Spring源码学习--Bean对象初始化（二）

水深无声 2022-06-13 05:38 198阅读 0赞

即使我们在不了解Spring对bean的初始化机制，我们也可以根据Java语言的特性猜测到其很有可能是通过反射机制来完成Bean的初始化操作，接下来我们一步一步的剖析Spring对Bean的初始化操作。

首先Spring会通过调用 getBean(String name)来获取Bean，在获取bean的过程中完成的Bean的初始化操作。

AbstractBeanFactory类中：

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

在doGetBean方法中会判断bean的类型单例、原型，如果是单例则需要判断是否已经初始化bean并添加到缓存中，如果是原型bean则需要重新初始化bean，由于Spring对beanName提供了别名机制，所有需要通过beanName获取最终的bean名称。

@SuppressWarnings("unchecked")
    	protected <T> T doGetBean(
    			final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
    			throws BeansException {
    
    		//查找name是否有别名，获取最终的beanName
    		final String beanName = transformedBeanName(name);
    		Object bean;
    
    		//如果bean是单例模式，首先尝试从缓存中获取
    		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 + "'");
    				}
    			}
    			//会判断bean是否是FactoryBean，如果不是直接返回sharedInstance，否则调用sharedInstance.getObject()方法返回bean
    			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    		}
    
    		else {
    			if (isPrototypeCurrentlyInCreation(beanName)) {
    				throw new BeanCurrentlyInCreationException(beanName);
    			}
    			//判断容器是否有服容器，如果有则首先尝试从父容器中获取
    			BeanFactory parentBeanFactory = getParentBeanFactory();
    			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
    				// Not found -> check parent.
    				String nameToLookup = originalBeanName(name);
    				if (args != null) {
    					// Delegation to parent with explicit args.
    					return (T) parentBeanFactory.getBean(nameToLookup, args);
    				}
    				else {
    					// No args -> delegate to standard getBean method.
    					return parentBeanFactory.getBean(nameToLookup, requiredType);
    				}
    			}
    
    			if (!typeCheckOnly) {
    				markBeanAsCreated(beanName);
    			}
    
    			try {
    				//根据beanName获取bean的元数据
    				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
    				checkMergedBeanDefinition(mbd, beanName, args);
    
    				// Guarantee initialization of beans that the current bean depends on.
    				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);
    						getBean(dep);
    					}
    				}
    
    				// bean是单例
    				if (mbd.isSingleton()) {
    					//获取bean，首先会暴露一个ObjectFactory，通过调用getObject来调用createBean(beanName, mbd, args)获取bean
    					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;
    							}
    						}
    					});
    					//会判断bean是否是FactoryBean，如果不是直接返回sharedInstance，否则调用sharedInstance.getObject()方法返回bean
    					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
    				}
    				//bean是原型
    				else if (mbd.isPrototype()) {
    					// It's a prototype -> create a new instance.
    					Object prototypeInstance = null;
    					try {
    						beforePrototypeCreation(beanName);
    						//初始化bean
    						prototypeInstance = createBean(beanName, mbd, args);
    					}
    					finally {
    						afterPrototypeCreation(beanName);
    					}
    					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
    				}
    				//bean是其他模式
    				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;
    			}
    		}
    		//省略代码..........
    		return (T) bean;
    	}

接下来我们看看在createBean(String beanName, RootBeanDefinition mbd, Object\[\] args)中进行的处理操作，在createBean中会通过BeanDefinition中获取类名resolvedClass（反射机制重要的一个参数），并且会调用resolveBeforeInstantiation在bean初始的过程中做一些预处理（简单来说就是BeanPostProcessor的实现类所做的处理操作，参考博客 [Spring源码学习--BeanPostProcessor][Spring_--BeanPostProcessor]）。

AbstractAutowireCapableBeanFactory：

@Override
    	protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
    		if (logger.isDebugEnabled()) {
    			logger.debug("Creating instance of bean '" + beanName + "'");
    		}
    		RootBeanDefinition mbdToUse = mbd;
    
    		//获取类名，通过反射机制来实例化类
    		Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
    		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
    			mbdToUse = new RootBeanDefinition(mbd);
    			//设置类名
    			mbdToUse.setBeanClass(resolvedClass);
    		}
    
    		// Prepare method overrides.
    		try {
    			mbdToUse.prepareMethodOverrides();
    		}
    		catch (BeanDefinitionValidationException ex) {
    			throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
    					beanName, "Validation of method overrides failed", ex);
    		}
    
    		try {
    			// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
    			//如果bean想要在初始化后使用之前做一些预处理操作
    			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
    			if (bean != null) {
    				return bean;
    			}
    		}
    		catch (Throwable ex) {
    			throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
    					"BeanPostProcessor before instantiation of bean failed", ex);
    		}
    		//初始化bean
    		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
    		if (logger.isDebugEnabled()) {
    			logger.debug("Finished creating instance of bean '" + beanName + "'");
    		}
    		return beanInstance;
    	}

doCreateBean中完成的处理操作就比较多了，简单来说调用了几个方法完成了bean的初始化操作：

createBeanInstance完成通过构造函数初始化bean的操作；

addSingletonFactory完成将初始化的bean提前暴露出去，这样就解决了单例bean非构造函数的循环引用问题；

populateBean完成bean的属性注入操作，通过set方法或者注解注入属性；

initializeBean完成了bean注入时设置的init-method方法的执行，同时在执行init-method之前会调用applyBeanPostProcessorsBeforeInitialization完成bean使用前的处理操作，调用applyBeanPostProcessorsAfterInitialization完成bean初始化后的操作；

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
    			throws BeanCreationException {
    		BeanWrapper instanceWrapper = null;
    		if (mbd.isSingleton()) {
    			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    		}
    		//初始化bean实例
    		if (instanceWrapper == null) {
    			instanceWrapper = createBeanInstance(beanName, mbd, args);
    		}
    		final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
    		Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
    		synchronized (mbd.postProcessingLock) {
    			if (!mbd.postProcessed) {
    				try {
    					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
    				}
    				catch (Throwable ex) {
    					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
    							"Post-processing of merged bean definition failed", ex);
    				}
    				mbd.postProcessed = true;
    			}
    		}
    
    		//将初始化的bean提前暴露出去，暴露一个ObjectFactory，这也是Spring解决单例bean非构造函数依赖的解决方法
    		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
    				isSingletonCurrentlyInCreation(beanName));
    		if (earlySingletonExposure) {
    			if (logger.isDebugEnabled()) {
    				logger.debug("Eagerly caching bean '" + beanName +
    						"' to allow for resolving potential circular references");
    			}
    			addSingletonFactory(beanName, new ObjectFactory<Object>() {
    				@Override
    				public Object getObject() throws BeansException {
    					return getEarlyBeanReference(beanName, mbd, bean);
    				}
    			});
    		}
    
    		// Initialize the bean instance.
    		Object exposedObject = bean;
    		try {
    			//初始化bean的各种注入或者setXX参数
    			populateBean(beanName, mbd, instanceWrapper);
    			if (exposedObject != null) {
    				//调用注入类的init-method方法
    				exposedObject = initializeBean(beanName, exposedObject, mbd);
    			}
    		}
    		catch (Throwable ex) {
    			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
    				throw (BeanCreationException) ex;
    			}
    			else {
    				throw new BeanCreationException(
    						mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
    			}
    		}
    
    		//
    		// Register bean as disposable.
    		try {
    			registerDisposableBeanIfNecessary(beanName, bean, mbd);
    		}
    		catch (BeanDefinitionValidationException ex) {
    			throw new BeanCreationException(
    					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    		}
    
    		return exposedObject;
    	}

createBeanInstance中会通过bean的构造函数或者默认构造函数来完成bean的初始化工作。

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
    		
    		//省略代码.............
    		
    		if (resolved) {
    			if (autowireNecessary) {
    				//通过构造函数初始化
    				return autowireConstructor(beanName, mbd, null, null);
    			}
    			else {
    				//使用默认构造函数初始化
    				return instantiateBean(beanName, mbd);
    			}
    		}
    
    		// Need to determine the constructor...
    		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    		if (ctors != null ||
    				mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
    				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
    			return autowireConstructor(beanName, mbd, ctors, args);
    		}
    
    		// No special handling: simply use no-arg constructor.
    		return instantiateBean(beanName, mbd);
    	}

instantiateBean中会调用bean初始策略InstantiationStrategy的实现类完成bean的初始化操作。

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
    		try {
    			Object beanInstance;
    			final BeanFactory parent = this;
    			if (System.getSecurityManager() != null) {
    				beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
    					@Override
    					public Object run() {
    						return getInstantiationStrategy().instantiate(mbd, beanName, parent);
    					}
    				}, getAccessControlContext());
    			}
    			else {
    				//调用bean初始化策InstantiationStrategy略初始化bean
    				beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
    			}
    			BeanWrapper bw = new BeanWrapperImpl(beanInstance);
    			initBeanWrapper(bw);
    			return bw;
    		}
    		catch (Throwable ex) {
    			throw new BeanCreationException(
    					mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
    		}
    	}

调用InstantiationStrategy的instantiate方法完成初始化操作。

SimpleInstantiationStrategy（InstantiationStrategy实现类）

@Override
    	public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
    		// Don't override the class with CGLIB if no overrides.
    		if (bd.getMethodOverrides().isEmpty()) {
    			Constructor<?> constructorToUse;
    			synchronized (bd.constructorArgumentLock) {
    				constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
    				if (constructorToUse == null) {
    					final Class<?> clazz = bd.getBeanClass();
    					if (clazz.isInterface()) {
    						throw new BeanInstantiationException(clazz, "Specified class is an interface");
    					}
    					try {
    						if (System.getSecurityManager() != null) {
    							constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() {
    								@Override
    								public Constructor<?> run() throws Exception {
    									return clazz.getDeclaredConstructor((Class[]) null);
    								}
    							});
    						}
    						else {
    							constructorToUse =	clazz.getDeclaredConstructor((Class[]) null);
    						}
    						bd.resolvedConstructorOrFactoryMethod = constructorToUse;
    					}
    					catch (Throwable ex) {
    						throw new BeanInstantiationException(clazz, "No default constructor found", ex);
    					}
    				}
    			}
    			//最终在BeanUtils中完成bean的初始化操作
    			return BeanUtils.instantiateClass(constructorToUse);
    		}
    		else {
    			// Must generate CGLIB subclass.
    			return instantiateWithMethodInjection(bd, beanName, owner);
    		}
    	}

最终我们发现bean的初始化操作是在BeanUtils中完成的，通过反射机制完成Bean的初始化操作。

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
    		Assert.notNull(ctor, "Constructor must not be null");
    		try {
    			ReflectionUtils.makeAccessible(ctor);
    			//通过反射机制初始化bean
    			return ctor.newInstance(args);
    		}
    		catch (InstantiationException ex) {
    			throw new BeanInstantiationException(ctor, "Is it an abstract class?", ex);
    		}
    		catch (IllegalAccessException ex) {
    			throw new BeanInstantiationException(ctor, "Is the constructor accessible?", ex);
    		}
    		catch (IllegalArgumentException ex) {
    			throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", ex);
    		}
    		catch (InvocationTargetException ex) {
    			throw new BeanInstantiationException(ctor, "Constructor threw exception", ex.getTargetException());
    		}
    	}

总结：简单来说Spring对bean的初始化操作就是根据反射机制通过构造函数进行初始化的。

接下来我们再用一篇介绍一下bean的属性注入相关的操作。

[Spring_--BeanPostProcessor]: http://blog.csdn.net/qq924862077/article/details/72347493