Copy-On-Write 机制

深藏阁楼爱情的钟 2022-05-17 08:11 157阅读 0赞

“写时复制”——要写的时候复制一份副本，往副本里面写，然后引用到这个副本，在引用到这个副本之前有读操作的话，读的还是之前的老版本，读写分离，写不影响读可以并发读读，读写，但是写写的时候还是冲突的，如果拷贝了N个副本，最后到底引用谁呢，引用某个副本，写到其他副本上的数据就丢失了，所以写要加锁，也就是一个一个来，串行写。

Java集合框架里采用CopyOnWrite 机制的就两个类，

![70][]

当然我们也可以定义自己的CopyOnWrite数据结构，

看下CopyOnWriteArrayList 的读写吧

读很直接，就读对象中当前引用的数组。

// Positional Access Operations
    
        @SuppressWarnings("unchecked")
        private E get(Object[] a, int index) {
            return (E) a[index];
        }
    
        /**
         * {@inheritDoc}
         *
         * @throws IndexOutOfBoundsException {@inheritDoc}
         */
        public E get(int index) {
            return get(getArray(), index);
        }

set add remove 就需要加锁了，而且操作的是copy的副本。

public E set(int index, E element) {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                Object[] elements = getArray();
                E oldValue = get(elements, index);
    
                if (oldValue != element) {
                    int len = elements.length;
                    Object[] newElements = Arrays.copyOf(elements, len);
                    newElements[index] = element;
                    setArray(newElements);
                } else {
                    // Not quite a no-op; ensures volatile write semantics
                    setArray(elements);
                }
                return oldValue;
            } finally {
                lock.unlock();
            }
        }

public boolean add(E e) {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                Object[] elements = getArray();
                int len = elements.length;
                Object[] newElements = Arrays.copyOf(elements, len + 1);
                newElements[len] = e;
                setArray(newElements);
                return true;
            } finally {
                lock.unlock();
            }

public E remove(int index) {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                Object[] elements = getArray();
                int len = elements.length;
                E oldValue = get(elements, index);
                int numMoved = len - index - 1;
                if (numMoved == 0)
                    setArray(Arrays.copyOf(elements, len - 1));
                else {
                    Object[] newElements = new Object[len - 1];
                    System.arraycopy(elements, 0, newElements, 0, index);
                    System.arraycopy(elements, index + 1, newElements, index,
                                     numMoved);
                    setArray(newElements);
                }
                return oldValue;
            } finally {
                lock.unlock();
            }
        }

迭代器采用快照的方式，所以迭代的是老版本，而且不支持写，只能读。

static final class COWIterator<E> implements ListIterator<E> {
            /** Snapshot of the array */
            private final Object[] snapshot;
            /** Index of element to be returned by subsequent call to next.  */
            private int cursor;
    
            private COWIterator(Object[] elements, int initialCursor) {
                cursor = initialCursor;
                snapshot = elements;
            }
    
            public boolean hasNext() {
                return cursor < snapshot.length;
            }
    
            public boolean hasPrevious() {
                return cursor > 0;
            }
    
            @SuppressWarnings("unchecked")
            public E next() {
                if (! hasNext())
                    throw new NoSuchElementException();
                return (E) snapshot[cursor++];
            }
    
            @SuppressWarnings("unchecked")
            public E previous() {
                if (! hasPrevious())
                    throw new NoSuchElementException();
                return (E) snapshot[--cursor];
            }
    
            public int nextIndex() {
                return cursor;
            }
    
            public int previousIndex() {
                return cursor-1;
            }
    
            /**
             * Not supported. Always throws UnsupportedOperationException.
             * @throws UnsupportedOperationException always; {@code remove}
             *         is not supported by this iterator.
             */
            public void remove() {
                throw new UnsupportedOperationException();
            }
    
            /**
             * Not supported. Always throws UnsupportedOperationException.
             * @throws UnsupportedOperationException always; {@code set}
             *         is not supported by this iterator.
             */
            public void set(E e) {
                throw new UnsupportedOperationException();
            }
    
            /**
             * Not supported. Always throws UnsupportedOperationException.
             * @throws UnsupportedOperationException always; {@code add}
             *         is not supported by this iterator.
             */
            public void add(E e) {
                throw new UnsupportedOperationException();
            }
    
            @Override
            public void forEachRemaining(Consumer<? super E> action) {
                Objects.requireNonNull(action);
                Object[] elements = snapshot;
                final int size = elements.length;
                for (int i = cursor; i < size; i++) {
                    @SuppressWarnings("unchecked") E e = (E) elements[i];
                    action.accept(e);
                }
                cursor = size;
            }
        }

这个类适用于读多写少的场景，并发情况下还可以适用Collections类提供的SynchronizedList 类

static class SynchronizedList<E>
            extends SynchronizedCollection<E>
            implements List<E> {
            private static final long serialVersionUID = -7754090372962971524L;
    
            final List<E> list;
    
            SynchronizedList(List<E> list) {
                super(list);
                this.list = list;
            }
            SynchronizedList(List<E> list, Object mutex) {
                super(list, mutex);
                this.list = list;
            }
    
            public boolean equals(Object o) {
                if (this == o)
                    return true;
                synchronized (mutex) {return list.equals(o);}
            }
            public int hashCode() {
                synchronized (mutex) {return list.hashCode();}
            }
    
            public E get(int index) {
                synchronized (mutex) {return list.get(index);}
            }
            public E set(int index, E element) {
                synchronized (mutex) {return list.set(index, element);}
            }
            public void add(int index, E element) {
                synchronized (mutex) {list.add(index, element);}
            }
            public E remove(int index) {
                synchronized (mutex) {return list.remove(index);}
            }
    
            public int indexOf(Object o) {
                synchronized (mutex) {return list.indexOf(o);}
            }
            public int lastIndexOf(Object o) {
                synchronized (mutex) {return list.lastIndexOf(o);}
            }
    
            public boolean addAll(int index, Collection<? extends E> c) {
                synchronized (mutex) {return list.addAll(index, c);}
            }
    
            public ListIterator<E> listIterator() {
                return list.listIterator(); // Must be manually synched by user
            }
    
            public ListIterator<E> listIterator(int index) {
                return list.listIterator(index); // Must be manually synched by user
            }
    
            public List<E> subList(int fromIndex, int toIndex) {
                synchronized (mutex) {
                    return new SynchronizedList<>(list.subList(fromIndex, toIndex),
                                                mutex);
                }
            }
    
            @Override
            public void replaceAll(UnaryOperator<E> operator) {
                synchronized (mutex) {list.replaceAll(operator);}
            }
            @Override
            public void sort(Comparator<? super E> c) {
                synchronized (mutex) {list.sort(c);}
            }
    
            
            private Object readResolve() {
                return (list instanceof RandomAccess
                        ? new SynchronizedRandomAccessList<>(list)
                        : this);
            }
        }

mutex 是继承自 SynchronizedCollection 的

final Object mutex;     // Object on which to synchronize

或者还可以使用过时的Vector。

（源码拷贝大法，瞧不起自己=。=）

随手推荐别人的文章：[并发编程网：聊聊并发-Java中的Copy-On-Write容器][-Java_Copy-On-Write]

[70]: /images/20220517/8a7d08a57c3e4f83af793d13c237e1ef.png
[-Java_Copy-On-Write]: http://ifeve.com/java-copy-on-write/