Java并发 - J.U.C并发容器类 list、set、queue-蒲公英云

1. List

ArrayList

本质就是一个数组

初识化大小默认为 10

/**

 * Default initial capacity.
 */
private static final int DEFAULT_CAPACITY = 10;

每次扩容后大小变为原大小的1.5倍

private void grow(int minCapacity) {

    // overflow-conscious code
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1);    // 扩容为1.5倍大小
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity);
}

使用 for(Object o : list) 迭代器进行迭代循环的时候不应该对列表 list 进行新增或者删除操作，否则会报ConcurrentModificationException 异常，原因是因为迭代过程中会检查变量数量和期望的数量是否一致。

如以下操作就会报错

int i=0;
        for (Object o : list) {
            if(i==0)  list.add("neco");
            i++;
        }

抛出异常的代码

final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }

LinkedList

本质就是一个链表，没有什么特殊的内容

里边的Node是支持双向链表的

private static class Node {

    E item;
    Node<E> next;
    Node<E> prev;
    Node(Node<E> prev, E element, Node<E> next) {
        this.item = element;
        this.next = next;
        this.prev = prev;
    }
}

CopyOnWriteArrayList

在写的时候复制了一份出来，然后重新写入数据

/**

 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return {@code true} (as specified by {@link Collection#add})
 */
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();
    }
}

确保了读写可以同步进行，但是可能会有脏读的情况
在多读少写的情况下可以使用

CopyOnWriteArrayList容器即写时复制的容器。和ArrayList比较，优点是并发安全，缺点有两个:
1、多了内存占用:写数据是copy一份完整的数据，单独进行操作。占用双份内存。
2、数据一致性:数据写完之后，其他线程不一定是马上读取到最新内容。

format_png

2. Set 集合

和List比较：不会重复

实现	原理	特点
HashSet	基于HashMap实现	非线程安全
CopyOnWriteArraySet	基于CopyOnWriteArrayList	线程安全
TreeSet	基于TreeMap	线程安全，有序，查询快

HashSet

内部的实现本质就是一个Map（因为key值不重复），但是只是使用了Key，对于value无所谓

/**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }
    /**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element <tt>e</tt> to this set if
     * this set contains no element <tt>e2</tt> such that
     * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns <tt>false</tt>.
     *
     * @param e element to be added to this set
     * @return <tt>true</tt> if this set did not already contain the specified
     * element
     */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }
    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

CopyOnWriteArraySet

内部的本质是一个 CopyOnWriteArrayList，通过判断是否存在来确定是否放入数据

/**
     * Creates an empty set.
     */
    public CopyOnWriteArraySet() {
        al = new CopyOnWriteArrayList<E>();
    }
    /**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element {@code e} to this set if
     * the set contains no element {@code e2} such that
     * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns {@code false}.
     *
     * @param e element to be added to this set
     * @return {@code true} if this set did not already contain the specified
     *         element
     */
    public boolean add(E e) {
        return al.addIfAbsent(e);
    }
    /**
     * Appends the element, if not present.
     *
     * @param e element to be added to this list, if absent
     * @return {@code true} if the element was added
     */
    public boolean addIfAbsent(E e) {
        Object[] snapshot = getArray();
        return indexOf(e, snapshot, 0, snapshot.length) >= 0 ? false :
            addIfAbsent(e, snapshot);
    }
    /**
     * A version of addIfAbsent using the strong hint that given
     * recent snapshot does not contain e.
     */
    private boolean addIfAbsent(E e, Object[] snapshot) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] current = getArray();
            int len = current.length;
            if (snapshot != current) {
                // Optimize for lost race to another addXXX operation
                int common = Math.min(snapshot.length, len);
                for (int i = 0; i < common; i++)
                    if (current[i] != snapshot[i] && eq(e, current[i]))
                        return false;
                if (indexOf(e, current, common, len) >= 0)
                        return false;
            }
            Object[] newElements = Arrays.copyOf(current, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

TreeSet

本质是一个TreeMap，但是也只用到了Key值，Value值没有什么意义。

/**
     * Constructs a new, empty tree set, sorted according to the
     * natural ordering of its elements.  All elements inserted into
     * the set must implement the {@link Comparable} interface.
     * Furthermore, all such elements must be <i>mutually
     * comparable</i>: {@code e1.compareTo(e2)} must not throw a
     * {@code ClassCastException} for any elements {@code e1} and
     * {@code e2} in the set.  If the user attempts to add an element
     * to the set that violates this constraint (for example, the user
     * attempts to add a string element to a set whose elements are
     * integers), the {@code add} call will throw a
     * {@code ClassCastException}.
     */
    public TreeSet() {
        this(new TreeMap<E,Object>());
    }
    /**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element {@code e} to this set if
     * the set contains no element {@code e2} such that
     * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns {@code false}.
     *
     * @param e element to be added to this set
     * @return {@code true} if this set did not already contain the specified
     *         element
     * @throws ClassCastException if the specified object cannot be compared
     *         with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     *         and this set uses natural ordering, or its comparator
     *         does not permit null elements
     */
    public boolean add(E e) {
        return m.put(e, PRESENT)==null;
    }
    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();