Spring源码解析:Bean的实例化与依赖注入(四)

在第三部分，我们介绍了BeanDefinition在IOC容器中的注册，本节将介绍Bean的实例化与依赖注入。

Bean的实例化与依赖注入是在用户第一次向IOC容器索要Bean时触发的（例外情况暂不考虑）。每一个BeanFactory都会有一个getBean的方法，现在我们就来看看DefaultListableBeanFactory的getBean方法。

一:getBean方法(DefaultListableBeanFactory)

public Object getBean(String name) throws BeansException {
return getBean(name, null, null);
}

其实所有的getBean最终都会委托doGetBean来处理的，源码如下:

/**
* Return an instance, which may be shared or independent, of the specified bean.
* @param name the name of the bean to retrieve
* @param requiredType the required type of the bean to retrieve
* @param args arguments to use if creating a prototype using explicit arguments to a
* static factory method. It is invalid to use a non-null args value in any other case.
* @param typeCheckOnly whether the instance is obtained for a type check,
* not for actual use
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
protected Object doGetBean(
final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException {

final String beanName = transformedBeanName(name);
Object bean = null;

// Eagerly check singleton cache for manually registered singletons.
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 + "'");
}
}
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);
}

// Check if bean definition exists in this factory.
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 parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}

if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}

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 (int i = 0; i < dependsOn.length; i++) {
String dependsOnBean = dependsOn[i];
getBean(dependsOnBean);
registerDependentBean(dependsOnBean, beanName);
}
}

// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory() {
public 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);
}

else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}

else {
String scopeName = mbd.getScope();
final Scope scope = (Scope) this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory() {
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);
}
}
}

// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return bean;
}

doGetBean的基本思想是根据BeanDefinition的信息生成Bean，这个方法的思路如下：

（1）如果在单例池中已经存在这个Bean，直接返回

（2）如果当前的BeanFactory中不存在这个Bean的BeanDefinition，那么就依次到这个BeanFactory的父工厂上去取，这里使用递归，直到找到最顶层的BeanFactory.

（3）递归地生成依赖的Bean

（4）根据当前工厂中的BeanDefinition完成Bean的实例化和依赖注入。

根据源码，我们可以看到真正的生成Bean的动作是发生在createBean方法中的，createBean是在DefaultListableBeanFactory的父类AbstractAutowireCapableBeanFactory中实现的，下面我们来看一下createBean的源码.

二:createBean的源码如下

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

AccessControlContext acc = AccessController.getContext();
return AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
// Make sure bean class is actually resolved at this point.
resolveBeanClass(mbd, beanName);

// Prepare method overrides.
try {
mbd.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}

try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
Object bean = resolveBeforeInstantiation(beanName, mbd);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}

Object beanInstance = doCreateBean(beanName, mbd, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
}, acc);
}

createBean里最核心的操作是doCreateBean，我们跟进doCreateBean里来看看其中的操作:

/**
* Actually create the specified bean. Pre-creation processing has already happened
* at this point, e.g. checking <code>postProcessBeforeInstantiation</code> callbacks.
* <p>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 arguments to use if creating a prototype using explicit arguments to a
* static factory method. This parameter must be <code>null</code> except in this case.
* @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(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = (BeanWrapper) 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() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}

// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
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)) {
String[] dependentBeans = getDependentBeans(beanName);
Set actualDependentBeans = new LinkedHashSet(dependentBeans.length);
for (int i = 0; i < dependentBeans.length; i++) {
String dependentBean = dependentBeans[i];
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 " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}

// Register bean as disposable.
registerDisposableBeanIfNecessary(beanName, bean, mbd);

return exposedObject;
}

在这里最核心的操作有两个，一个是createBeanInstance,这个方法主要是用于完成Bean的实例化，可能采用工厂方法，也可能采用构造方法new等，具体的操作根据BeanDefinition中的配置信息。另一个是populateBean方法，这个方法主要完成了依赖注入的功能。

createBean方法源码解析:

/**
* 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 arguments to use if creating a prototype using explicit arguments to a
* static factory method. It is invalid to use a non-null args value in any other case.
* @return BeanWrapper for the new instance
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
* @see #instantiateBean
*/
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// Make sure bean class is actually resolved at this point.
Class beanClass = resolveBeanClass(mbd, beanName);

if (mbd.getFactoryMethodName() != null)  {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}

// Shortcut when re-creating the same bean...
if (mbd.resolvedConstructorOrFactoryMethod != null) {
if (mbd.constructorArgumentsResolved) {
return autowireConstructor(beanName, mbd, null, args);
}
else {
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);
}

在这里我们可以看到，根据BeanDefinition中的配置信息，生成Bean的实例的方式不同，有的采用工厂方法的方式，有的采用带参的构造方法的模式，有的采用无参的构造方法的模式。我们进入instantiateBean中来看看具体的代码。

/**
* Instantiate the given bean using its default constructor.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @return BeanWrapper for the new instance
*/
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
try {
Object beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}

这里使用配置的实例化策略来完成实例化工作，根据源码我们可以知道使用的CglibSubclassingInstantiationStrategy()来生成bean的代理。

下面，我们继续来看populateBean是如何完成Bean的依赖注入的

/**
* Populate the bean instance in the given BeanWrapper with the property values
* from the bean definition.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @param bw BeanWrapper with bean instance
*/
protected void populateBean(String beanName, AbstractBeanDefinition mbd, BeanWrapper bw) {
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;
}
}

// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
boolean continueWithPropertyPopulation = true;

if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (Iterator it = getBeanPostProcessors().iterator(); it.hasNext();) {
BeanPostProcessor beanProcessor = (BeanPostProcessor) it.next();
if (beanProcessor instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) beanProcessor;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}

if (!continueWithPropertyPopulation) {
return;
}

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);
if (hasInstAwareBpps) {
for (Iterator it = getBeanPostProcessors().iterator(); it.hasNext(); ) {
BeanPostProcessor beanProcessor = (BeanPostProcessor) it.next();
if (beanProcessor instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) beanProcessor;
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}

applyPropertyValues(beanName, mbd, bw, pvs);
}

我们进入applyPropertyValues来看看Spring是如何真正实现依赖注入的

/**
* Apply the given property values, resolving any runtime references
* to other beans in this bean factory. Must use deep copy, so we
* don't permanently modify this property.
* @param beanName the bean name passed for better exception information
* @param mbd the merged bean definition
* @param bw the BeanWrapper wrapping the target object
* @param pvs the new property values
*/
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs == null || pvs.isEmpty()) {
return;
}

MutablePropertyValues mpvs = null;
List original = null;

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;
}
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 (Iterator it = original.iterator(); it.hasNext();) {
PropertyValue pv = (PropertyValue) it.next();
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 (originalValue instanceof TypedStringValue && convertible) {
pv.setConvertedValue(convertedValue);
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
if (mpvs != null && !resolveNecessary) {
mpvs.setConverted();
}

// Set our (possibly massaged) deep copy.
try {
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}

从这里，我们可以看出真正对属性值进行解析处理注入的实际BeanDefinitionValueResolver的resolveValueIfNecessary方法.

/**
* Given a PropertyValue, return a value, resolving any references to other
* beans in the factory if necessary. The value could be:
* <li>A BeanDefinition, which leads to the creation of a corresponding
* new bean instance. Singleton flags and names of such "inner beans"
* are always ignored: Inner beans are anonymous prototypes.
* <li>A RuntimeBeanReference, which must be resolved.
* <li>A ManagedList. This is a special collection that may contain
* RuntimeBeanReferences or Collections that will need to be resolved.
* <li>A ManagedSet. May also contain RuntimeBeanReferences or
* Collections that will need to be resolved.
* <li>A ManagedMap. In this case the value may be a RuntimeBeanReference
* or Collection that will need to be resolved.
* <li>An ordinary object or <code>null</code>, in which case it's left alone.
* @param argName the name of the argument that the value is defined for
* @param value the value object to resolve
* @return the resolved object
*/
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 ref = ((RuntimeBeanNameReference) value).getBeanName();
if (!this.beanFactory.containsBean(ref)) {
throw new BeanDefinitionStoreException(
"Invalid bean name '" + ref + "' in bean reference for " + argName);
}
return ref;
}
else if (value instanceof BeanDefinitionHolder) {
// Resolve BeanDefinitionHolder: contains BeanDefinition with name and aliases.
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;
return resolveInnerBean(argName, "(inner bean)", bd);
}
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 (Iterator it = original.entrySet().iterator(); it.hasNext();) {
Map.Entry propEntry = (Map.Entry) it.next();
Object propKey = propEntry.getKey();
Object propValue = propEntry.getValue();
if (propKey instanceof TypedStringValue) {
propKey = ((TypedStringValue) propKey).getValue();
}
if (propValue instanceof TypedStringValue) {
propValue = ((TypedStringValue) propValue).getValue();
}
copy.put(propKey, propValue);
}
return copy;
}
else if (value instanceof TypedStringValue) {
// Convert value to target type here.
TypedStringValue typedStringValue = (TypedStringValue) value;
try {
Class resolvedTargetType = resolveTargetType(typedStringValue);
if (resolvedTargetType != null) {
return this.typeConverter.convertIfNecessary(typedStringValue.getValue(), resolvedTargetType);
}
else {
// No target type specified - no conversion necessary...
return typedStringValue.getValue();
}
}
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 {
// No need to resolve value...
return value;
}
}

在这里我们可以看到对set,map,list,properties等的解析，具体的动作请进入相关的方法来分析查看，这里不做详述了。