字段的值为expect，则将其更新为update
        public final boolean compareAndSet(T obj, long expect, long update) {
            accessCheck(obj);
            return U.compareAndSetLong(obj, offset, expect, update);
        }
        
        // 如果obj中目标字段的值为expect，则将其更新为update
        public final boolean weakCompareAndSet(T obj, long expect, long update) {
            accessCheck(obj);
            return U.compareAndSetLong(obj, offset, expect, update);
        }
        
        /*▲ 比较并更新 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /*▼ 增/减 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        // 将对象obj内的目标字段原子地增加delta，并返回旧值
        public final long getAndAdd(T obj, long delta) {
            accessCheck(obj);
            return U.getAndAddLong(obj, offset, delta);
        }
        
        // 将对象obj内的目标字段原子地增加delta，并返回新值
        public final long addAndGet(T obj, long delta) {
            return getAndAdd(obj, delta) + delta;
        }
        
        // 将对象obj内的目标字段原子地递增，并返回旧值
        public final long getAndIncrement(T obj) {
            return getAndAdd(obj, 1);
        }
        
        // 将对象obj内的目标字段原子地递增，并返回新值
        public final long incrementAndGet(T obj) {
            return getAndAdd(obj, 1) + 1;
        }
        
        // 将对象obj内的目标字段原子地递减，并返回旧值
        public final long getAndDecrement(T obj) {
            return getAndAdd(obj, -1);
        }
        
        // 将对象obj内的目标字段原子地递减，并返回新值
        public final long decrementAndGet(T obj) {
            return getAndAdd(obj, -1) - 1;
        }
        
        /*▲ 增/减 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /**
         * Checks that target argument is instance of cclass.  On
         * failure, throws cause.
         */
        // 检查[对象]obj是否为cclass类的实例，不是的话会抛异常
        private final void accessCheck(T obj) {
            if(!cclass.isInstance(obj)) {
                throwAccessCheckException(obj);
            }
        }
        
        /**
         * Throws access exception if accessCheck failed due to
         * protected access, else ClassCastException.
         */
        // 负责抛异常
        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()));
            }
        }
    }
    
    // 如果JVM【不支持】long的无锁compareAndSet
    private static final class LockedUpdater<T> extends AtomicLongFieldUpdater<T> {
        private static final Unsafe U = Unsafe.getUnsafe();
        
        // 待访问的非静态字段的JVM偏移地址
        private final long offset;
        
        /** class holding the field */
        // 待访问字段所在的类
        private final Class<T> tclass;
        
        /**
         * if field is protected, the subclass constructing updater, else
         * the same as tclass
         */
        // 如果从不在同一个包中的子类caller中访问tclass，则cclass==caller，否则，cclass==tclass
        private final Class<?> cclass;
        
        
        
        /*▼ 构造器 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        LockedUpdater(final Class<T> tclass, final String fieldName, final Class<?> caller) {
            final Field field;
            final int modifiers;
            
            try {
                // 返回tclass中指定名称的字段，但不包括父类/父接口中的字段
                field = AccessController.doPrivileged(new PrivilegedExceptionAction<Field>() {
                    public Field run() throws NoSuchFieldException {
                        return tclass.getDeclaredField(fieldName);
                    }
                });
                
                // 获取字段修饰符
                modifiers = field.getModifiers();
                
                // 确保caller可以访问tclass中具有modifiers修饰符的成员，否则会抛异常
                ReflectUtil.ensureMemberAccess(caller, tclass, null, modifiers);
                
                ClassLoader ccl = caller.getClassLoader();
                ClassLoader cl = tclass.getClassLoader();
                
                if((ccl != null) && (ccl != cl) && ((cl == null) || !isAncestor(cl, ccl))) {
                    // 使用系统安全管理器检查当前类对tclass所在的包的访问权限
                    ReflectUtil.checkPackageAccess(tclass);
                }
            } catch(PrivilegedActionException pae) {
                throw new RuntimeException(pae.getException());
            } catch(Exception ex) {
                throw new RuntimeException(ex);
            }
            
            // 字段类型必须为long
            if(field.getType() != long.class) {
                throw new IllegalArgumentException("Must be long type");
            }
            
            // 修饰符必须带有volatile
            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.offset = U.objectFieldOffset(field);
        }
        
        /*▲ 构造器 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /*▼ 获取值 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        // 获取对象obj中的目标字段值
        public final long get(T obj) {
            accessCheck(obj);
            synchronized(this) {
                return U.getLong(obj, offset);
            }
        }
        
        /*▲ 获取值 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /*▼ 设置值 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        // 设置对象obj中的目标字段的值为newValue
        public final void set(T obj, long newValue) {
            accessCheck(obj);
            synchronized(this) {
                U.putLong(obj, offset, newValue);
            }
        }
        
        // 设置对象obj中的目标字段的值为newValue
        public final void lazySet(T obj, long newValue) {
            set(obj, newValue);
        }
        
        /*▲ 设置值 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /*▼ 比较并更新 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        // 如果obj中目标字段的值为expect，则将其更新为update
        public final boolean compareAndSet(T obj, long expect, long update) {
            accessCheck(obj);
            synchronized(this) {
                long v = U.getLong(obj, offset);
                if(v != expect) {
                    return false;
                }
                U.putLong(obj, offset, update);
                return true;
            }
        }
        
        // 如果obj中目标字段的值为expect，则将其更新为update
        public final boolean weakCompareAndSet(T obj, long expect, long update) {
            return compareAndSet(obj, expect, update);
        }
        
        /*▲ 比较并更新 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /**
         * Checks that target argument is instance of cclass.  On
         * failure, throws cause.
         */
        // 检查[对象]obj是否为cclass类的实例，不是的话会抛异常
        private final void accessCheck(T obj) {
            if(!cclass.isInstance(obj)) {
                throw accessCheckException(obj);
            }
        }
        
        /**
         * Returns access exception if accessCheck failed due to
         * protected access, else ClassCastException.
         */
        // 负责抛异常
        private final RuntimeException accessCheckException(T obj) {
            if(cclass == tclass) {
                return new ClassCastException();
            } else {
                return new RuntimeException(new IllegalAccessException("Class " + cclass.getName() + " can not access a protected member of class " + tclass.getName() + " using an instance of " + obj.getClass().getName()));
            }
        }
    }
}