spring5.x源码解析之加载bean过程前奏--Environment

上篇文章里最后我们提到了spring会获取当前所处环境后调用它的resolveRequiredPlaceholders
方法，也就是解析我们传递路径参数中的占位符，那么它是如何确定当前处于什么环境的呢？

AbstractRefreshableConfigApplicationContext类会调用它的抽象父类AbstractApplicationContext
类中的getEnvironment方法：

/*** Return the {@code Environment} for this application context in configurable* form, allowing for further customization.* <p>If none specified, a default environment will be initialized via* {@link #createEnvironment()}.*/
* Return the {@code Environment} for this application context in configurable
* form, allowing for further customization.
* <p>If none specified, a default environment will be initialized via
* {@link #createEnvironment()}.
*/
@Override
public ConfigurableEnvironment getEnvironment() {
if (this.environment == null) {
  this.environment = createEnvironment();
}
return this.environment;
}

这个方法的注释很清晰，意思就是以可配置的格式返回当前应用程序上下文的环境对象。我们看到这个方法
已经重写了，它的定义其实是在

是干嘛的呢？我们看官方文档：

Interface indicating a component that contains and exposes an {@link Environment} reference.
是一个接口，它可以用来表明一个组件包含并暴露Environment的引用。我们看它的定义就知道了：

```Java
  public interface EnvironmentCapable {
  	/**
  	 * Return the {@link Environment} associated with this component.
  	 */
     //返回跟这个组件关联的环境
  	Environment getEnvironment();

  }

All Spring application contexts are EnvironmentCapable, and the interface is used primarily
for performing {@code instanceof} checks in framework methods that accept BeanFactory
instances that may or may not actually be ApplicationContext instances in order to interact
with the environment if indeed it is available.

所有的Spring应用上下文都是带EnvironmentCapable接口的(可装配Environment)，而且接口主要用于在框架方法中执行instanceof检查，
这些方法接受BeanFactory实例，实例可能是ApplicationContext实例，也可能不是，如果是带EnvironmentCapable那么就可以访问Environment了。

As mentioned, {@link org.springframework.context.ApplicationContext ApplicationContext}
extends EnvironmentCapable, and thus exposes a {@link #getEnvironment()} method; however,
{@link org.springframework.context.ConfigurableApplicationContext ConfigurableApplicationContext}
redefines {@link org.springframework.context.ConfigurableApplicationContext#getEnvironment
getEnvironment()} and narrows the signature to return a {@link ConfigurableEnvironment}.
The effect is that an Environment object is ‘read-only’ until it is being accessed from
a ConfigurableApplicationContext, at which point it too may be configured。

总体意思就是ApplicationContext继承了EnvironmentCapable接口，但是ConfigurableApplicationContext重新定义了getEnvironment方法，
而且返回了一个ConfigurableEnvironment，影响就是Environment对象变只读了，除非从ConfigurableApplicationContext访问，
这个时候它就是可配置的了。

我们在文章一开始就看到AbstractApplicationContext类实现了ConfigurableApplicationContext接口，所以我们看到在getEnvironment方法
中我们可以自己配置environment变量了。那么这个createEnvironment内部到底做了些什么呢？

下回再分解！