Spring原始碼分析之IoC（二）

public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
logger.warn("Exception encountered during context initialization - cancelling refresh attempt", ex);
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
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首先會執行prepareRefresh方法，該方法為重新整理上下文做準備，進入該方法：

protected void prepareRefresh() {
this.startupDate = System.currentTimeMillis();
this.active.set(true);
if (logger.isInfoEnabled()) {
logger.info("Refreshing " + this);
}
// Initialize any placeholder property sources in the context environment
initPropertySources();
// Validate that all properties marked as required are resolvable
// see ConfigurablePropertyResolver#setRequiredProperties
getEnvironment().validateRequiredProperties();
// Allow for the collection of early ApplicationEvents,
// to be published once the multicaster is available...
this.earlyApplicationEvents = new LinkedHashSet<ApplicationEvent>();
}
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該方法首先會設定開始重新整理時間startupDate、活動標誌active，然後執行initPropertySources方法初始化屬性源，進入initPropertySources方法看下：

protected void initPropertySources() {
// For subclasses: do nothing by default.
}
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該方法是個空方法，子類可以重寫該方法，達到擴充套件目的。

然後，會執行validateRequiredProperties方法：

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;
}
}
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該方法會校驗系統環境中設定的RequiredProperties屬性是否為空，如果為空則丟擲異常。比如說我們設定了屬性值aaa：getEnvironment().setRequiredProperties(“aaa”)，但是在系統環境中沒有該屬性則會拋異常。例子：

@Test
public void testMyApplicationContext() {
ApplicationContext applicationContext = new MyApplicationContext();
}
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此時，啟動就會報錯：

org.springframework.core.env.MissingRequiredPropertiesException: The following properties were declared as required but could not be resolved: [aaa]
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我們在系統環境中設定該屬性，就不會丟擲該異常：

@Test
public void testMyApplicationContext() {
Properties properties = System.getProperties();
properties.setProperty("aaa", "bbb");
ApplicationContext applicationContext = new MyApplicationContext();
}
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prepareRefresh方法最後建立一個LinkedHashSet<ApplicationEvent> ，並賦值給earlyApplicationEvents，到此prepareRefresh方法就結束了。

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