Java同步容器_Vector
synchronized boolean add(E object) void add(int location, E object) synchronized boolean addAll(Collection<? extends E> collection) synchronized boolean addAll(int location, Collection<? extends E> collection) synchronized void addElement(E object) synchronized int capacity() void clear() synchronized Object clone() boolean contains(Object object) synchronized boolean containsAll(Collection<?> collection) synchronized void copyInto(Object[] elements) synchronized E elementAt(int location) Enumeration<E> elements() synchronized void ensureCapacity(int minimumCapacity) synchronized boolean equals(Object object) synchronized E firstElement() E get(int location) synchronized int hashCode() synchronized int indexOf(Object object, int location) int indexOf(Object object) synchronized void insertElementAt(E object, int location) synchronized boolean isEmpty() synchronized E lastElement() synchronized int lastIndexOf(Object object, int location) synchronized int lastIndexOf(Object object) synchronized E remove(int location) boolean remove(Object object) synchronized boolean removeAll(Collection<?> collection) synchronized void removeAllElements() synchronized boolean removeElement(Object object) synchronized void removeElementAt(int location) synchronized boolean retainAll(Collection<?> collection) synchronized E set(int location, E object) synchronized void setElementAt(E object, int location) synchronized void setSize(int length) synchronized int size() synchronized List<E> subList(int start, int end) synchronized <T> T[] toArray(T[] contents) synchronized Object[] toArray() synchronized String toString() synchronized void trimToSize()
可以看到Vector中很多方法都是同步的。。
Vector的数据结构和ArrayList差不多,它包含了3个成员变量:elementData , elementCount, capacityIncrement。
(01) elementData 是"Object[]类型的数组",它保存了添加到Vector中的元素。elementData是个动态数组,如果初始化Vector时,没指定动态数组的>大小,则使用默认大小10。随着Vector中元素的增加,Vector的容量也会动态增长,capacityIncrement是与容量增长相关的增长系数,具体的增长方式,请参考源码分析中的ensureCapacity()函数。
(02) elementCount 是动态数组的实际大小。
(03) capacityIncrement 是动态数组的增长系数。如果在创建Vector时,指定了capacityIncrement的大小;则,每次当Vector中动态数组容量增加时,增加的大小都是capacityIncrement。
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); }
通过源码可以看到如何确定Vector如何确定新的容量
当capacityIncrement大于零时,oldCapacity + capacityIncrement;
当capacityIncrement小于零时,oldCapacity + oldCapacity ;
vector容器保证线程安全性的方法就是把每个对外暴露的存取方法用synchronized关键字同步化,这样会带来两个问题:性能不能保证,复合操作的原子性不能保证。看代码:
package container; import java.util.Vector; /** * Created with IntelliJ IDEA. * User: ASUS * Date: 14-8-7 * Time: 下午1:05 * To change this template use File | Settings | File Templates. */ public class VectorTest { public static Object getLast(Vector vector) { int lastIndex = vector.size() - 1; return vector.get(lastIndex); } public static void deleteLast(Vector vector) { int lastIndex = vector.size() - 1; vector.remove(lastIndex); } public static void main(String args[]) { final Vector vector = new Vector(); for (int n = 0; n < 100; n++) { vector.add("vector" + n); } Runnable task1 = new Runnable() { @Override public void run() { System.out.println(getLast(vector)); } }; Runnable task2 = new Runnable() { @Override public void run() { deleteLast(vector); } }; Thread t1 = new Thread(task1); Thread t2 = new Thread(task2); t2.setName("thread-2"); //thread2删除最后一元素 t2.start(); t1.setName("thread-1"); //thread1得到最后一个元素 t1.start(); } }
上面的代码,在多线程环境下是不安全的,虽然get和remove操作是同步的,但getLast和deleteLast不是原子操作,是一个复合操作。
如图所示,如果发生这样的执行顺序
结果肯定不是我们所期望的。
如果客户端是一组复合操作,它就没法同步了,依然需要客户端做额外同步。所以要保证getLast和deleteLast这种复合操作的原子性
public static Object getLast0(Vector vector) { //保证原子操作的同步锁(持有容器的锁,但是降低了并发性) synchronized (vector) { int lastIndex = vector.size() - 1; return vector.get(lastIndex); } } public static void deleteLast0(Vector vector) { synchronized (vector) { int lastIndex = vector.size() - 1; vector.remove(lastIndex); } }
=====END=====