面经手册 · 第30篇《关于 Spring 中 getBean 的全流程源码解析》

@Test
public void test_getBean() {
    BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config.xml");
    UserDao userDao = beanFactory.getBean("userDao", UserDao.class);
    logger.info("获取 Bean：{}", userDao);
}

@Override
public <T> T getBean(String name, Class<T> requiredType) throws BeansException { 
    // getBean 就像你的领导其实没做啥，都在 doGetBean 里
	return doGetBean(name, requiredType, null, false);
}

protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
		throws BeansException {     
    
    // 处理别名BeanName、处理带&符的工厂BeanName
	final String beanName = transformedBeanName(name);
	Object bean;  

	// 先尝试从缓存中获取Bean实例，这个位置就是三级缓存解决循环依赖的方法
	Object sharedInstance = getSingleton(beanName);   

	if (sharedInstance != null && args == null) {
		if (logger.isDebugEnabled()) {
			if (isSingletonCurrentlyInCreation(beanName)) {
				logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
						"' that is not fully initialized yet - a consequence of a circular reference");
			}
			else {
				logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
			}
		}        
        
        // 1. 如果 sharedInstance 是普通的 Bean 实例，则下面的方法会直接返回
        // 2. 如果 sharedInstance 是工厂Bean类型，则需要获取 getObject 方法，可以参考关于 FactoryBean 的实现类 
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}
	else {
		
        // 循环依赖有三种，setter注入、多实例和构造函数，Spring 只能解决 setter 注入，所以这里是 Prototype 则会抛出异常
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}    

		// 1. 父 bean 工厂存在
        // 2. 当前 bean 不存在于当前bean工厂，则到父工厂查找 bean 实例
		BeanFactory parentBeanFactory = getParentBeanFactory();
		if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
			// 获取 name 对应的 beanName，如果 name 是以 & 开头，则返回 & + beanName
			String nameToLookup = originalBeanName(name);         
            
            // 根据 args 参数是否为空，调用不同的父容器方法获取 bean 实例
			if (args != null) {
				return (T) parentBeanFactory.getBean(nameToLookup, args);
			}
			else {
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			}
		}       

        // 1. typeCheckOnly，用于判断调用 getBean 方法时，是否仅是做类型检查
        // 2. 如果不是只做类型检查，就会调用 markBeanAsCreated 进行记录
		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}
		try {    
    
            // 从容器 getMergedLocalBeanDefinition 获取 beanName 对应的 GenericBeanDefinition，转换为 RootBeanDefinition
			final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); 
            // 检查当前创建的 bean 定义是否为抽象 bean 定义
			checkMergedBeanDefinition(mbd, beanName, args);
			
            // 处理使用了 depends-on 注解的依赖创建 bean 实例
			String[] dependsOn = mbd.getDependsOn();
			if (dependsOn != null) {
				for (String dep : dependsOn) {   
                    // 监测是否存在 depends-on 循环依赖，若存在则会抛出异常
					if (isDependent(beanName, dep)) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
					}       
                    
                    // 注册依赖记录
					registerDependentBean(dep, beanName);
					try {    
					    // 加载 depends-on 依赖（dep 是 depends-on 缩写）
						getBean(dep);
					}
					catch (NoSuchBeanDefinitionException ex) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
					}
				}
			}  

			// 创建单例 bean 实例
			if (mbd.isSingleton()) {    

			    // 把 beanName 和 new ObjectFactory 匿名内部类传入回调
				sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
					@Override
					public Object getObject() throws BeansException {
						try {    
                            // 创建 bean
							return createBean(beanName, mbd, args);
						}
						catch (BeansException ex) {
							// 创建失败则销毁
							destroySingleton(beanName);
							throw ex;
						}
					}
				});
				bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}      
            // 创建其他类型的 bean 实例
			else if (mbd.isPrototype()) {
				// It's a prototype -> create a new instance.
				Object prototypeInstance = null;
				try {
					beforePrototypeCreation(beanName);
					prototypeInstance = createBean(beanName, mbd, args);
				}
				finally {
					afterPrototypeCreation(beanName);
				}
				bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			}
			else {
				String scopeName = mbd.getScope();
				final Scope scope = this.scopes.get(scopeName);
				if (scope == null) {
					throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
				}
				try {
					Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
						@Override
						public Object getObject() throws BeansException {
							beforePrototypeCreation(beanName);
							try {
								return createBean(beanName, mbd, args);
							}
							finally {
								afterPrototypeCreation(beanName);
							}
						}
					});
					bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
				}
				catch (IllegalStateException ex) {
					throw new BeanCreationException(beanName,
							"Scope '" + scopeName + "' is not active for the current thread; consider " +
							"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
							ex);
				}
			}
		}
		catch (BeansException ex) {
			cleanupAfterBeanCreationFailure(beanName);
			throw ex;
		}
	}
	// 如果需要类型转换，这里会进行操作
	if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
		try {
			return getTypeConverter().convertIfNecessary(bean, requiredType);
		}
		catch (TypeMismatchException ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Failed to convert bean '" + name + "' to required type '" +
						ClassUtils.getQualifiedName(requiredType) + "'", ex);
			}
			throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
		}
	}    

    // 返回 Bean
	return (T) bean;
}

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

public String canonicalName(String name) {
	String canonicalName = name;
	// Handle aliasing...
	String resolvedName;
	do {
		resolvedName = this.aliasMap.get(canonicalName);
		if (resolvedName != null) {
			canonicalName = resolvedName;
		}
	}
	while (resolvedName != null);
	return canonicalName;
}

protected boolean isDependent(String beanName, String dependentBeanName) {
	synchronized (this.dependentBeanMap) {
		return isDependent(beanName, dependentBeanName, null);
	}

private boolean isDependent(String beanName, String dependentBeanName, Set<String> alread
	if (alreadySeen != null && alreadySeen.contains(beanName)) {
		return false;
	}
	String canonicalName = canonicalName(beanName);
	Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
	if (dependentBeans == null) {
		return false;
	}
	if (dependentBeans.contains(dependentBeanName)) {
		return true;
	}
	for (String transitiveDependency : dependentBeans) {
		if (alreadySeen == null) {
			alreadySeen = new HashSet<String>();
		}
		alreadySeen.add(beanName);
		if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) {
			return true;
		}
	}
	return false;
}

public void registerDependentBean(String beanName, String dependentBeanName) {
	String canonicalName = canonicalName(beanName);  

	synchronized (this.dependentBeanMap) {
		Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
		if (dependentBeans == null) {
			dependentBeans = new LinkedHashSet<String>(8);
			this.dependentBeanMap.put(canonicalName, dependentBeans);
		}
		dependentBeans.add(dependentBeanName);
	}   

	synchronized (this.dependenciesForBeanMap) {
		Set<String> dependenciesForBean = this.dependenciesForBeanMap.get(dependentBeanName
		if (dependenciesForBean == null) {
			dependenciesForBean = new LinkedHashSet<String>(8);
			this.dependenciesForBeanMap.put(dependentBeanName, dependenciesForBean);
		}
		dependenciesForBean.add(canonicalName);
	}
}

if (mbd.isSingleton()) {
	sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
		@Override
		public Object getObject() throws BeansException {
			try {
				return createBean(beanName, mbd, args);
			}
			catch (BeansException ex) {
				destroySingleton(beanName);
				throw ex;
			}
		}
	});
	bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
	Assert.notNull(beanName, "'beanName' 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 + "'");
			}
			beforeSingletonCreation(beanName);
			boolean newSingleton = false;
			boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
			if (recordSuppressedExceptions) {
				this.suppressedExceptions = new LinkedHashSet<Exception>();
			}
			try {
				singletonObject = singletonFactory.getObject();
				newSingleton = true;
			}
			catch (IllegalStateException ex) {
				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 != NULL_OBJECT ? singletonObject : null);
	}
}

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // 从 singletonObjects 获取实例，singletonObjects 中缓存的实例都是完全实例化好的 bean，可以直接使用
	Object singletonObject = this.singletonObjects.get(beanName);
    // 如果 singletonObject 为空，则没有创建或创建中
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        // 加锁
		synchronized (this.singletonObjects) {
            // 单例缓存池中，没有当前beanName
			singletonObject = this.earlySingletonObjects.get(beanName);
			if (singletonObject == null && allowEarlyReference) {
				ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
				if (singletonFactory != null) {
                    // 加入到三级缓存，暴漏早期对象用于解决三级缓存
					singletonObject = singletonFactory.getObject();  
					this.earlySingletonObjects.put(beanName, singletonObject);
					this.singletonFactories.remove(beanName);
				}
			}
		}
	}
	return (singletonObject != NULL_OBJECT ? singletonObject : null);
}

protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
    
    // 如果 beanName 以 & 开头，但 beanInstance 却不是 FactoryBean，则会抛出异常
	if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
		throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
	}

    // 这里判断就是这个 bean 是不是 FactoryBean，不是就直接返回了
	if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
		return beanInstance;
	}   

	Object object = null;
	if (mbd == null) {    
        // 如果 mbd 为空，则从缓存加载 bean（FactoryBean 生成的单例 bean 实例会缓存到 factoryBeanObjectCache 集合中，方便使用）
		object = getCachedObjectForFactoryBean(beanName);
	}   

	if (object == null) {
		// 到这，beanInstance 是 FactoryBean 类型，所以就强转了
		FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
		// mbd 为空且判断 containsBeanDefinition 是否包含 beanName
		if (mbd == null && containsBeanDefinition(beanName)) {    
            // 合并 BeanDefinition
			mbd = getMergedLocalBeanDefinition(beanName);
		}    
		boolean synthetic = (mbd != null && mbd.isSynthetic());   
        // 调用 getObjectFromFactoryBean 获取实例 
		object = getObjectFromFactoryBean(factory, beanName, !synthetic);
	}
	return object;
}

@Test
public void test_alias() {
    BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config-alias.xml");
    UserService userService = beanFactory.getBean("userService-alias02", UserService.class);
    logger.info("获取 Bean 通过别名：{}", userService);
}

@Test
public void test_depends_on() {
    BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config-depends-on.xml");
    UserService userService = beanFactory.getBean(UserService.class, "userService");
    logger.info("获取 Bean：{}", userService.getUserDao());
}

@Test
public void test_factory_bean() {
    BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config-factory-bean.xml");
    UserDao userDao = beanFactory.getBean("userDao", UserDao.class);
    logger.info("获取 Bean：{}", userDao);
}