关于Ordered接口，用过的人可能知道，这里我谈下自己的理解。也希望各位大神能给予指点。
源码如下：
/**
* Interface that can be implemented by objects that should be
* orderable, for example in a Collection.
*
* <p>The actual order can be interpreted as prioritization, with
* the first object (with the lowest order value) having the highest
* priority.
*
* <p>Note that there is a 'priority' marker for this interface:
* {@link PriorityOrdered}. Order values expressed by PriorityOrdered
* objects always apply before order values of 'plain' Ordered values.
*
* @author Juergen Hoeller
* @since 07.04.2003
* @see OrderComparator
* @see org.springframework.core.annotation.Order
*/
public interface Ordered {
/**
* Useful constant for the highest precedence value.
* @see java.lang.Integer#MIN_VALUE
*/
int HIGHEST_PRECEDENCE = Integer.MIN_VALUE;
/**
* Useful constant for the lowest precedence value.
* @see java.lang.Integer#MAX_VALUE
*/
int LOWEST_PRECEDENCE = Integer.MAX_VALUE;
/**
* Return the order value of this object, with a
* higher value meaning greater in terms of sorting.
* <p>Normally starting with 0, with <code>Integer.MAX_VALUE</code>
* indicating the greatest value. Same order values will result
* in arbitrary positions for the affected objects.
* <p>Higher values can be interpreted as lower priority. As a
* consequence, the object with the lowest value has highest priority
* (somewhat analogous to Servlet "load-on-startup" values).
* @return the order value
*/
int getOrder();
}
当然这里并不是无故提出这个接口来看的。事由是由PropertyPlaceholderConfigurer这个类的加载顺序导致的。
这个类是实现了Ordered接口并且有这样一句话
private int order = Ordered.LOWEST_PRECEDENCE; // default: same as non-Ordered
来说明PropertyPlaceholderConfigurer加载的顺序，如果你细心的话，Ordered上面的一个注释你会看到。
大概的翻译就是值越小加载优先级越高，而普通的对象我们定义的是没有实现Ordered接口的，即是他所说的
'plain' Ordered values.这样的对象的加载优先级比实现Ordered接口优先级最低的还要低。就是说虽然
PropertyPlaceholderConfigurer类的优先级是实现Ordered接口中最低的，但也比plain Object的优先级高。所以我们在
定义PropertyPlaceholderConfigurer的时候，可以把位置放后面而先用其值。如图：
