Spring源码解读之创建bean过程中的加载类、实例化前、实例化

        在spring获取一个bean时，如果是非懒加载的bean,会先在单例池中获取，如果没有获取到，则需要进行bean的创建。那么bean的创建需要经过哪些步骤呢？具体步骤查看下一文了解Spring底层整体脉络-CSDN博客 bean的创建过程这一章节。本文会对bean创建过程中的类加载、实例化前、实例化后的源码进行解读；

        读此文章前，最好先了解一下bean的创建的主体源码，参考Spring源码解读之创建bean-CSDN博客。

bean创建的主线代码:

AbstractAutowireCapableBeanFactory.java 的createBean这个方法。

/**
	 * Central method of this class: creates a bean instance,
	 * populates the bean instance, applies post-processors, etc.
	 * @see #doCreateBean
	 */
	@Override
	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		if (logger.isTraceEnabled()) {
			logger.trace("Creating instance of bean '" + beanName + "'");
		}
		RootBeanDefinition mbdToUse = mbd;

		// Make sure bean class is actually resolved at this point, and
		// clone the bean definition in case of a dynamically resolved Class
		// which cannot be stored in the shared merged bean definition.
		// 马上就要实例化Bean了，确保beanClass被加载了，进行类加载
		Class resolvedClass = resolveBeanClass(mbd, beanName);
		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
			mbdToUse = new RootBeanDefinition(mbd);
			mbdToUse.setBeanClass(resolvedClass);
		}

		// Prepare method overrides.
		try {//和@lookUp这个注解相关   就是要做一些属性覆盖的操作
			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.
			// 实例化前
			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);
		}

		try {
			//创建bean
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			if (logger.isTraceEnabled()) {
				logger.trace("Finished creating instance of bean '" + beanName + "'");
			}
			return beanInstance;
		}
		catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
			// A previously detected exception with proper bean creation context already,
			// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
		}
	}

1. 类加载

        当spring要创建一个bean时，先要进行类加载

1. 在bean创建的主线代码createBean方法中会调用

Class resolvedClass = resolveBeanClass(mbd, beanName);

该方法主线逻辑就是判断类有没有被加载，如果已经加载了，则直接返回类；如果没有被加载，则进行类加载。

点击进入该方法。源码解读如下：

/**
	 * Resolve the bean class for the specified bean definition,
	 * resolving a bean class name into a Class reference (if necessary)
	 * and storing the resolved Class in the bean definition for further use.
	 * @param mbd the merged bean definition to determine the class for
	 * @param beanName the name of the bean (for error handling purposes)
	 * @param typesToMatch the types to match in case of internal type matching purposes
	 * (also signals that the returned {@code Class} will never be exposed to application code)
	 * @return the resolved bean class (or {@code null} if none)
	 * @throws CannotLoadBeanClassException if we failed to load the class
	 */
	@Nullable
	protected Class resolveBeanClass(RootBeanDefinition mbd, String beanName, Class... typesToMatch)
			throws CannotLoadBeanClassException {

		try {
			// 如果beanClass被加载了 直接返回该类
			if (mbd.hasBeanClass()) {
				return mbd.getBeanClass();
			}

			// 如果beanClass没有被加载
			if (System.getSecurityManager() != null) {//查看是否开启了安全管理器
				return AccessController.doPrivileged((PrivilegedExceptionAction>)
						() -> doResolveBeanClass(mbd, typesToMatch), getAccessControlContext());
			}
			else {//没有开启安全管理器 加载类
				return doResolveBeanClass(mbd, typesToMatch);
			}
		}
		catch (PrivilegedActionException pae) {
			ClassNotFoundException ex = (ClassNotFoundException) pae.getException();
			throw new CannotLoadBeanClassException(mbd.getResourceDescription(), beanName, mbd.getBeanClassName(), ex);
		}
		catch (ClassNotFoundException ex) {
			throw new CannotLoadBeanClassException(mbd.getResourceDescription(), beanName, mbd.getBeanClassName(), ex);
		}
		catch (LinkageError err) {
			throw new CannotLoadBeanClassException(mbd.getResourceDescription(), beanName, mbd.getBeanClassName(), err);
		}
	}

2. 如果类没有被加载，resolveBeanClass这个方法则会调用

doResolveBeanClass(mbd, typesToMatch);

这个方法，这个方法主要逻辑就是获取类加载器，然后进行加载类；

其源码解读如下：

@Nullable
	private Class doResolveBeanClass(RootBeanDefinition mbd, Class... typesToMatch)
			throws ClassNotFoundException {

		//1. 获取对应的类加载器
		ClassLoader beanClassLoader = getBeanClassLoader();
		ClassLoader dynamicLoader = beanClassLoader;
		boolean freshResolve = false;

		if (!ObjectUtils.isEmpty(typesToMatch)) {
			// When just doing type checks (i.e. not creating an actual instance yet),
			// use the specified temporary class loader (e.g. in a weaving scenario).
			ClassLoader tempClassLoader = getTempClassLoader();
			if (tempClassLoader != null) {
				dynamicLoader = tempClassLoader;
				freshResolve = true;
				if (tempClassLoader instanceof DecoratingClassLoader) {
					DecoratingClassLoader dcl = (DecoratingClassLoader) tempClassLoader;
					for (Class typeToMatch : typesToMatch) {
						dcl.excludeClass(typeToMatch.getName());
					}
				}
			}
		}

		String className = mbd.getBeanClassName();
		if (className != null) {
			// 解析Spring表达式，有可能直接返回了一个Class对象
			Object evaluated = evaluateBeanDefinitionString(className, mbd);
			if (!className.equals(evaluated)) {//可以暂时忽略不看   是我们定义bean时可能会用spring的表达式定义bean,很少用这种形式
				// A dynamically resolved expression, supported as of 4.2...
				if (evaluated instanceof Class) {//解析得到的对象已经是一个class对象  说明已经被加载  直接返回
					return (Class) evaluated;
				}
				else if (evaluated instanceof String) {//解析得到的值还是一个字符串 说明类还没有被加载  需要被加载
					className = (String) evaluated;
					freshResolve = true;
				}
				else {
					throw new IllegalStateException("Invalid class name expression result: " + evaluated);
				}
			}
			if (freshResolve) {//2. 加载类
				// When resolving against a temporary class loader, exit early in order
				// to avoid storing the resolved Class in the bean definition.
				if (dynamicLoader != null) {
					try {
						//加载类
						return dynamicLoader.loadClass(className);
					}
					catch (ClassNotFoundException ex) {
						if (logger.isTraceEnabled()) {
							logger.trace("Could not load class [" + className + "] from " + dynamicLoader + ": " + ex);
						}
					}
				}
				return ClassUtils.forName(className, dynamicLoader);
			}
		}

		// Resolve regularly, caching the result in the BeanDefinition...
		return mbd.resolveBeanClass(beanClassLoader);
	}

2.1 那么我们看一下，类加载器是如何获取的：我们根据上述代码中

	ClassLoader beanClassLoader = getBeanClassLoader();

一路点入，找到获取类加载器的源码；ClassUtils.java中的

public static ClassLoader getDefaultClassLoader()

具体源码解读如下：

/**
	 * Return the default ClassLoader to use: typically the thread context
	 * ClassLoader, if available; the ClassLoader that loaded the ClassUtils
	 * class will be used as fallback.
	 * 
Call this method if you intend to use the thread context ClassLoader
	 * in a scenario where you clearly prefer a non-null ClassLoader reference:
	 * for example, for class path resource loading (but not necessarily for
	 * {@code Class.forName}, which accepts a {@code null} ClassLoader
	 * reference as well).
	 * @return the default ClassLoader (only {@code null} if even the system
	 * ClassLoader isn't accessible)
	 * @see Thread#getContextClassLoader()
	 * @see ClassLoader#getSystemClassLoader()
	 */
	@Nullable
	public static ClassLoader getDefaultClassLoader() {
		ClassLoader cl = null;

		// 1. 优先获取线程中的类加载器
		try {
			cl = Thread.currentThread().getContextClassLoader();
		}
		catch (Throwable ex) {
			// Cannot access thread context ClassLoader - falling back...
		}

		// 2. 线程中类加载器为null的情况下，获取加载ClassUtils类的类加载器
		if (cl == null) {
			// No thread context class loader -> use class loader of this class.
			cl = ClassUtils.class.getClassLoader();
			if (cl == null) {
				// getClassLoader() returning null indicates the bootstrap ClassLoader
				// 加入ClassUtils是被Bootstrap类加载器加载的，则获取系统类加载器
				try {
					cl = ClassLoader.getSystemClassLoader();
				}
				catch (Throwable ex) {
					// Cannot access system ClassLoader - oh well, maybe the caller can live with null...
				}
			}
		}
		return cl;
	}

2.2 获取到了类加载器后，上述中doResolveBeanClass这个方法中

dynamicLoader.loadClass(className);

进行加载类；

2. 实例化前

        在spring加载完要创建的bean的类后，在对象实例化前会有一些操作，源码注释中写到：给bean的后置处理器一个产生代理对象代替将要创建的实例的机会。

        bean创建的主线代码:AbstractAutowireCapableBeanFactory.java 的createBean这个方法会调用以下代码；

	// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
			// 实例化前
			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);

点击进入resolveBeforeInstantiation这个方法

 具体源码解读如下：

/**
	 * Apply before-instantiation post-processors, resolving whether there is a
	 * before-instantiation shortcut for the specified bean.
	 * @param beanName the name of the bean
	 * @param mbd the bean definition for the bean
	 * @return the shortcut-determined bean instance, or {@code null} if none
	 */
	@Nullable
	protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
		Object bean = null;
		if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
			// Make sure bean class is actually resolved at this point.
			// synthetic表示合成，如果某些Bean是合成的，那么则不会经过BeanPostProcessor的处理
			//hasInstantiationAwareBeanPostProcessors()   判断类有没有继承BeanPostProcessors相关的子类
			if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
				Class targetType = determineTargetType(beanName, mbd);
				if (targetType != null) {
					//执行具体实例化前的操作
					bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
					if (bean != null) {
						bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
					}
				}
			}
			mbd.beforeInstantiationResolved = (bean != null);
		}
		return bean;
	}

3. 实例化

        在spring给bean的后置处理器一个产生代理对象代替将要创建的实例的机会，也就是实例化前的操作完成后，spring会创建出一个普通对象，即bean的实例化。

        bean创建的主线代码:AbstractAutowireCapableBeanFactory.java 的createBean这个方法会调用以下代码；

Object beanInstance = doCreateBean(beanName, mbdToUse, args);

doCreateBean这个方法就是创建一个bean的详细过程，比如bean的实例化、bean的实例化后、bean的依赖注入等。该方法源码解读如下：

/**
	 * Actually create the specified bean. Pre-creation processing has already happened
	 * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
	 * 
Differentiates between default bean instantiation, use of a
	 * factory method, and autowiring a constructor.
	 * @param beanName the name of the bean
	 * @param mbd the merged bean definition for the bean
	 * @param args explicit arguments to use for constructor or factory method invocation
	 * @return a new instance of the bean
	 * @throws BeanCreationException if the bean could not be created
	 * @see #instantiateBean
	 * @see #instantiateUsingFactoryMethod
	 * @see #autowireConstructor
	 */
	protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		// 实例化bean
		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			// 有可能在本Bean创建之前，就有其他Bean把当前Bean给创建出来了（比如依赖注入过程中）
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
			// 创建Bean实例
			// createBeanInstance(beanName, mbd, args)  包含推断构造方法
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		Object bean = instanceWrapper.getWrappedInstance();
		Class beanType = instanceWrapper.getWrappedClass();
		if (beanType != NullBean.class) {
			mbd.resolvedTargetType = beanType;
		}

		// 后置处理合并后的BeanDefinition
		// Allow post-processors to modify the merged bean definition.
		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;
			}
		}

		// 为了解决循环依赖提前缓存单例创建工厂
		// Eagerly cache singletons to be able to resolve circular references
		// even when triggered by lifecycle interfaces like BeanFactoryAware.
		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");
			}
			// 循环依赖-添加到三级缓存
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

		// Initialize the bean instance.
		Object exposedObject = bean;
		try {
			// 属性填充
			populateBean(beanName, mbd, instanceWrapper);

			// 初始化
			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);
			}
		}

		if (earlySingletonExposure) {
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				}
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
					// beanName被哪些bean依赖了，现在发现beanName所对应的bean对象发生了改变，那么则会报错
					String[] dependentBeans = getDependentBeans(beanName);
					Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					for (String dependentBean : dependentBeans) {
						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
							actualDependentBeans.add(dependentBean);
						}
					}
					if (!actualDependentBeans.isEmpty()) {
						throw new BeanCurrentlyInCreationException(beanName,
								"Bean with name '" + beanName + "' has been injected into other beans [" +
								StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
								"] in its raw version as part of a circular reference, but has eventually been " +
								"wrapped. This means that said other beans do not use the final version of the " +
								"bean. This is often the result of over-eager type matching - consider using " +
								"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
					}
				}
			}
		}

		// Register bean as disposable.
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}

此处，我们着重看一下bean是如何被实例化的。

点击上述方法中

instanceWrapper = createBeanInstance(beanName, mbd, args);

进入该方法，该方法主要包含了bean实例化的几种方式以及推断构造方法的实现，具体源码解读如下：

/**
	 * Create a new instance for the specified bean, using an appropriate instantiation strategy:
	 * factory method, constructor autowiring, or simple instantiation.
	 * @param beanName the name of the bean
	 * @param mbd the bean definition for the bean
	 * @param args explicit arguments to use for constructor or factory method invocation
	 * @return a BeanWrapper for the new instance
	 * @see #obtainFromSupplier
	 * @see #instantiateUsingFactoryMethod
	 * @see #autowireConstructor
	 * @see #instantiateBean
	 */
	protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
		// Make sure bean class is actually resolved at this point.
		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());
		}

		// BeanDefinition中添加了Supplier，则调用Supplier来得到对象
		Supplier instanceSupplier = mbd.getInstanceSupplier();
		if (instanceSupplier != null) {
			return obtainFromSupplier(instanceSupplier, beanName);
		}

		// @Bean对应的BeanDefinition
		if (mbd.getFactoryMethodName() != null) {
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}

		// Shortcut when re-creating the same bean...
		// 一个原型BeanDefinition，会多次来创建Bean，那么就可以把该BeanDefinition所要使用的构造方法缓存起来，避免每次都进行构造方法推断
		boolean resolved = false;
		boolean autowireNecessary = false;
		if (args == null) {
			synchronized (mbd.constructorArgumentLock) {
				if (mbd.resolvedConstructorOrFactoryMethod != null) {
					resolved = true;
					// autowireNecessary表示有没有必要要进行注入，比如当前BeanDefinition用的是无参构造方法，那么autowireNecessary为false，否则为true，表示需要给构造方法参数注入值
					autowireNecessary = mbd.constructorArgumentsResolved;
				}
			}
		}
		if (resolved) {
			// 如果确定了当前BeanDefinition的构造方法，那么看是否需要进行对构造方法进行参数的依赖注入（构造方法注入）
			if (autowireNecessary) {
				// 方法内会拿到缓存好的构造方法的入参
				return autowireConstructor(beanName, mbd, null, null);
			}
			else {
				// 构造方法已经找到了，但是没有参数，那就表示是无参，直接进行实例化
				return instantiateBean(beanName, mbd);
			}
		}

		// 如果没有找过构造方法，那么就开始找了

		// Candidate constructors for autowiring?
		// 提供一个扩展点，可以利用SmartInstantiationAwareBeanPostProcessor来控制用beanClass中的哪些构造方法
		// 比如AutowiredAnnotationBeanPostProcessor会把加了@Autowired注解的构造方法找出来，具体看代码实现会更复杂一点
		Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);

		// 如果推断出来了构造方法，则需要给构造方法赋值，也就是给构造方法参数赋值，也就是构造方法注入
		// 如果没有推断出来构造方法，但是autowiremode为AUTOWIRE_CONSTRUCTOR，则也可能需要给构造方法赋值，因为不确定是用无参的还是有参的构造方法
		// 如果通过BeanDefinition指定了构造方法参数值，那肯定就是要进行构造方法注入了
		// 如果调用getBean的时候传入了构造方法参数值，那肯定就是要进行构造方法注入了
		if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
			return autowireConstructor(beanName, mbd, ctors, args);
		}

		// Preferred constructors for default construction?
		ctors = mbd.getPreferredConstructors();
		if (ctors != null) {
			return autowireConstructor(beanName, mbd, ctors, null);
		}

		// No special handling: simply use no-arg constructor.
		// 不匹配以上情况，则直接使用无参构造方法
		return instantiateBean(beanName, mbd);
	}