Java并发 - J.U.C并发容器类 list、set、queue-阿里云开发者社区

Java并发 - J.U.C并发容器类 list、set、queue

2024-03-14 61

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简介： Queue API阻塞是通过 condition 来实现的，可参考 Java 并发 - Lock 接口ArrayBlockingQueue 阻塞LinkedBlockingQueue 阻塞ArrayQueue 非阻塞LinkedQueue 非阻塞

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 next;
  Node prev;
  
  Node(Node prev, E element, Node 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。

CopyOnWriteArrayList

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 {<a class="at-user" href="//bbs.h3c.com/user/1458404942316748801">@link </a>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();

SET 接口没有所谓的有序还是无序。 TreeSet 是有序的，此有序是说读取数据的顺序和插入数据的顺序一样。 HashSet 无序？此无序说的是读取数据的顺序不一定和插入数据的顺序一样。