spring5.x源码分析之自定义PropertySources

这两天太忙了没时间更新，上一期我们聊到spring在解析资源配置文件的时候会去获取当前上下文环境，
如果环境变量

```Java
   /**
	 * Create and return a new {@link StandardEnvironment}.
	 * <p>Subclasses may override this method in order to supply
	 * a custom {@link ConfigurableEnvironment} implementation.
	 */
	protected ConfigurableEnvironment createEnvironment() {
		//StandardEnvironment 构造函数 会 使他的抽象父类 也执行构造函数
		return new StandardEnvironment();
	}

注释写的很清晰，作用就是创建一个

那我们再来看看StandardEnvironment内部做了什么，它继承自```AbstractEnvironment```,
然后有两个私有静态final变量:

```Java
  /** System environment property source name: {@value}. */
	public static final String SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME = "systemEnvironment";

	/** JVM system properties property source name: {@value}. */
	public static final String SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME = "systemProperties";

这两个变量 一个是 操作系统环境变量，另一个是JVM系统环境变量，他们干嘛用呢，我们看到StandardEnvironment
还定义了一个方法

```Java
    /**
    * Customize the set of property sources with those appropriate for any standard（适用于任何标准的）
    * Java environment:
    * <ul>
    * <li>{@value #SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME}
    * <li>{@value #SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME}
    * </ul>
    * <p>Properties present in {@value #SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME} will
    * take precedence over those in {@value #SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME}.
    * @see AbstractEnvironment#customizePropertySources(MutablePropertySources)
    * @see #getSystemProperties()
    * @see #getSystemEnvironment()
    */
    @Override
    protected void customizePropertySources(MutablePropertySources propertySources) {

    //https://blog.csdn.net/rtuujnncc/article/details/78248733
    //addLast最低优先级
    //https://www.cnblogs.com/Baronboy/p/6030443.html
    propertySources.addLast(new MapPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties()));
    propertySources.addLast(new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
    }

1. MapPropertySource 是 System.getProperties()
2. SystemEnvironmentPropertySource 是System.getenv()

这个方法是继承自它的抽象类

StandardEnvironment```的时候

会先调用```AbstractEnvironment```的构造函数，函数内部会调用customizePropertySources方法，并且会把
propertySources参数传入。

```Java
  //可变的属性源集合
	private final MutablePropertySources propertySources = new MutablePropertySources();
	//把propertySources放入 Resolver中
	private final ConfigurablePropertyResolver propertyResolver =
			new PropertySourcesPropertyResolver(this.propertySources);

      /**
	 * Create a new {@code Environment} instance, calling back to
	 * {@link #customizePropertySources(MutablePropertySources)} during construction to
	 * allow subclasses to contribute or manipulate(操作) {@link PropertySource} instances as
	 * appropriate.
	 * @see #customizePropertySources(MutablePropertySources)
	 */
	//调用 子类的customizePropertySources
	public AbstractEnvironment() {
		customizePropertySources(this.propertySources);
	}

customizePropertySources也允许子类去重写，这样就允许子类去操作这个propertySources变量了，在
Spring中我们看到很多这样的设计，包括接口和抽象类的运用到处都是。关于这个自定义propertySources有什么用
我们下回再解！