Java并发编程之同步容器

// 通过synchronized为add方法上锁
public synchronized boolean add(E e) {
  modCount++;
  ensureCapacityHelper(elementCount + 1);
  elementData[elementCount++] = e;
  return true;
}

public Vector(Collection<? extends E> c) {
	// 这里通过toArray将传来的集合 转为己有
  elementData = c.toArray();
  elementCount = elementData.length;
  // c.toArray might (incorrectly) not return Object[] (see 6260652)
  if (elementData.getClass() != Object[].class)
    elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
}

SynchronizedList(List<E> list) {
  super(list);
  // 这里只是指向传来的list，不转为己有，后面的相关操作还是基于原有的list集合
  this.list = list;
}

final Collection<E> c;  // Backing Collection
// 这个就是锁的对象
final Object mutex;     // Object on which to synchronize

SynchronizedCollection(Collection<E> c) {
  this.c = Objects.requireNonNull(c);
// 初始化为 this
  mutex = this;
}

SynchronizedCollection(Collection<E> c, Object mutex) {
  this.c = Objects.requireNonNull(c);
  this.mutex = Objects.requireNonNull(mutex);
}

public E next() {
  synchronized (Vector.this) {
    checkForComodification();
    int i = cursor;
    if (i >= elementCount)
      throw new NoSuchElementException();
    cursor = i + 1;
    return elementData(lastRet = i);
  }
}

public Iterator<E> iterator() {
  // 这里会直接实现类的迭代器（比如ArrayList，它里面的迭代器肯定是没上锁的）
  return c.iterator(); // Must be manually synched by user!
}

public class SyncCollectionDemo {
    
    private List<Integer> listNoSync;

    public SyncCollectionDemo() {
        this.listNoSync = new ArrayList<>();
    }

    public void addNoSync(int temp){
        listNoSync.add(temp);
    }

    public static void main(String[] args) throws InterruptedException {
        SyncCollectionDemo demo = new SyncCollectionDemo();
				// 创建10个线程
        for (int i = 0; i < 10; i++) {
					// 每个线程执行100次添加操作
          new Thread(()->{
                for (int j = 0; j < 1000; j++) {
                    demo.addNoSync(j);
                }
            }).start();
        }
    }
}

// 扩容方法
private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
				// 这里可以看到，每次扩容增加一半的容量
  			int newCapacity = oldCapacity + (oldCapacity >> 1);
        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);
    }

/**
 * <p>
 *  同步容器类：为什么要有它
 * </p>
 *
 * @author: JavaLover
 * @time: 2021/5/3
 */
public class SyncCollectionDemo {

    private List<Integer> listSync;

    public SyncCollectionDemo() {
      	// 这里包装一个空的ArrayList
        this.listSync = Collections.synchronizedList(new ArrayList<>());
    }

    public void addSync(int j){
      	// 内部是同步操作: synchronized (mutex) {return c.add(e);}
        listSync.add(j);
    }

    public static void main(String[] args) throws InterruptedException {
        SyncCollectionDemo demo = new SyncCollectionDemo();

        for (int i = 0; i < 10; i++) {
            new Thread(()->{
                for (int j = 0; j < 100; j++) {
                    demo.addSync(j);
                }
            }).start();
        }

        TimeUnit.SECONDS.sleep(1);
      	// 输出1000
        System.out.println(demo.listSync.size());
    }
}

final void checkForComodification() {
  // modCount：容器的长度变化次数， expectedModCount：期望的容器的长度变化次数
  if (modCount != expectedModCount)
    throw new ConcurrentModificationException();
}

public E remove(int index) {
        rangeCheck(index);
				// 1. 这里修改了 modCount
        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
              	// 1. 这里调用上面介绍的list.romove，修改modCount
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
              	// 2. 这里再同步更新 expectedModCount
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }