Spring源码阅读3-bean获取-下

上篇说到了通过CglibSubclassingInstantiationStrategy类的instantiate方法创建对象实例，然后使用BeanWrapperImpl对象进行包装，初始化后返回。CglibSubclassingInstantiationStrategy类继承自SimpleInstantiationStrategy，而instantiate调用的就是父类SimpleInstantiationStrategy的方法，我们看下源码：

@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>() {
								@Override
								public Constructor run() throws Exception {
									return clazz.getDeclaredConstructor((Class[]) null);
								}
							});
						}
						else {
							constructorToUse =	clazz.getDeclaredConstructor((Class[]) null);
						}
						bd.resolvedConstructorOrFactoryMethod = constructorToUse;
					}
					catch (Exception ex) {
						throw new BeanInstantiationException(clazz, "No default constructor found", ex);
					}
				}
			}
			return BeanUtils.instantiateClass(constructorToUse);
		}
		else {
			// Must generate CGLIB subclass.
			return instantiateWithMethodInjection(bd, beanName, owner);
		}
	}

然后使用默认构造函数初始化对象，前提是没有methodOvrrides，不需要使用CGLIB动态代理生成子类。

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

这里也看一下CGLIB的生成代理对象的过程：方法在SimpleInstantiationStrategy的子类CglibSubclassingInstantiationStrategy中被重写得以实现，SimpleInstantiationStrategy内此方法只是抛出了not support的异常。

	@Override
	protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner) {
		return instantiateWithMethodInjection(bd, beanName, owner, null);
	}

	@Override
	protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner,
			Constructor ctor, Object... args) {

		// Must generate CGLIB subclass...
		return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
	}

真正的实现方法在CglibSubclassingInstantiationStrategy的内部类CglibSubclassCreator中，源码如下：

	/**
		 * Create a new instance of a dynamically generated subclass implementing the
		 * required lookups.
		 * @param ctor constructor to use. If this is {@code null}, use the
		 * no-arg constructor (no parameterization, or Setter Injection)
		 * @param args arguments to use for the constructor.
		 * Ignored if the {@code ctor} parameter is {@code null}.
		 * @return new instance of the dynamically generated subclass
		 */
		public Object instantiate(Constructor ctor, Object... args) {
			Class subclass = createEnhancedSubclass(this.beanDefinition);
			Object instance;
			if (ctor == null) {
				instance = BeanUtils.instantiate(subclass);
			}
			else {
				try {
					Constructor enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
					instance = enhancedSubclassConstructor.newInstance(args);
				}
				catch (Exception ex) {
					throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
							"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
				}
			}
			// SPR-10785: set callbacks directly on the instance instead of in the
			// enhanced class (via the Enhancer) in order to avoid memory leaks.
			Factory factory = (Factory) instance;
			factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
					new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
					new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
			return instance;
		}

		/**
		 * Create an enhanced subclass of the bean class for the provided bean
		 * definition, using CGLIB.
		 */
		private Class createEnhancedSubclass(RootBeanDefinition beanDefinition) {
			Enhancer enhancer = new Enhancer();
			enhancer.setSuperclass(beanDefinition.getBeanClass());
			enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
			enhancer.setCallbackFilter(new MethodOverrideCallbackFilter(beanDefinition));
			enhancer.setCallbackTypes(CALLBACK_TYPES);
			return enhancer.createClass();
		}
	}

从这里可以看出，处理的还是上面我们说到过的两种情况：

	private static final Class[] CALLBACK_TYPES = new Class[]
				{NoOp.class, LookupOverrideMethodInterceptor.class, ReplaceOverrideMethodInterceptor.class};

言归正传，初始化对象后，我们又回到了AbstractAutowireCapableBeanFactory类的doCreateBean方法，为了方便阅读，再次帖一下这里的源码：

	protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
		Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

		// Allow post-processors to modify the merged bean definition.
		synchronized (mbd.postProcessingLock) {
			if (!mbd.postProcessed) {
				applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				mbd.postProcessed = true;
			}
		}

		// 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.isDebugEnabled()) {
				logger.debug("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			addSingletonFactory(beanName, new ObjectFactory