AtomicReferenceArray.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.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.util.Arrays;
import java.util.function.BinaryOperator;
import java.util.function.UnaryOperator;

/**
 * An array of object references in which elements may be updated
 * atomically.  See the {@link VarHandle} specification for
 * descriptions of the properties of atomic accesses.
 *
 * @param <E> The base class of elements held in this array
 *
 * @author Doug Lea
 * @since 1.5
 */
// 引用类型数组（原子性）
public class AtomicReferenceArray<E> implements Serializable {
    private static final long serialVersionUID = -6209656149925076980L;
    
    private final Object[] array; // must have exact type Object[]
    private static final VarHandle AA = MethodHandles.arrayElementVarHandle(Object[].class);
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a new AtomicReferenceArray of the given length, with all
     * elements initially null.
     *
     * @param length the length of the array
     */
    public AtomicReferenceArray(int length) {
        array = new Object[length];
    }
    
    /**
     * Creates a new AtomicReferenceArray with the same length as, and
     * all elements copied from, the given array.
     *
     * @param array the array to copy elements from
     *
     * @throws NullPointerException if array is null
     */
    public AtomicReferenceArray(E[] array) {
        // Visibility guaranteed by final field guarantees
        this.array = Arrays.copyOf(array, array.length, Object[].class);
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 获取值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the current value of the element at index {@code i},
     * with memory effects as specified by {@link VarHandle#getVolatile}.
     *
     * @param i the index
     *
     * @return the current value
     */
    // 获取下标i处的元素
    @SuppressWarnings("unchecked")
    public final E get(int i) {
        return (E) AA.getVolatile(array, i);
    }
    
    /**
     * Returns the current value of the element at index {@code i},
     * with memory semantics of reading as if the variable was declared
     * non-{@code volatile}.
     *
     * @param i the index
     *
     * @return the value
     *
     * @since 9
     */
    // 获取下标i处的元素
    public final E getPlain(int i) {
        return (E) AA.get(array, i);
    }
    
    /**
     * Returns the current value of the element at index {@code i},
     * with memory effects as specified by {@link VarHandle#getOpaque}.
     *
     * @param i the index
     *
     * @return the value
     *
     * @since 9
     */
    // 获取下标i处的元素
    public final E getOpaque(int i) {
        return (E) AA.getOpaque(array, i);
    }
    
    /**
     * Returns the current value of the element at index {@code i},
     * with memory effects as specified by {@link VarHandle#getAcquire}.
     *
     * @param i the index
     *
     * @return the value
     *
     * @since 9
     */
    // 获取下标i处的元素
    public final E getAcquire(int i) {
        return (E) AA.getAcquire(array, i);
    }
    
    /*▲ 获取值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 设置值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Sets the element at index {@code i} to {@code newValue},
     * with memory effects as specified by {@link VarHandle#setVolatile}.
     *
     * @param i        the index
     * @param newValue the new value
     */
    // 将下标i处的元素赋值为newValue
    public final void set(int i, E newValue) {
        AA.setVolatile(array, i, newValue);
    }
    
    /**
     * Sets the element at index {@code i} to {@code newValue},
     * with memory semantics of setting as if the variable was
     * declared non-{@code volatile} and non-{@code final}.
     *
     * @param i        the index
     * @param newValue the new value
     *
     * @since 9
     */
    // 将下标i处的元素赋值为newValue
    public final void setPlain(int i, E newValue) {
        AA.set(array, i, newValue);
    }
    
    /**
     * Sets the element at index {@code i} to {@code newValue},
     * with memory effects as specified by {@link VarHandle#setOpaque}.
     *
     * @param i        the index
     * @param newValue the new value
     *
     * @since 9
     */
    // 将下标i处的元素赋值为newValue
    public final void setOpaque(int i, E newValue) {
        AA.setOpaque(array, i, newValue);
    }
    
    /**
     * Sets the element at index {@code i} to {@code newValue},
     * with memory effects as specified by {@link VarHandle#setRelease}.
     *
     * @param i        the index
     * @param newValue the new value
     *
     * @since 9
     */
    // 将下标i处的元素赋值为newValue
    public final void setRelease(int i, E newValue) {
        AA.setRelease(array, i, newValue);
    }
    
    /**
     * Sets the element at index {@code i} to {@code newValue},
     * with memory effects as specified by {@link VarHandle#setRelease}.
     *
     * @param i        the index
     * @param newValue the new value
     *
     * @since 1.6
     */
    // 将下标i处的元素赋值为newValue
    public final void lazySet(int i, E newValue) {
        AA.setRelease(array, i, newValue);
    }
    
    /**
     * Atomically sets the element at index {@code i} to {@code
     * newValue} and returns the old value,
     * with memory effects as specified by {@link VarHandle#getAndSet}.
     *
     * @param i        the index
     * @param newValue the new value
     *
     * @return the previous value
     */
    // 获取下标i处的元素值，随后将该元素赋值为newValue
    @SuppressWarnings("unchecked")
    public final E getAndSet(int i, E newValue) {
        return (E) AA.getAndSet(array, i, newValue);
    }
    
    /*▲ 设置值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 比较并更新 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Atomically sets the element at index {@code i} to {@code newValue}
     * if the element's current value {@code == expectedValue},
     * with memory effects as specified by {@link VarHandle#compareAndSet}.
     *
     * @param i             the index
     * @param expectedValue the expected value
     * @param newValue      the new value
     *
     * @return {@code true} if successful. False return indicates that
     * the actual value was not equal to the expected value.
     */
    // 如果下标i处元素值是expectedValue，则将其更新为newValue
    public final boolean compareAndSet(int i, E expectedValue, E newValue) {
        return AA.compareAndSet(array, i, expectedValue, newValue);
    }
    
    /**
     * Possibly atomically sets the element at index {@code i} to
     * {@code newValue} if the element's current value {@code == expectedValue},
     * with memory effects as specified by {@link VarHandle#weakCompareAndSetPlain}.
     *
     * @param i             the index
     * @param expectedValue the expected value
     * @param newValue      the new value
     *
     * @return {@code true} if successful
     *
     * @see #weakCompareAndSetPlain
     * @deprecated This method has plain memory effects but the method
     * name implies volatile memory effects (see methods such as
     * {@link #compareAndExchange} and {@link #compareAndSet}).  To avoid
     * confusion over plain or volatile memory effects it is recommended that
     * the method {@link #weakCompareAndSetPlain} be used instead.
     */
    // 如果下标i处元素值是expectedValue，则将其更新为newValue
    @Deprecated(since = "9")
    public final boolean weakCompareAndSet(int i, E expectedValue, E newValue) {
        return AA.weakCompareAndSetPlain(array, i, expectedValue, newValue);
    }
    
    /**
     * Possibly atomically sets the element at index {@code i} to
     * {@code newValue} if the element's current value {@code == expectedValue},
     * with memory effects as specified by {@link VarHandle#weakCompareAndSetPlain}.
     *
     * @param i             the index
     * @param expectedValue the expected value
     * @param newValue      the new value
     *
     * @return {@code true} if successful
     *
     * @since 9
     */
    // 如果下标i处元素值是expectedValue，则将其更新为newValue
    public final boolean weakCompareAndSetPlain(int i, E expectedValue, E newValue) {
        return AA.weakCompareAndSetPlain(array, i, expectedValue, newValue);
    }
    
    /**
     * Possibly atomically sets the element at index {@code i} to
     * {@code newValue} if the element's current value {@code == expectedValue},
     * with memory effects as specified by
     * {@link VarHandle#weakCompareAndSetAcquire}.
     *
     * @param i             the index
     * @param expectedValue the expected value
     * @param newValue      the new value
     *
     * @return {@code true} if successful
     *
     * @since 9
     */
    // 如果下标i处元素值是expectedValue，则将其更新为newValue
    public final boolean weakCompareAndSetAcquire(int i, E expectedValue, E newValue) {
        return AA.weakCompareAndSetAcquire(array, i, expectedValue, newValue);
    }
    
    /**
     * Possibly atomically sets the element at index {@code i} to
     * {@code newValue} if the element's current value {@code == expectedValue},
     * with memory effects as specified by
     * {@link VarHandle#weakCompareAndSetRelease}.
     *
     * @param i             the index
     * @param expectedValue the expected value
     * @param newValue      the new value
     *
     * @return {@code true} if successful
     *
     * @since 9
     */
    // 如果下标i处元素值是expectedValue，则将其更新为newValue
    public final boolean weakCompareAndSetRelease(int i, E expectedValue, E newValue) {
        return AA.weakCompareAndSetRelease(array, i, expectedValue, newValue);
    }
    
    /**
     * Possibly atomically sets the element at index {@code i} to
     * {@code newValue} if the element's current value {@code == expectedValue},
     * with memory effects as specified by
     * {@link VarHandle#weakCompareAndSet}.
     *
     * @param i             the index
     * @param expectedValue the expected value
     * @param newValue      the new value
     *
     * @return {@code true} if successful
     *
     * @since 9
     */
    // 如果下标i处元素值是expectedValue，则将其更新为newValue
    public final boolean weakCompareAndSetVolatile(int i, E expectedValue, E newValue) {
        return AA.weakCompareAndSet(array, i, expectedValue, newValue);
    }
    
    /**
     * Atomically sets the element at index {@code i} to {@code newValue}
     * if the element's current value, referred to as the <em>witness
     * value</em>, {@code == expectedValue},
     * with memory effects as specified by
     * {@link VarHandle#compareAndExchange}.
     *
     * @param i             the index
     * @param expectedValue the expected value
     * @param newValue      the new value
     *
     * @return the witness value, which will be the same as the
     * expected value if successful
     *
     * @since 9
     */
    // 如果下标i处元素值是expectedValue，则将其更新为newValue，返回旧值
    public final E compareAndExchange(int i, E expectedValue, E newValue) {
        return (E) AA.compareAndExchange(array, i, expectedValue, newValue);
    }
    
    /**
     * Atomically sets the element at index {@code i} to {@code newValue}
     * if the element's current value, referred to as the <em>witness
     * value</em>, {@code == expectedValue},
     * with memory effects as specified by
     * {@link VarHandle#compareAndExchangeAcquire}.
     *
     * @param i             the index
     * @param expectedValue the expected value
     * @param newValue      the new value
     *
     * @return the witness value, which will be the same as the
     * expected value if successful
     *
     * @since 9
     */
    // 如果下标i处元素值是expectedValue，则将其更新为newValue，返回旧值
    public final E compareAndExchangeAcquire(int i, E expectedValue, E newValue) {
        return (E) AA.compareAndExchangeAcquire(array, i, expectedValue, newValue);
    }
    
    /**
     * Atomically sets the element at index {@code i} to {@code newValue}
     * if the element's current value, referred to as the <em>witness
     * value</em>, {@code == expectedValue},
     * with memory effects as specified by
     * {@link VarHandle#compareAndExchangeRelease}.
     *
     * @param i             the index
     * @param expectedValue the expected value
     * @param newValue      the new value
     *
     * @return the witness value, which will be the same as the
     * expected value if successful
     *
     * @since 9
     */
    // 如果下标i处元素值是expectedValue，则将其更新为newValue，返回旧值
    public final E compareAndExchangeRelease(int i, E expectedValue, E newValue) {
        return (E) AA.compareAndExchangeRelease(array, i, expectedValue, newValue);
    }
    
    /*▲ 比较并更新 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ lambda操作 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Atomically updates (with memory effects as specified by {@link
     * VarHandle#compareAndSet}) the element at index {@code i} 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 i              the index
     * @param updateFunction a side-effect-free function
     *
     * @return the previous value
     *
     * @since 1.8
     */
    // 对下标i处的元素应用updateFunction操作，返回旧值
    public final E getAndUpdate(int i, UnaryOperator<E> updateFunction) {
        E prev = get(i), next = null;
        for(boolean haveNext = false; ; ) {
            if(!haveNext)
                next = updateFunction.apply(prev);
            if(weakCompareAndSetVolatile(i, prev, next))
                return prev;
            haveNext = (prev == (prev = get(i)));
        }
    }
    
    /**
     * Atomically updates (with memory effects as specified by {@link
     * VarHandle#compareAndSet}) the element at index {@code i} 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 i              the index
     * @param updateFunction a side-effect-free function
     *
     * @return the updated value
     *
     * @since 1.8
     */
    // 对下标i处的元素应用updateFunction操作，返回新值
    public final E updateAndGet(int i, UnaryOperator<E> updateFunction) {
        E prev = get(i), next = null;
        for(boolean haveNext = false; ; ) {
            if(!haveNext)
                next = updateFunction.apply(prev);
            if(weakCompareAndSetVolatile(i, prev, next))
                return next;
            haveNext = (prev == (prev = get(i)));
        }
    }
    
    /**
     * Atomically updates (with memory effects as specified by {@link
     * VarHandle#compareAndSet}) the element at index {@code i} 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 of the element at index {@code i}
     * as its first argument, and the given update as the second
     * argument.
     *
     * @param i                   the index
     * @param x                   the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     *
     * @return the previous value
     *
     * @since 1.8
     */
    // 对下标i处的元素应用accumulatorFunction操作，返回旧值
    public final E getAndAccumulate(int i, E x, BinaryOperator<E> accumulatorFunction) {
        E prev = get(i), next = null;
        for(boolean haveNext = false; ; ) {
            if(!haveNext)
                next = accumulatorFunction.apply(prev, x);
            if(weakCompareAndSetVolatile(i, prev, next))
                return prev;
            haveNext = (prev == (prev = get(i)));
        }
    }
    
    /**
     * Atomically updates (with memory effects as specified by {@link
     * VarHandle#compareAndSet}) the element at index {@code i} 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 of the element at index {@code i}
     * as its first argument, and the given update as the second
     * argument.
     *
     * @param i                   the index
     * @param x                   the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     *
     * @return the updated value
     *
     * @since 1.8
     */
    // 对下标i处的元素应用accumulatorFunction操作，返回新值
    public final E accumulateAndGet(int i, E x, BinaryOperator<E> accumulatorFunction) {
        E prev = get(i), next = null;
        for(boolean haveNext = false; ; ) {
            if(!haveNext)
                next = accumulatorFunction.apply(prev, x);
            if(weakCompareAndSetVolatile(i, prev, next))
                return next;
            haveNext = (prev == (prev = get(i)));
        }
    }
    
    /*▲ lambda操作 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Returns the length of the array.
     *
     * @return the length of the array
     */
    // 获取数组长度
    public final int length() {
        return array.length;
    }
    
    
    
    
    /**
     * Returns the String representation of the current values of array.
     *
     * @return the String representation of the current values of array
     */
    public String toString() {
        int iMax = array.length - 1;
        if(iMax == -1)
            return "[]";
        
        StringBuilder b = new StringBuilder();
        b.append('[');
        for(int i = 0; ; i++) {
            b.append(get(i));
            if(i == iMax)
                return b.append(']').toString();
            b.append(',').append(' ');
        }
    }
    
    
    
    /**
     * Reconstitutes the instance from a stream (that is, deserializes it).
     *
     * @param s the stream
     *
     * @throws ClassNotFoundException if the class of a serialized object
     *                                could not be found
     * @throws java.io.IOException    if an I/O error occurs
     */
    private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
        // Note: This must be changed if any additional fields are defined
        Object a = s.readFields().get("array", null);
        if(a == null || !a.getClass().isArray())
            throw new java.io.InvalidObjectException("Not array type");
        if(a.getClass() != Object[].class)
            a = Arrays.copyOf((Object[]) a, Array.getLength(a), Object[].class);
        Field arrayField = java.security.AccessController.doPrivileged((java.security.PrivilegedAction<Field>) () -> {
            try {
                Field f = AtomicReferenceArray.class.getDeclaredField("array");
                f.setAccessible(true);
                return f;
            } catch(ReflectiveOperationException e) {
                throw new Error(e);
            }
        });
        try {
            arrayField.set(this, a);
        } catch(IllegalAccessException e) {
            throw new Error(e);
        }
    }
    
}