引言

spring加载xml的对象信息解析实例化成各个bean的过程我在这里就不细讲了，毕竟从头开始看很容易绕晕大家，反而让大家觉得这并不需要写。我们姑且认为spring已经加载好了各类对象信息封装成BeanDefinition，并已经实例化存储在了某个地方。不管是懒汉还是饿汉，都要经历反射出对象实例，然后初始化，我们先从spring中比较好理解的地方来入手IOC，那就是IOC中的bean在实例化之后的初始化操作。主要涉及到BeanPostProcessor，InitializingBean这两个的应用。

IOC的bean初始化

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {Object wrappedBean = bean;if (mbd == null || !mbd.isSynthetic()) {wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);}invokeInitMethods(beanName, wrappedBean, mbd);if (mbd == null || !mbd.isSynthetic()) {wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);}return wrappedBean;}
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为了方便阅读，我删除了部分不相干的代码。 AbstractAutowireCapableBeanFactory#initializeBean就是初始化IOC容器中的Bean的主要方法，从这个方法入手，来看看IOC加载Bean到底做了什么？ 将实例化好的bean传入该方法：

--> 调用BeanPostProcessor的postProcessBeforeInitialization方法
--> 调用bean实例的初始化方法
--> 调用BeanPostProcessor的postProcessAfterInitialization方法
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以下三段代码就是这三个过程：

@Overridepublic Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {result = beanProcessor.postProcessBeforeInitialization(result, beanName);if (result == null) {return result;}}return result;}
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循环所有实现了BeanPostProcessor类的bean，并执行相应的对象初始化之前的方法postProcessBeforeInitialization

protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd)throws Throwable {boolean isInitializingBean = (bean instanceof InitializingBean);if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {if (logger.isDebugEnabled()) {logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");}if (System.getSecurityManager() != null) {try {AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {@Overridepublic Object run() throws Exception {((InitializingBean) bean).afterPropertiesSet();return null;}}, getAccessControlContext());}catch (PrivilegedActionException pae) {throw pae.getException();}}else {((InitializingBean) bean).afterPropertiesSet();}}if (mbd != null) {String initMethodName = mbd.getInitMethodName();if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&!mbd.isExternallyManagedInitMethod(initMethodName)) {invokeCustomInitMethod(beanName, bean, mbd);}}}
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该段代码给一些实现了InitializingBean的bean进行初始化操作。

注意：

1：spring为bean提供了两种初始化的方式，需要实现InitializingBean接口，重写afterPropertiesSet方法，或者在配置文件中同过init-method指定，两种方式可以同时使用，但是会先执行afterPropertiesSet再执行init-method。下文还有demo证明。

2：实现InitializingBean接口是直接调用afterPropertiesSet方法，比通过反射调用init-method指定的方法效率相对来说要高点。但是init-method方式消除了对spring的依赖

3：如果调用afterPropertiesSet方法时出错，则不调用init-method指定的方法。

@Overridepublic Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {result = beanProcessor.postProcessAfterInitialization(result, beanName);if (result == null) {return result;}}return result;}
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再次循环所有实现了BeanPostProcessor类的bean，并执行相应的对象初始化之前的方法postProcessAfterInitialization。顺便提一下，以后要写的Spring AOP的底层处理也是通过实现BeanPostProcessor来执行代理包装逻辑的。

IOC的bean注入

我们的ioc对象初始化好了，接下来就要看看最关键的依赖注入了。先思考一个问题，spring在什么时候把对象注入进去的？这里先不解释，我们看这段源码:

protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {if (pvs instanceof MutablePropertyValues) {...}else {original = Arrays.asList(pvs.getPropertyValues());}TypeConverter converter = getCustomTypeConverter();if (converter == null) {converter = bw;}BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);// Create a deep copy, resolving any references for values.List<PropertyValue> deepCopy = new ArrayList<PropertyValue>(original.size());boolean resolveNecessary = false;for (PropertyValue pv : original) {if (pv.isConverted()) {deepCopy.add(pv);}else {String propertyName = pv.getName();Object originalValue = pv.getValue();//在这里解析并塞入了注入的对象Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);Object convertedValue = resolvedValue;boolean convertible = bw.isWritableProperty(propertyName) &&!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);if (convertible) {convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);}// Possibly store converted value in merged bean definition,// in order to avoid re-conversion for every created bean instance.if (resolvedValue == originalValue) {if (convertible) {pv.setConvertedValue(convertedValue);}deepCopy.add(pv);}else if (convertible && originalValue instanceof TypedStringValue &&!((TypedStringValue) originalValue).isDynamic() &&!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {pv.setConvertedValue(convertedValue);deepCopy.add(pv);}else {resolveNecessary = true;deepCopy.add(new PropertyValue(pv, convertedValue));}}}}
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我还是删了很多其他逻辑，这里主要的一句话就是Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue); 在这里分析并塞入了注入对象的关联。具体怎么操作还得看自己去翻阅。接下来我用最简化的代码方式来展示我的寻找过程：

AbstractBeanFactory类
@Override
public Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);
}
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我们肯定找到拿到每个bean的如果，如果不存在就会创建。

AbstractBeanFactory类
@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)throws BeansException {// Create bean instance.if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {// Explicitly remove instance from singleton cache: It might have been put there// eagerly by the creation process, to allow for circular reference resolution.// Also remove any beans that received a temporary reference to the bean.destroySingleton(beanName);throw ex;}}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}
}
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然后再进入createBean()，它的实现是在 AbstractAutowireCapableBeanFactory 当中：

AbstractAutowireCapableBeanFactory类
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {Object beanInstance = doCreateBean(beanName, mbdToUse, args);return beanInstance;
}
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AbstractAutowireCapableBeanFactory类
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);}if (instanceWrapper == null) {instanceWrapper = createBeanInstance(beanName, mbd, args);}// Initialize the bean instance.Object exposedObject = bean;try {populateBean(beanName, mbd, instanceWrapper);if (exposedObject != null) {exposedObject = initializeBean(beanName, exposedObject, mbd);}}
}
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重点关注 createBeanInstance() 和 populateBean() 这两个方法。其中，createBeanInstance方法生成了Bean所包含的Java对象:

AbstractAutowireCapableBeanFactory类
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {if (resolved) {if (autowireNecessary) {return autowireConstructor(beanName, mbd, null, null);}else {return instantiateBean(beanName, mbd);}}
}
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protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {Object beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);BeanWrapper bw = new BeanWrapperImpl(beanInstance);initBeanWrapper(bw);return bw;
}
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重点关注 getInstantiationStrategy()这个方法，可以看到instantiateBean方法的功能实现是通过调用getInstantiationStrategy().instantiate方法实现的。 getInstantiationStrategy方法的作用是获得实例化的策略对象，也就是指通过哪种方案进行实例化的过程。继续跟踪下去我们可以发现，Spring当中提供了两种实例化方案： BeanUtils和Cglib方式。BeanUtils实现机制是通过Java的反射机制，Cglib是一个第三方类库采用的是一种字节码加强方式机制。Spring中采用的默认实例化策略是Cglib。

接下来就是重头戏建立bean的依赖关系了。我们回到doCreateBean方法中的populateBean(beanName, mbd, instanceWrapper);

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {applyPropertyValues(beanName, mbd, bw, pvs);
}
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有没有发现，我们已经来到我开始讲的applyPropertyValues方法了？ 下面我们就不看源码了，我们用反射来把value注入进去，这样更容易理解。

try {Method declaredMethod = bean.getClass().getDeclaredMethod("set" + propertyValue.getName().substring(0, 1).toUpperCase()+ propertyValue.getName().substring(1), value.getClass());declaredMethod.setAccessible(true);declaredMethod.invoke(bean, value);
} catch (NoSuchMethodException e) {Field declaredField = bean.getClass().getDeclaredField(propertyValue.getName());declaredField.setAccessible(true);declaredField.set(bean, value);
}
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拿到set参数的方法，如果出现异常，表示并没有写set方法，则就粗暴的方法塞入field。（此处并不是源码，是我根据源码理解写的比较通俗易懂的方式）

这是我觉得在繁多的spring源码中找出一段目标代码先看到，更容易让人能够跟着思路走下去。

题外话：

synchronized (this.dependenciesForBeanMap) {Set<String> dependenciesForBean =this.dependenciesForBeanMap.computeIfAbsent(dependentBeanName, k -> new LinkedHashSet<>(8));dependenciesForBean.add(canonicalName);}
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在看IOC源码的时候看到Map的computeIfAbsent让我恍然大悟，我来用以前怎么写这段代码的来解释这段代码什么意思如下：

Set<String> dependenciesForBean = this.dependenciesForBeanMap.get(dependentBeanName);
if (dependenciesForBean != null){dependenciesForBean = new LinkedHashSet<>(8);
}
dependenciesForBean.add(dependentBeanName);
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这是jdk1.8才支持的lambda写法，是不是代码更简洁？

再来分享一种简洁的方法，计算每个学生的总分记录到map中：

List<Student> students = new ArrayList<>();
students.add(new Student("张三", "语文", 18));
students.add(new Student("张三", "数学", 20));
Map<String, Integer> resultMap = new HashMap<>();
for (Student student : students) {resultMap.merge(student.getName(), student.getScore(), (a, b) -> b + a);
}
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