【Spring源碼這樣讀】-細扒ApplicationContext之super(parent)

之前我們粗略的過了一遍IOC加載流程，現在我們開始細扒一下這個流程，不過我們不再去講XmlBeanFactory，這裏直接講ApplicationContext，本文主要聊聊初始化的時候，super(parent)到底做了什麼。

ClassPathXmlApplicationContext類圖

深扒源碼之前，一定要做好參考點，這裏我們使用類圖來做一個參考。

super涉及的代碼

super這個方法對應的代碼並不多，如果我們要去跟的話，只需要仔細一點，瞭解他的層級結構，基本問題不大。這裏把所有源碼都拿出來了

public ClassPathXmlApplicationContext(ApplicationContext parent) {
    super(parent);
}

public AbstractXmlApplicationContext(@Nullable ApplicationContext parent) {
    super(parent);
}

public AbstractRefreshableConfigApplicationContext(@Nullable ApplicationContext parent) {
    super(parent);
}

public AbstractRefreshableApplicationContext(@Nullable ApplicationContext parent) {
    super(parent);
}

public AbstractApplicationContext(@Nullable ApplicationContext parent) {
    this();
    this.setParent(parent);
}

public AbstractApplicationContext() {
	this.resourcePatternResolver = getResourcePatternResolver();
}

protected ResourcePatternResolver getResourcePatternResolver() {
    return new PathMatchingResourcePatternResolver(this);
}

public PathMatchingResourcePatternResolver(ResourceLoader resourceLoader) {
	Assert.notNull(resourceLoader, "ResourceLoader must not be null");
	this.resourceLoader = resourceLoader;
}

public void setParent(@Nullable ApplicationContext parent) {
	this.parent = parent;
	if (parent != null) {
		Environment parentEnvironment = parent.getEnvironment();
		if (parentEnvironment instanceof ConfigurableEnvironment) {
			getEnvironment().merge((ConfigurableEnvironment) parentEnvironment);
		}
	}
}

從我們的源碼上來看，結合我們的類圖，不難發現，其實super(parent)一直在調用父類的方法，直到AbstractApplicationContext，然後纔有真正的操作代碼。

AbstractApplicationContext中具體給super(parent)做了什麼

先來看看this()，this()實際就是我們當前類裏面的無參構造方法，最終其實也是給我們初始化了一個ClassPathXmlApplicationContext。這一點我們可以跟一下初始化的代碼

public Resource getResource(String location) {
	return getResourceLoader().getResource(location);
}

public Resource getResource(String location) {
	Assert.notNull(location, "Location must not be null");

	for (ProtocolResolver protocolResolver : getProtocolResolvers()) {
		Resource resource = protocolResolver.resolve(location, this);
		if (resource != null) {
			return resource;
		}
	}

	if (location.startsWith("/")) {
		return getResourceByPath(location);
	}
	else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
		return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
	}
	else {
		try {
			// Try to parse the location as a URL...
			URL url = new URL(location);
			return (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
		}
		catch (MalformedURLException ex) {
			// No URL -> resolve as resource path.
			return getResourceByPath(location);
		}
	}
}

最終是按照我們的路徑的開頭判斷返回哪種resource

setParent(parent)到底做了些啥

setParent(parent);對應的代碼並不多,也是比較簡單明瞭的

public void setParent(@Nullable ApplicationContext parent) {
	this.parent = parent;
	if (parent != null) {
		Environment parentEnvironment = parent.getEnvironment();
		if (parentEnvironment instanceof ConfigurableEnvironment) {
			getEnvironment().merge((ConfigurableEnvironment) parentEnvironment);
		}
	}
}

這幾部代碼就做了一件事情：保存父容器，並將父容器的環境與當前容器環境合併。