使用简单对象作为锁对象比Java中任何其他更复杂的对象都好？

我使用对象作为锁，例如成员函数

public class Test {
    private Object a_obj = new Object();
    private ArrayList a_List = new ArrayList();

    public void test() {
        synchronized (a) {
            // do whatever you want with the a_List here
        }
    }
}

但如果我这样做：

public class Test {
    private ArrayList a_List = new ArrayList();

    public void test() {
        synchronized (a_List) {
            // do whatever you want with the a_List here
        }
    }
}

这两者有什么区别