jdk並發包 CopyOnWriteArrayList源碼分析

標籤：os   io   java   ar   sp   on   c   new   ad   

CopyOnWriteArrayList是jdk1.5並法包裡面用於處理高並發下，讀多寫少的情況下，降低鎖等待的集合類。下面對該類實現做一個簡要的分析

1，首先CopyOnWriteArrayList是實現了List介面，對=List介面的相關方法進行了實現。

2，下面的它的add方法，會首先加鎖，然後copy原List內部的數組，然後對新數組長度加1後釋放鎖。由於數組copy速度很快，切在讀多寫少的情況下鎖開銷比較少

    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();        }    }

2，它的迭代器COWIterator不是快速失敗的，下面是它的源碼

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++];        }

3，下面是它的remove方法，加鎖原理同add方法

    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();        }    }

jdk並發包 CopyOnWriteArrayList源碼分析