Spring 原始碼(9)Spring Bean的建立過程的前期準備

玲丶蹊發表於2022-05-07

回顧總結

到目前為止,Spring原始碼中AbstractApplicationContext#refresh方法的已經解讀到第11個方法finishBeanFactoryInitialization,前10個方法介紹了:

  • BeanFactory的準備,建立,重新整理,個性化BeanFactory的擴充套件點,自定義屬性解析;
  • 環境資訊Environment的載入(包括環境變數、系統變數等);
  • BeanDefinition的載入,解析,自定義xml的方式;
  • BeanFactoryPostProcessor的註冊與執行流程,BeanDefinitionRegistryPostProcessor的解析,ConfigurationClassPostProcessorSpring註解的解析過程(@Component、@PropertySources、@PropertySource、@ComponentScans、@ComponentScan、@Import等註解的解析),Spring Boot 是如何通過@Configuration+@Import + ImportSelector進行自動裝配的等;
  • BeanPostProcessor的註冊流程;
  • 國際化,Spring事件驅動的載入執行過程;

finishBeanFactoryInitialization 解析過程

接下來開始解析SpringBean的建立過程,上原始碼:

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
  // Initialize conversion service for this context.
  if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
      beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
    // 設定轉換服務,轉換服務用來對屬性值進行解析的
    beanFactory.setConversionService(
      beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
  }

  // Register a default embedded value resolver if no BeanFactoryPostProcessor
  // (such as a PropertySourcesPlaceholderConfigurer bean) registered any before:
  // at this point, primarily for resolution in annotation attribute values.
  // 如果之前沒有註冊過任何 BeanFactoryPostProcessor(例如 PropertySourcesPlaceholderConfigurer bean),
  // 則註冊一個預設的嵌入值解析器:此時,主要用於解析註釋屬性值。
  if (!beanFactory.hasEmbeddedValueResolver()) {
    beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
  }

  // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
  String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
  for (String weaverAwareName : weaverAwareNames) {
    getBean(weaverAwareName);
  }

  // Stop using the temporary ClassLoader for type matching.
  beanFactory.setTempClassLoader(null);

  // Allow for caching all bean definition metadata, not expecting further changes.
  // 允許快取所有 bean 定義後設資料,而不是期望進一步的更改
  beanFactory.freezeConfiguration();

  // Instantiate all remaining (non-lazy-init) singletons.
  // 例項化所有剩餘的(非惰性初始化)單例
  beanFactory.preInstantiateSingletons();
}
  • 判斷是否存在轉換服務,有就設定
  • 判斷是否有內建的值解析器,沒有就建立一個處理佔位符的解析器
  • 例項化LoadTimeWeaverAware,進行早期的Bean的建立
  • 停止使用臨時的類載入器
  • 凍結BeanDefinition的後設資料資訊,防止被修改
  • 開始例項化所有的單例bean物件

除了beanFactory.preInstantiateSingletons() 方法,其他都是Bean建立的準備,接下來一個一個分析,首先是轉換服務的設定。

轉換服務ConversionService的初始化

方法一開始設定了一個轉換服務,這個轉換服務在Spring中還是非常的重要的,比如我們xml中配置一個String 型別的屬性值,但是在Bean的定義中是一個Integer型別的,這時Spring就會自動幫我們轉出來,他是怎麼做的呢?

Spring中有幾個比較重要的介面:

  • Converer 用於將物件S轉換為物件T
  • ConverterFactory 一個轉換工廠,能夠將物件S轉成一類物件R的子集T,比如可以將字串S轉換為TInteger、Long等)Number型別R的子集
  • GenericConverter支援多種型別之間互相轉換。

Spring轉換器介面ConversionService 的預設實現是DefaultConversionService,這個預設的轉換器實現中,內建了很多的轉換器,比如:

public static void addDefaultConverters(ConverterRegistry converterRegistry) {
  addScalarConverters(converterRegistry);
  addCollectionConverters(converterRegistry);

  converterRegistry.addConverter(new ByteBufferConverter((ConversionService) converterRegistry));
  converterRegistry.addConverter(new StringToTimeZoneConverter());
  converterRegistry.addConverter(new ZoneIdToTimeZoneConverter());
  converterRegistry.addConverter(new ZonedDateTimeToCalendarConverter());

  converterRegistry.addConverter(new ObjectToObjectConverter());
  converterRegistry.addConverter(new IdToEntityConverter((ConversionService) converterRegistry));
  converterRegistry.addConverter(new FallbackObjectToStringConverter());
  converterRegistry.addConverter(new ObjectToOptionalConverter((ConversionService) converterRegistry));
}

public static void addCollectionConverters(ConverterRegistry converterRegistry) {
  ConversionService conversionService = (ConversionService) converterRegistry;
  // 陣列轉集合
  converterRegistry.addConverter(new ArrayToCollectionConverter(conversionService));
  // 集合轉陣列
  converterRegistry.addConverter(new CollectionToArrayConverter(conversionService));

  converterRegistry.addConverter(new ArrayToArrayConverter(conversionService));
  converterRegistry.addConverter(new CollectionToCollectionConverter(conversionService));
  converterRegistry.addConverter(new MapToMapConverter(conversionService));
  // 陣列轉字串
  converterRegistry.addConverter(new ArrayToStringConverter(conversionService));
  converterRegistry.addConverter(new StringToArrayConverter(conversionService));

  converterRegistry.addConverter(new ArrayToObjectConverter(conversionService));
  converterRegistry.addConverter(new ObjectToArrayConverter(conversionService));

  converterRegistry.addConverter(new CollectionToStringConverter(conversionService));
  converterRegistry.addConverter(new StringToCollectionConverter(conversionService));

  converterRegistry.addConverter(new CollectionToObjectConverter(conversionService));
  converterRegistry.addConverter(new ObjectToCollectionConverter(conversionService));

  converterRegistry.addConverter(new StreamConverter(conversionService));
}

private static void addScalarConverters(ConverterRegistry converterRegistry) {
  converterRegistry.addConverterFactory(new NumberToNumberConverterFactory());

  converterRegistry.addConverterFactory(new StringToNumberConverterFactory());
  converterRegistry.addConverter(Number.class, String.class, new ObjectToStringConverter());

  converterRegistry.addConverter(new StringToCharacterConverter());
  converterRegistry.addConverter(Character.class, String.class, new ObjectToStringConverter());

  converterRegistry.addConverter(new NumberToCharacterConverter());
  converterRegistry.addConverterFactory(new CharacterToNumberFactory());

  converterRegistry.addConverter(new StringToBooleanConverter());
  converterRegistry.addConverter(Boolean.class, String.class, new ObjectToStringConverter());

  converterRegistry.addConverterFactory(new StringToEnumConverterFactory());
  converterRegistry.addConverter(new EnumToStringConverter((ConversionService) converterRegistry));

  converterRegistry.addConverterFactory(new IntegerToEnumConverterFactory());
  converterRegistry.addConverter(new EnumToIntegerConverter((ConversionService) converterRegistry));

  converterRegistry.addConverter(new StringToLocaleConverter());
  converterRegistry.addConverter(Locale.class, String.class, new ObjectToStringConverter());

  converterRegistry.addConverter(new StringToCharsetConverter());
  converterRegistry.addConverter(Charset.class, String.class, new ObjectToStringConverter());

  converterRegistry.addConverter(new StringToCurrencyConverter());
  converterRegistry.addConverter(Currency.class, String.class, new ObjectToStringConverter());

  converterRegistry.addConverter(new StringToPropertiesConverter());
  converterRegistry.addConverter(new PropertiesToStringConverter());

  converterRegistry.addConverter(new StringToUUIDConverter());
  converterRegistry.addConverter(UUID.class, String.class, new ObjectToStringConverter());
}

可以說是非常的豐富的,基本上常見都是Spring提供了,非常貼心。

那麼怎麼使用呢?

不懂當然是上官網:https://docs.spring.io/spring-framework/docs/current/reference/html/core.html#core-convert ,這裡可以看到我們只需要將ConversionServiceFactoryBean 配置到Spring容器中就可以了,Spring內建的轉換器就可以工作了,非常方便。

ConversionServiceFactoryBean實現了FactoryBean介面和InitializingBean 介面,而InitializingBean#afterPropertiesSet是初始化Bean過程中需要執行的。ConversionServiceFactoryBean原始碼中:

@Override
public void afterPropertiesSet() {
  this.conversionService = createConversionService();
  ConversionServiceFactory.registerConverters(this.converters, this.conversionService);
}

protected GenericConversionService createConversionService() {
  return new DefaultConversionService();
}

// 建構函式
public DefaultConversionService() {
  // 新增預設的轉換器
  addDefaultConverters(this);
}

可以看到這個ConversionServiceFactroyBean就是用來初始化轉換器的,並且這個類還提供了擴充套件,可以自定義轉換器加入到轉換器集合中。

自定義轉換器

自定義String轉Integer型別的轉換器:

/**
 * @author <a href="https://www.cnblogs.com/redwinter/">redwinter</a>
 * @since 1.0
 **/
public class StringToIntegerConverter implements Converter<String,Integer> , ConditionalConverter {
	@Override
	public Integer convert(String source) {
		return NumberUtils.parseNumber(source,Integer.class);
	}

	@Override
	public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
		System.out.println(sourceType.getType());
		System.out.println(targetType.getType());
		return true;
	}
}

邏輯非常簡單,直接呼叫Spring提供的工具類進行轉換。

配置xml:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	   xmlns:context="http://www.springframework.org/schema/context"
	   xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">

	<context:component-scan base-package="com.redwinter.selfconverter"/>
	<!--配置轉化器-->
	<bean class="org.springframework.context.support.ConversionServiceFactoryBean">
		<property name="converters">
			<set>
				<bean class="com.redwinter.selfconverter.StringToIntegerConverter"/>
			</set>
		</property>
	</bean>

</beans>

建立轉換器客戶端:

/**
 * @author <a href="https://www.cnblogs.com/redwinter/">redwinter</a>
 * @since 1.0
 **/
@Service
public class MyConverter {

	private final ConversionService conversionService;

	public MyConverter(ConversionService conversionService) {
		this.conversionService = conversionService;
	}

	public void test(String source){
		System.out.println(conversionService.convert(source, Integer.class));
	}
}

建立測試:

/**
 * @author <a href="2360564660@qq.com">redwinter</a>
 * @since 1.0
 **/
public class FactoryBeanTest {

	@Test
	public void test(){
		MyClassPathXmlApplicationContext context = new MyClassPathXmlApplicationContext("spring-factory.xml");
		MyConverter myConverter = context.getBean(MyConverter.class);
		myConverter.test("12345");
	}
}

輸出:

class java.lang.String
class java.lang.Integer
12345

分析完轉換服務,接下來分析 值解析器的新增。

預設的值解析器

// 省略程式碼.....
// Register a default embedded value resolver if no BeanFactoryPostProcessor
// (such as a PropertySourcesPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
// 如果之前沒有註冊過任何 BeanFactoryPostProcessor(例如 PropertySourcesPlaceholderConfigurer bean),
// 則註冊一個預設的嵌入值解析器:此時,主要用於解析註釋屬性值。
if (!beanFactory.hasEmbeddedValueResolver()) {
  beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// 省略程式碼.....


首先判斷了容器中是否存在嵌入的值解析器,如果沒有就新增一個進去,這裡新增進去的是StringValueResolver,點選resolvePlaceHolders方法進去,最終會在AbstractPropertyResolver#resolvePlaceholders中建立一個PropertyPlaceholderHelper

private PropertyPlaceholderHelper createPlaceholderHelper(boolean ignoreUnresolvablePlaceholders) {
  // 字首為 ${ ,字尾為 },值的分隔符為 : ,比如 ${username:zhansan} username沒有的話,後面的為預設的值
  return new PropertyPlaceholderHelper(this.placeholderPrefix, this.placeholderSuffix,
                                       this.valueSeparator, ignoreUnresolvablePlaceholders);
}

如果已經註冊過一個BFPP的佔位符解析器的話,就不需要在註冊了,BFPP的佔位符解析器就是PropertySourcesPlaceholderConfigurer ,專門用於解析佔位符的,比如在xml中或者yaml中,配置類似於${jdbc.username} 這種格式的,就會被解析器解析。PropertySourcesPlaceholderConfigurer 這個解析器實現了BeanFactoryPostProcessor介面,能夠對BeanDefinition進行處理,當然也可以對屬性值進行處理。

分析完值解析器,繼續往下分析。

Bean建立前的其他準備

// 省略程式碼.....
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
// 在prepareBeanFactory 準備BeanFactory時設定進去的,如果存在,則開始早期Bean的建立
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
  getBean(weaverAwareName);
}

// Stop using the temporary ClassLoader for type matching.
// 停止使用臨時的類載入器,這裡也是在準備BeanFactory時設定進去的
beanFactory.setTempClassLoader(null);

// Allow for caching all bean definition metadata, not expecting further changes.
// 允許快取所有 bean 定義後設資料,而不是期望進一步的更改
beanFactory.freezeConfiguration();
// 省略程式碼.....

這裡從容器中獲取了AOP的織入,如果有的話就開始進行早期的Bean的建立;然後停止了臨時的類載入器;然後就是凍結BeanDefinition的後設資料資訊。

public void freezeConfiguration() {
  this.configurationFrozen = true;
  this.frozenBeanDefinitionNames = StringUtils.toStringArray(this.beanDefinitionNames);
}

點選進來,其實就是設定了標識,防止後期對BeanDefinition的修改。

這前面的幾個判斷和方法實際上都是Bean建立的準備工作,接下來開始分析preInstantiateSingletons 預例項化所有的單例Bean

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