Spring源码剖析——依赖注入实现原理

2016年08月06日 09:35:00 阅读数：31760 标签： 更多

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版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/lisongjia123/article/details/52134396

引言

  在之前的这篇文章中，已经详细介绍了在Spring当中我们配置的Bean是怎样被Spring解析和管理的，我们配置的那些Bean经过 载入 、解析 和 注册 这三个过程后，在框架内部被抽象封装成BeanDefinition这种类型，最终所有的BeanDefinition交由BeanFactory当中的definitionMap统一管理起来(如果对该过程还不是很清楚可以先看之前的文章)。走到这一步，Spring还不能提供给我们一个完整可用的Bean，因为虽然所有的Bean已经在Spring经过解析和转化过程被集中组织起来，但此时这些BeanDefinition对象都是彼此分离的，并没有建立起依赖关系，也就是我们常说的 依赖注入 还没有发生。今天我们就来研究一下Spring中依赖注入的实现原理是怎样的。

依赖注入介绍

  在分析原理之前我们先回顾下依赖注入的概念：

我们常提起的依赖注入(Dependency Injection)和控制反转(Inversion of Control)是同一个概念。具体含义是:当某个角色(可能是一个Java实例，调用者)需要另一个角色(另一个Java实例，被调用者)的协助时，在 传统的程序设计过程中，通常由调用者来创建被调用者的实例。但在Spring里，创建被调用者的工作不再由调用者来完成，因此称为控制反转;创建被调用者 实例的工作通常由Spring容器来完成，然后注入调用者，因此也称为依赖注入。

  其实简单的说，依赖注入起到的作用就是讲对象之间的依赖关系从原先的代码中解耦出来，通过配置文件或注解等方式加上Spring框架的处理让我们对依赖关系灵活集中的进行管理。

源码剖析

  之前的文章分析到了调用getBean获得Bean对象之前的步骤，而我们调用getBean后获得已经是完成依赖关系注入的完整Bean对象，所以完成依赖注入的奥秘就发生在getBean当中，要分析我们便从getBean这个入口开始吧。

getBean触发的依赖注入

  先看看 getBean() 方法的源码，方法的实现是在 AbstractBeanFactory.java 中

getBean方法

//---------------------------------------------------------------------    // Implementation of BeanFactory interface    //---------------------------------------------------------------------    @Override    public Object getBean(String name) throws BeansException { return doGetBean(name, null, null, false); } @Override public 
     
       T getBean(String name, Class
      
        requiredType) throws BeansException { return doGetBean(name, requiredType, null, false); } @Override public Object getBean(String name, Object... args) throws BeansException { return doGetBean(name, null, args, false); } public 
       
         T getBean(String name, Class
        
          requiredType, Object... args) throws BeansException { return doGetBean(name, requiredType, args, false); }
        
       
      
     

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  几个重载的getBean方法的实现都调用了 doGetBean 这个方法，继续看 doGetBean()

doGetBean方法

//这里是实际取得Bean的地方，也是触发依赖注入的地方    protected 
     
       T doGetBean(            final String name, final Class
      
        requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException { final String beanName = transformedBeanName(name); Object bean; //对于Singleton的Bean先检查是否已经创建，若已经创建过直接返回，无需再次进行创建 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 + "'"); } } //这个方法是用于FactoryBean的创建Bean的相关处理 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } else { // Fail if we're already creating this bean instance: // We're assumably within a circular reference. if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } // 对BeanDefinition进行检查，看是否能在当前工厂取得需要的Bean。如果当前工厂没有取到，则到双亲BeanFactory中去取 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Bean名称获得BeanDefinition final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); checkMergedBeanDefinition(mbd, beanName, args); // 获取当前Bean所有依赖的Bean，这是个递归的过程，直到取到一个没有任何依赖的Bean为止 String[] dependsOn = mbd.getDependsOn(); if (dependsOn != null) { for (String dependsOnBean : dependsOn) { if (isDependent(beanName, dependsOnBean)) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'"); } registerDependentBean(dependsOnBean, beanName); getBean(dependsOnBean); } } // Create bean instance. if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, new ObjectFactory
       
      
     

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  getBean是依赖注入的起点，之后会调用createBean，下面我们分析createBean的代码。

创建完整Bean的流程

createBean() 的实现是在 AbstractAutowireCapableBeanFactory 当中

creaBean方法

protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {        if (logger.isDebugEnabled()) {            logger.debug("Creating instance of bean '" + beanName + "'"); } RootBeanDefinition mbdToUse = mbd; Class
      resolvedClass = resolveBeanClass(mbd, beanName); if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) { mbdToUse = new RootBeanDefinition(mbd); mbdToUse.setBeanClass(resolvedClass); } try { mbdToUse.prepareMethodOverrides(); } catch (BeanDefinitionValidationException ex) { throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(), beanName, "Validation of method overrides failed", ex); } try { 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); } //创建过程委托给了doCreateBean方法 Object beanInstance = doCreateBean(beanName, mbdToUse, args); if (logger.isDebugEnabled()) { logger.debug("Finished creating instance of bean '" + beanName + "'"); } return beanInstance; }

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doCreateBean方法

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) { // 在Bean对象基础上做了一层包装，它持有Bean对象应用 BeanWrapper instanceWrapper = null; if (mbd.isSingleton()) { instanceWrapper = this.factoryBeanInstanceCache.remove(beanName); } //创建Bean的地方，createBeanInstance完成创建过程 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

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  看了上面的代码后，我们应该重点关注 createBeanInstance() 和 populateBean() 这两个方法。其中，createBeanInstance方法生成了Bean所包含的Java对象，代码如下

createBeanInstance方法

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {        // 确认需要创建Bean实例的类可以实例化        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()); } //使用工厂方法进行实例化 if (mbd.getFactoryMethodName() != null) { return instantiateUsingFactoryMethod(beanName, mbd, args); } // Shortcut when re-creating the same bean... boolean resolved = false; boolean autowireNecessary = false; if (args == null) { synchronized (mbd.constructorArgumentLock) { if (mbd.resolvedConstructorOrFactoryMethod != null) { resolved = true; autowireNecessary = mbd.constructorArgumentsResolved; } } } if (resolved) { if (autowireNecessary) { return autowireConstructor(beanName, mbd, null, null); } else { return instantiateBean(beanName, mbd); } } // 使用构造函数进行实例化 Constructor
     [] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName); if (ctors != null || mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR || mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) { return autowireConstructor(beanName, mbd, ctors, args); } // 默认构造函数进行实例化 return instantiateBean(beanName, mbd); }

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  跟随上面的步伐，上面实例化的时候调用了instantiateBean方法，该方法代码如下：

instantiateBean方法

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

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  我们重点关注 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。

  分析到这里我们已经知道了实例化Bean对象的流程，现在已经是万事具备，只欠东风，就剩下对这些建立好的Bean对象建立联系了。

依赖关系的处理

  我们要分析依赖关系是怎样注入到Bean对象当中，就需要回到前面的 populateBean() 方法，这个方法实现是在 AbstractAutowireCapableBeanFactory中

populateBean方法

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {//获得BeanDefinition中设置的property信息，简单理解依赖注入的过程就是对这些property赋值的过程，而只是property信息在BeanDefinition的解析过程中        PropertyValues pvs = mbd.getPropertyValues();        if (bw == null) { if (!pvs.isEmpty()) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance"); } else { // Skip property population phase for null instance. return; } } boolean continueWithPropertyPopulation = true; if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) { for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) { continueWithPropertyPopulation = false; break; } } } } if (!continueWithPropertyPopulation) { return; } //开始依赖注入的过程，先处理Autowire的注入，根据自动注入模式调用相应的处理方法 if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) { MutablePropertyValues newPvs = new MutablePropertyValues(pvs); // Add property values based on autowire by name if applicable. if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) { autowireByName(beanName, mbd, bw, newPvs); } // Add property values based on autowire by type if applicable. if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) { autowireByType(beanName, mbd, bw, newPvs); } pvs = newPvs; } boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors(); boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE); if (hasInstAwareBpps || needsDepCheck) { PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching); if (hasInstAwareBpps) { for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName); if (pvs == null) { return; } } } } if (needsDepCheck) { checkDependencies(beanName, mbd, filteredPds, pvs); } } //对属性进行注入 applyPropertyValues(beanName, mbd, bw, pvs); }

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  我们接着看 applyPropertyValues() 这个方法，了解具体的对属性解析然后注入的过程

applyPropertyValues方法

protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {        if (pvs == null || pvs.isEmpty()) { return; } MutablePropertyValues mpvs = null; List
     
       original; if (System.getSecurityManager() != null) { if (bw instanceof BeanWrapperImpl) { ((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext()); } } if (pvs instanceof MutablePropertyValues) { mpvs = (MutablePropertyValues) pvs; if (mpvs.isConverted()) { // Shortcut: use the pre-converted values as-is. try { bw.setPropertyValues(mpvs); return; } catch (BeansException ex) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Error setting property values", ex); } } original = mpvs.getPropertyValueList(); } else { original = Arrays.asList(pvs.getPropertyValues()); } TypeConverter converter = getCustomTypeConverter(); if (converter == null) { converter = bw; } //解析BeanDefinition中的信息 BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter); // Create a deep copy, resolving any references for values. List
      
        deepCopy = new ArrayList
       
        (original.size()); boolean resolveNecessary = false; for (PropertyValue pv : original) { if (pv.isConverted()) { deepCopy.add(pv); } else { String propertyName = pv.getName(); Object originalValue = pv.getValue(); 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)); } } } if (mpvs != null && !resolveNecessary) { mpvs.setConverted(); } //依赖注入发生的地方，BeanWrapperImpl中实现了这个setPropertyValues方法 try { bw.setPropertyValues(new MutablePropertyValues(deepCopy)); } catch (BeansException ex) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Error setting property values", ex); } }
       
      
     

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  上面的代码中通过 BeanDefinitionResolver 来对BeanDefinition进行解析，我们可以看下该上面解析过程中调用的主要处理方法 resolveValueIfNecessary()。

resolveValueIfNecessary方法

public Object resolveValueIfNecessary(Object argName, Object value) {        // We must check each value to see whether it requires a runtime reference        // to another bean to be resolved.        if (value instanceof RuntimeBeanReference) { RuntimeBeanReference ref = (RuntimeBeanReference) value; return resolveReference(argName, ref); } else if (value instanceof RuntimeBeanNameReference) { String refName = ((RuntimeBeanNameReference) value).getBeanName(); refName = String.valueOf(doEvaluate(refName)); if (!this.beanFactory.containsBean(refName)) { throw new BeanDefinitionStoreException( "Invalid bean name '" + refName + "' in bean reference for " + argName); } return refName; } else if (value instanceof BeanDefinitionHolder) { // 解析BeanDefinitionHolder:包含BeanDefinition的名称和别名等信息 BeanDefinitionHolder bdHolder = (BeanDefinitionHolder) value; return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition()); } else if (value instanceof BeanDefinition) { // Resolve plain BeanDefinition, without contained name: use dummy name. BeanDefinition bd = (BeanDefinition) value; String innerBeanName = "(inner bean)" + BeanFactoryUtils.GENERATED_BEAN_NAME_SEPARATOR + ObjectUtils.getIdentityHexString(bd); return resolveInnerBean(argName, innerBeanName, bd); } else if (value instanceof ManagedArray) { // May need to resolve contained runtime references. ManagedArray array = (ManagedArray) value; Class
      elementType = array.resolvedElementType; if (elementType == null) { String elementTypeName = array.getElementTypeName(); if (StringUtils.hasText(elementTypeName)) { try { elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader()); array.resolvedElementType = elementType; } catch (Throwable ex) { // Improve the message by showing the context. throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Error resolving array type for " + argName, ex); } } else { elementType = Object.class; } } return resolveManagedArray(argName, (List
     ) value, elementType); } else if (value instanceof ManagedList) { // May need to resolve contained runtime references. return resolveManagedList(argName, (List
     ) value); } else if (value instanceof ManagedSet) { // May need to resolve contained runtime references. return resolveManagedSet(argName, (Set
     ) value); } else if (value instanceof ManagedMap) { // May need to resolve contained runtime references. return resolveManagedMap(argName, (Map
     ) value); } else if (value instanceof ManagedProperties) { Properties original = (Properties) value; Properties copy = new Properties(); for (Map.Entry
     
       propEntry : original.entrySet()) { Object propKey = propEntry.getKey(); Object propValue = propEntry.getValue(); if (propKey instanceof TypedStringValue) { propKey = evaluate((TypedStringValue) propKey); } if (propValue instanceof TypedStringValue) { propValue = evaluate((TypedStringValue) propValue); } copy.put(propKey, propValue); } return copy; } else if (value instanceof TypedStringValue) { // Convert value to target type here. TypedStringValue typedStringValue = (TypedStringValue) value; Object valueObject = evaluate(typedStringValue); try { Class
       resolvedTargetType = resolveTargetType(typedStringValue); if (resolvedTargetType != null) { return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType); } else { return valueObject; } } catch (Throwable ex) { // Improve the message by showing the context. throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Error converting typed String value for " + argName, ex); } } else { return evaluate(value); } }
     

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  在配置Bean的属性的时候，属性可能有多种类型，我们再进行注入的时候，不同的属性类型我们不可能一概而论的进行处理，集合类型的属性和非集合类型具备很大的差别，对不同的类型应该有不同的解析处理过程，故该方法流程中首先判断value的类型然后在分别调用 resolveManagedList()、 resolveManagedSet() 、 resolveManagedMap()等方法进行具体的解析。为了掌握大体的主线，这里我们就不详细展示这几个方法源码，感兴趣的可以自己下来分析。

  在完成这个解析过程后，已经为依赖注入准备好了条件，下面的 setPropertyValue() 方法是真正设置属性依赖的地方，该方法的实现是在BeanWrapper的实现类BeanWrapperImpl中，其代码如下:

setPropertyValue方法

protected void setPropertyValue(PropertyTokenHolder tokens, PropertyValue pv) throws BeansException {        String propertyName = tokens.canonicalName;        String actualName = tokens.actualName;        if (tokens.keys != null) { // Apply indexes and map keys: fetch value for all keys but the last one. PropertyTokenHolder getterTokens = new PropertyTokenHolder(); getterTokens.canonicalName = tokens.canonicalName; getterTokens.actualName = tokens.actualName; getterTokens.keys = new String[tokens.keys.length - 1]; System.arraycopy(tokens.keys, 0, getterTokens.keys, 0, tokens.keys.length - 1); Object propValue; try { //取得注入对象的引用 propValue = getPropertyValue(getterTokens); } catch (NotReadablePropertyException ex) { throw new NotWritablePropertyException(getRootClass(), this.nestedPath + propertyName, "Cannot access indexed value in property referenced " + "in indexed property path '" + propertyName + "'", ex); } // Set value for last key. String key = tokens.keys[tokens.keys.length - 1]; if (propValue == null) { // null map value case if (isAutoGrowNestedPaths()) { int lastKeyIndex = tokens.canonicalName.lastIndexOf('['); getterTokens.canonicalName = tokens.canonicalName.substring(0, lastKeyIndex); propValue = setDefaultValue(getterTokens); } else { throw new NullValueInNestedPathException(getRootClass(), this.nestedPath + propertyName, "Cannot access indexed value in property referenced " + "in indexed property path '" + propertyName + "': returned null"); } } //判断注入的是否为数组 if (propValue.getClass().isArray()) { PropertyHandler ph = getLocalPropertyHandler(actualName); Class
      requiredType = propValue.getClass().getComponentType(); int arrayIndex = Integer.parseInt(key); Object oldValue = null; try { if (isExtractOldValueForEditor() && arrayIndex < Array.getLength(propValue)) { oldValue = Array.get(propValue, arrayIndex); } Object convertedValue = convertIfNecessary(propertyName, oldValue, pv.getValue(), requiredType, ph.nested(tokens.keys.length)); int length = Array.getLength(propValue); if (arrayIndex >= length && arrayIndex < this.autoGrowCollectionLimit) { Class
      componentType = propValue.getClass().getComponentType(); Object newArray = Array.newInstance(componentType, arrayIndex + 1); System.arraycopy(propValue, 0, newArray, 0, length); setPropertyValue(actualName, newArray); propValue = getPropertyValue(actualName); } Array.set(propValue, arrayIndex, convertedValue); } catch (IndexOutOfBoundsException ex) { throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName, "Invalid array index in property path '" + propertyName + "'", ex); } } //是否为list else if (propValue instanceof List) { PropertyHandler ph = getPropertyHandler(actualName); Class
      requiredType = ph.getCollectionType(tokens.keys.length); List

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  到了这里，就完成了对Bean各种属性的依赖注入的过程，整个过程我们只要把握住 getBean 这个依赖注入的入口，一步步分析下去，就可以完成对整个依赖注入的把握。