spring中的MutablePropertySources类设计

刚看到MutablePropertySources这个类中有一段示例感觉蛮有意思的：

* Customize the set of {@link PropertySource} objects to be searched by this
  * {@code Environment} during calls to {@link #getProperty(String)} and related
  * methods.
  *
  * <p>Subclasses that override this method are encouraged(鼓励) to add property
  * sources using {@link MutablePropertySources#addLast(PropertySource)} such that
  * further subclasses may call {@code super.customizePropertySources()} with
  * predictable(可预测的) results. For example:
  * public class Level1Environment extends AbstractEnvironment {
  *     @Override
  *     protected void customizePropertySources(MutablePropertySources propertySources) {
            //没有来自基类的操作 no-op from base class
  *         super.customizePropertySources(propertySources);
  *         propertySources.addLast(new PropertySourceA(...));
  *         propertySources.addLast(new PropertySourceB(...));
  *     }
  * }

  * public class Level2Environment extends Level1Environment {
  *     @Override
  *     protected void customizePropertySources(MutablePropertySources propertySources) {
            //从超类中全部 添加 add all from superclass
  *         super.customizePropertySources(propertySources);
  *         propertySources.addLast(new PropertySourceC(...));
  *         propertySources.addLast(new PropertySourceD(...));
  *     }
  * }
  * </pre>
  * In this arrangement, properties will be resolved against sources A, B, C, D in that
  * order. That is to say that property source "A" has precedence over property source
  * "D". If the {@code Level2Environment} subclass wished to give property sources C
  * and D higher precedence than A and B, it could simply call
  * {@code super.customizePropertySources} after, rather than before adding its own:
  * <pre class="code">
  * public class Level2Environment extends Level1Environment {
  *     @Override
  *     protected void customizePropertySources(MutablePropertySources propertySources) {
  *         propertySources.addLast(new PropertySourceC(...));
  *         propertySources.addLast(new PropertySourceD(...));
  *         super.customizePropertySources(propertySources);
            // add all from superclass
  *     }
  * }

也就是继承AbstractEnvironment类最主要目的是要通过customizePropertySources添加属性资源，而且在子类中调用积累的customizePropertySources将决定属性的优先级关系。

除了这些操作，我们还可以像之前的文章中说得那样可以通过ApplicationContextInitializer中的initialize方法中去获取MutableProperties然后再调用addLast或其他方法。