SynchronizedStack -- tomcat8同步栈

同步栈（安全栈）：

org.apache.tomcat.util.collections.SynchronizedStack
通过stack栈锁来控制栈中获取的类T。通过push、pop和clear方法操作栈对象。栈初始化大小是128，没有上限。

初始化：

public SynchronizedStack() {
    this(DEFAULT_SIZE：128, DEFAULT_LIMIT：-1);
}

压入栈：

public synchronized boolean push(T obj) {
    index++;
    if (index == size) {
        if (limit == -1 || size < limit) {
            expand();
        } else {
            index--;
            return false;
        }
    }
    stack[index] = obj;
    return true;
}

private void expand() {
    int newSize = size * 2;
    if (limit != -1 && newSize > limit) {
        newSize = limit;
    }
    Object[] newStack = new Object[newSize];
    System.arraycopy(stack, 0, newStack, 0, size);
    // This is the only point where garbage is created by throwing away the
    // old array. Note it is only the array, not the contents, that becomes
    // garbage.
    stack = newStack;
    size = newSize;
}

获取栈对象：

public synchronized T pop() {
    if (index == -1) {
        return null;
    }
    T result = (T) stack[index];
    stack[index--] = null;
    return result;
}

清除栈释放资源：

public synchronized void clear() {
    if (index > -1) {
        for (int i = 0; i < index + 1; i++) {
            stack[i] = null;
        }
    }
    index = -1;
}

举例：