Spring实例化源码解析之Bean的实例化(十二)

前言

本章开始分析finishBeanFactoryInitialization(beanFactory)方法，直译过来就是完成Bean工厂的初始化，这中间就是非lazy单例Bean的实例化流程。ConversionService在第十章已经提前分析了。重点就是最后一句，我们的bean实例化分析就从这里开始。

本章主要是实例化流程的分析，不会太深入到细节

java 复制代码

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		// ConversionService（转换服务）在spring框架中用于处理类型转换的任务。它提供了一种统一的方式来执行各种类型之间的转换操作，
		// 包括字符串到其他类型的转换、日期和时间的转换、数字类型的转换等。
		if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
				beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
			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.
		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.
		beanFactory.freezeConfiguration();

		// Instantiate all remaining (non-lazy-init) singletons.
		beanFactory.preInstantiateSingletons();
	}

预处理单例bean

preInstantiateSingletons() 是 Spring 容器的一个方法，它用于预实例化所有的单例（Singleton）Bean。

java 复制代码

@Override
	public void preInstantiateSingletons() throws BeansException {
		if (logger.isTraceEnabled()) {
			logger.trace("Pre-instantiating singletons in " + this);
		}

		// Iterate over a copy to allow for init methods which in turn register new bean definitions.
		// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
		List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

		// Trigger initialization of all non-lazy singleton beans...
		for (String beanName : beanNames) {
			// 通过bean的名称去获取bean的定义信息
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			// 不是抽象类&&是单例的&&不是懒加载
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				if (isFactoryBean(beanName)) {
					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
					if (bean instanceof FactoryBean) {
						FactoryBean<?> factory = (FactoryBean<?>) bean;
						boolean isEagerInit;
						if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
							isEagerInit = AccessController.doPrivileged(
									(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
									getAccessControlContext());
						}
						else {
							isEagerInit = (factory instanceof SmartFactoryBean &&
									((SmartFactoryBean<?>) factory).isEagerInit());
						}
						if (isEagerInit) {
							getBean(beanName);
						}
					}
				}
				else {
					getBean(beanName);
				}
			}
		}

		// Trigger post-initialization callback for all applicable beans...
		for (String beanName : beanNames) {
			Object singletonInstance = getSingleton(beanName);
			if (singletonInstance instanceof SmartInitializingSingleton) {
				StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
						.tag("beanName", beanName);
				SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
				if (System.getSecurityManager() != null) {
					AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
						smartSingleton.afterSingletonsInstantiated();
						return null;
					}, getAccessControlContext());
				}
				else {
					smartSingleton.afterSingletonsInstantiated();
				}
				smartInitialize.end();
			}
		}
	}

这个预处理单例bean的实例化其实非常简单，我们简要分析一下整体流程：

获取到当前所有的beanDefinitionNames，用于迭代。
循环所有的beanDefinitionNames，也就是beanNamess
获取beanName的定义信息，用于后续判断
如果是抽象类，或者不是单例或者是懒加载bean就不处理。
其他情况下会进入getBean逻辑，这个方法我们经常用来从 BeanFactory 中获取一个 Bean，而初始化的过程也封装到了这个方法里。

getBean/doGetBean

Spring源码中很多时候真正做事的是do开头的，个人经验。

java 复制代码

@Override
	public Object getBean(String name) throws BeansException {
		return doGetBean(name, null, null, false);
	}

transformedBeanName

获取beanName，如果是FactoryBean是会以&开头，也就是说这里返回的就是单纯的beanName

java 复制代码

String beanName = transformedBeanName(name);

	public static String transformedBeanName(String name) {
		Assert.notNull(name, "'name' must not be null");
		if (!name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
			return name;
		}
		return transformedBeanNameCache.computeIfAbsent(name, beanName -> {
			do {
				beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
			}
			while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
			return beanName;
		});
	}

getSingleton

这里就是常说的三级缓存。在实例化的时候先要去三级缓存中查看是否存在了，需不需要走后续的实例化流程。

java 复制代码

Object sharedInstance = getSingleton(beanName);

@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		// Quick check for existing instance without full singleton lock
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			singletonObject = this.earlySingletonObjects.get(beanName);
			if (singletonObject == null && allowEarlyReference) {
				synchronized (this.singletonObjects) {
					// Consistent creation of early reference within full singleton lock
					singletonObject = this.singletonObjects.get(beanName);
					if (singletonObject == null) {
						singletonObject = this.earlySingletonObjects.get(beanName);
						if (singletonObject == null) {
							ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
							if (singletonFactory != null) {
								singletonObject = singletonFactory.getObject();
								this.earlySingletonObjects.put(beanName, singletonObject);
								this.singletonFactories.remove(beanName);
							}
						}
					}
				}
			}
		}
		return singletonObject;
	}

从上述代码中，我们知道有三个Map对象，也就是我们常说的三级缓存，实际上就是以下这三个Map。

java 复制代码

/** Cache of singleton objects: bean name to bean instance. */
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

	/** Cache of singleton factories: bean name to ObjectFactory. */
	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

	/** Cache of early singleton objects: bean name to bean instance. */
	private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

一级缓存：singletonObjects缓存的是单例对象，key是beanName value是bean的实例。
二级缓存：earlySingletonObjects缓存的是早期的单例对象，key是beanName value是bean的实例。
三级缓存：singletonFactories缓存的是ObjectFactory，key是beanName value是ObjectFactory。

回到getSingleton方法，首先spring尝试通过beanName从一级缓存中获取，如果一级缓存中不存在，并且当前bean并不是正在创建中的话直接返回null。只有当bean正在创建中的时候才回去从二级缓存中获取，如果二级缓存中获取不到再从三级缓存中去获取。此处我们先大概有这么一个概念，后续会在循环依赖章节详细描述。

如果当前获取到的sharedInstance不为空且不是FacotyBean的话就直接返回回去了。如果为空继续走后续的逻辑。

dependsOn

根据BeanDefinition获取是否有依赖的bean，如果有的话先把其他bean实例化完成。@DependsOn的注解的作用就在这里可以体现。

java 复制代码

String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
						registerDependentBean(dep, beanName);
						try {
							getBean(dep);
						}
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

singletonsCurrentlyInCreation

如果是单例的，就会调用 getSingleton(String beanName, ObjectFactory<?> singletonFactory)方法。

第一个参数就是beanName
第二个参数是一个FunctionalInterface

java 复制代码

if (mbd.isSingleton()) {
					// 先执行getSingleton方法，然后在代码中会调用createBean方法
					sharedInstance = getSingleton(beanName, () -> {
						try {
							return createBean(beanName, mbd, args);
						}
						catch (BeansException ex) {
							destroySingleton(beanName);
							throw ex;
						}
					});
					beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}

执行getSingleton方法，代码如下

java 复制代码

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(beanName, "Bean name must not be null");
		synchronized (this.singletonObjects) {
			Object singletonObject = this.singletonObjects.get(beanName);
			if (singletonObject == null) {
				if (this.singletonsCurrentlyInDestruction) {
					throw new BeanCreationNotAllowedException(beanName,
							"Singleton bean creation not allowed while singletons of this factory are in destruction " +
							"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
				}
				// 对象是否正在创建中 singletonsCurrentlyInCreation，避免循环依赖的一种操作
				beforeSingletonCreation(beanName);
				boolean newSingleton = false;
				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = new LinkedHashSet<>();
				}
				try {
					// 在这里调用外出的createBean返回的就是springbean，然后后续执行缓存的相关操作
					singletonObject = singletonFactory.getObject();
					newSingleton = true;
				}
				catch (IllegalStateException ex) {
					// Has the singleton object implicitly appeared in the meantime ->
					// if yes, proceed with it since the exception indicates that state.
					singletonObject = this.singletonObjects.get(beanName);
					if (singletonObject == null) {
						throw ex;
					}
				}
                catch (BeanCreationException ex) {
					if (recordSuppressedExceptions) {
						for (Exception suppressedException : this.suppressedExceptions) {
							ex.addRelatedCause(suppressedException);
						}
					}
					throw ex;
				}
				finally {
					if (recordSuppressedExceptions) {
						this.suppressedExceptions = null;
					}
					afterSingletonCreation(beanName);
				}
				if (newSingleton) {
					addSingleton(beanName, singletonObject);
				}
			}
			return singletonObject;
		}
	}

首先这个放回加了一个synchronized的锁，锁的是一级缓存对象，也就是说对象的实例化是线程安全的，不让其在实例化的时候出现重复创建。获取锁之后会再从缓存中去获取当前beanName是否存在实例，如果没有的话会执行beforeSingletonCreation(beanName)。

java 复制代码

protected void beforeSingletonCreation(String beanName) {
		if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}
	}

此处的核心就是singletonsCurrentlyInCreation集合的add方法，就是提前曝光的地方。

java 复制代码

private final Set<String> singletonsCurrentlyInCreation =
			Collections.newSetFromMap(new ConcurrentHashMap<>(16));

直接就是会调用传入的FunctionalInterface，也就是执行createBean方法。

java 复制代码

		try {
					// 在这里调用外出的createBean返回的就是springbean，然后后续执行缓存的相关操作
					singletonObject = singletonFactory.getObject();
					newSingleton = true;
				}

createBean/doCreateBean

在执行doCreateBean的时候会调用构造方法

java 复制代码

if (instanceWrapper == null) {
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}

然后判断beanDefinition信息是单例的，并且是允许循环依赖的（默认为允许）并且当前的bean是正在创建中就会进入if的逻辑。

addSingletonFactory,从名字开发猜测是否和我们的三级缓存有关联。

java 复制代码

boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
			if (logger.isTraceEnabled()) {
				logger.trace("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

addSingletonFactory方法的参数如下：

beanName，bean的名称
ObjectFactory，函数式接口，之前看getSingleton(name)方法的时候也知道，会调用这个getObject方法来获取对象放入二级缓存中。

一级缓存中不存在就将这个ObjectFactory放入三级缓存，同时清理掉二级缓存和加入已注册的有序集合中。

java 复制代码

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(singletonFactory, "Singleton factory must not be null");
		synchronized (this.singletonObjects) {
			if (!this.singletonObjects.containsKey(beanName)) {
				
				this.singletonFactories.put(beanName, singletonFactory);
				
				this.earlySingletonObjects.remove(beanName);
				
				this.registeredSingletons.add(beanName);
			}
		}
	}

populateBean

填充bean，也就是属性注入。这里会去解析bean的依赖，从而继续去走getBean的逻辑来让当前bean实例化完整。

java 复制代码

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
		if (bw == null) {
			if (mbd.hasPropertyValues()) {
				throw new BeanCreationException(
						mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
			}
			else {
				// property mean 属性
				// Skip property population phase for null instance.
				return;
			}
		}

		// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
		// state of the bean before properties are set. This can be used, for example,
		// to support styles of field injection.
		// 在属性填充之前给任意InstantiationAwareBeanPostProcessors一个机会去修改bean的状态
		// 后置处理器可以被使用在例如:支持字段注入的样式.
		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
			for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
				if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
					return;
				}
			}
		}


     	PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

		int resolvedAutowireMode = mbd.getResolvedAutowireMode();
		if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
			MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
			// Add property values based on autowire by name if applicable.
			if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
				autowireByName(beanName, mbd, bw, newPvs);
			}
			// Add property values based on autowire by type if applicable.
			if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
				autowireByType(beanName, mbd, bw, newPvs);
			}
			pvs = newPvs;
		}

		boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
		boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);

		PropertyDescriptor[] filteredPds = null;
		if (hasInstAwareBpps) {
			if (pvs == null) {
				pvs = mbd.getPropertyValues();
			}
			// 实例化感知的BeanPostProcessor
			for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
				// 默认的情况下resolvedAutowireMode是为0的，所以默认是AutoWiredAnnotationBeanPostProcessor进行自动装配
  				PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
				if (pvsToUse == null) {
					if (filteredPds == null) {
						filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
					}
					// 在这里面回去校验属性的版本信息（BeanNotOfRequiredTypeException）
					/**
					 *Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException:
					 * Error creating bean with name 'b': Unsatisfied dependency expressed through field 'a';
					 * nested exception is org.springframework.beans.factory.BeanNotOfRequiredTypeException:
					 * Bean named 'a' is expected to be of type 'com.qhyu.cloud.circlarRefrence.A' but was actually of type 'com.sun.proxy.$Proxy34'
					 */
					pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
					if (pvsToUse == null) {
						return;
					}
				}
				pvs = pvsToUse;
			}
		}
		if (needsDepCheck) {
			if (filteredPds == null) {
				filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
			}
			checkDependencies(beanName, mbd, filteredPds, pvs);
		}

		if (pvs != null) {
			// 属性填充的最后回对spel进行解析
			applyPropertyValues(beanName, mbd, bw, pvs);
		}
	}

initializeBean

如果 bean 实现了 BeanNameAware、BeanClassLoaderAware 或 BeanFactoryAware 接口，回调invokeAwareMethods(beanName, bean)

处理 bean 中定义的 init-method，或者如果 bean 实现了 InitializingBean 接口，调用 afterPropertiesSet() 方法

java 复制代码

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
		if (System.getSecurityManager() != null) {
			AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
				invokeAwareMethods(beanName, bean);
				return null;
			}, getAccessControlContext());
		}
		else {
			invokeAwareMethods(beanName, bean);
		}

		Object wrappedBean = bean;
		if (mbd == null || !mbd.isSynthetic()) {
			// 先执行beanpostProcessor的before
			// 通过BeanPostProcessor在bean初始化之前做点事情
			wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
		}

		try {
			// 再执行initmethods
			// 调用bean的初始化方法进行初始化
			invokeInitMethods(beanName, wrappedBean, mbd);
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					(mbd != null ? mbd.getResourceDescription() : null),
					beanName, "Invocation of init method failed", ex);
		}
		if (mbd == null || !mbd.isSynthetic()) {
			// 再执行BeanPostProcessor的After方法
			// 通过BeanPostProcessor在bean初始化之后做点事情
			// aop的切入点
			wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
		}

		return wrappedBean;
	}

getSingleton

如果创建没有任何问题，newSingleton = true，继续执行 addSingleton(beanName, singletonObject)方法

java 复制代码

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "Bean name must not be null");
    synchronized (this.singletonObjects) {
       Object singletonObject = this.singletonObjects.get(beanName);
       if (singletonObject == null) {
          if (this.singletonsCurrentlyInDestruction) {
             throw new BeanCreationNotAllowedException(beanName,
                   "Singleton bean creation not allowed while singletons of this factory are in destruction " +
                   "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
          }
          if (logger.isDebugEnabled()) {
             logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
          }
          // 对象是否正在创建中 singletonsCurrentlyInCreation，避免循环依赖的一种操作
          beforeSingletonCreation(beanName);
          boolean newSingleton = false;
          boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
          if (recordSuppressedExceptions) {
             this.suppressedExceptions = new LinkedHashSet<>();
          }
          try {
             // 在这里调用外出的createBean返回的就是springbean，然后后续执行缓存的相关操作
             singletonObject = singletonFactory.getObject();
             newSingleton = true;
          }
          catch (IllegalStateException ex) {
             // Has the singleton object implicitly appeared in the meantime ->
             // if yes, proceed with it since the exception indicates that state.
             singletonObject = this.singletonObjects.get(beanName);
             if (singletonObject == null) {
                throw ex;
             }
          }
          catch (BeanCreationException ex) {
             if (recordSuppressedExceptions) {
                for (Exception suppressedException : this.suppressedExceptions) {
                   ex.addRelatedCause(suppressedException);
                }
             }
             throw ex;
          }
          finally {
             if (recordSuppressedExceptions) {
                this.suppressedExceptions = null;
             }
             afterSingletonCreation(beanName);
          }
          if (newSingleton) {
             addSingleton(beanName, singletonObject);
          }
       }
       return singletonObject;
    }
}

将创建好的bean放入一级缓存，移除一二级缓存。

java 复制代码

protected void addSingleton(String beanName, Object singletonObject) {
		// 线程安全的
		synchronized (this.singletonObjects) {
			// 放入一级缓存
			this.singletonObjects.put(beanName, singletonObject);
			// 移除三级缓存
			this.singletonFactories.remove(beanName);
			// 移除二级缓存
			this.earlySingletonObjects.remove(beanName);
			// 已注册的map中把新初始化的bean放入
			this.registeredSingletons.add(beanName);
		}
	}

总结

整个bean的初始化流程就如下图所示，下面的这个图是一个最简单的单例bean的实例化流程，不涉及到循环依赖问题，循环依赖和三级缓存将在下一章详细分析。本章主要了解bean实例化的整体流程，了解和熟悉spring工作模式。