[Java知识库]Spring IoC原理(三) prepareRefresh()方法

Spring IoC原理(三) prepareRefresh()方法

一、prepareRefresh()方法分析

protected void prepareRefresh() {
	// Switch to active.
	this.startupDate = System.currentTimeMillis();
	this.closed.set(false);
	this.active.set(true);

	if (logger.isDebugEnabled()) {
		if (logger.isTraceEnabled()) {
			logger.trace("Refreshing " + this);
		}
		else {
			logger.debug("Refreshing " + getDisplayName());
		}
	}

	// 替换PropertySources
	initPropertySources();

	// 验证属性
	getEnvironment().validateRequiredProperties();

	// 存储预刷新的事件监听器
	if (this.earlyApplicationListeners == null) {
		this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);
	}
	else {
		// 将本地应用监听器重置为预刷新状态
		this.applicationListeners.clear();
		this.applicationListeners.addAll(this.earlyApplicationListeners);
	}


	// 当multicaster启用时，允许将应用启动之间的事件进行发布
	this.earlyApplicationEvents = new LinkedHashSet<>();
}

二、initPropertySources()

??首先先来看一下initPropertySources()方法的源码，从源码中可以看到注明对于子类默认情况下不执行任何操作，但可以发现该方法被三个子类进行重写，如图2-1所示。要了解initPropertySources()方法的作用，先要了解什么是PropertySource。

/**
 * Replace any stub property sources with actual instances.
 * @see org.springframework.core.env.PropertySource.StubPropertySource
 * @see org.springframework.web.context.support.WebApplicationContextUtils#initServletPropertySources
 */
protected void initPropertySources() {
	// For subclasses: do nothing by default.
}

图2-1 重写initPropertySources()方法的子类

??为了便于操作PropertySource对象，Spring中有PropertySources接口与MutablePropertySources类，PropertySources提供了一个类型为PropertySource的迭代器，提供了一些MutablePropertySources在PropertySources基础上进行了功能上的扩展， 提供一些类似add、replace方法。

public abstract class PropertySource<T> {

	protected final Log logger = LogFactory.getLog(getClass());

	protected final String name;

	protected final T source;

	public PropertySource(String name, T source) {
		Assert.hasText(name, "Property source name must contain at least one character");
		Assert.notNull(source, "Property source must not be null");
		this.name = name;
		this.source = source;
	}

	// 省略部分代码

	public static class StubPropertySource extends PropertySource<Object> {

		public StubPropertySource(String name) {
			super(name, new Object());
		}
		
		@Override
		@Nullable
		public String getProperty(String name) {
			return null;
		}
	}

	static class ComparisonPropertySource extends StubPropertySource {

		private static final String USAGE_ERROR =
				"ComparisonPropertySource instances are for use with collection comparison only";

		public ComparisonPropertySource(String name) {
			super(name);
		}

		@Override
		public Object getSource() {
			throw new UnsupportedOperationException(USAGE_ERROR);
		}

		@Override
		public boolean containsProperty(String name) {
			throw new UnsupportedOperationException(USAGE_ERROR);
		}

		@Override
		@Nullable
		public String getProperty(String name) {
			throw new UnsupportedOperationException(USAGE_ERROR);
		}
	}

}

三、validateRequiredProperties()

??在调用validateRequiredProperties()首先调用getEnvironment()方法，可以看一下getEnvironment()方法的源码，若当前环境为空则创建一个标准环境StandardEnvironment，然后将返回当前环境。

public ConfigurableEnvironment getEnvironment() {
	if (this.environment == null) {
		this.environment = createEnvironment();
	}
	return this.environment;
}

??接着调用返回的环境StandardEnvironment基类AbstractEnvironment中的validateRequiredProperties()方法进行属性验证。在上下文环境创建前可通过ConfigurablePropertyResolver的setRequiredProperties()方法对必要属性进行设置并存储在requiredProperties中，validate RequiredProperties()方法会遍历requiredProperties中的所有属性检测是否进行相应的设置，若存在未设置的属性则会抛出异常。

public void validateRequiredProperties() {
	MissingRequiredPropertiesException ex = new MissingRequiredPropertiesException();
	for (String key : this.requiredProperties) {
		if (this.getProperty(key) == null) {
			ex.addMissingRequiredProperty(key);
		}
	}
	if (!ex.getMissingRequiredProperties().isEmpty()) {
		throw ex;
	}
}