Spring Bean生命週期之Bean的範例化詳解

2022-03-04 16:00:37

前言

上一節說到了BeanDefinition的合併過程，這節該說Bean的範例化過程了。根據AbstractAutowireCapableBeanFactory#createBean原始碼邏輯可將範例化過程分為範例化前階段、範例化過程、範例化後階段。

範例化前階段

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {
		//省略無關程式碼
		try {
	       // 這裡就是我們分析的重點了 ⭐️
			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
			//⭐️ 注意這個邏輯：如果postProcessBeforeInstantiation方法返回非null 則將返回值作為建立的Bean。並中斷正常的建立流程
			if (bean != null) {
				return bean;
			}
		}
		catch (Throwable ex) {
			//省略異常資訊
		}
		try {
		    //真正建立Bean的邏輯 範例化Bean物件，為Bean屬性賦值等，這裡暫不展開
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			//省略紀錄檔輸出
			return beanInstance;
		}
		catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {//省略異常資訊
	}

resolveBeforeInstantiation這個方法在BeanPostProcessor淺析這一節分析過了這裡不再具體展開了。

如果InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation 方法返回null，那麼將不會中斷正常Bean建立過程。
下面就來到的Bean範例化部分了。

範例化階段

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
		//解析BeanClass，在BeanDefinition中類資訊是以字串形式展現，這裡解析到字串後 會將其載入為Class
		Class<?> beanClass = resolveBeanClass(mbd, beanName);
		if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
			throw new BeanCreationException(mbd.getResourceDescription(), beanName,
					"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
		}
		//如果設定了 Supplier 回撥，則使用給定的回撥方法初始化策略，通過獲取Supplier#get得到範例化物件 
		Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
		if (instanceSupplier != null) {
			return obtainFromSupplier(instanceSupplier, beanName);
		}
 		//如果工廠方法不為空，則使用工廠方法初始化
		if (mbd.getFactoryMethodName() != null) {
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}
		
		boolean resolved = false;
		boolean autowireNecessary = false;
		if (args == null) {
           //加鎖
			synchronized (mbd.constructorArgumentLock) {
            //條件成立 說明建構函式或FactoryMethod已經被解析並被快取，可直接利用建構函式解析
           //與後面的SimpleInstantiationStrategy#instantiate呼應
				if (mbd.resolvedConstructorOrFactoryMethod != null) {
					resolved = true;
					autowireNecessary = mbd.constructorArgumentsResolved;
				}
			}
		}
    //如果已經被解析過
		if (resolved) {
          //條件成立 使用建構函式注入
			if (autowireNecessary) {
				return autowireConstructor(beanName, mbd, null, null);
			}
			else {
        //使用預設建構函式解析
				return instantiateBean(beanName, mbd);
			}
		}
    // 使用SmartInstantiationAwareBeanPostProcessor 找到候選的建構函式 用於注入
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    // AutowireMode==AUTOWIRE_CONSTRUCTOR 條件成立 說明使用基於建構函式的注入方式 (預設是AUTOWIRE_NO,需要動態檢測)
    // mbd.hasConstructorArgumentValues() 條件成立 說明建構函式中擁有引數
		if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
      //基於建構函式自動注入
			return autowireConstructor(beanName, mbd, ctors, args);
		}
		// 如果mbd中設定了建構函式 則使用它進行注入
		ctors = mbd.getPreferredConstructors();
		if (ctors != null) {
			return autowireConstructor(beanName, mbd, ctors, null);
		}
		// 使用預設建構函式範例化
		return instantiateBean(beanName, mbd);
	}

上面將doCreateBean精簡一下，只暴露出我們比較關係的部分。一目瞭然，Bean的範例化過程就藏在createBeanInstance方法中。

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
		//解析BeanClass，在BeanDefinition中類資訊是以字串形式展現，這裡解析到字串後 會將其載入為Class
		Class<?> beanClass = resolveBeanClass(mbd, beanName);
		if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
			throw new BeanCreationException(mbd.getResourceDescription(), beanName,
					"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
		}
		//如果設定了 Supplier 回撥，則使用給定的回撥方法初始化策略，通過獲取Supplier#get得到範例化物件 
		Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
		if (instanceSupplier != null) {
			return obtainFromSupplier(instanceSupplier, beanName);
		}
 		//如果工廠方法不為空，則使用工廠方法初始化
		if (mbd.getFactoryMethodName() != null) {
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}
		
		boolean resolved = false;
		boolean autowireNecessary = false;
		if (args == null) {
           //加鎖
			synchronized (mbd.constructorArgumentLock) {
            //條件成立 說明建構函式或FactoryMethod已經被解析並被快取，可直接利用建構函式解析
           //與後面的SimpleInstantiationStrategy#instantiate呼應
				if (mbd.resolvedConstructorOrFactoryMethod != null) {
					resolved = true;
					autowireNecessary = mbd.constructorArgumentsResolved;
				}
			}
		}
    //如果已經被解析過
		if (resolved) {
          //條件成立 使用建構函式注入
			if (autowireNecessary) {
				return autowireConstructor(beanName, mbd, null, null);
			}
			else {
        //使用預設建構函式解析
				return instantiateBean(beanName, mbd);
			}
		}
    // 使用SmartInstantiationAwareBeanPostProcessor 找到候選的建構函式 用於注入
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    // AutowireMode==AUTOWIRE_CONSTRUCTOR 條件成立 說明使用基於建構函式的注入方式 (預設是AUTOWIRE_NO,需要動態檢測)
    // mbd.hasConstructorArgumentValues() 條件成立 說明建構函式中擁有引數
		if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
      //基於建構函式自動注入
			return autowireConstructor(beanName, mbd, ctors, args);
		}
		// 如果mbd中設定了建構函式 則使用它進行注入
		ctors = mbd.getPreferredConstructors();
		if (ctors != null) {
			return autowireConstructor(beanName, mbd, ctors, null);
		}
		// 使用預設建構函式範例化
		return instantiateBean(beanName, mbd);
	}

分析了上面的原始碼之後，我們試著總結一下上面程式碼主要完成的事情：

1、如果mbd設定了instanceSupplier回撥，則使用instanceSupplier去初始化BeanDefinition

2、如果mbd設定了工廠方法，則使用工廠方法區初始化BeanDefinition

3、範例化BeanDefinition

如果mbd已經被解析過了，則根據快取選擇使用有參建構函式注入還是預設建構函式注入
如果mbd沒有被解析過，找到mbd中候選的建構函式(一個類可能有多個建構函式)，再根據一些限定條件選擇是基於有參建構函式初始化還是預設建構函式初始化

針對第1點，其實就是lambda8 supplier介面的使用，不再介紹。

針對第3點，其實就是通過反射機制建立範例物件，最終呼叫了SimpleInstantiationStrategy#instantiate方法

針對第2點舉例說明下 工廠方法與靜態工廠生成Bean的兩種形式，再來展開說下instantiateUsingFactoryMethod原始碼。

設定Xml檔案

<?xml version="1.0" encoding="UTF-8"?>
<beans
        xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">
    <bean id="peopleFactory" class="com.wojiushiwo.factorymethod.PeopleFactory"/>
    <!--實體方法-->
    <bean id="instanceMethod" factory-bean="peopleFactory" factory-method="createPeople"/>
		<!--靜態方法-->
    <bean id="staticFactoryMethod" class="com.wojiushiwo.factorymethod.People" factory-method="createPeople"/>
</beans>

//實體物件
@Data
public class People implements Serializable {
    private String name;
    private Integer age;
    public People() {
    }
    public static People createPeople() {
        People people = new People();
        people.setAge(18);
        people.setName("我就是我");
        return people;
    }
}
//People工廠類
public class PeopleFactory {
    public People createPeople() {
        return People.createPeople();
    }
}
public class FactoryMethodDemo {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context=new ClassPathXmlApplicationContext("META-INF/spring.xml");
        context.refresh();
        People people = (People) context.getBean("staticFactoryMethod");
        System.out.println(people);
		People people = (People) context.getBean("instanceMethod");
        System.out.println(people);		
        context.close();
    }
}

public BeanWrapper instantiateUsingFactoryMethod(
			String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {
		BeanWrapperImpl bw = new BeanWrapperImpl();
		this.beanFactory.initBeanWrapper(bw);
		Object factoryBean;
		Class<?> factoryClass;
		boolean isStatic;
		//獲取FactoryBeanName，實體方法與靜態工廠方法的區別就在於有沒有FactoryBeanName
		String factoryBeanName = mbd.getFactoryBeanName();
		if (factoryBeanName != null) {
      //如果存在FactoryBeanName，則說明是實體方法
			if (factoryBeanName.equals(beanName)) {
				throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
						"factory-bean reference points back to the same bean definition");
			}
      //獲取當前factoryBeanName名稱的Bean
			factoryBean = this.beanFactory.getBean(factoryBeanName);
			if (mbd.isSingleton() && this.beanFactory.containsSingleton(beanName)) {
				throw new ImplicitlyAppearedSingletonException();
			}
      //獲取工廠類Class
			factoryClass = factoryBean.getClass();
      //標記為非靜態
			isStatic = false;
		}
		else {
			// 走到這裡，說明是靜態工廠方法
			if (!mbd.hasBeanClass()) {
				throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
						"bean definition declares neither a bean class nor a factory-bean reference");
			}
      //factoryBean設定為null
			factoryBean = null;
      //獲取工廠類Class，這裡使用BeanDefinition作為其工廠類
			factoryClass = mbd.getBeanClass();
      //標記為非靜態
			isStatic = true;
		}
		Method factoryMethodToUse = null;
		ArgumentsHolder argsHolderToUse = null;
		Object[] argsToUse = null;
		//explicitArgs 這個是getBean方法傳遞過來的，一般為null
		if (explicitArgs != null) {
			argsToUse = explicitArgs;
		}
		else {
			Object[] argsToResolve = null;
      //加鎖
			synchronized (mbd.constructorArgumentLock) {
        //獲取被解析的工廠方法
				factoryMethodToUse = (Method) mbd.resolvedConstructorOrFactoryMethod;
        //條件成立 說明工廠方法已經被解析過了，並存到了mbd中快取起來了
				if (factoryMethodToUse != null && mbd.constructorArgumentsResolved) {
					// Found a cached factory method...
					argsToUse = mbd.resolvedConstructorArguments;
					if (argsToUse == null) {
						argsToResolve = mbd.preparedConstructorArguments;
					}
				}
			}
			if (argsToResolve != null) {
				argsToUse = resolvePreparedArguments(beanName, mbd, bw, factoryMethodToUse, argsToResolve, true);
			}
		}
		if (factoryMethodToUse == null || argsToUse == null) {
      //獲取工廠類
			factoryClass = ClassUtils.getUserClass(factoryClass);
			//獲取類中的方法
			Method[] rawCandidates = getCandidateMethods(factoryClass, mbd);
			List<Method> candidateList = new ArrayList<>();
			for (Method candidate : rawCandidates) {
        //如果方法修飾符包含static，並且方法名稱是設定的FactoryMethod，則新增到候選集合中
				if (Modifier.isStatic(candidate.getModifiers()) == isStatic && mbd.isFactoryMethod(candidate)) {
					candidateList.add(candidate);
				}
			}
		  //如果候選集合有1個元素 並且BeanDefinition中未設定構造引數 (explicitArgs一般都為null )
			if (candidateList.size() == 1 && explicitArgs == null && !mbd.hasConstructorArgumentValues()) {
        //獲取方法
				Method uniqueCandidate = candidateList.get(0);
        //如果方法引數為空
				if (uniqueCandidate.getParameterCount() == 0) {
					mbd.factoryMethodToIntrospect = uniqueCandidate;
					synchronized (mbd.constructorArgumentLock) {
            //將下面這些全快取到mbd中，下次直接用(與createBeanInstance方法呼應上了)
						mbd.resolvedConstructorOrFactoryMethod = uniqueCandidate;
						mbd.constructorArgumentsResolved = true;
						mbd.resolvedConstructorArguments = EMPTY_ARGS;
					}
          //範例化bd，設定到BeanWrapper中
					bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, uniqueCandidate, EMPTY_ARGS));
					return bw;
				}
			}
			//程式走到這裡 大概率是BeanDefinition中設定了構造引數
			Method[] candidates = candidateList.toArray(new Method[0]);
      //按照修飾符及方法引數 進行排序
			AutowireUtils.sortFactoryMethods(candidates);
			ConstructorArgumentValues resolvedValues = null;
      //是否建構函式注入
			boolean autowiring = (mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
			int minTypeDiffWeight = Integer.MAX_VALUE;
			Set<Method> ambiguousFactoryMethods = null;
      //最小引數數量 預設是0
			int minNrOfArgs;
			if (explicitArgs != null) {
				minNrOfArgs = explicitArgs.length;
			}
			else {
        //走到這裡 說明explicitArgs未被設定引數
        //如果bd設定了構造引數，則從bd中解析引數
				if (mbd.hasConstructorArgumentValues()) {
					ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
					resolvedValues = new ConstructorArgumentValues();
          //得到解析的最小引數數量
					minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
				}
				else {
					minNrOfArgs = 0;
				}
			}
			LinkedList<UnsatisfiedDependencyException> causes = null;
			//下面主要是推斷引數、FactoryMethod，程式碼比較長 就先不分析了
			for (Method candidate : candidates) {
				Class<?>[] paramTypes = candidate.getParameterTypes();
				if (paramTypes.length >= minNrOfArgs) {
					ArgumentsHolder argsHolder;
					if (explicitArgs != null) {
						// Explicit arguments given -> arguments length must match exactly.
						if (paramTypes.length != explicitArgs.length) {
							continue;
						}
						argsHolder = new ArgumentsHolder(explicitArgs);
					}
					else {
						// Resolved constructor arguments: type conversion and/or autowiring necessary.
						try {
							String[] paramNames = null;
							ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
							if (pnd != null) {
								paramNames = pnd.getParameterNames(candidate);
							}
							argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw,
									paramTypes, paramNames, candidate, autowiring, candidates.length == 1);
						}
						catch (UnsatisfiedDependencyException ex) {
							if (logger.isTraceEnabled()) {
								logger.trace("Ignoring factory method [" + candidate + "] of bean '" + beanName + "': " + ex);
							}
							// Swallow and try next overloaded factory method.
							if (causes == null) {
								causes = new LinkedList<>();
							}
							causes.add(ex);
							continue;
						}
					}
					int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
							argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
					// Choose this factory method if it represents the closest match.
					if (typeDiffWeight < minTypeDiffWeight) {
						factoryMethodToUse = candidate;
						argsHolderToUse = argsHolder;
						argsToUse = argsHolder.arguments;
						minTypeDiffWeight = typeDiffWeight;
						ambiguousFactoryMethods = null;
					}
					// Find out about ambiguity: In case of the same type difference weight
					// for methods with the same number of parameters, collect such candidates
					// and eventually raise an ambiguity exception.
					// However, only perform that check in non-lenient constructor resolution mode,
					// and explicitly ignore overridden methods (with the same parameter signature).
					else if (factoryMethodToUse != null && typeDiffWeight == minTypeDiffWeight &&
							!mbd.isLenientConstructorResolution() &&
							paramTypes.length == factoryMethodToUse.getParameterCount() &&
							!Arrays.equals(paramTypes, factoryMethodToUse.getParameterTypes())) {
						if (ambiguousFactoryMethods == null) {
							ambiguousFactoryMethods = new LinkedHashSet<>();
							ambiguousFactoryMethods.add(factoryMethodToUse);
						}
						ambiguousFactoryMethods.add(candidate);
					}
				}
			}
			if (factoryMethodToUse == null) {
				if (causes != null) {
					UnsatisfiedDependencyException ex = causes.removeLast();
					for (Exception cause : causes) {
						this.beanFactory.onSuppressedException(cause);
					}
					throw ex;
				}
				List<String> argTypes = new ArrayList<>(minNrOfArgs);
				if (explicitArgs != null) {
					for (Object arg : explicitArgs) {
						argTypes.add(arg != null ? arg.getClass().getSimpleName() : "null");
					}
				}
				else if (resolvedValues != null) {
					Set<ValueHolder> valueHolders = new LinkedHashSet<>(resolvedValues.getArgumentCount());
					valueHolders.addAll(resolvedValues.getIndexedArgumentValues().values());
					valueHolders.addAll(resolvedValues.getGenericArgumentValues());
					for (ValueHolder value : valueHolders) {
						String argType = (value.getType() != null ? ClassUtils.getShortName(value.getType()) :
								(value.getValue() != null ? value.getValue().getClass().getSimpleName() : "null"));
						argTypes.add(argType);
					}
				}
				String argDesc = StringUtils.collectionToCommaDelimitedString(argTypes);
				//丟擲異常
			}
			else if (void.class == factoryMethodToUse.getReturnType()) {
				//丟擲異常
			}
			else if (ambiguousFactoryMethods != null) {
				//丟擲異常
			}
			if (explicitArgs == null && argsHolderToUse != null) {
				mbd.factoryMethodToIntrospect = factoryMethodToUse;
				argsHolderToUse.storeCache(mbd, factoryMethodToUse);
			}
		}
		Assert.state(argsToUse != null, "Unresolved factory method arguments");
    //範例化bd 設定到BeanWrapper中
		bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, factoryMethodToUse, argsToUse));
		return bw;
	}

至此經過createBeanInstance方法就為我們建立了一個範例物件，但是現在這個物件屬性還未被賦值。

範例化後階段

範例物件建立之後，就來到了物件屬性賦值過程了，我們大致看一下populateBean方法，觀察下InstantiationAwareBeanPostProcessor對屬性賦值過程的影響

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
		//省略無關程式碼
		boolean continueWithPropertyPopulation = true;
		//條件一 synthetic預設值是false 一般都會成立
		//條件二 成立的話 說明存在InstantiationAwareBeanPostProcessor
		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
			for (BeanPostProcessor bp : getBeanPostProcessors()) {
				if (bp instanceof InstantiationAwareBeanPostProcessor) {
					InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
					//在BeanPostProcessor淺析中分析到此方法時說過，若Bean範例化後回撥不返回true 則對屬性賦值過程產生影響。以下程式碼就是說明
					if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
						continueWithPropertyPopulation = false;
						break;
					}
				}
			}
		}
		//ibp.postProcessAfterInstantiation=false時 屬性賦值過程終止
		if (!continueWithPropertyPopulation) {
			return;
		}

總結

本篇文章就到這裡了，希望能夠給你帶來幫助，也希望您能夠多多關注it145.com的更多內容!

Spring Bean生命週期之Bean的範例化詳解

目錄

前言

範例化前階段

範例化階段

範例化後階段

總結

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