Java Vector源码学习

Vector

public class Vector extends AbstractList implements List, RandomAccess, Cloneable, java.io.Serializable

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一：自动扩容机制

The Vector class implements a growable array of objects. Like an array, it contains components that can be accessed using an integer index. However, the size of a Vector can grow or shrink as needed to accommodate adding and removing items after the Vector has been created.

1. 初始化

   底层为对象数组protected Object[] elementData;

   protected int elementCount; 元素个数

   protected int capacityIncrement;增长个数

       public Vector(int initialCapacity, int capacityIncrement) {
           super();
           if (initialCapacity < 0)
               throw new IllegalArgumentException("Illegal Capacity: "+
                                                  initialCapacity);
           this.elementData = new Object[initialCapacity];
           this.capacityIncrement = capacityIncrement;
       }
   
       public Vector(int initialCapacity) {
           this(initialCapacity, 0);
       }
   
       public Vector() {
           this(10);
       }
   

2. add

   public synchronized boolean add(E e) {
       modCount++;
       ensureCapacityHelper(elementCount + 1);
       elementData[elementCount++] = e;
       return true;
   }
   

   判断是否分配

   private void ensureCapacityHelper(int minCapacity) {
       // overflow-conscious code
       if (minCapacity - elementData.length > 0)
           grow(minCapacity);
   }
   

   关键：capacityIncrement》0时，每次增长capacityIncrement个元素；

   ​ capacityIncrement《=0时，2倍扩增；

   private void grow(int minCapacity) {
       // overflow-conscious code
       int oldCapacity = elementData.length;
       int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                        capacityIncrement : oldCapacity);
       if (newCapacity - minCapacity < 0)
           newCapacity = minCapacity;
       if (newCapacity - MAX_ARRAY_SIZE > 0)
           newCapacity = hugeCapacity(minCapacity);
       elementData = Arrays.copyOf(elementData, newCapacity);
   }
   

3. remove： 并不会调节底层数组的大小

   public synchronized E remove(int index) {
       modCount++;
       if (index >= elementCount)
           throw new ArrayIndexOutOfBoundsException(index);
       E oldValue = elementData(index);
   
       int numMoved = elementCount - index - 1;
       if (numMoved > 0)
           System.arraycopy(elementData, index+1, elementData, index,
                            numMoved);
       elementData[--elementCount] = null; // Let gc do its work
   
       return oldValue;
   }
   

   clear实现

   public synchronized void removeAllElements() {
       modCount++;
       // Let gc do its work
       for (int i = 0; i < elementCount; i++)
           elementData[i] = null;
   
       elementCount = 0;
   }
   

   真正减少底层数组容量

   public synchronized void trimToSize() {
       modCount++;
       int oldCapacity = elementData.length;
       if (elementCount < oldCapacity) {
           elementData = Arrays.copyOf(elementData, elementCount);
       }
   }
   

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二：线程安全

jdk1.0 Vector ，jdk1.2 ArrayList

Vector 将可变状态封装在对象内部，通过对象锁对所有访问可变状态的代码进行同步，使得在该对象上不会发生并发访问。

对比ArrayList可以发现，增删改查所有方法都加了synchronized 关键字。

    public synchronized int size() {
        return elementCount;
    }

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三： Fail-Fast 机制

Vector返回的Iterator也是fail-fast。

public synchronized Iterator iterator() {
  return new Itr();
}
private class Itr implements Iterator {
    int cursor;       // index of next element to return
    int lastRet = -1; // index of last element returned; -1 if no such
    int expectedModCount = modCount;

    public boolean hasNext() {
        // Racy but within spec, since modifications are checked
        // within or after synchronization in next/previous
        return cursor != elementCount;
    }
    public E next() {
        synchronized (Vector.this) {
            checkForComodification();
            int i = cursor;
            if (i >= elementCount)
                throw new NoSuchElementException();
            cursor = i + 1;
            return elementData(lastRet = i);
        }
    }
}

Vector创建了Iterator之后，进行增删操作，modCount变化。-》ConcurrentModificationException

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四：总结

1. Vector的问题

   1）过多的同步，每个方法都有synchronized；

   2）复合操作，并不安全，需要额外加锁。

   if(!vector.contains(e)){
     vector.add(e);
   }
   

2. Vector vs ArrayList

   1. ArrayList扩增方式为1.5倍。

      Vector扩容方式可以自设每次加几个，或者默认2倍扩容。

   2. Vector线程安全

      ArrayList线程不安全

   3. Vector 为jdk1.0版本，并不推荐；

      不需要线程安全时，推荐选择ArrayList；

      需要线程安全时，List list = Collections.synchronizedList(new ArrayList(...));

      Collections.synchronizedList虽然在增删改查方法都有synchronized；但是有好处；

      (1) Vector 同步方法加锁的是this对象，没办法控制锁定的对象。

      SynchronizedList的同步，使用的是synchronized代码块对mutex对象加锁，这个mutex对象还能够通过构造函数传进来，也就是说我们可以指定锁定的对象。

      (2)对于迭代需要加大锁，Iterator.remove()的小锁并不需要，额外开销；synchronizedList.iterator()没synchronized关键字，自己加大锁，Iterator.remove()没有锁；

      // 对迭代过程加锁
              synchronized (vector) {
                  for (Integer i : vector) {
                  }
              }
      

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@梦工厂 2018.3.7