jdk：CopyOnWriteArrayList

解决问题

如何理解CopyOnWrite？

关键词

数组、new数组、复制、数组更新、数组引用更新

code

CopyOnWriteArrayList.set(int index, E element)

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();    }}

CopyOnWriteArrayList.iterator()

 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;    }    ...}

List<String> sl = new CopyOnWriteArrayList<String>();sl.add("a");sl.add("b");sl.add("c");/*此处用作遍历的COWIterator中的数组引用 始终都没有被更新过*/for(String s : sl){    System.out.println(s);    sl.set(1,s+"1");    /*每次更新都会new一个新的数组，然后将CopyOnWriteArrayList中数组的引用指向该数组*/    System.out.println(sl);}/*a[a, a1, c]b[a, b1, c]c[a, c1, c]遍历过程中List内容已经更新，但是依然是 a b c*/